Compact engineering of path-entangled sources from a monolithic quadratic nonlinear photonic crystal

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.

Direct generation of high brightness path entangled N00N states using structured crystals and shaped pump beams

Optical N00N states are N-photon path entangled states with important applications in quantum metrology. However, their use was limited till now owing to the difficulties of generating them in an efficient and robust manner. Here we propose and experimentally demonstrate two new simple, compact and robust schemes to generate path entangled N00N states with N = 2 that emerge directly from the nonlinear interaction. The first scheme is based on shaping the pump beam, and the second scheme is based on modulating the nonlinear coefficient of the crystal. These new methods exhibit high coincidence count rates for the detection of a N00N state, reaching record value of 2 × 10⁵ coincidences per second. We observe super-resolution by measuring the second order correlation on the generated N = 2 state in an interferometric setup, showing the distinct fringe periodicity at half of the optical wavelength. Our findings may pave the way towards scalable and efficient sources for super-resolved quantum metrology applications and for the generation of bright squeezed vacuum states.

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.

Inverse Design of Ferroelectric-Order in Perovskite Crystal for Self-Powered Ultraviolet Photodetection

It has always been a hot topic to design an orderly mesoscopic structure in functional materials to tailor the macroscopic properties or realize new functions. The existence of domains in ferroelectric materials has been proven to affect the macroscopic properties, being actively studied in nonlinear optical conversion and piezoelectric effects. However, the high-efficiency photoelectric conversion capability of ferroelectric crystals has not yet been explored. Here, the authors study the orderly arrangement of ferroelectric order in KTa_{1- x} Nb_x O₃ (KTN) perovskite crystals, and design the "head-to-head" domains by tuning the Curie temperature T_c , thereby generating abundant charged domain walls and robust conductive channels for electrons and holes. An ultrahigh ultraviolet photoresponsivity is achieved in the KTN crystal under zero bias voltage, being about four orders magnitude higher than that of the well-known ferroelectric materials. The substantial improvement can be attributed to the judiciously designed ferroelectric order, as demonstrated by the conductive atomic force microscopy. In addition, KTN detector exhibits high stability and reliability after high-temperature and fatigue treatment. KTN crystal features giant photoresponsivity, high electric-optical coefficient, and large χ⁽²⁾ nonlinearity concurrently, indicating its great potential for application of all-optical devices on photonic chips.

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.

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.

Hot-spots and gain enhancement in a doubly pumped parametric down-conversion process

We experimentally investigate the parametric down-conversion process in a nonlinear bulk crystal, driven by two non-collinear pump modes. The experiment shows the emergence of bright hot-spots in modes shared by the two pumps, similar to the phenomenology recently observed in 2D nonlinear photonic crystals. By exploiting the spatial walk-off between the two extraordinary pump modes, we have been able to recreate a peculiar resonance condition, reported by a local enhancement of the parametric gain, which corresponds to a transition from a three-mode to a four-mode coupling. From a quantum point of view, this opens the way to the generation of multimode entangled states of light, such as tripartite or quadripartite states, in simple bulk nonlinear sources.

Second Harmonic Generation Susceptibilities from Symmetry Adapted Wannier Functions

Elucidating the orbital level origin of second harmonic generation (SHG) in materials and identifying the local contributions is a long-standing challenge. We report a first principles approach for the SHG where the contributions from individual orbitals or atoms can be evaluated via symmetry adapted Wannier functions without semiempirical parameters. We apply this method to the common SHG materials KBe_{2}BO_{3}F_{2}, KCaCO_{3}F, and β-BaB_{2}O_{4}, and show that the orbitals on noncentrosymmetric sublattices are responsible for SHG effect and the energies of these orbitals control the magnitude.

Compact polarization-entangled photon-pair source based on a dual-periodically-poled Ti:LiNbO3 waveguide

We present an experimental realization of a compact and reliable way to build a nondegenerate polarization-entangled photon-pair source based on a dual-periodically-poled $ {\rm Ti}:{{\rm LiNbO}_3} $Ti:LiNbO₃ waveguide, which is in the telecommunication window and compatible with the fiber quantum networks. The dual-periodic structure allows two inherently concurrent quasiphase-matching spontaneous parametric down-conversion processes pumped by a single laser beam, hence enabling our source to be compact and stable. We show that our source has a high brightness of $ B = 1.22{\rm } \times {\rm }{10^7}\;{\rm pairs}/(\rm s \times mW \times nm) $B=1.22×10⁷pairs/(s×mW×nm). With quantum state tomography, we estimate an entanglement fidelity of $ 0.945 \pm 0.003 $0.945±0.003. A violation of Clauser-Horne-Shimony-Holt inequality with $ S = 2.75 \pm 0.03 $S=2.75±0.03 is also demonstrated.

Efficient parametric generation in a nonlinear photonic crystal pumped by a dual beam

We investigate parametric down-conversion in a hexagonally poled nonlinear photonic crystal, pumped by a dual pump with a transverse modulation that matches the periodicity of the χ ⁽²⁾ nonlinear grating. A peculiar feature of this resonant configuration is that the two pumps simultaneously generate photon pairs over an entire branch of modes, via quasi-phase matching with both fundamental vectors of the reciprocal lattice of the nonlinearity. The parametric gain of these modes depends thus coherently on the sum of the two pump amplitudes and can be controlled by varying their relative intensities and phases. We find that a significant enhancement of the source conversion efficiency, comparable to that of one-dimensionally poled crystals, can be achieved by a dual symmetric pump. We also show how the four-mode coupling arising among shared modes at resonance can be tailored by changing the dual pump parameters.

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

Nonlinear beam shaping refers to spatial reconfiguration of a light beam at a new frequency, which can be achieved by using nonlinear photonic crystals (NPCs). Direct nonlinear beam shaping has been achieved to convert second-harmonic waves into focusing spots, vortex beams, and diffraction-free beams. However, previous nonlinear beam shaping configurations in one-dimensional and two-dimensional (2D) NPCs generally suffer from low efficiency because of unfulfilled phase-matching condition. Here, we present efficient generations of second-harmonic vortex and Hermite-Gaussian beams in the recently-developed three-dimensional (3D) lithium niobate NPCs fabricated by using a femtosecond-laser-engineering technique. Since 3D χ⁽²⁾ modulations can be designed to simultaneously fulfill the requirements of nonlinear wave-front shaping and quasi-phase-matching, the conversion efficiency is enhanced up to two orders of magnitude in a tens-of-microns-long 3D NPC in comparison to the 2D case. Efficient nonlinear beam shaping paves a way for its applications in optical communication, super-resolution imaging, high-dimensional entangled source, etc.

Generation of light with desirable amplitude and phase profiles with nonlinear wavefront shaping is of great interest for optical technologies. Here, the authors demonstrate efficient nonlinear beam shaping using three-dimensional lithium niobate photonic crystals fabricated using a femtosecond-laser-engineering technique.

Hybrid Plasmonic-Ferroelectric Architectures for Lasing and SHG Processes at the Nanoscale

Coherent light sources providing sub-wavelength confined modes are in ever more demand to face new challenges in a variety of disciplines. Scalability and cost-effective production of these systems are also highly desired. The use of ferroelectrics in functional optical platforms, on which plasmonic arrangements can be formed, is revealed as a simple and powerful method to develop coherent light sources with improved and novel functionalities at the nanoscale. Two types of sources with sub-diffraction spatial confinement and improved performances are presented: i) plasmon-assisted solid-state nanolasers based on the interaction between metallic nanostructures and optically active rare earth doped ferroelectric crystals and ii) nonlinear radiation sources based on quadratic frequency mixing processes that are enhanced by means of localized surface plasmon (LSP) resonances. The mechanisms responsible for the intensification of the radiation-matter interaction processes by LSP resonances are discussed in each case. The challenges, potential applications, and future perspectives of the field are highlighted.

Compact generation of a two-photon multipath Dicke state from a single χ(2) nonlinear photonic crystal

Multipartite quantum entanglement is a powerful resource for enriching the functionality of quantum computation and quantum communication. In this Letter, we propose a new method to generate a two-photon multipath Dicke state with concurrent spontaneous parametric downconversion processes from a single periodically poled nonlinear photonic crystal. We design the poling structure to produce a three-path Dicke state where three quasi-phase-matching conditions are fulfilled simultaneously by a hybrid one- and two-dimensionally poled nonlinear photonic crystal. We use genuine multipartite entanglement concurrence to quantify the entanglement of the Dicke state. Using a more complicated poling configuration like multiple-periodically poled two-dimensional nonlinear photonic crystal, we can also produce four-path, five-path, or multipath Dicke states by a single crystal. The multiple-periodically poled two-dimensional nonlinear photonic crystal provides a new method, to the best of our knowledge, for the integrated generation of multipartite quantum light sources.

Quasi-phase-matched second harmonic generation of long-range surface plasmon polaritons

In this paper, we experimentally demonstrate the second harmonic generation of long-range surface plasmon polaritons via quasi-phase matching in lithium niobate. After depositing a 9/13 nm thick Au film on periodically poled lithium niobate, TiO₂ of about 2.3 μm in thickness is evaporated on the sample as a refractive-index-matching material. This dielectric (periodically poled lithium niobate)-metal(Au)-dielectric(TiO₂) sandwich structure can support the transmission of long-range surface plasmon polaritons through it. By designing a moderate ferroelectric domain period of periodically poled lithium niobate, the phase mismatch between the fundamental wave and second harmonic wave of the long-range surface plasmon polaritons can be compensated and a second harmonic wave can be generated effectively. This can be used to provide integrated plasmonic devices with attractive applications in quantum and classic information processing.

Generation of path-polarization hyperentanglement using quasi-phase-matching in quasi-periodic nonlinear photonic crystal

A compact scheme for the generation of path-polarization entangled photon pairs is proposed by using a quasi-periodic nonlinear photonic crystal to simultaneously accomplish four spontaneous parametric down-conversion processes. Moreover, we report experimental scheme to measure the polarization entanglement and path entanglement separately and theoretically get numerical results that verify some predictions about the hyperentanglement. This method can be expanded for the generation of multi-partite and two-photon path-polarization hyperentanglement in a single quasi-periodic nonlinear photonic crystal structure. This compact quantum light source can be used as a significant ingredient in quantum information science.

Rigorous intensity and phase-shift manipulation in optical frequency conversion

A simple method is employed to investigate the nonlinear frequency conversion in optical superlattices (OSL) with pump depletion. Four rigorous phase-matching conditions for different purposes are obtained directly from the nonlinear coupled equations, and the resulting OSL domain structures are generally aperiodic rather than periodic. With this method, not only the intensity but also the phase-shift of the harmonic waves can be manipulated at will. The second-harmonic generation of Gaussian beam is further investigated. This work may provide a guidance for the practical applications of designing nonlinear optical devices with high conversion efficiency.

Scheme for generating distillation-favorable continuous-variable entanglement via three concurrent parametric down-conversions in a single χ(2) nonlinear photonic crystal

We propose to generate a single-mode-squeezing two-mode squeezed vacuum state via a single χ⁽²⁾ nonlinear photonic crystal. The state is favorable for existing Gaussian entanglement distillation schemes, since local squeezing operations can enhance the final entanglement and the success probability. The crystal is designed for enabling three concurrent quasi-phase-matching parametric-down conversions, and hence relieves the auxiliary on-line bi-side local squeezing operations. The compact source opens up a way for continuous-variable quantum technologies and could find more potential applications in future large-scale quantum networks.

On-chip generation and manipulation of entangled photons based on reconfigurable lithium-niobate waveguide circuits

A consequent tendency toward high-performance quantum information processing is to develop the fully integrated photonic chip. Here, we report the on-chip generation and manipulation of entangled photons based on reconfigurable lithium-niobate waveguide circuits. By introducing a periodically poled structure into the waveguide circuits, two individual photon-pair sources with a controllable electro-optic phase shift are produced within a Hong-Ou-Mandel interferometer, resulting in a deterministically separated identical photon pair. The state is characterized by 92.9±0.9% visibility Hong-Ou-Mandel interference. The photon flux reaches ∼1.4×10(7)  pairs nm-1 mW-1. The whole chip is designed to contain nine similar units to produce identical photon pairs spanning the telecom C and L band by the flexible engineering of nonlinearity. Our work presents a scenario for on-chip engineering of different photon sources and paves the way to fully integrated quantum technologies.

Concurrent optical parametric down-conversion in χ(2) nonlinear photonic crystals

We experimentally investigated concurrent parametric downconversion processes in a two-dimensional hexagonally poled lithium tantalate crystal. The substantial enhancement of parametric gain was observed when concurrent processes shared a common parametric beam. Both degenerate and nondegenerate concurrent parametric downconversion processes were studied. Analyses of the spatial forms and output angles showed a strong dependence on the working temperature, during which a well-defined beamlike parametric output was observed. Our results will stimulate the design for coherent high-gain generation of multiple parametric beams and also shed light on the compact engineering of path-entanglement with specific spatial forms based on concurrent spontaneous parametric downconversion processes.