Mid-Infrared Frequency Generation via Intermodal Difference Frequency Generation in AlGaAs-On-Insulator Waveguides

We investigate theoretically mid-infrared (MIR) generation via difference frequency generation in multimode AlGaAs-on insulator (AlGaAs-OI) waveguides. The large refractive index difference between the AlGaAs core and the silica cladding shrinks the modes size down to the sub-μm2 scale, and, together with AlGaAs strong second-order nonlinear polarization, empowers strong nonlinear effects. As a result, efficient MIR generation is obtained in few-cm long waveguides with sub-μm2 transverse section, where higher order modes are exploited to achieve the phase-matching condition. These observations suggest that multimode AlGaAs-OI waveguides could represent a novel promising platform for on-chip, compact MIR sources.

Resonance-forbidden second-harmonic generation in nonlinear photonic crystals

Second harmonic generation through nonlinear nano-photonic structures is important in both classical and quantum applications. It is commonly expected that the second harmonic frequency can always be generated as long as appropriate quadratic nonlinearity is provided by the material and the phase-matching condition is satisfied. Here, we present an anomaly to this common wisdom by showing that second-harmonic dipoles generated in a nonlinear photonic crystal slab can be completely nonradiative. As a result, no energy is transferred from the fundamental frequency to the second harmonic even when the phase-matching condition is satisfied – a phenomenon we call “resonance-forbidden second-harmonic generation”. Through numerical simulation, we identify two mechanisms that can achieve this phenomenon: symmetry protection and parameter tuning. The finite-size effect and the topological origin of this phenomenon are also discussed.

Broadband infrared light source by simultaneous parametric down-conversion

Spontaneous parametric down-conversion is an essential tool for a quantum light source in the infrared region ranging 2–5 µm for the purpose of material identification, chemical analysis, and gas sensing. So far, photon pairs from the process in a nonlinear crystal have low tunability and a narrow spectral range because of the phase-matching condition. Here, we propose a novel type of spontaneous parametric down-conversion processes that overcomes these challenges, where two photon pairs are simultaneously produced in the visible and infrared regions in periodically poled stoichiometric lithium tantalite. It allows broadband and tunable generation of infrared photon pairs that can be employed as an alternative light source for quantum infrared spectroscopy.

Deterministic generation of parametrically driven dissipative Kerr soliton

We theoretically study the nature of parametrically driven dissipative Kerr soliton (PD-DKS) in a doubly resonant degenerate micro-optical parametric oscillator (DR-DμOPO) with the cooperation of χ (2) and χ (3) nonlinearities. Lifting the assumption of close-to-zero group velocity mismatch (GVM) that requires extensive dispersion engineering, we show that there is a threshold GVM above which single PD-DKS in DR-DμOPO can be generated deterministically. We find that the exact PD-DKS generation dynamics can be divided into two distinctive regimes depending on the phase matching condition. In both regimes, the perturbative effective third-order nonlinearity resulting from the cascaded quadratic process is responsible for the soliton annihilation and the deterministic single PD-DKS generation. We also develop the experimental design guidelines for accessing such deterministic single PD-DKS state. The working principle can be applied to different material platforms as a competitive ultrashort pulse and broadband frequency comb source architecture at the mid-infrared spectral range.

Broadband terahertz wave generation from an epsilon-near-zero material

Broadband light sources emitting in the terahertz spectral range are highly desired for applications such as noninvasive imaging and spectroscopy. Conventionally, THz pulses are generated by optical rectification in bulk nonlinear crystals with millimetre thickness, with the bandwidth limited by the phase-matching condition. Here we demonstrate broadband THz emission via surface optical rectification from a simple, commercially available 19 nm-thick indium tin oxide (ITO) thin film. We show an enhancement of the generated THz signal when the pump laser is tuned around the epsilon-near-zero (ENZ) region of ITO due to the pump laser field enhancement associated with the ENZ effect. The bandwidth of the THz signal generated from the ITO film can be over 3 THz, unrestricted by the phase-matching condition. This work offers a new possibility for broadband THz generation in a subwavelength thin film made of an ENZ material, with emerging physics not found in existing nonlinear crystals.

Laser Pulse Shortening via Zero-Dispersion Phase Matching of Parametric Raman Interactions in Crystals

We propose and study the conditions of zero-dispersion phase matching for parametric Raman interactions in birefringent crystals differing by anisotropy of zero-dispersion wavelength and allowing for the spectral tuning of the zero-dispersion phase-matching condition. We choose a highly birefringent crystal of calcite having a wide zero-dispersion anisotropy range for the demonstration of new effects of laser pulse shortening in parametric Raman lasers with spectrally tunable zero-dispersion phase matching. We demonstrate the anti-Stokes (1168 nm) and multi-Stokes (1629 nm) picosecond pulse shortening and self-separation of single 80-ps ultra-short pulse from the zero-dispersion phase-matched parametric Raman lasers that are based on the calcite crystal without using any electro-optical device.

Robust Geometries for Second-Harmonic-Generation in Microrings Exhibiting a 4-Bar Symmetry

Microring resonators made of materials with a zinc-blend or diamond lattice allow exploiting their 4-bar symmetry to achieve quasi-phase matching condition for second-order optical nonlinearities. However, fabrication tolerances impose severe limits on the quasi-phase matching condition, which in turn degrades the generation efficiency. Here, we present a method to mitigate these limitations. As an example, we studied the geometry and the pump wavelength conditions to induce the second-harmonic generation in silicon-based microrings with a second-order susceptibility χzxy(2)≠0. We found the best compromises between performances and experimental requirements, and we unveil a strategy to minimize the impacts of fabrication defects. The method can be easily transferred to other material systems.

Angle-Sensitive Detector Based on Silicon-On-Insulator Photodiode Stacked with Surface Plasmon Antenna

We present a pixel-level angle sensitive detector composed of silicon-on-insulator (SOI) photodiode (PD) stacked with a gold surface plasmon (SP) antenna to affect the direction of the incoming light. The surface plasmons are excited in the grating-type SP antenna and enhance the diffraction efficiency of the grating. The diffracted light is coupled strongly with the propagation light in the SOI waveguide when the phase matching condition is satisfied. The phase matching takes place at a specific angle of light incidence, and the discrimination of the light based on the incident angle is achieved. As spatial patterns in the polar coordinate of the elevation-azimuth angles (θ, ϕ) of the incident light, we present the phase matching condition theoretically, the absorption efficiency in the SOI by simulation, and also the quantum efficiency of the SOI PD experimentally for different SP antennas of one-dimensional (1D) line-and-space (L/S) and two-dimensional (2D) hole array gratings under various polarization angles. 1D grating offers a polarization sensitive angle detection and 2D grating exhibits angle detection in two orthogonal directions, enabling a polarization independent angle sensitivity. A good agreement among the theory, simulation, and experiment are attained. The proposed device features relatively high quantum efficiency as an angle-sensitive pixel (ASP) and gives wider opportunities in applications such as three-dimensional (3D) imaging, depth-of-field extension, and lensless imaging.

Terahertz Birefringence and Dichroism of KTA Crystal

For the first time, we present the spectra of all three components of the refractive index and absorption coefficient of the KTiOAsO4 (KTA) crystal measured by the means of terahertz time-domain spectroscopy in the range of 0.3–2.1 THz. The dispersion of the refractive index is approximated and presented in the form of the Sellmeier equation. We observe a large birefringence ΔnZ-X ≈ 0.76 and dichroism which is attributed to a strong absorption peak in the vicinity of ~1.23–1.25 THz for the Z-axis. However, the crystal can be considered as almost uniaxial due to a close value of nX and nY as well as αX ≈ αY in the region below 0.5 THz. Moreover, KTA crystals can satisfy the phase-matching condition in principal XZ-plane for THz emission on difference frequency generation mechanism. Therefore, the crystal could be considered as an efficient candidate for terahertz wave generator under intense laser pump.

Understanding of Collective Atom Phase Control in Modified Photon Echoes for a Near-Perfect Storage Time-Extended Quantum Memory

A near-perfect storage time-extended photon echo-based quantum memory protocol has been analyzed by solving the Maxwell–Bloch equations for a backward scheme in a three-level system. The backward photon echo scheme is combined with a controlled coherence conversion process via controlled Rabi flopping to a third state, where the control Rabi flopping collectively shifts the phase of the ensemble coherence. The propagation direction of photon echoes is coherently determined by the phase-matching condition between the data (quantum) and the control (classical) pulses. Herein, we discuss the classical controllability of a quantum state for both phase and propagation direction by manipulating the control pulses in both single and double rephasing photon echo schemes of a three-level system. Compared with the well-understood uses of two-level photon echoes, the Maxwell–Bloch equations for a three-level system have a critical limitation regarding the phase change when interacting with an arbitrary control pulse area.