Strongly Enhanced Second Harmonic Generation in a Thin Film Lithium Niobate Heterostructure Cavity

Miniaturized guided-mode resonance laser based on a one-dimensional finite heterostructure cavity

Lasers based on the resonant nanostructures have attracted much attention due to their low threshold and compact dimensions. Guided-mode resonance (GMR) structures have been studied in lasing configurations because of their optical field enhancement and convenient free space excitation. However, the GMR inherently requires a larger footprint and is not suitable for high-density packaging. Here, we present numerical evidence of a miniaturized laser implemented in a one-dimensional finite heterostructure cavity (FHC). A GMR resonator and distributed Bragg reflectors are integrated to create the FHC, which enables the efficient coupling and localization of the electric field. Numerical findings indicate that the threshold is approximately 22.5 µJ/cm2, while the emission region is confined within a length of 5.4 µm. In addition, by adjusting the coupling strength, it is capable to achieve controllable lasing emission. The proposed structure provides a compact source for high-capacity optical communications, sensing, and quantum information processing.

A Multi-Scale Approach to Simulate the Nonlinear Optical Response of Molecular Nanomaterials

Nonlinear optics is essential for many recent photonic technologies. Here, a novel multi-scale approach is introduced to simulate the nonlinear optical response of molecular nanomaterials combining ab initio quantum-chemical and classical Maxwell-scattering computations. In this approach, the first hyperpolarizability tensor is computed with time-dependent density-functional theory and incorporated into a multi-scattering formalism that considers the optical interaction between neighboring molecules. Such incorporation is achieved by a novel object: the Hyper-Transition(T)-matrix. With this object at hand, the nonlinear optical response from single molecules and also from entire photonic devices can be computed, including the full tensorial and dispersive nature of the optical response of the molecules, as well as the optical interaction between different molecules as, for example, in the lattice of a molecular crystal. To demonstrate the applicability of the novel approach, the generation of a second-harmonic signal from a thin film of an Urea molecular crystal is computed and compared to more traditional simulations. Furthermore, an optical cavity is designed, which enhances the second-harmonic response of the molecular film up to more than two orders of magnitude. This approach is highly versatile and accurate and can be the working horse for the future exploration of nonlinear photonic molecular materials in structured photonic environments.

Fabrication of 100-nm-period domain structure in lithium niobate on insulator

Lithium niobate on insulator (LNOI) is a powerful platform for integrated photonic circuits. Recently, advanced applications in nonlinear and quantum optics require to controllably fabricate nano-resolution domain structures in LNOI. Here, we report on the fabrication of stable domain structures with sub-100 nm feature size through piezoelectric force microscopy (PFM) tip poling in a z-cut LNOI. In experiment, the domain dot with an initial diameter of 80 nm and the domain line with an initial width of 50 nm can survive after a storage of more than 3 months. Particularly, we demonstrate the successful fabrication of 1D stable domain array with a period down to 100 nm and a duty cycle of ∼50%. Our method paves the way to precisely manipulate frequency conversion and quantum entanglement on an LNOI chip.

Q-switched vortex waveguide laser generation based on LNOI thin films with implanted Ag nanoparticles

Lithium-niobate-on-insulator (LNOI) thin films have gained significant attention in integrated photonics due to their exceptional crystal properties and wide range of applications. In this paper, we propose a novel approach to realize a Q-switched vortex waveguide laser by incorporating integrated lithium niobate thin films with embedded silver nanoparticles (Ag:LNOI) as a saturable absorber. The saturable absorption characteristics of Ag:LNOI are investigated using a home-made Z-scan system. Additionally, we integrate Ag:LNOI as a saturable absorber into a Nd:YAG "ear-like" cladding waveguide platform, which is prepared via femtosecond laser direct writing. By combining this setup with helical phase plates for phase modulation in the resonator, we successfully achieve a passive Q-switched vortex laser with a high repetition rate and narrow pulse duration in the near-infrared region. This work demonstrates the potential applications of LNOI thin films towards on-chip integration of vortex waveguide laser sources.

Polarization Engineering of Entangled Photons from a Lithium Niobate Nonlinear Metasurface

Complex polarization states of photon pairs are indispensable in various quantum technologies. Conventional methods for preparing desired two-photon polarization states are realized through bulky nonlinear crystals, which can restrict the versatility and tunability of the generated quantum states due to the fixed crystal nonlinear susceptibility. Here we present a solution using a nonlinear metasurface incorporating multiplexed silica metagratings on a lithium niobate film of 300 nm thickness. We fabricate two orthogonal metagratings on a single substrate with an identical resonant wavelength, thereby enabling the spectral indistinguishability of the emitted photons, and we demonstrate in experiments that the two-photon polarization states can be shaped by the metagrating orientation. Leveraging this essential property, we formulate a theoretical approach for generating arbitrary polarization-entangled qutrit states by combining three metagratings on a single metasurface, allowing the encoding of the desired quantum states or information. Our findings enable miniaturized optically controlled quantum devices by using ultrathin metasurfaces as polarization-entangled photon sources.

Efficient second- and higher-order harmonic generation from LiNbO3 metasurfaces

Lithium niobate (LiNbO3) is a material that has drawn great interest in nonlinear optics because of its large nonlinear susceptibility and wide transparency window. However, for complex nonlinear processes such as high-harmonic generation (HHG), which involves frequency conversion over a wide frequency range, it can be extremely challenging for a bulk LiNbO3 crystal to fulfill the phase-matching conditions. LiNbO3 metasurfaces with resonantly enhanced nonlinear light-matter interaction at the nanoscale may circumvent such an issue. Here, we experimentally demonstrate efficient second-harmonic generation (SHG) and HHG from a LiNbO3 metasurface enhanced by guided-mode resonance. We observe a high normalized SHG efficiency of 5.1 × 10-5 cm2 GW-1. Moreover, with the alleviated above-gap absorption of the material, we demonstrate HHG up to the 7th order with the shortest generated wavelength of 226 nm. This work may provide a pathway towards compact coherent white-light sources with frequency spanning into the deep ultraviolet region for applications in spectroscopy and imaging.

χ(2) nonlinear photonics in integrated microresonators

Second-order (χ⁽²⁾) optical nonlinearity is one of the most common mechanisms for modulating and generating coherent light in photonic devices. Due to strong photon confinement and long photon lifetime, integrated microresonators have emerged as an ideal platform for investigation of nonlinear optical effects. However, existing silicon-based materials lack a χ⁽²⁾ response due to their centrosymmetric structures. A variety of novel material platforms possessing χ⁽²⁾ nonlinearity have been developed over the past two decades. This review comprehensively summarizes the progress of second-order nonlinear optical effects in integrated microresonators. First, the basic principles of χ⁽²⁾ nonlinear effects are introduced. Afterward, we highlight the commonly used χ⁽²⁾ nonlinear optical materials, including their material properties and respective functional devices. We also discuss the prospects and challenges of utilizing χ⁽²⁾ nonlinearity in the field of integrated microcavity photonics.

Giant Bulk Electrophotovoltaic Effect in Heteronodal-Line Systems

The realization of a giant and continuously tunable second-order photocurrent is desired for many nonlinear optical (NLO) and optoelectronic applications, which remains a great challenge. Here, based on a two-band model, we propose a concept of the bulk electrophotovoltaic effect, that is, an out-of-plane external electric field (E_{ext}) that can continuously tune in-plane shift current along with its sign flip in a heteronodal-line (HNL) system. While strong linear optical transition around the nodal loop may potentially generate giant shift current, an E_{ext} can effectively control the radius of the nodal loop, which can continuously modulate the shift-vector components inside and outside the nodal loop holding opposite signs. This concept has been demonstrated in the HNL HSnN/MoS_{2} system using first-principles calculations. The HSnN/MoS_{2} heterobilayer can not only produce a shift-current conductivity with magnitude that is one to two orders larger than other reported systems, but it can also realize a giant bulk electrophotovoltaic effect. Our finding opens new routes to create and manipulate NLO responses in 2D materials.

Efficient Second- and Third-Harmonic Generations in Er3+/Fe2+-Doped Lithium Niobate Single Crystal with Engineered Surficial Cylindrical Hole Arrays

Herein, significant enhancement of second- and third-harmonic generation efficiencies in a 1 mol% Er³⁺ and 0.07 mol% Fe²⁺-doped lithium niobate single-crystal plate were achieved after ablating periodic cylindrical pit arrays on the surface. Enhanced absorption and reduced transmittance of light were measured when the incident light signal passed through the patterned sample. Enhanced photoluminescence and two-photon-pumped upconversion emission spectra were also explored to obtain more details on the efficiency gains. The excitation-energy-dependent second-harmonic generation efficiency was measured, and an enhancement as high as 20-fold was calculated. The conversion efficiency of second-harmonic generation is 1 to 3 orders higher than that from other lithium niobite metasurfaces and nanoantennas. This work provides a convenient and effective method to improve the nonlinear conversion efficiency in a thin lithium niobite plate, which is desirable for applying to integrated optical devices.

Giant Second Harmonic Generation from Membrane Metasurfaces

Metasurfaces have emerged as a fascinating framework for nonlinear optics, which have advantages of a compact footprint and unprecedented flexibility in manipulating light. But their nonlinear responses are generally limited by the short interaction lengths with light. Therefore, further enhancement is highly desired for building high-efficiency nonlinear devices. Here, we experimentally demonstrate a record high second harmonic generation (SHG) efficiency of 2.0 × 10^-4 using lithium niobate (LN) membrane metasurfaces. Benefiting from the large refractive index contrast in the vertical direction and high fabrication quality, distinct spectral resonances and tight field confinements in the LN layer were achieved. Strong SHG peaks resulting from pump resonances of the metasurfaces were observed. Our nonlinear efficiency is more than 2 orders of magnitude larger than previously reported LN metasurfaces. The results inspire a way to improve the efficiency of nonlinear metasurfaces for ultracompact nonlinear light sources in applications of nonlinear holography, Li-Fi, beam shaping, etc.

Self-Powered Lithium Niobate Thin-Film Photodetectors

Thin-film lithium niobate platform, namely lithium-niobate-on-insulator (LNOI), brings new opportunities for integrated photonics, taking advantages from both outstanding crystalline properties and special structural features. The excellent properties of LNOI have triggered development of a variety of on-chip photonic devices for light generation and manipulation. However, as an indispensable component for photonic circuit with full functionalities, the thin-film photodetector lacks in portfolios of LNOI-based devices due to standing obstacles of low electrical conductivity and poor photoelectric conversion ability. Here, a self-powered broadband LNOI photodetector based on enhanced photovoltaic effect, benefitting from encapsulated plasmonic nanoparticles and doped silver ions, is reported. Maximum responsivity of 0.25 A W^-1 and detectivity (1.56 × 10¹⁴ Jones) are achieved. First-principle calculations and electric-field simulation reveal intrinsic mechanisms and crucial roles of plasmonic nanoparticles and silver ions on photocurrent generation and collection. This work opens an avenue to develop high-performance on-chip LNOI photodetectors, offering a conceivable means toward multiple-functional photonic circuits.

Femtosecond laser writing of lithium niobate ferroelectric nanodomains

Lithium niobate (LiNbO₃) is viewed as a promising material for optical communications and quantum photonic chips^1,2. Recent breakthroughs in LiNbO₃ nanophotonics have considerably boosted the development of high-speed electro-optic modulators^3-5, frequency combs^6,7 and broadband spectrometers⁸. However, the traditional method of electrical poling for ferroelectric domain engineering in optic^9-13, acoustic^14-17 and electronic applications^18,19 is limited to two-dimensional space and micrometre-scale resolution. Here we demonstrate a non-reciprocal near-infrared laser-writing technique for reconfigurable three-dimensional ferroelectric domain engineering in LiNbO₃ with nanoscale resolution. The proposed method is based on a laser-induced electric field that can either write or erase domain structures in the crystal, depending on the laser-writing direction. This approach offers a pathway for controllable nanoscale domain engineering in LiNbO₃ and other transparent ferroelectric crystals, which has potential applications in high-efficiency frequency mixing^20,21, high-frequency acoustic resonators^14-17 and high-capacity non-volatile ferroelectric memory^19,22.

Extreme enhancement of the quality (Q)-factor and mode field intensity in cavity-resonator gratings

In this paper, dielectric Cavity-Resonant Integrated-Grating Filters (CRIGFs) are numerically optimized to achieve extremely high-quality factors, by optimizing the cavity in/out-coupling rate and by introducing apodizing mode-matching sections to reduce scattering losses. Q-factors ranging between 0.1 and 50 million are obtained and two different domains are distinguished, as a function of the perturbation parameter which controls the cavity in/out-coupling rate. When the cavity coupling Q-factor is lower than the Q-factor of the uncoupled Fabry-Perot cavity, corresponding to the over-coupling regime, the reflectivity response exhibits a high resonance maximum. On the contrary, in the under-coupling regime the resonant reflectivity maximum is much weaker since the scattering losses of the uncoupled cavity dominate. Between these two domains, the so-called critical coupling condition leads to very strong field enhancement inside the device, reaching up to 10⁴ times the incident field amplitude. This theoretical work paves the way towards the practical implementation of CRIGFs with much higher Q-factors than currently demonstrated, potentially reaching performance on a par with other resonators such as photonic crystal cavities or whispering gallery mode resonators. These results can serve to optimize the design of narrow-band planar grating filters, particularly for application in non-linear optics.