Efficient nonlinear beam shaping in three-dimensional lithium niobate nonlinear photonic crystals

On-axis structured beams generation via moiré and Mie resonant metallo-dielectric moiré gratings

Structured beams carrying orbital angular momentum have been generated conventionally using spiral phase plates, fork gratings, and metasurfaces. Spiral phase plates are non-planar, fork gratings do not produce structured beams on the axis, and metasurfaces need subwavelength unit cell level design. In this work, we show a method to generate on-axis structured beams, at the zeroth order of a diffraction grating with experimentally relevant efficiency using moiré patterned binary gratings that are compatible with planar fabrication, do not need subwavelength unit cell level design, and can be used with a spatial light modulator. By logically superposing two binary forked gratings, we create a moiré pattern that enables on-axis structured beam generation at the zeroth order of the diffraction grating. We demonstrate, using experiments and simulations, the generation of on-axis zeroth order structured beams using spatial light modulator based reconfigurable moiré gratings and Mie resonant metallo-dielectric standalone moiré gratings, showcasing the versatility of this approach in different configurations. Simulations and experiments demonstrate that the on-axis structured beam is generated by the moiré pattern within the gratings, and its shape is determined by the topological charges of the overlapping binary forked gratings. Additionally, we demonstrate color-selective on-axis structured beam generation at the zeroth order of the grating, where the color-selectivity of the on-axis structured beam depends on the grating period and arises due to Mie resonance in standalone nanofabricated metallo-dielectric moiré gratings. The on-axis structured beam generation at the zeroth order of the grating using the proposed method may have several applications, including sensing and optical trapping.

Advancing inorganic electro-optical materials for 5 G communications: from fundamental mechanisms to future perspectives

In the 5 G era, the demand for high-capacity and fast fiber-optic communication underscores the importance of inorganic optical materials with high electro-optical (EO) coefficients, rapid responses, and stability for efficient electro-optical modulators. The exploration of novel EO materials and their applications remains in the early stages. At present, research mainly focuses on the performance of EO materials and devices. However, the EO coefficients of different preparation methods for the same material and different materials vary significantly. Currently, a crucial gap lies in understanding the link between the EO effect and ferroelectric polarization, hindering advancements in ferroelectric material optimization. This article offers a comprehensive insight into the EO effect, initially discussing ferroelectric polarization and its relationship to the phenomenon. It then reviews standard inorganic ABO3 metal oxide ferroelectric ceramics and thin films, followed by an examination of emerging ferroelectrics such as HfO2-based polymorph ferroelectrics and ZnO/AlN-based materials. The article concludes by addressing the challenges in investigating ferroelectric EO mechanisms and provides an outlook on the future of EO material research, including a review of the latest developments in EO effect mechanisms and their optimization for light modulation, as well as an exploration of potential areas for high-performance EO materials research.

Na-Hybridized LiNbO3 Nanocrystal-Glass Composites for Ultra-Short Optical Pulse Detection

LiNbO3 nanocrystal-glass composites (LiNbO3-NGC), characterized by its unique 3D random domain structure, have shown great promise for significant applications, such as femtosecond pulse monitoring and full-color 3D displays. However, the nonlinear response of the LiNbO3-NGC is greatly suppressed by the defects, and effective manipulation of these defects remains a long-standing challenge. In this study, a Na-hybridization strategy is proposed to control defects in the LiNbO3-NGC to enhance its nonlinear properties and realizing its practical application for ultrashort optical pulse monitoring. The findings reveal that the incorporation of Na ions effectively reduces the defects within the composite, resulting in significantly improved nonlinear effects. By using this hybridized LiNbO3-NGC, the transverse second-harmonic generation is achieved. An ultrashort optical pulse system is also constructed and successfully applied it for real-time quantitative measurement of the duration, distribution, and front tilting of optical pulses in the 10-15 s scale. These results not only present an excellent example about defect engineering in nonlinear LiNbO3-NGC but also point to practical applications for the measurement of extreme physical parameters.

Vortex solitons in quasi-phase-matched photonic crystals with competing quadratic and cubic nonlinearity

We introduce a methodology for investigating vortex solitons (VSs) within quasi-phase-matched (QPM) photonic crystals, featuring competing quadratic and cubic nonlinearities. The photonic crystal is introduced with a checkerboard structure, which is feasible through contemporary technological advancements. The VS family is constructed as quadrupole and 8-pole configurations, with the quadrupole solitons displaying rhombus and square modes depending on different phase-matching conditions. Notably, an intriguing transformation from the quadrupole to the 8-pole configuration can be achieved by modulating the cubic nonlinear coefficient. Additionally, the square-shaped VSs would transfer to rhombus patterns by adjusting the power and size of the checkerboard cell in the framework of cubic nonlinearity. This work presents a versatile and powerful tool for exploring and manipulating vortex solitons in QPM photonic structures, with potential applications in optical signal processing, optical communications, and nonlinear optics research.

Electro-optically Modulated Nonlinear Metasurfaces

Electrically reconfigurable nonlinear metasurfaces provide dynamic control over nonlinear phenomena such as second-harmonic generation (SHG), unlocking novel applications in signal processing, light switching, and sensing. Previous methods, like electric-field-induced SHG in plasmonic metasurfaces and Stark-tuned nonlinearities in quantum well metasurfaces, face limitations due to weak SHG responses from metals and mid-infrared constraints of quantum wells, respectively. Addressing the need for efficient SHG control in the visible and near-infrared ranges, we present a novel approach using the electro-optic (EO) effect to modulate SHG. By leveraging the exceptional EO and SHG properties of lithium niobate (LN), we integrate the EO effect with SHG within a metasurface framework for the first time. Our LN metasurface achieves an 11.3% modulation depth in SHG amplitude under a ±50 V alternating voltage. These results open new avenues for reconfigurable photonic applications. including tunable nonlinear light sources, quantum optics, and nonlinear information processing.

Ultra-smooth processing of lithium niobate for outstanding mid-infrared transmittance

The fabrication of anti-reflection (AR) subwavelength structures (SWSs) of lithium niobate (LN) is a challenging but rewarding task in mid-infrared LN laser systems. However, there are still some issues with the high-quality processing and fabrication of bifacial AR SWSs. Herein, a novel, to the best of our knowledge, approach to the fabrication of SWSs was proposed, which includes femtosecond laser ablation followed by wet etching and thermal annealing. The fabricated structures exhibit high surface quality (Ra = 0.08 nm) and uniformity. According to the experimental and simulated results, the transmittance of the mid-infrared AR SWSs with a period of 1.8 µm could be improved from 78% to 87% in the 3.6-5 µm band. Furthermore, the double-sided construction enabled a transmittance of up to 90%. The results have great potential in the promotion of the development of mid-infrared laser systems and LN-based photonics.

Helically twisted nonlinear photonic crystals

Nonlinear photonic crystals with a helical structure in the second-order nonlinear coefficient (χ(2)) are fabricated using infrared femtosecond laser poling in ferroelectric Sr0.61Ba0.39Nb2O6 crystals. The quasi-orbital angular momentum of the helical χ(2) structure can be imprinted on the interacting photons during nonlinear optical processes, allowing the topological charge of the generated photons at new frequencies to be controlled. Here we study the case of a double-helix nonlinear photonic structure for the generation of a second-harmonic vortex beam from a Gaussian pump beam without phase singularity. The conservation law for orbital angular momentum in the second-harmonic process is also verified, with the topological charge of the pump photons being fully compensated by the double-helix structure. The flexible control of light carrying orbital angular momentum (OAM) at new frequencies will find important applications in both classical and quantum photonics, such as nonlinear wavefront shaping and multidimensional entanglement of photons.

Efficient generation of octave-separating orbital angular momentum beams via forked grating array in lithium niobite crystal

The concept of orbital angular momentum (OAM) of light has not only advanced fundamental physics research but also yielded a plethora of practical applications, benefitting from the abundant methods for OAM generation based on linear, nonlinear and combined schemes. The combined scheme could generate octave-separating OAM beams, potentially increasing the channels for optical communication and data storage. However, this scheme faces a challenge in achieving high conversion efficiency. In this work, we have demonstrated the generation of multiple OAM beams at both fundamental frequency and second harmonic (SH) wavelengths using a three-dimensional forked grating array with both spatial χ (1) and χ (2) distributions in a lithium niobate nonlinear photonic crystal platform. The enhancements of the fundamental and SH OAM beams have been achieved by employing linear Bragg diffraction and nonlinear Bragg diffraction, respectively, i.e., quasi-phase matching. The experimental results show that OAM beams with variable topological charges can be enhanced at different diffraction orders via wavelength or angle tuning, achieving conversion efficiencies of 60.45 % for the linear OAM beams and 1.08 × 10-4 W -1 for the nonlinear ones. This work provides a promising approach for parallel detection of OAM states in optical communications, and extends beyond OAM towards the control of structured light via cascaded linear and nonlinear processes.

Lithium niobate on insulator - fundamental opto-electronic properties and photonic device prospects

Lithium niobate on insulator (LNOI) combines a variety of optoelectronic properties and can meet practical performance requirements that are uncommon in optoelectronic materials. This review introduces the fundamentals and the photonic device concepts that arise from the LNOI materials platform. Firstly, the nonlinear optical response of LNOI is presented, including birefringent phase matching (BPM), modal phase matching (MPM), and quasi-phase matching (QPM). The tunable properties are also introduced, including electro-optical (EO), thermo-optical (TO), and acousto-optical (AO) effects. The structures of nonlinear optical devices, such as ridge waveguides (including periodically polarized inversion waveguides), Mach-Zehnder interferometer (MZI) modulators and micro-resonators (such as disks and rings) are demonstrated. Finally, the future of LNOI devices is discussed. In the already mature and developed optoelectronic material systems, it is rare to find one particular material system supporting so many basic optical components, photonic devices and optoelectronic devices as LNOI does in the field of integrated photonic chips.

Nonlocal erasing and writing of ferroelectric domains using a femtosecond laser in lithium niobate

We experimentally demonstrate the highly-efficient nonlocal erasing and writing of ferroelectric domains using a femtosecond laser in lithium niobate. Based on the induction of a focused infrared femtosecond laser without any relative displacement or additional treatment, the original multiple ferroelectric domains can be either erased (erasing operation) or elongated (writing operation) simultaneously in the crystal, depending on the laser focusing depth and the laser pulse energy. In the erasing operation, the original multiple ferroelectric domains can be cleared completely by just one laser induction, while in the writing operation, the average length of the ferroelectric domains can be elongated up to 235 µm by three laser inductions. A model has been proposed in which a thermoelectric field and a space charge field are used cooperatively to successfully explain the mechanism of nonlocal erasing and writing. This method greatly improves the efficiency and flexibility of tailoring ferroelectric domain structures, paving the way to large-scale all-optical industrial production for nonlinear photonic crystals and nonvolatile ferroelectric domain wall memories.

Second harmonic generation of visible vortex laser based on a waveguide-grating emitter in LBO

In this work, we propose a practical solution to visible vortex laser emission at 532 nm based on second harmonic generation (SHG) in a well-designed waveguide-grating structure. Such an integrated structure is fabricated by femtosecond laser direct writing (FsLDW) in an LBO crystal. Confocal micro-Raman spectroscopy is employed for detailed analysis of FsLDW-induced localized crystalline damage. By optical excitation at 1064 nm, the guiding properties, SHG performance, as well as vortex laser generation of the waveguide-grating hybrid structure are systematically studied. Our results indicate that FsLDW waveguide-grating emitter is a reliable design holding great promise for nonlinear vortex beam generation in integrated optics.

Poling-assisted hydrofluoric acid wet etching of thin-film lithium niobate

Thin-film lithium niobate (TFLN) has been extensively investigated for a wide range of applications due to continuous advancements in its fabrication methods. The recent emergence of high-fidelity ferroelectric domain poling of TFLN provides an opportunity for achieving a precise pattern control of ferroelectric domains and a subsequent pattern transfer to the TFLN layer using hydrofluoric acid (HF). In this work, we present, to the best of our knowledge, the first demonstration of z-cut TFLN microdisks using a poling-assisted HF wet etching approach. By applying intense electric fields, we are able to induce a domain inversion in the TFLN with a designed microdisk pattern. A HF solution is subsequently utilized to transfer the inverted domain pattern to the TFLN layer with the selective etching of -z LN, ultimately revealing the microdisks.

Second-harmonic flat-top beam shaping via a three-dimensional nonlinear photonic crystal

We experimentally extend the nonlinear Gaussian to flat-top beam shaping from one to two dimensions through a three-dimensional nonlinear photonic crystal. Employing a near-infrared femtosecond laser, we induce a modification inside lithium niobate to achieve a second-order nonlinear optical coefficient modulation in three dimensions. The flat-topped truncation of wavefront has been adjusted in a mutual perpendicular coordinate separately. Among the generated flat-topped beams, the optimal flatness is 97.1%, and the nonlinear conversion efficiency is 10-2 at the peak power of 37 kW with the interaction length of 630 µm. By adding an extra dimension, our work simultaneously enables full-wavefront flat-top distribution and nonlinear frequency conversion.

Arbitrary fabrication of complex lithium niobate three-dimensional microstructures for second harmonic generation enhancement

Lithium niobate (LN) crystal plays important roles in future integrated photonics, but it is still a great challenge to efficiently fabricate three-dimensional micro-/nanostructures on it. Here, a femtosecond laser direct writing-assisted liquid back-etching technology (FsLDW-LBE) is proposed to achieve the three-dimensional (3D) microfabrication of lithium niobate (LN) with high surface quality (Ra = 0.422 nm). Various 3D structures, such as snowflakes, graphic arrays, criss-cross arrays, and helix arrays, have been successfully fabricated on the surface of LN crystals. As an example, a microcone array was fabricated on LN crystals, which showed a strong second harmonic signal enhancement with up to 12 times bigger than the flat lithium niobate. The results indicate that the method provides a new approach for the microfabrication of lithium niobate crystals for nonlinear optics.

Large Field-of-View Nonlinear Holography in Lithium Niobate

A nonlinear holographic technique is capable of processing optical information in the newly generated optical frequencies, enabling fascinating functions in laser display, security storage, and image recognition. One popular nonlinear hologram is based on a periodically poled lithium niobate (LN) crystal. However, due to the limitations of traditional fabrication techniques, the pixel size of the LN hologram is typically several micrometers, resulting in a limited field-of-voew (FOV) of several degrees. Here, we experimentally demonstrate an ultra-high-resolution LN hologram by using the laser poling technique. The minimal pixel size reaches 200 nm, and the FOV is extended above 120° in our experiments. The image distortions at large view angles are effectively suppressed through the Fourier transform. The FOV is further improved by combining multiple diffraction orders of SH fields. The ultimate FOV under our configuration is decided by a Fresnel transmission. Our results pave the way for expanding the applications of nonlinear holography to wide-view imaging and display.

Semidiscrete optical vortex droplets in quasi-phase-matched photonic crystals

What we believe is a new scheme for producing semidiscrete self-trapped vortices ("swirling photon droplets") in photonic crystals with competing quadratic (χ(2)) and self-defocusing cubic (χ(3)) nonlinearities is proposed. The photonic crystal is designed with a striped structure, in the form of spatially periodic modulation of the χ(2) susceptibility, which is imposed by the quasi-phase-matching technique. Unlike previous realizations of semidiscrete optical modes in composite media, built as combinations of continuous and arrayed discrete waveguides, the semidiscrete vortex "droplets" are produced here in the fully continuous medium. This work reveals that the system supports two types of semidiscrete vortex droplets, viz., onsite- and intersite-centered ones, which feature, respectively, odd and even numbers of stripes, N. Stability areas for the states with different values of N are identified in the system's parameter space. Some stability areas overlap with each other, giving rise to the multistability of states with different N. The coexisting states are mutually degenerate, featuring equal values of the Hamiltonian and propagation constant. An experimental scheme to realize the droplets is outlined, suggesting new possibilities for the long-distance transmission of nontrivial vortex beams in nonlinear media.

3D Imprinting of Voxel-Level Structural Colors in Lithium Niobate Crystal

Advanced coloration methods are of pivotal importance in science, technology, and engineering. However, 3D structural colors that are critical for emerging multidimensional information representation and recording are rarely achievable. Here, a facile voxel-level programmable 3D structural coloration in the bulk lithium niobate (LiNbO3 ) crystal is reported. This is achieved by engineering wavelength-selective interference between ordinary (O) and extraordinary (E) light in the crystal matrix. To induce effective phase contrast between O and E light for establishing the highly localized interference across the visible band, the presence of a pulse-internal-coupling effect is revealed in the single-pulse ultrafast laser-crystal interaction and an ultrafast-laser-induced micro-amorphization (MA) strategy is thus developed to manipulate local matrix structure. Consequently, micro-nanoscale colorful voxels can be fast inscribed into any spatial position of the crystal matrix in one step. It is demonstrated that the colors can be flexibly manipulated and quickly extracted in 3D space. Multidimensional MA-color data storage with large capacity, high writing and readout speed, long lifetime, and excellent stability under harsh conditions is achieved. The present principle enables multifunctional 3D structural coloration devices inside high-refractive-index transparent dielectrics and can serve as a general platform to innovate next-generation information optics.

Nonlinear generation of an optical bottle beam in domain-engineered ferroelectric crystals

Nonlinear wavefront shaping in periodically poled ferroelectric crystals has received great attention because it offers a convenient way to generate a structured light beam at new frequencies. In contrast to structurally uniform beams like Laguerre-Gaussian or Hermite-Gaussian modes, here we demonstrate the possibility to generate a spatially varied optical bottle beam via a frequency doubling process in a domain-engineered Sr0.61Ba0.39Nb2O6 (SBN) crystal. The nonlinear holography method was employed to design the modulation pattern of the second-order nonlinear coefficient χ(2), and the femtosecond laser poling was used to imprint the χ(2) pattern into the SBN crystal via ferroelectric domain inversion. The second harmonic bottle beam with zero intensity in its center that is surrounded in all three dimensions by light was observed with the incidence of a fundamental Gaussian beam. These results are useful for nonlinear generation and control of structured light at new frequencies, which has important applications in nonlinear photonics and quantum optics.

Femtosecond-laser-assisted high-aspect-ratio nanolithography in lithium niobate

We report the successful fabrication of high-aspect-ratio lithium niobate (LN) nanostructures by using femtosecond-laser-assisted chemical etching. In this technique, a 1 kHz femtosecond laser is first used to induce local modifications inside the LN crystal. Then, selective chemical wet etching is conducted using a buffered oxide etch (BOE) solution. The etching rate in the laser-modified area reaches 2 μm h-1, which is enhanced by a factor of ∼660 in comparison to previous reports without laser irradiation. Such high selectivity in chemical etching helps realize high-performance maskless nanolithography in lithium niobate. In the experiment, we have fabricated high-quality LN nanohole arrays. The nanohole size reaches ∼100 nm and its aspect ratio is above 40 : 1. The minimal period of the LN hole array is 300 nm. Our work paves a way to fabricate LN nano-integrated devices for advanced optic and electronic applications.

Laser nanoprinting of 3D nonlinear holograms beyond 25000 pixels-per-inch for inter-wavelength-band information processing

Nonlinear optics provides a means to bridge between different electromagnetic frequencies, enabling communication between visible, infrared, and terahertz bands through χ(2) and higher-order nonlinear optical processes. However, precisely modulating nonlinear optical waves in 3D space remains a significant challenge, severely limiting the ability to directly manipulate optical information across different wavelength bands. Here, we propose and experimentally demonstrate a three-dimensional (3D) χ(2)-super-pixel hologram with nanometer resolution in lithium niobate crystals, capable of performing advanced processing tasks. In our design, each pixel consists of properly arranged nanodomain structures capable of completely and dynamically manipulating the complex-amplitude of nonlinear waves. Fabricated by femtosecond laser writing, the nonlinear hologram features a pixel diameter of 500 nm and a pixel density of approximately 25000 pixels-per-inch (PPI), reaching far beyond the state of the art. In our experiments, we successfully demonstrate the novel functions of the hologram to process near-infrared (NIR) information at visible wavelengths, including dynamic 3D nonlinear holographic imaging and frequency-up-converted image recognition. Our scheme provides a promising nano-optic platform for high-capacity optical storage and multi-functional information processing across different wavelength ranges.

Influences of focusing conditions on optical poling in lithium niobate using a 1035 nm femtosecond fiber laser

In this study, we demonstrate the optical induction of ferroelectric domain inversion on the -z surface of lithium niobate crystals using a 1035 nm femtosecond fiber laser. We investigate the effects of laser power and focusing parameters on domain morphology. The results revealed that domains exhibit an irregular random distribution at low power, whereas regularly arranged domains were formed near the crystal damage threshold. Furthermore, with sufficiently high power, lens focusing enables the fabrication of uniformly sized domains exceeding 100 µm. The results suggest potential applications for controlling domain morphology and the duty cycle, and fabricating ferroelectric functional devices.

Asymmetric wavefront shaping with nonreciprocal 3D nonlinear detour phase hologram

Asymmetric control of light with nonlinear material is of great significance in the design of novel micro-photonic components, such as asymmetric imaging devices and nonreciprocal directional optical filters. However, the use of nonlinear photonic crystals for asymmetric optical transmission, to the best of our knowledge, is still an untouched area of research. Herein we propose the 3D nonlinear detour phase holography for realizing asymmetric SH wavefront shaping by taking advantage of the dependence of the SH phase on the propagation direction of the excitation beam. With the proposed method, the designed nonreciprocal 3D nonlinear detour phase hologram yields SH phases with opposite signs for the forward and backward transmission situations. Moreover, the quasi-phase-matching scheme and orbital angular momentum conservation in the asymmetric SH wavefront shaping process are also discussed. This study conceptually extends the 2D nonlinear detour phase holography into 3D space to build the nonreciprocal 3D nonlinear detour phase hologram for achieving SH twin-image elimination and asymmetric SH wavefront shaping, offering new possibilities for the design of nonreciprocal optical devices.

Nanoscale multi-beam lithography of photonic crystals with ultrafast laser

Photonic crystals are utilized in many noteworthy applications like optical communications, light flow control, and quantum optics. Photonic crystal with nanoscale structure is important for the manipulation of light propagation in visible and near-infrared range. Herein, we propose a novel multi beam lithography method to fabricate photonic crystal with nanoscale structure without cracking. Using multi-beam ultrafast laser processing and etching, parallel channels with subwavelength gap are obtained in yttrium aluminum garnet crystal. Combining optical simulation based on Debye diffraction, we experimentally show the gap width of parallel channels can be controlled at nanoscale by changing phase holograms. With the superimposed phase hologram designing, functional structures of complicated channel arrays distribution can be created in crystal. Optical gratings of different periods are fabricated, which can diffract incident light in particular ways. This approach can efficiently manufacture nanostructures with controllable gap, and offer an alternative to the fabrication of complex photonic crystal for integrated photonics applications.

Tunable phase-mismatched mid-infrared difference-frequency generation between 6 and 17 µm in CdTe

In parametric conversion, phase-matching techniques such as birefringence and quasi phase-matching (PM) with the designed crystal angle or periodically poled polarities are employed to fulfill the requirement of momentum conservation. However, directly using phase-mismatched interactions in nonlinear media with large quadratic nonlinear coefficient remains unheeded. Here, for the first time to the best of our knowledge, we study the phase-mismatched difference-frequency generation (DFG) in an isotropic cadmium telluride (CdTe) crystal, with the comparison of other DFG processes based on birefringence-PM, quasi-PM, and random-quasi-PM. Long-wavelength mid-infrared (LWMIR) phase-mismatched DFG with an ultra-broadband spectral tuning range of 6-17 µm based on CdTe is demonstrated. Thanks to the giant quadratic nonlinear coefficient (∼109 pm/V) and good figure of merit in the parametric process, the output power up to 100 µW is obtained, which is comparable to or even better than the DFG output from a polycrystalline ZnSe with the same thickness facilitated by random-quasi-PM. A proof-of-concept demonstration in gas sensing of CH₄ and SF₆ is conducted based on the phase-mismatched DFG as a typical application. Our results demonstrate the feasibility of phase-mismatched parametric conversion in producing useful LWMIR power and ultra-broadband tunability in a simple and convenient way without the necessity of controlling the polarization, phase-matching angle, or pole periods, which could find applications in the fields of spectroscopy and metrology.

Direct generation of spatially entangled qudits using quantum nonlinear optical holography

Nonlinear holography shapes the amplitude and phase of generated new harmonics using nonlinear processes. Classical nonlinear holography influenced many fields in optics, from information storage, demultiplexing of spatial information, and all-optical control of accelerating beams. Here, we extend the concept of nonlinear holography to the quantum regime. We directly shape the spatial quantum correlations of entangled photon pairs in two-dimensional patterned nonlinear photonic crystals using spontaneous parametric down conversion, without any pump shaping. The generated signal-idler pair obeys a parity conservation law that is governed by the nonlinear crystal. Furthermore, the quantum states exhibit quantum correlations and violate the Clauser-Horne-Shimony-Holt inequality, thus enabling entanglement-based quantum key distribution. Our demonstration paves the way for controllable on-chip quantum optics schemes using the high-dimensional spatial degree of freedom.

High-Power LiNbO3 Domain-Wall Nanodevices

Wide band gap semiconductors keep on pushing the limits of power electronic devices to higher switching speeds and higher operating temperatures, including diodes and transistors on low-cost Si substrates. Alternatively, erasable conducting walls created within ferroelectric single-crystal films integrated on the Si platform have emerged as a promising gateway to adaptive nanoelectronics in sufficient output power, where the repetitive creation of highly charged domain walls (DWs) is particularly important to increase the wall current density. Here, we observe large conduction of the head-to-head DW at an optimized inclination angle of 15° within a LiNbO₃ single crystal that is 3-4 orders of magnitude higher than that of the tail-to-tail DW. The wall conduction is diode-like with a linear current density of higher than 1 mA/μm and an on/off ratio of larger than 10⁶ under the application of a repetitive switching voltage pulse in time less than 10 ns and an endurance number of higher than 10⁵. The high-power diodes can not only perform direct data processing in high-density nonvolatile DW memories in fast operation speeds and low-energy consumption but also function as sensors in compact electromechanical systems, selectors in phase-change memory and resistive random-access memory, and half-wave/full-wave rectifiers in modern nanocircuits in dimensions approaching the thickness of the depletion layer below which the tradition p-n junction malfunctions.

Ferroelectric domain engineering with femtosecond pulses of different wavelengths

Direct femtosecond laser writing of ferroelectric domain structures has been an indispensable technique for engineering the second-order optical nonlinearity of materials in three dimensions. It utilizes localized thermoelectric field motivated by nonlinear absorption at the position of laser focus to manipulate domains. However, the impact of laser wavelengths, which is pivotal in nonlinear absorption, on the inverted domains is still sketchy. Herein, the light-induced ferroelectric domain inversion is experimentally studied. It is shown that the domain inversions can be achieved over a broad spectral range, but the optical threshold for domain inversion varies dramatically with the laser wavelength, which can be explained by considering the physical mechanism of femtosecond laser poling and nonlinear absorption properties of the crystal. Meanwhile, the effects of other laser processing parameters are also experimentally investigated. Our findings are useful to guide the fabrication of high-performance optical and electronic devices based on ferroelectric domains.

Thermally assisted fabrication of nonlinear photonic structures in lithium niobate with femtosecond laser pulses

Engineered domain structures play an essential role in nonlinear optics for quasi-phase-matched parametric processes. Pyroelectric field-assisted domain inversion with focused femtosecond laser pulses is a promising approach to create arbitrary two-dimensional nonlinear photonic structures in a large volume without externally applied electrical fields. We fabricate lattices of ferroelectric domains by patterning lithium niobate crystals with femtosecond laser pulses and then heating them to elevated temperatures. After cooling to room temperature, domains form below and above the laser-induced seeds. We investigate the effect of temperature and seed spacing on the number and size of inverted domains. In a temperature range of 220 °C-300 °C all domains are inverted in a two-dimensional lattice with periods of 15 µm × 6.3 µm. Smaller lattice periods result in a smaller fraction of inverted domains. Measurements with conducting, nonconducting, and short-circuited crystal surfaces reveal the influence of surface charges during the domain formation process. From the obtained domain widths and spacings, we calculate the effective nonlinear coefficient of quasi-phase-matched second-harmonic generation in two-dimensional nonlinear photonic structures.

Effect of spatial variation of the duty cycle in transverse second-harmonic generation

Transverse second-harmonic generation, in which the emission angles of the second harmonic are determined by the spatial modulation of the quadratic nonlinearity, has important applications in nonlinear optical imaging, holography, and beam shaping. Here we study the role of the local duty cycle of the nonlinearity on the light intensity distribution in transverse second-harmonic generation, taking the generation of perfect vortices in periodically poled ferroelectric crystal as an example. We show, theoretically and experimentally, that spatial variations of the nonlinearity modulation must be accompanied by the corresponding changes of the width of inverted ferroelectric domains, to ensure uniformity of the light intensity distribution in the generated second harmonic. This work provides a fundamental way to achieve high-quality transverse second-harmonic generation and, hence, opens more possibilities in applications based on harmonic generation and its control.

Ferroelectric Domain Reversal Dynamics in LiNbO3 Optical Superlattice Investigated with a Real-Time Monitoring System

The optical superlattice structure derived from a periodic poling process endows ferroelectric crystals with tunable optical property regulation, which has become one of the most efficient strategies for fabricating high-efficiency optical devices. Achieving a precise superlattice structure has been the main barrier for preparation of specific optical applications due to the unclear dynamics of domain structure regulation. Herein, a real-time monitoring system for the in situ observation of periodic poling of lithium niobate is established to investigate ferroelectric domain reversal dynamics. The formation of reversed domain nuclei, growth, and expansion of the domain are monitored, which is highly related to domain growth dynamics. The nucleation and growth of domain are discussed combined with the monition of domain reversal and the variation of local electric field distribution along with finite element analysis. An electrode configuration with multiholes is proposed to use the local electric field more efficiently and controllably, which could achieve a higher domain nucleus density with high uniformity. Two-mm-thick periodically poled LiNbO3 crystals with high quality are achieved. A nonlinear light conversion from 1064.2 to 3402.4 nm is realized by the single-resonance optical parameter oscillator with a nonlinear optical efficiency up to 26.2%.

Manipulating Orbital Angular Momentum Entanglement in Three-Dimensional Spiral Nonlinear Photonic Crystals

We propose and theoretically investigate two-photon orbital angular momentum (OAM) correlation through spontaneous parameter down-conversion (SPDC) processes in three-dimensional (3D) spiral nonlinear photonic crystals (NPCs). By properly designing the NPC structure, one can feasibly modulate the OAM-correlated photon pair, which provides a potential platform to realize high-dimensional entanglement for quantum information processing and quantum communications.

Femtosecond laser writing of waveguides in zinc oxide crystals: fabrication and mode modulation

We report for the first time on optical waveguides in zinc oxide (ZnO) crystals fabricated by femtosecond laser direct writing. The confocal Raman microscopy under 488 nm laser excitation is used to investigate the micro-modifications of the laser irradiation, and guiding properties are studied via the end-face coupling at 632.8 nm. The mode modulation has been achieved by the adjustment of laser writing parameters. A minimum propagation loss of ∼6 dB/cm is obtained for the double-line waveguide structures. A Y-branch waveguide beam splitter is also fabricated, reaching a splitting ratio of nearly 1:1. The original optical properties in the guiding region have been well preserved, according to the confocal Raman investigation, which suggests potential applications of the ZnO waveguides for integrated photonics and nonlinear optics.

Observation of topological Anderson phase in laser-written quasi-periodic waveguide arrays

We report on the experimental observation of the topological Anderson phase in one-dimensional quasi-periodical waveguide arrays produced by femtosecond laser writing. The evanescently coupled waveguides are with alternating coupling constants, constructing photonic lattices analogous to the Su-Schrieffer-Heeger model. Dynamic tuning of the interdimer hopping amplitudes of the waveguide array generates the quasi-periodic disorder of the coupling constants for the model. As light propagates in the corresponding photonic waveguides, it exhibits different modes depending on the magnitude of the disorder. The topological Anderson phase is observed as the disorder is sufficiently strong, which corresponds to the zero-energy mode in its spectrum. The experimental results are consistent with the theoretical simulations, confirming the existence of the disorder-driven topological phase from a trivial band in the photonic lattice.

Manipulating the radial components of LG pump beam for ultrahigh-dimensional maximally entangled orbital angular momentum states

High-dimensional maximally entangled orbital angular momentum (OAM) states are a promising resource for enhancing information capacity and robustness in quantum communication. However, it still lacks an effective method to increase the state dimensionality. Here, we theoretically propose an efficient scheme to generate maximally entangled OAM states of ultrahigh dimensionality by manipulating the radial components of a Laguerre-Gaussian (LG) pump beam. By optimizing the complex amplitudes of multiple radial modes of the LG pump light, one can feasibly achieve 101-dimensional OAM-based maximally entangled states. Our scheme has potential applications in high capacity quantum communication networks.

On-Chip Optical Beam Manipulation with an Electrically Tunable Lithium-Niobate-on-Insulator Metasurface

Photonic integrated circuits (PICs) have garnered increasing attention because of their high efficiency in information processing. Recently, lithium niobate on insulator (LNOI) has become a new platform for PICs with excellent properties. Several tunable devices such as on-chip tunable devices that utilize the electric-optic effect of LN have been reported. However, an on-chip electrically tunable beam modulator that can focus or deflect the wave has not yet been developed. In this study, we designed an electrically tunable LNOI metasurface for on-chip optical beam manipulation. With a carefully designed local phase profile, we realized the tunable focusing and reflection functions on the chip. As the bias voltage varies, the focusing length can be shifted up to 19.9 μm (~13λ), whereas the focusing efficiency remains greater than 72%. A continuously tunable deflection can also be achieved efficiently within a range of 0–45°. The beam modulator enhances the ability to manipulate light on LNOI chips, which is expected to promote the development of integrated on-chip photonics.

Nonlinear wavefront engineering with metasurface decorated quartz crystal

In linear optical processes, compact and effective wavefront shaping techniques have been developed with the artificially engineered materials and devices in the past decades. Recently, wavefront shaping of light at newly generated frequencies was also demonstrated using nonlinear photonic crystals and metasurfaces. However, the nonlinear wave-shaping devices with both high nonlinear optical efficiency and high wave shaping efficiency are difficult to realize. To circumvent this constraint, we propose the idea of metasurface decorated optical crystal to take the best aspects of both traditional nonlinear crystals and photonic metasurfaces. In the proof-of-concept experiment, we show that a silicon nitride metasurface decorated quartz crystal can be used for the wavefront shaping of the second harmonic waves generated in quartz. With this crystal-metasurface hybrid platform, the nonlinear vortex beam generation and nonlinear holography were successfully demonstrated. The proposed methodology may have important applications in nonlinear structured light generation, super-resolution imaging, and optical information processing, etc.

Storing and retrieving multiple images in 3D nonlinear photonic crystals

A nonlinear hologram enables to record the amplitude and phase of a waveform by spatially modulating the second order nonlinear coefficient, so that when a pump laser illuminates it, this waveform is reconstructed at the second harmonic frequency. The concept was now extended to enable the generation of multiple waveforms from a single hologram, with potential applications in high density storage, quantum optics, and optical microscopy.

Quasi-phase-matching-division multiplexing holography in a three-dimensional nonlinear photonic crystal

Nonlinear holography has recently emerged as a novel tool to reconstruct the encoded information at a new wavelength, which has important applications in optical display and optical encryption. However, this scheme still struggles with low conversion efficiency and ineffective multiplexing. In this work, we demonstrate a quasi-phase-matching (QPM) -division multiplexing holography in a three-dimensional (3D) nonlinear photonic crystal (NPC). 3D NPC works as a nonlinear hologram, in which multiple images are distributed into different Ewald spheres in reciprocal space. The reciprocal vectors locating in a given Ewald sphere are capable of fulfilling the complete QPM conditions for the high-efficiency reconstruction of the target image at the second-harmonic (SH) wave. One can easily switch the reconstructed SH images by changing the QPM condition. The multiplexing capacity is scalable with the period number of 3D NPC. Our work provides a promising strategy to achieve highly efficient nonlinear multiplexing holography for high-security and high-density storage of optical information.

Manipulating Nonlinear Optical Response via Domain Control in Nanocrystal-in-Glass Composites

Nanocrystal-in-glass (NIG) is an exciting class of composites, because it can not only combine the advantages of crystal and glass materials but also potentially generate new physical phenomenon in a cooperative manner. Herein, the nonlinear light-matter interaction processes in a broad range of NIG composites homogeneously embedded with LiNbO₃ are investigated. It is shown that, by rational control of the organization manner of crystal and glass phases, second-harmonic generation (SHG) can be precisely tuned. Importantly, an unusual SHG phenomenon, transverse SHG (TSHG), can be realized in the special region of the microstructure map combined with the features of high loading, nanoscale size, and homogenous distribution of nanocrystals. Furthermore, NIG composites exhibit broadband optical response, allowing TSHG in a wide waveband region to be achieved. Based on the above effects, the applications of the constructed NIG composite for precise measurement of the group velocity and duration of ultrashort optical pulses with femtosecond time scales are demonstrated. Indeed, the findings outline a fundamental principle to design NIG configurations for creating new properties, providing new directions for expanding the scope of NIG functional materials.

Nanopatterning with Photonic Nanojets: Review and Perspectives in Biomedical Research

Nanostructured surfaces and devices offer astounding possibilities for biomedical research, including cellular and molecular biology, diagnostics, and therapeutics. However, the wide implementation of these systems is currently limited by the lack of cost-effective and easy-to-use nanopatterning tools. A promising solution is to use optical methods based on photonic nanojets, namely, needle-like beams featuring a nanometric width. In this review, we survey the physics, engineering strategies, and recent implementations of photonic nanojets for high-throughput generation of arbitrary nanopatterns, along with applications in optics, electronics, mechanics, and biosensing. An outlook of the potential impact of nanopatterning technologies based on photonic nanojets in several relevant biomedical areas is also provided.

Nonlinear detour phase holography

Nonlinear photonic crystals are capable of highly efficient nonlinear wavefront manipulation, providing a promising platform for compact and large-scale integrated nonlinear devices. However, the current nonlinear encoding methods for nonlinear photonic crystals inherently require a number of disordered and complex microstructures, which are quite challenging in a real fabrication process. Herein we propose and experimentally demonstrate a nonlinear detour phase method for nonlinear wavefront manipulation in nonlinear photonic crystals. With the proposed method, the designed nonlinear detour phase hologram only requires a set of basic building blocks with simple shapes, which are easy to fabricate by using the femtosecond laser writing technique. The second-harmonic hologram is demonstrated by designing the nonlinear detour phase patterns, and the quasi-phase-matching scheme in the second-harmonic holographic imaging process is also discussed. This study conceptually extends the conventional detour phase method into the nonlinear regime, offering new possibilities for compact nonlinear micro-devices with multi-functions.

Microstructure and domain engineering of lithium niobate crystal films for integrated photonic applications

Recently, integrated photonics has attracted considerable interest owing to its wide application in optical communication and quantum technologies. Among the numerous photonic materials, lithium niobate film on insulator (LNOI) has become a promising photonic platform owing to its electro-optic and nonlinear optical properties along with ultralow-loss and high-confinement nanophotonic lithium niobate waveguides fabricated by the complementary metal-oxide-semiconductor (CMOS)-compatible microstructure engineering of LNOI. Furthermore, ferroelectric domain engineering in combination with nanophotonic waveguides on LNOI is gradually accelerating the development of integrated nonlinear photonics, which will play an important role in quantum technologies because of its ability to be integrated with the generation, processing, and auxiliary detection of the quantum states of light. Herein, we review the recent progress in CMOS-compatible microstructure engineering and domain engineering of LNOI for integrated lithium niobate photonics involving photonic modulation and nonlinear photonics. We believe that the great progress in integrated photonics on LNOI will lead to a new generation of techniques. Thus, there remains an urgent need for efficient methods for the preparation of LNOI that are suitable for large-scale and low-cost manufacturing of integrated photonic devices and systems.

High-Order Nonlinear Spin-Orbit Interaction on Plasmonic Metasurfaces

In linear optics, the angular momentum of light can be easily manipulated through the optical spin-orbit interaction (SOI) in structured media such as liquid crystals, metasurfaces, and forked gratings. Similarly, metasurfaces can be used to generate nonlinear optical beams with both custom-defined spin angular momentum (SAM) and orbital angular momentum (OAM) states. However, it has been limited to a low-order process in which only a Gaussian-shaped fundamental wave is used. In this work, the high-order nonlinear optical SOI effect on metasurfaces is demonstrated through the generation of multiple angular momentum states in nonlinear waves. This is achieved by exploiting the degrees of freedom provided by both the SAM and the OAM states of the fundamental wave (FW) and the topological charges of the plasmonic metasurfaces. The mechanism of both intrinsic and extrinsic contributions to the OAM of the nonlinear waves is revealed. High-order nonlinear SOI on metasurfaces offers new opportunities for realizing ultracompact nonlinear vortex beams.

Three-dimensional nonlinear photonic crystal in naturally grown potassium-tantalate-niobate perovskite ferroelectrics

Since quasi-phase-matching of nonlinear optics was proposed in 1962, nonlinear photonic crystals were rapidly developed by ferroelectric domain inversion induced by electric or light poling. The three-dimensional (3D) periodical rotation of ferroelectric domains may add feasible modulation to the nonlinear coefficients and break the rigid requirements for the incident light and polarization direction in traditional quasi-phase-matching media. However, 3D rotating ferroelectric domains are difficult to fabricate by the direct external poling technique. Here, we show a natural potassium-tantalate-niobate (KTN) perovskite nonlinear photonic crystal with spontaneous Rubik's cube-like domain structures near the Curie temperature of 40 °C. The KTN crystal contains 3D ferroelectric polarization distributions corresponding to the reconfigured second-order susceptibilities, which can provide rich reciprocal vectors to compensate for the phase mismatch along an arbitrary direction and polarization of incident light. Bragg diffraction and broadband second-harmonic generation are also presented. This natural nonlinear photonic crystal directly meets the 3D quasi-phase-matching condition without external poling and establishes a promising platform for all-optical nonlinear beam shaping and enables new optoelectronic applications for perovskite ferroelectrics.

Phase-Matching Controlled Orbital Angular Momentum Conversion in Periodically Poled Crystals

Nonlinear interactions between light waves can exchange energy, linear momentum, and angular momentum. The direction of energy flow between frequency components is usually determined by the conventional phase-matching condition related to the linear momentum. However, the transfer law of orbital angular momentum (OAM) during frequency conversion remains to be elucidated. Here, we demonstrate experimentally that OAM transfer depends strongly on the phase-matching condition defined by both linear and orbital angular momenta. Under different phase-matching configurations, the second-harmonic wave exhibits variable OAM spectral characteristics such as the presence of just a single value or of odd orders only. Our results pave the way toward unveiling the underlying mechanism of nonlinear conversion of OAM states.

Laguerre-Gaussian transform for rotating image processing

In practical applications, it is often crucial to track high-speed rotating objects. However, the traditional Fourier transform techniques are not applicable under such circumstances because the Fourier spectrum of a rotating object is changing. Here, we propose a Laguerre-Gaussian (LG) transform to analyze the rotating object. The rotation provides a feasible way to acquire a LG-mode spectrum, which does not change even the object working at a high rotating speed. By analyzing the LG spectrum, one can perform image processing such as reconstruction, edge enhancement, and pattern replication. The LG transform makes it convenient for real-time monitoring of industrial and astronomical objects.

Second-harmonic computer-generated holographic imaging through monolithic lithium niobate crystal by femtosecond laser micromachining

The computer-generated holography technique is a powerful tool for three-dimensional display, beam shaping, optical tweezers, ultrashort pulse laser parallel processing, and optical encryption. We have realized nonlinear holography in ferroelectric crystals by utilizing spatial light modulators in our previous works. Here, we demonstrate an improved method to realize second-harmonic (SH) holographic imaging through a monolithic lithium niobate crystal based on binary computer-generated holograms (CGHs). The CGH patterns were encoded with the detour phase method and fabricated by femtosecond laser micromachining. By the use of the birefringence phase-matching process in the longitudinal direction, bright nonlinear holograms can be obtained in the far-field. The realization of SH holography through monolithic crystal opens wide possibilities in the field of high power laser nonlinear holographic imaging.

Edge-enhanced optical parametric generation in periodically poled LiNbO3

We demonstrate enhanced optical parametric gains occurring at the edge of periodically poled LiNbO₃ (PPLN) regions. Experiments performed in MgO-doped PPLN samples, pumped at 532 nm with parametric signal outputs around 800 nm and 1550 nm, exhibit good agreement with numerical simulations of the nonlinear wave dynamics in the system, based on the assumption of an average refractive index increase Δn = 5.3×10^-5 in the PPLN region. Excitation in proximity to the PPLN edge with a pump power of 8.1 mW results in a 3.6-fold output power increase with respect to parametric generation inside the PPLN area.

Pushing periodic-disorder-induced phase matching into the deep-ultraviolet spectral region: theory and demonstration

Nonlinear frequency conversion is a ubiquitous technique that is used to obtain broad-range lasers and supercontinuum coherent sources. The phase-matching condition (momentum conservation relation) is the key criterion but a challenging bottleneck in highly efficient conversion. Birefringent phase matching (BPM) and quasi-phase matching (QPM) are two feasible routes but are strongly limited in natural anisotropic crystals or ferroelectric crystals. Therefore, it is in urgent demand for a general technique that can compensate for the phase mismatching in universal nonlinear materials and in broad wavelength ranges. Here, an additional periodic phase (APP) from order/disorder alignment is proposed to meet the phase-matching condition in arbitrary nonlinear crystals and demonstrated from the visible region to the deep-ultraviolet region (e.g., LiNbO₃ and quartz). Remarkably, pioneering 177.3-nm coherent output is first obtained in commercial quartz crystal with an unprecedented conversion efficiency above 1‰. This study not only opens a new roadmap to resuscitate those long-neglected nonlinear optical crystals for wavelength extension, but also may revolutionize next-generation nonlinear photonics and their further applications.

Polarization sensitive microstructures fabricated on lithium niobate surfaces by using femtosecond laser pulses

Polarization sensitive microstructures with different morphologies were induced by irradiating dual lithium niobate crystals with femtosecond laser pulses. An upper lithium niobate crystal served as a mask plate to tailor light field, which led to the formation of crater and arc-shaped structures on the surface of a lower lithium niobate crystal. In single-shot irradiation, the orientation and morphology of resultant microstructures can be tailored by controlling the focusing position, because focus splitting took place when a focused laser light propagated through dual lithium niobate crystals. In scanning, the width and morphology of laser scan lines can be governed using various combinations of focusing position and scanning direction. Furthermore, large-area micro/nanostructures with different topography features were successfully fabricated on the crystal surface and their absorption spectra indicated that the absorptance in the visible wavelength range was strongly dependent on fabricated micro/nanostructures. This new type of structured lithium niobate surfaces can be potentially applied in optical and photonic devices.