Tunable Third-harmonic Generation Research Articles

Tunable third harmonic generation based on high-Q polarization-controlled hybrid phase-change metasurface

Abstract Dielectric metasurfaces have made significant advancements in the past decade for enhancing light–matter interaction at the nanoscale. Particularly, bound states in the continuum (BICs) based on dielectric metasurfaces have been employed to enhance nonlinear harmonic generation. However, conventional nonlinear metasurfaces are typically fixed in their operating wavelength after fabrication. In this work, we numerically demonstrate tunable third harmonic generation (THG) by integrating a dielectric metasurface with the phase-change material Ge2Sb2Te5 (GST). The hybrid phase-change metasurface can support two BICs with different electromagnetic origins, which are transformed into two high-Q quasi-BICs through the introduction of structural asymmetry. The two quasi-BICs are selectively excited by controlling the polarization of incident light, and their wavelengths are tunable due to the phase transition of GST. Notably, the efficiency of THG is significantly enhanced at the fundamental wavelengths corresponding to the two quasi-BICs, and the operating wavelength for THG enhancement can be dynamically tuned through the GST phase transition. Furthermore, the wavelength of THG enhancement can be further tuned by manipulating the polarization of pump light. Additionally, a high-Q analog of electromagnetically induced transparency is numerically achieved through the interaction between a low-Q Mie resonance and a quasi-BIC mode, which also improves the THG efficiency. The high-Q polarization-controlled hybrid phase-change metasurface holds promise for applications in dynamically tunable nonlinear optical devices.

From 393 to 1167 nm Tunable Third-Harmonic Generation in a Mid-Infrared Tellurite Optical Fiber

From 393 to 1167 nm Tunable Third-Harmonic Generation in a Mid-Infrared Tellurite Optical Fiber

Broadband‐Tunable Third‐Harmonic Generation Using Phase‐Change Chalcogenides

Despite remarkable progress in passive nonlinear nanophotonics, dynamically controlled ultracompact nonlinear optical sources with high efficiency remain elusive. Nonvolatility, large refractive index contrast between amorphous and crystalline phases, low thermal threshold for crystallization, and particularly high‐third‐order nonlinear optical susceptibility of phase‐change alloy Ge2Sb2Te5 (GST) make it a promising candidate for active subwavelength metaphotonic structures for realization of third‐harmonic generation (THG) devices. Herein, a GST‐based asymmetric Fabry–Perot cavity is numerically designed and experimentally demonstrated to show a dynamically reconfigurable structure with a large shift of the THG resonant band. In addition, continuous resonant spectral shifting bridged by a precisely controlled semicrystalline phase of GST is realized. Tunable THG with the fundamental wavelength ranging from 1150 to 1400 nm is presented, which corresponds to a broadband THG source in the violet‐blue visible wavelength range. Herein, the potential of GST subwavelength structures as a reliable platform for realization of the broadband‐tunable frequency‐conversion sources for applications such as THG microscopy and optical communication is indicated.

Multistate Tuning of Third Harmonic Generation in Fano‐Resonant Hybrid Dielectric Metasurfaces

AbstractHybrid dielectric metasurfaces have emerged as a promising approach to enhancing near field confinement and thus high optical nonlinearity by utilizing low loss dielectric rather than relatively high loss metallic resonators. A wider range of applications can be realized if more design dimensions can be provided from material and fabrication perspectives to allow dynamic control of light. Here, tunable third harmonic generation (THG) via hybrid metasurfaces with phase change material Ge2Sb2Te5 (GST) deposited on top of amorphous silicon metasurfaces is demonstrated. Fano resonance is excited to confine the incident light inside the hybrid metasurfaces, and an experimental quality factor (Q‐factor ≈ 125) is achieved at the fundamental pump wavelength around 1210 nm. Not only the switching between a turn‐on state of Fano resonance in the amorphous state of GST and a turn‐off state in its crystalline state are demonstrated, but also gradual multistate tuning of THG emission at its intermediate states. A high THG conversion efficiency of η = 2.9 × 10−6% is achieved, which is 32 times more than that of a GST‐based Fabry–Pèrot cavity under a similar pump laser power. Experimental results show the potential of exploring GST‐based hybrid dielectric metasurfaces for tunable nonlinear optical devices.

Fano-resonant silicon photonic crystal slab for efficient third-harmonic generation.

Strong light-matter interactions in resonant photonic nanostructures open up opportunities for enhancing nonlinear responses. In this work, by applying Fano resonances, we experimentally demonstrate efficient third-harmonic generation (THG) obtained with 2D silicon photonic crystal slabs (PCSs) thanks to the field enhancement in the dielectric layer. A 160-fold enhancement of THG is observed in the silicon PCS compared to the unpatterned silicon film. Through slightly changing the radius of the PCS, tunable THG on a single chip is obtained, paving a way for the optical manipulation of harmonic generation.

Broadband, electrically tunable third-harmonic generation in graphene.

Optical harmonic generation occurs when high intensity light (>1010 W m-2) interacts with a nonlinear material. Electrical control of the nonlinear optical response enables applications such as gate-tunable switches and frequency converters. Graphene displays exceptionally strong light-matter interaction and electrically and broadband tunable third-order nonlinear susceptibility. Here, we show that the third-harmonic generation efficiency in graphene can be increased by almost two orders of magnitude by controlling the Fermi energy and the incident photon energy. This enhancement is due to logarithmic resonances in the imaginary part of the nonlinear conductivity arising from resonant multiphoton transitions. Thanks to the linear dispersion of the massless Dirac fermions, gate controllable third-harmonic enhancement can be achieved over an ultrabroad bandwidth, paving the way for electrically tunable broadband frequency converters for applications in optical communications and signal processing.

Tunable Third-Harmonic Generation from Polaritons in the Ultrastrong Coupling Regime

Strong interparticle interactions between polaritons have traditionally stemmed from their exciton component. In this work, we impart a strong third-order nonlinearity to a polaritonic mode by embedding a nonlinear polymethine dye within a high-Q all-metal microcavity. We demonstrate nonlinear microcavities operating in the ultrastrong coupling regime with a normalized coupling ratio of 62%, the highest reported to date. When pumping the lower polariton branch, we observe tunable third-harmonic generation spanning the entire visible spectrum, with internal conversion enhancements more than 3 orders of magnitude larger than in bare films. Transfer matrix calculations indicate that the observed enhancements are consistent with the enhanced pump electric field.

Widely tunable third-harmonic generation in a tellurite microstructured optical fiber.

We designed and fabricated a tellurite (76.5TeO(2)-6Bi(2)O(3)-11.5Li(2)O-6ZnO, mol. %) microstructured optical fiber (TMOF) with four air holes for widely tunable third-harmonic generation (THG). The loss of the TMOF is ~0.2 dB/m at 1550 nm. Widely tunable THG from ~567 to 902 nm is obtained when the TMOF is pumped by an optical parametric oscillator with the pump wavelength changing from ~1700 to 2700 nm. The mechanism of THG in this work is further investigated through the third-harmonic signal pattern, which is due to the high nonlinearity of the TMOF and the high pump power, not from the phase-matching process between the fundamental mode and the high-order TH mode.

Tunable third-harmonic generation in a chalcogenide-tellurite hybrid optical fiber with high refractive index difference

A chalcogenide-tellurite hybrid optical fiber with a step-index structure is fabricated by the rod-in-tube drawing technique. The core is made of 15Ge-3Ga-12Sb-70S (mol. %) glass, and the cladding is made of 78TeO₂-5ZnO-12Li₂O-5Bi₂O₃ (mol. %) glass. The refractive index difference Δn=0.24. Tunable third-harmonic generation from 568 to 869 nm is observed when the optical fiber is pumped by an optical parametric oscillator with the pump wavelength changing from 1700 to 2600 nm.

Third-harmonic generation in an elliptical-core ZBLAN fluoride fiber

We demonstrate third-harmonic generation (THG) in an elliptical-core ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fluoride fiber, for the first time to our best knowledge. Linearly polarized THG around 523 nm is obtained when pumped by a pulse laser at 1560 nm. The extinction ratios of average power and peak power are ~6.7 and ~6.8 dB, respectively, in a 10 m long fiber. The extinction ratios are improved to ~11.1 and ~12.6 dB, respectively, when the fiber length is cut to 35 cm. Tunable THG from 605 to 740 nm is observed when pumped by an optical parametric oscillator with the pump wavelength changed from 1800 to 2200 nm.

Tunable third-harmonic generation in a solid-core tellurite glass fiber

A solid-core tellurite glass fiber with 1.8 dB/m loss at 1.55 μm was made by using the built-in casting preform fabrication method and rod-in-tube fiber drawing technique. Pumping a 10 cm fiber piece with picosecond pulses of 3-5×10(12) W/cm(2), 0.1% of the fundamental power limited by the coherence length of 0.3-5 μm was converted into visible third-harmonic power tunable over a broad near-IR wavelength ranging from 1500 to 1680 nm. Frequency conversion from the mid-IR to near-IR was found to be even more efficient due to the longer coherence lengths of 12-20 μm in the wavelength range of 2200-2500 nm.

Pressure-controlled phase matching to third harmonic in Ar-filled hollow-core photonic crystal fiber

We report tunable third-harmonic generation (THG) in an Ar-filled hollow-core photonic crystal fiber, pumped by broadband <2 microJ, 30 fs pulses from an amplified Ti:sapphire laser system. The overall dispersion is precisely controlled by balancing the negative dielectric susceptibility of the waveguide against the positive susceptibility of the gas. We demonstrate THG to a higher-order guided mode and show that the phase-matched UV wavelength is tunable by adjusting the gas pressure.

Third-order optical spectroscopy of polythiophene

Tunable third-harmonic generation and nondegenerate four-wave mixing experiments were performed on polythiophene thin films to produce six independent nonlinear-optical spectra. An essential states model consisting of three electronic excited states has been compared with the spectra. Inclusion of a Raman state could improve the fit to two of the spectra.

Third-Order Nonlinearities in Organic Polymers and the Role of Microscopic Cascading of Second-Order Hyperpolarizabilities

Abstract We investigate the third-order nonlinearity of DANS and DAN2 side-chain polymers by tunable third-harmonic generation and nonlinear distributed coupling. The large third-order susceptibility can be interpreted in terms of microscopic cascading of second-order hyperpolarizabilities.