Large Second-order Nonlinearity Research Articles

Enhanced coherent microwave-to-optics conversion based on second-order nonlinearity

An efficient microwave-optical frequency conversion scheme is proposed based on the second-order nonlinearity in the atomic system. Due to the magnetic transition driven by a microwave field, the second-order nonlinearity builds up and three-wave mixing process appears. It is interesting to find that a detuned auxiliary transition causes a large second-order nonlinearity, which contributes to the increase of conversion efficiency. The simulation results show that the conversion efficiency could be increased by five times. Other effects on the conversion efficiency are also discussed.

Understanding the Correlation Between Structure and Entangled Photon Pair Properties with Metal–Organic Frameworks

Spontaneous parametric down conversion (SPDC) is a quantum second-order nonlinear optical process where the photons generated are frequently used in quantum information processing. Materials with large second-order nonlinearities (χ(2)) can be used as entangled photon sources with a high brightness. The source brightness scales as the square of the effective nonlinearity (deff), which is an intrinsic property of the material. Understanding material factors that can significantly alter this intrinsic property is useful in developing new materials that are SPDC efficient. In our work we focus on understanding factors affecting the entangled photon pair properties, such as the arrangements of ligands within the Zn(3-ptz)2 metal–organic framework (MOF) crystal and temperature. We find that the arrangement and alignment of the pyridine rings in the crystal structure significantly affect the deff and birefringence (Δn). Smaller pyridine ring alignments relative to the optic c-axis increase the Δn, which in turn leads to larger photon pair correlation times (τc) in coincidence measurements. Our work has significant implications in understanding the effect of ligand arrangement on deff and τc for any MOF crystal structure, providing a tool to rationalize the optimization of MOF crystals for the development of efficient nonlinear optical devices.

Electric-field-induced quasi-phase-matched three-wave mixing in silicon-based superlattice-on-insulator integrated circuits

We present a theoretical investigation, based on the tight-binding Hamiltonian, of efficient electric-field-induced three-waves mixing (EFIM) in an undoped lattice-matched short-period superlattice (SL) that integrates quasi-phase-matched (QPM) SL straight waveguides and SL racetrack resonators on an opto-electronic chip. Periodically reversed DC voltage is applied to electrode segments on each side of the strip waveguide. The spectra of χxxxx(3) and of the linear susceptibility have been simulated as a function of the number of the atomic monolayers for “non-relaxed” heterointerfaces, and by considering all the transitions between valence and conduction bands. The large obtained values ofχxxxx(3) make the (ZnS)3/(Si2)3 short-period SL a good candidate for realizing large effective second-order nonlinearity, enabling future high-performance of the SLOI PICs and OEICs in the 1000-nm and 2000-nm wavelengths ranges. We have made detailed calculations of the efficiency of second-harmonic generation and of the performances of the optical parametric oscillator (OPO). The results indicate that the (ZnS)N/(Si2)M QPM is competitive with present PPLN technologies and is practical for classical and quantum applications.

Full characterization of spontaneous parametric down conversion in non-ideal quarter-wavelength semiconductor Bragg reflection waveguide

Entangled photons are important for testing foundations of quantum physics and are at the heart of quantum technology. Integrated photonics has overwhelming dominance in terms of density and performance, making it a promise route for scalable quantum information processing. AlGaAs-based materials having large second-order non-linearities, direct bandgap and strong electro-optical effect can offer distinct advantages in quantum light source. Here we report a non-ideal quarter-wavelength Bragg reflection waveguide for generating three types of spontaneous parametric down-conversion processes. A general solution to the dispersion equation is derived and employed for designing high efficiency devices by taking into account the influence of core layer aluminium concentration. We further design and fabricate a Bragg reflection waveguide sample based on the analysis, and experimentally characterize its phase matching types and spectral brightness. Our work paves the path for the development of portable quantum light sources.

Efficient Low Threshold Frequency Conversion in AlGaAs-On-Insulator Waveguides

A design study is presented for an efficient, compact and robust device to convert the frequency of single-photons from the near-infrared to the telecom C-band. The material platform aluminum gallium arsenide (AlGaAs)-on-insulator, with its relatively large second-order nonlinearity, is used to create highly confined optical modes. This platform can feasibly incorporate single-photon emitters such as indium arsenide (InAs) on gallium arsenide (GaAs), paving the way towards direct integration of single-photon sources and nonlinear waveguides on the same chip. In this design study, single-pass difference-frequency generation (DFG) producing C-band single-photons is enabled via form birefringent phase-matching between a 930 nm single-photon pump and continuous wave (CW) idler at 2,325 nm. In particular the idler and single-photons are combined with an on-chip directional coupler, and then tapered to a single waveguide where the three modes are phase-matched. The design is studied at a special case, showing high fabrication tolerances, and an internal conversion efficiency up to 41%.

Large second-order nonlinearity in asymmetric metallic quantum wells

Investigation of new plasmonic material platforms with large optical nonlinearity is crucial for the continued development of nonlinear optics and its applications. Here, we report an enhanced second-order nonlinear effect in metallic quantum wells (QWs), where the intersubband transition plays a dominant role. Centrosymmetry in these metallic QWs is broken by forming multilayers with chemically and structurally distinct barrier oxides above and below a metal nanofilm. For Au-based QWs, we show that a large χ(2) around 229.6 pm/V in the near infrared was achieved in an asymmetric metallic QW of SiO2|Au|HfO2 on a fused silica substrate.

Thermal annealing study of the mid-infrared aluminum nitride on insulator (AlNOI) photonics platform

Aluminum nitride on insulator (AlNOI) photonics platform has great potential for mid-infrared applications thanks to the large transparency window, piezoelectric property, and second-order nonlinearity of AlN. However, the deployment of AlNOI platform might be hindered by the high propagation loss. We perform thermal annealing study and demonstrate significant loss improvement in the mid-infrared AlNOI photonics platform. After thermal annealing at 400°C for 2 hours in ambient gas environment, the propagation loss is reduced by half. Bend loss and taper coupling loss are also investigated. The performance of multimode interferometer, directional coupler, and add/drop filter are improved in terms of insertion loss, quality factor, and extinction ratio. Fourier-transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction spectroscopy suggest the loss improvement is mainly attributed to the reduction of extinction coefficient in the silicon dioxide cladding. Apart from loss improvement, appropriate thermal annealing also helps in reducing thin film stress.

Silicophosphate films doped with organic compounds for nonlinear optical applications

Doped organic compounds are of great interest for optical communications and high-density optical data storage due to their large second-order nonlinearity and high-speed response. A promising approach in the fabrication of new materials for second-order nonlinear optics is sol–gel technique. The present paper reports on the sol–gel synthesis of some pyrazolone derivative doped SiO2–P2O5 films. The precursors used for sol gel synthesis were tetraethylorthosilicate and phosphoric acid (H3PO4). The organic compound was added under continuous stirring in the precursors’ mixture. The structure of the deposited azo-derivatives doped thin films was examined by Fourier transform infrared spectroscopy, and atomic force microscopy and their optical properties by UV–VIS spectroscopy. The nonlinear optical efficiencies due to the interaction of the NLO-active chromophores with the inorganic matrix has a significant influence on the second harmonic generation capabilities that was measured using a femtosecond Ti:sapphire laser. The properties of the films were investigated and correlated with the concentration of the organic dopant and the thermal treatment temperature.

Optical nonlinearity in Cu₂CdSnS₄ and α/β-Cu₂ZnSiS₄: diamond-like semiconductors with high laser-damage thresholds.

Cu2CdSnS4 and α/β-Cu2ZnSiS4 meet several criteria for promising nonlinear optical materials for use in the infrared (IR) region. Both are air-stable, crystallize in noncentrosymmetric space groups, and possess high thermal stabilities. Cu2CdSnS4 and α/β-Cu2ZnSiS4 display wide ranges of optical transparency, 1.4-25 and 0.7-25 μm, respectively, and have relatively large second-order nonlinearity as well as phase matchability for wide regions in the IR. The laser-damage threshold (LDT) for Cu2CdSnS4 is 0.2 GW/cm(2), whereas α/β-Cu2ZnSiS4 has a LDT of 2.0 GW/cm(2) for picosecond near-IR excitation. Both compounds also exhibit efficient third-order nonlinearity. Electronic structure calculations provide insight into the variation in properties.

An investigation into second harmonic generation by Si-rich SiNx thin films deposited by RF sputtering over a wide range of Si concentrations

The second-order nonlinearity of Si-rich SiNx thin films prepared by RF sputtering was investigated. It was found that the second-order nonlinear coefficient (deff) increases with increasing Si concentration in a wide range of Si concentrations. The observed maximum value of deff was 5.9 pm V−1, which is twice that of the β-BaB2O4 crystal. X-ray photoelectron spectroscopy analysis in the Si 2p energy region revealed that the films consist of Si species with different valence states. Strong correlation was observed between the deff values and the XPS peak intensity of Si0 species, suggesting that Si clusters are the most probable origin of the large second-order nonlinearity.

Second harmonic generation from Ge doped SiO2 (Gex(SiO2)1−x) thin films grown by sputtering

Second-order nonlinear optical properties of sputter-deposited Ge-doped SiO2 thin films were investigated. It was shown that the second-order nonlinearity of SiO2, which vanishes in the electric-dipole approximation due to the centrosymmetric structure, can be significantly enhanced by Ge doping. The observed maximum value of d33 was 8.2 pm/V, which is 4 times larger than d22 of β-BaB2O4 crystal. Strong correlation was observed between the deff values and the electron spin resonance signals arising from GePb centers, suggesting that GePb centers are the most probable origin of the large second-order nonlinearity.

Investigation of local strain distribution and linear electro-optic effect in strained silicon waveguides

We present detailed investigations of the local strain distribution and the induced second-order optical nonlinearity within strained silicon waveguides cladded with a Si₃N₄ strain layer. Micro-Raman Spectroscopy mappings and electro-optic characterization of waveguides with varying width w(WG) show that strain gradients in the waveguide core and the effective second-order susceptibility χ(2)(yyz) increase with reduced w(WG). For 300 nm wide waveguides a mean effective χ(2)(yyz) of 190 pm/V is achieved, which is the highest value reported for silicon so far. To gain more insight into the origin of the extraordinary large optical second-order nonlinearity of strained silicon waveguides numerical simulations of edge induced strain gradients in these structures are presented and discussed.

Generating mid-IR octave-spanning supercontinua and few-cycle pulses with solitons in phase-mismatched quadratic nonlinear crystals

We discuss a novel method for generating octave-spanning supercontinua and few-cycle pulses in the important mid-IR wavelength range. The technique relies on strongly phase-mismatched cascaded second-harmonic generation (SHG) in mid-IR nonlinear frequency conversion crystals. Importantly we here investigate the so-called noncritical SHG case, where no phase matching can be achieved but as a compensation the largest quadratic nonlinearities are exploited. A self-defocusing temporal soliton can be excited if the cascading nonlinearity is larger than the competing material self-focusing nonlinearity, and we define a suitable figure of merit to screen a wide range of mid-IR dielectric and semiconductor materials with large effective second-order nonlinearities $d_{\rm eff}$. The best candidates have simultaneously a large bandgap and a large $d_{\rm eff}$. We show selected realistic numerical examples using one of the promising crystals: in one case soliton pulse compression from 50 fs to 15 fs (1.5 cycles) at $3.0\mic$ is achieved, and at the same time a 3-cycle dispersive wave at $5.0\mic$ is formed that can be isolated using a long-pass filter. In another example we show that extremely broadband supercontinua can form spanning the near-IR to the end of the mid-IR (nearly 4 octaves).

Broadband terahertz pulse emission from ZnGeP_2

Optical rectification is demonstrated in (110)-cut ZnGeP2 (ZGP) providing broadband terahertz (THz) generation. The source is compared to both GaP and GaAs over a wavelength range of 1150 nm to 1600 nm and peak-intensity range of 0.5 GW/cm2 to 40 GW/cm2. ZGP peak-to-peak field amplitude is larger than in the other materials due to either lower nonlinear absorption or larger second-order nonlinearity. This material is well suited for broadband THz generation across a wide range of infrared excitation wavelengths.

New Thiolated Nitrophenylhydrazone Crystals for Nonlinear Optics

We investigate new 4-nitophenylhydrazone crystals with thiolated electron donors for nonlinear optical applications. The methylthiolated (SM) electron donor and the biphenyl sulfane (SB) electron donor, which promotes a herringbone packing motif, are incorporated with an acentric nonlinear optical core, nitrophenylhydrazone (NPH), which promotes a λ-shaped packing motif in the crystalline state. SM-NPH (4-(methylthio)benzaldehyde-4-nitrophenylhydrazone) crystals show head-to-tail hydrogen-bonded chains, which outweigh the usual λ-shaped packing of the nitrophenylhydrazone group. SB-NPH crystals (4-(phenylthio)benzaldehyde-4-nitrophenylhydrazone) showsimultaneous λ-shaped packing, based on the hydrogen bonds of nitrophenylhydrazone with the nitro electron acceptor group, and the herringbone-shaped packing of biphenyl sulfane groups. Polar SM-NPH chains pack in a centrosymmetric crystal structure, while SB-NPH crystals exhibit a large macroscopic second-order nonlinearity resulting from an acentric molecular ordering of two rotational isomers. In addition, SB-NPH crystals undergo an irreversible phase transition before melting, with the second phase being also acentric with a strong second harmonic generation efficiency.

Microscopic cascading of second-order molecular nonlinearity: new design principles for enhancing third-order nonlinearity

Herein, we develop a phenomenological model for microscopic cascading and substantiate it with ab initio calculations. It is shown that the concept of local microscopic cascading of a second-order nonlinearity can lead to a third-order nonlinearity, without introducing any new loss mechanisms that could limit the usefulness of our approach. This approach provides a new molecular design protocol, in which the current great successes achieved in producing molecules with extremely large second-order nonlinearity can be used in a supra molecular organization in a preferred orientation to generate very large third-order response magnitudes. The results of density functional calculations for a well-known second-order molecule, (para)nitroaniline, show that a head-to-tail dimer configuration exhibits enhanced third-order nonlinearity, in agreement with the phenomenological model which suggests that such an arrangement will produce cascading due to local field effects.

Large second-order nonlinearity in thermally poled Ge–Sb–Cd–S chalcogenide glass

After thermally poled, second-harmonic generation was observed from 0.85GeS 2·0.1Sb 2S 3·0.05CdS chalcogenide glass by using the Maker fringe method, and the second-order nonlinearity (SON) susceptibility, χ (2) , is 9 pm/V at maximum. The stability of SON is excellent and its theoretical decaying time according to the thermally stimulated depolarization current (TSDC) measurement is as long as 654 days. The origin of SON, the effects of poling parameters and the reasons of angle variation of maximum SH intensity in Maker fringe, were discussed based on dipole reorientation model. With its large and stable SON, this novel nonlinear chalcogenide glass is a very promising material that can be applied to many optical fields in the future.

A STUDY ON THE SECOND-ORDER NONLINEAR OPTICAL PROPERTIES OF AZO-DYE CHROMOPHORES CONTAINING THE ELECTRON-ACCEPTOR GROUP

Two new azo-dye chromophores containing different electron-acceptor groups were synthesized and characterized by 1 H NMR, FT-IR and UV-vis spectra, respectively. The second-order nonlinearity of azo-dyes doped films in poly (methylmethacrylate) and the first hyperpolarizability of the azo molecules were investigated by the Maker Fringe method and the solvatochromic method, respectively. Experimental results showed that both of the two dyes exhibited large second-order nonlinearity (d33 > 30 pm/v) and the stronger electron-acceptor groups in dye molecules would induce larger second-order nonlinearity. This was confirmed by the measurement of the first hyperpolarizability.

Superior characteristics of organic chalcone single crystals as efficient nonlinear optical material

High-quality biaxial single crystals of chalcone derivatives were grown by solution growth technique. Their molecular structures were designed to possess large second-order nonlinearities by choosing proper donor/acceptor groups while retaining a high transparency in the visible and infrared spectral regions. The second-order nonlinear optical coefficients, determined by applying the Maker fringe technique, were found to be much larger than those of LiB3O5, KTiOPO4, KH2PO4, and urea. The advantages, such as easy synthesis and crystal growth, low cutoff wavelength (<450nm), large optical nonlinearity, and high damage threshold (>7.2GW∕cm2), make these organic crystals promising for efficient frequency doubling.

Creating large second-order nonlinearity in twin-hole optical fibre with core at the centre of the two holes

A twin-hole optical fibre with the fibre core located at the centre of the two holes was thermally poled to induce second-order nonlinearity (SON) in the core. The SON spatial distribution was measured with second-harmonic microscopy. It was found that the SON was distributed in a narrow layer, which moved towards the cathode for longer poling time. The whole fibre core became second-harmonic active after the SON layer had reached the core–cladding interface with a maximum value of 0.26 pm/V induced under a poling condition of 3.5 kV, 320°C and 40 min.