Single CdTe Nanowire Optical Correlator for Femtojoule Pulses

Ultracompact single-layer optical MEMS accelerometer based on evanescent coupling through silicon nanowaveguides

In this paper, a novel optical MEMS accelerometer is proposed based on evanescent coupling between parallel silicon nanowaveguides. The coupling length between nanowaveguides changes due to the input acceleration, leading to a great change of coupling efficiency. As a result, the applied acceleration can be obtained by measuring the transmission of waveguiding light. Simulation results with optical displacement sensing sensitivity of 32.83%/[Formula: see text]m within measurement range of 1.68 g is obtained. This design shows high compactness with no need of assembly, suggesting great potential in applications such as integrated photonic circuits.

Self-frequency-conversion nanowire lasers

Semiconductor nanowires (NWs) could simultaneously provide gain medium and optical cavity for performing nanoscale lasers with easy integration, ultracompact footprint, and low energy consumption. Here, we report III-V semiconductor NW lasers can also be used for self-frequency conversion to extend their output wavelengths, as a result of their non-centrosymmetric crystal structure and strongly localized optical field in the NWs. From a GaAs/In_0.16Ga_0.84As core/shell NW lasing at 1016 nm, an extra visible laser output at 508 nm is obtained via the process of second-harmonic generation, as confirmed by the far-field polarization dependence measurements and numerical modeling. From another NW laser with a larger diameter which supports multiple fundamental lasing wavelengths, multiple self-frequency-conversion lasing modes are observed due to second-harmonic generation and sum-frequency generation. The demonstrated self-frequency conversion of NW lasers opens an avenue for extending the working wavelengths of nanoscale lasers, even to the deep ultraviolet and THz range.

Manipulating Nonlinear Optical Response via Domain Control in Nanocrystal-in-Glass Composites

Nanocrystal-in-glass (NIG) is an exciting class of composites, because it can not only combine the advantages of crystal and glass materials but also potentially generate new physical phenomenon in a cooperative manner. Herein, the nonlinear light-matter interaction processes in a broad range of NIG composites homogeneously embedded with LiNbO₃ are investigated. It is shown that, by rational control of the organization manner of crystal and glass phases, second-harmonic generation (SHG) can be precisely tuned. Importantly, an unusual SHG phenomenon, transverse SHG (TSHG), can be realized in the special region of the microstructure map combined with the features of high loading, nanoscale size, and homogenous distribution of nanocrystals. Furthermore, NIG composites exhibit broadband optical response, allowing TSHG in a wide waveband region to be achieved. Based on the above effects, the applications of the constructed NIG composite for precise measurement of the group velocity and duration of ultrashort optical pulses with femtosecond time scales are demonstrated. Indeed, the findings outline a fundamental principle to design NIG configurations for creating new properties, providing new directions for expanding the scope of NIG functional materials.

Efficient Frequency Mixing of Guided Surface Waves by Atomically Thin Nonlinear Crystals

Monolayer transition metal dichalcogenides possess considerable second-order nonlinear coefficients but a limited efficiency of frequency conversion due to the short interaction length with light under the typical direct illumination. Here, we demonstrate an efficient frequency mixing of the guided surface waves on a monolayer tungsten disulfide (WS₂) by simultaneously lifting the temporal and spatial overlap of the guided wave and the nonlinear crystal. Three orders-of-magnitude enhancement of the conversion efficiency was achieved in the counter-propagating excitation configuration. Also, the frequency-mixing signals are highly collimated, with the emission direction and polarization controlled, respectively, by the pump frequencies and the rotation angle of WS₂ relative to the propagation direction of the guided waves. These results indicate that the rules of nonlinear frequency conversion are applicable even when the crystal is scaled down to the ultimate single-layer limit. This study provides a versatile platform to enhance the nonlinear optical response of 2D materials and favor the scalable generation of a coherent light source and entangled photon pairs.

Self-phase modulation in single CdTe nanowires

We measure the transmission of near-infrared ps pulses through single CdTe nanowires. Benefitting from the strong light confinement and large effective nonlinearity of these nanowires, a significant spectral broadening of ∼ 5 nm and nonlinear phase shift of a few π due to self-phase modulation (SPM) is observed experimentally at coupled peak power of a dozen W with a propagating length down to several hundred µms. A nonlinear-index coefficient (n2) as high as (9.5 ± 1.4) × 10^-17 m²/W at 1550 nm is extracted from transmission spectra, corresponding to a nonlinear parameter (γ) of ∼ 1050 W^-1m^-1. The simulations indicate a spectral broadening more than 1.5 µm in single nanowire when pumped by fs pulses in anomalous dispersion regime. The obtained results suggest that, CdTe nanowire is promising in developing ultracompact nonlinear optical devices for microphotonic circuits.

A pulsewidth measurement technology based on carbon-nanotube saturable absorber

We demonstrate a proof-of-concept saturable absorption based pulsewidth measurement (SAPM) by exploring the intensity dependent nonlinear transmission (i.e., saturable absorption) of low-dimensional material (LDM) carbon nanotubes. A minimum pulse energy of 75 fJ is experimentally detected with an average-power-peak-power product (P_av⋅ P_pk) of 5.44×10^-7 W² near 1550 nm. A minimum detectable pulse energy of 10 fJ with a P_av⋅ P_pk of 1.3×10^-9 W² is estimated with further optimization. The nanometer-level thickness and femtosecond-level decay time of LDMs allow ultrafast light interaction on a very small footprint, which potentially supports chip-scale characterization of ultrafast pulses with minimum distortion.

Hybrid Three-Dimensional Spiral WSe2 Plasmonic Structures for Highly Efficient Second-Order Nonlinear Parametric Processes

Two-dimensional (2D) layered materials, with large second-order nonlinear susceptibility, are currently growing as an ideal candidate for fulfilling tunable nanoscale coherent light through the second-order nonlinear optical parametric processes. However, the atomic thickness of 2D layered materials leads to poor field confinement and weak light-matter interaction at nanoscale, resulting in low nonlinear conversion efficiency. Here, hybrid three-dimensional (3D) spiral WSe₂ plasmonic structures are fabricated for highly efficient second harmonic generation (SHG) and sum-frequency generation (SFG) based on the enhanced light-matter interaction in hybrid plasmonic structures. The 3D spiral WSe₂, with AA lattice stacking, exhibits efficient SH radiation due to the constructive interference of nonlinear polarization between the neighboring atomic layers. Thus, extremely high external SHG conversion efficiency (about 2.437×10⁻⁵) is achieved. Moreover, the ease of phase-matching condition combined with the enhanced light-matter interaction in hybrid plasmonic structure brings about efficient SHG and SFG simultaneously. These results would provide enlightenment for the construction of typical structures for efficient nonlinear processes.

CdTe microwires as mid-infrared optical waveguides

Cadmium telluride (CdTe) has been proven to be an attractive mid-infrared (MIR) material with a large refractive index (~2.68 at 4.5 μm) and broadband transparency (~1 to 25 μm). CdTe microwires (MWs) with diameters from a few to about ten micrometers were fabricated by a thermal evaporation process. MIR light was coupled into and guided through individual MWs. Excellent optical waveguiding properties of these MWs are experimentally obtained within MIR spectral range (up to 8.6 μm), with waveguiding losses from 1.3 to 13 dB/cm. Our results show that CdTe MWs can be used as wavelength or subwavelength-width waveguides for MIR microphotonics or circuits.

Efficient higher-order nonlinear optical effects in CdSe nanowaveguides

Stimulating higher-order nonlinear optical (HO-NLO) response from individual semiconductor nanostructures is challenging due to the low nonlinear coefficients and the small number of molecules within the nanostructures. In this work, we demonstrate efficient third harmonic generation and multi-photon luminescence in CdSe nanowaveguides by means of evanescent wave coupling technique. Under appropriate conditions, a coupling efficiency of 70% can be achieved from an optical microfiber to a single CdSe nanowaveguide, leading to the enhanced HO-NLO effects. Provided a high signal-to-noise ratio, we thus observe a fourth order excitation power dependence of 3-photon luminescence, and we attribute it to surface defect mechanism based on the recombination of free carriers. This work provides an alternative for efficient excitation for HO-NLO, which also makes these hard-to-produce signals more feasible in the applications of nonlinear optical devices.