High Second-order Nonlinearity Research Articles

Electro-Optical Comb Envelope Engineering Based on Mode Crossing.

Resonator-enhanced electro-optical (EO) combs could generate a series of comb lines with high coherence and stability. Recently, EO comb based on thin-film lithium niobate (TFLN) has begun to show great potential thanks to the high second-order nonlinearity coefficient of lithium niobate crystal. Here we demonstrate that EO comb envelope engineering based on mode crossing induced a quality factor reduction in the TFLN racetrack microcavity both in the numerical simulation and experiment. Our method paves the way for the generation of EO combs with an arbitrary envelope.

Utilizing Se vacancies as electronic traps to synergize impedance matching and dipole polarization with ultrathin strategy to boost Fe-Se electromagnetic wave absorption

The electronic structure, transport properties and magnetic properties of the material can be adjusted by regulating the formation and concentration of anionic vacancies. In particular, materials based on transition metal chulides (TMC) have high second-order nonlinearity and atomic thin thickness, and anionic vacancy engineering can effectively achieve the optimization of material properties. However, the impact of anion vacancy engineering on dipole polarization has not been fully resolved and clarified yet, and it cannot be solely relied upon semi-empirical methods. Therefore, it is of utmost importance to deeply understand the intrinsic mechanism between vacancies and electromagnetic (EM) parameters. This knowledge will provide crucial guidance for the development of advanced electromagnetic wave (EMW) absorption materials. In this study, a novel approach is proposed for the first time, which involves the creation of Se vacancies at the heterogeneous interface formed by Fe-Se compounds. Remarkably, the generation of Se vacancies leads to an increased electron transfer from Fe to Se atoms, resulting in the formation of numerous dipolarization centers. This effect significantly enhances the material’s polarization loss capacity. Furthermore, the presence of Se vacancies acts as traps for nearby free electrons, reducing the material’s conductivity, weakening the skin effect, and improving impedance matching. The precise modulation of Se vacancies in Fe-Se compounds has led to the achievement of ultrathin characteristics and significantly enhanced EM absorption capabilities. With a thickness of only 1.5 mm, the effective absorption bandwidth (EAB) reaches 5.42 GHz, covering the Ku band, which represents a remarkable increase of 53.5 % compared to unmodified Fe-Se compounds. At a thickness of 1.3 mm, the minimum reflection loss (RL) reaches −42.4 dB, demonstrating an impressive improvement of 41.3 %. Moreover, simulations of the radar cross section (RCS) indicate that the material possesses a robust microwave attenuation ability, underscoring its great potential for practical applications.

Single-photon detection and cryogenic reconfigurability in lithium niobate nanophotonic circuits

Lithium-Niobate-On-Insulator (LNOI) is emerging as a promising platform for integrated quantum photonic technologies because of its high second-order nonlinearity and compact waveguide footprint. Importantly, LNOI allows for creating electro-optically reconfigurable circuits, which can be efficiently operated at cryogenic temperature. Their integration with superconducting nanowire single-photon detectors (SNSPDs) paves the way for realizing scalable photonic devices for active manipulation and detection of quantum states of light. Here we demonstrate integration of these two key components in a low loss (0.2 dB/cm) LNOI waveguide network. As an experimental showcase of our technology, we demonstrate the combined operation of an electrically tunable Mach-Zehnder interferometer and two waveguide-integrated SNSPDs at its outputs. We show static reconfigurability of our system with a bias-drift-free operation over a time of 12 hours, as well as high-speed modulation at a frequency up to 1 GHz. Our results provide blueprints for implementing complex quantum photonic devices on the LNOI platform.

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

Boosting second-order optical nonlinear frequency conversion over subwavelength thickness has long been pursued through optical resonance in micro- and nanophotonics. However, the availability of thin film materials with high second-order nonlinearity is limited to III-V semiconductors, which have low transparency in the visible. Here, we experimentally demonstrated strongly enhanced second harmonic generation in one-dimensional heterostructure cavities on thin film lithium niobate. A guided-mode resonance resonator and distributed Bragg reflectors are combined for both efficient coupling and electromagnetic field localization. Over 1200 times second harmonic generation enhancement is experimentally realized compared with flat thin film lithium niobate through optimizing the trade-off between quality factor and mode volume, leading to a record high normalized conversion efficiency of 2.03×10^{-5} cm^{2}/GW under 1.92 MW/cm^{2} pump intensity. Our approach could inspire the miniaturization and integration of compact resonant nonlinear photonic devices on thin film lithium niobate.

Integrated lithium niobate photonics

Abstract Lithium niobate (LiNbO3) on insulator (LNOI) is a promising material platform for integrated photonics due to single crystal LiNbO3 film’s wide transparent window, high refractive index, and high second-order nonlinearity. Based on LNOI, the fast-developing ridge-waveguide fabrication techniques enabled various structures, devices, systems, and applications. We review the basic structures including waveguides, cavities, periodically poled LiNbO3, and couplers, along with their fabrication methods and optical properties. Treating those basic structures as building blocks, we review several integrated devices including electro-optic modulators, nonlinear optical devices, and optical frequency combs with each device’s operating mechanism, design principle and methodology, and performance metrics. Starting from these integrated devices, we review how integrated LNOI devices boost the performance of LiNbO3’s traditional applications in optical communications and data center, integrated microwave photonics, and quantum optics. Beyond those traditional applications, we also review integrated LNOI devices’ novel applications in metrology including ranging system and frequency comb spectroscopy. Finally, we envision integrated LNOI photonics’ potential in revolutionizing nonlinear and quantum optics, optical computing and signal processing, and devices in ultraviolet, visible, and mid-infrared regimes. Beyond this outlook, we discuss the challenges in integrated LNOI photonics and the potential solutions.

Improving lower bounds on the second-order nonlinearity of three classes of Boolean functions

Boolean functions used in symmetric-key cryptosystems must have high second-order nonlinearity to withstand several known attacks and some potential attacks which may exist but are not yet efficient and might be improved in the future. The second-order nonlinearity of Boolean functions also plays an important role in coding theory, since the maximal second-order nonlinearity of all Boolean functions in n variables equals the covering radius of the Reed–Muller code with length 2n and order r. It is well-known that providing a tight lower bound on the second-order nonlinearity of a general Boolean function with high algebraic degree is a hard task, excepting a few special classes of Boolean functions. In this paper, we improve the lower bounds on the second-order nonlinearity of three classes of Boolean functions of the form fi(x)=Tr1n(xdi) in n variables for i=1,2 and 3, where Tr1n denotes the absolute trace mapping from F2n to F2 and di’s are of the form (1) d1=2m+1+3 and n=2m, (2) d2=2m+2m+12+1, n=2m with odd m, and (3) d3=22r+2r+1+1 and n=4r with even r.

Enhanced nonlinear optical effects in organic frustum-shaped microresonators

We discuss photonic effects in arrays of frustum-shaped organic microstructures prepared from organic orange dye composed on a solid substrate by self-assembling technique. We demonstrate that such structures reveal strong amplification of second-order nonlinear optical response, including optical second harmonic generation (SHG) and two-photon fluorescence (TPF), as compared to that of a continuous film. This is associated with a strong light localization in microstructures composed of high refractive index material. The TPF and SHG power dependencies show that the observed effects are governed by high second-order nonlinearity of the dye damped by a strong absorption. FDTD calculations confirm that the mechanism of the light localization inside organic frustums can be in the form of whispering gallery modes excitation.

High second-order nonlinearity by p-n junction formation in plasma enhanced chemical vapor deposition deposited hydrogenated amorphous silicon thin films

High second-order nonlinearity as high as 115.9 pm/V was obtained through the formation of p-n junction using plasma-enhanced CVD deposition of hydrogenated amorphous silicon thin films on fused silica substrate without additional electrical poling to induce second harmonic generation in the isotropic amorphous materials. No second harmonic generation was observed for films deposited with only p or n type amorphous silicon. The built-in electrostatic field Edc of p-n junction was observed by measuring the I-V curve of the designed p-i-n layered films, and was found to be the main source of high second-order nonlinearity.

High power room-temperature design of terahertz quantum cascade laser based on difference frequency generation

Terahertz (THz) quantum cascade lasers (QCLs) are key elements for high-power terahertz beam generation for integrated applications. In this study, we design a highly nonlinear THz-QCL active region in order to increase the output power of the device especially at lower THz frequencies based on difference frequency generation (DFG) process. It has been shown that the output power increases for a 3.2THz structure up to 1.2μW at room temperature in comparison with the reported power of P=0.3μW in [1]. The mid-IR wavelengths associated with this laser are λ1=12.12μm and λ2=13.93μm, which are mixed in a medium with high second-order nonlinearity. A similar approach has been used to design an active region with THz frequency of 1.8THz. The output power of this structure reaches to 1μW at room temperature where the mid-IR wavelengths are λ1=12.05μm, λ2=12.99μm.

Efficient second-order nonlinear optical chromophores based on dithienothiophene and thienothiophene bridges

Two series of push–pull systems bearing dithienothiophene or thienothiophene as (part of) π-conjugated spacer have been synthesized. These compounds combine high second-order molecular nonlinearities with good thermal stabilities, showing one of the thienothiophene (TT) derivative the highest μβ0 value found for a 4H-pyranylidene-containing merocyanine.

一种具有高二阶非线性椭圆芯中空光纤

一种具有高二阶非线性椭圆芯中空光纤

On lower bounds of second-order nonlinearities of cubic bent functions constructed by concatenating Gold functions

We consider the problem of identifying the classes of Boolean functions having high second-order nonlinearities. In this paper, we demonstrate that the cubic bent functions obtained by Leander and McGuire (J. Combin. Theory Ser. A, 116 (2009), pp. 960–970), which are concatenations of the quadratic Gold functions, possess high second-order nonlinearities.

The problem of achieving high second-order nonlinearities in glasses: The role of electronic conductivity in poling of high index glasses

Efficient thermal poling of electronically conducting glass is prevented by the inherent difficulty to record a large electrostatic field within such glasses. To overcome this limitation, a waveguide/substrate configuration has been proposed, in which the glass for poling was deposited as a film of appropriate thickness on a substrate chosen for its higher ionic conductivity. Owing to this configuration, the poling voltage drops entirely across the glass film, allowing high electrostatic field to be recorded in spite of the high electronic conductivity of the glass. The proposed method was demonstrated here in the case of bismuth-zinc-borate glasses, which possess high potential for poling because of their high intrinsic χ (3). A four-fold enhancement of χ (2) compared to bulk glass, from ~ 0.5 to ~ 2 pm/V, is demonstrated. It is also shown that the χ (2) values obtained are the highest sustainable by the glass limited by the onset of nonlinear conductivity. The waveguide/substrate configuration intrinsically allows obtaining perfect overlap of the poling induced second-order nonlinearity with the guiding region of the waveguide. An equivalent RC-circuit model describing the poled glass reveals that the value of the poling-induced second-order nonlinearity is strongly dependent on the ratio β between ionic and electronic conductivity. The most promising glass systems for poling are found to be the ones displaying the highest product χ (3) β. This work is performed on bismuth-zinc-borate heavy metal oxide glasses but the waveguide/substrate configuration proposed here is likely to be equally successful in enhancing the second-order nonlinearity in high χ (3) electronic conducting glasses such as for example telluride and chalcogenide glasses.

Synthesis and stability studies of conformationally locked 4-(diarylamino)aryl- and 4-(dialkylamino)phenyl-substituted second-order nonlinear optical polyene chromophores

A series of chromophores with high second-order nonlinearities has been synthesized; the chromphores consist of triarylamine or dialkylarylamine donors linked by a conformationally locked polyene bridge to a dicyanomethylidene acceptor. The use of bridges of this type, combined with the replacement of dialkylarylamine with triarylamine donors, leads to high thermal stability without adverse affects on the nonlinear optical properties of the chromophores.

Optical parametric generation in CGA crystal

We have demonstrated optical parametric generation (OPG) in CGA crystal known for its extremely high second-order nonlinearity (236 pm/V). A single-pass OPG was pumped by 10 μJ, 600 fs pulses at λ=5 μm and the output was tunable in the 7–18 μm range. Efficient second harmonic generation of 9–11 μm radiation with an internal conversion efficiency of 62% has also been demonstrated.

High second-order optical nonlinearities in thermally poled sol-gel silica

Silica glass samples prepared by a sol-gel process, the mixture of silica sol for gelation and colloidal silica particles, have been thermally poled in vacuum by continuous high voltage (8–12 kV) at elevated temperature (280 °C). High second-order nonlinearities (>1 pm/V), located under the anodic surface, have been measured and the values are higher than those obtained in fused silica glass, poled under the same conditions. A model for thermal poling is suggested which explains the experimental results in sol-gel silica and the difference between thermal poling of sol-gel and fused silica. The granular structure and the boundaries of sol-gel silica seem to play a major role in establishing the electrical properties of the depletion layer during and after poling.

High second-order nonlinearities in poled silicate fibers

Effective quadratic nonlinearities as high as 0.2 pm/V are reported for the first time to our knowledge in poled germanosilicate fibers. This value is ~200 times higher than previously achieved in these fibers. The presence of Ge is found to enhance the efficacy of both thermal (in combination with OH doping) and electron-beam poling in silica.

High second-order nonlinearities induced in lead silicate glass by electron-beam irradiation

A new technique for inducing a large permanent second-order susceptibility in lead silicate glass is reported. The procedure involves implanting electrons by irradiating the glass with an electron beam. Second-order nonlinearities chi((2)) as high as 0.7 pm/V are obtained.

Anisotropic second-order nonlinearities of organic monolayers.

Monolayers prepared from hemicyanine chromophores of high second-order nonlinearity (\ensuremath{\beta}=1.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}26}$ esu at resonance) have been used to study anisotropies of the concomitant ${\mathrm{\ensuremath{\chi}}}^{(2)}$-susceptibility tensor. All components of this third-rank tensor have been determined by Fourier analysis of the anisotropic coverage density ${\mathit{N}}_{\mathit{s}}$(\ensuremath{\varphi}) with respect to the angle of rotation about surface normal. Depending on the preparation both ${\mathit{C}}_{\mathit{s}}$ and ${\mathit{C}}_{2\mathit{v}}$ symmetry could be identified.