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1. Plasma-assisted light bullets and wavelength scaling of laser filamentation in the long-wavelength infrared

   Grynko, Rostislav; Nagar, Garima; Shim, Bonggu (October 2018, APS Division of Plasma Physics Meeting 2018)

   We model laser filamentation in ZnSe in the mid-infrared (Mid-IR, wavelengths λ = 4 and 6 μm) and the long-wavelength infrared (LWIR, λ = 8 and 10 μm) using carrier-resolved unidirectional pulse propagation equations (UPPE). We predict an unprecedented propagation regime at λ = 8 μm that supports light bullets, which are spatio-temporally non-spreading electromagnetic pulses. Furthermore, in contrast to the previous report in air in the mid-IR, we predict that LWIR light bullets in solids critically rely on plasma-mediated dispersion, which dynamically evolves during multiphoton and tunneling ionization as peak plasma densities reach ρ 6.6 ×10^18 cm-3 . Finally, the plasma-assisted light bullets propagate with sub-cycle pulse durations and peak intensities I = 1.1 ×10^12 W /cm^2 , making them useful for high-harmonic generation and attosecond pulse generation. 

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2. Sub-two-cycle gigawatt-peak-power LWIR OPA for ultrafast nonlinear spectroscopy of condensed state materials

   https://doi.org/10.1364/OL.500550  

   Leshchenko, Vyacheslav; Li, Sha; Agostini, Pierre; DiMauro, Louis_F (September 2023, Optics Letters)

   The application of high-power, few-cycle, long-wave infrared (LWIR, 8–20 µm) pulses in strong-field physics is largely unexplored due to the lack of suitable sources. However, the generation of intense pulses with >6 µm wavelength range is becoming increasingly feasible with the recent advances in high-power ultrashort lasers in the middle-infrared range that can serve as a pump for optical parametric amplifiers (OPA). Here we experimentally demonstrate the feasibility of this approach by building an OPA pumped at 2.4 µm that generates 93 µJ pulses at 9.5 µm, 1 kHz repetition rate with sub-two-cycle pulse duration, 1.6 GW peak power, and excellent beam quality. The results open a wide range of applications in attosecond physics (especially for studies of condensed phase samples), remote sensing, and biophotonics. 

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3. Narrow-bandgap titanium sesquioxide with resonant metasurfaces for enhanced infrared absorption

   https://doi.org/10.1063/5.0240125  

   Babicheva, Viktoriia E; Lock, Evgeniya; Kim, Heungsoo (December 2024, Applied Physics Letters)

   We report on the structural, chemical, and optical properties of titanium sesquioxide Ti2O3 thin films on single-crystal sapphire substrates by pulsed laser deposition. The thin film of Ti2O3 on sapphire exhibits light absorption of around 25%–45% in the wavelength range of 2–10 μm. Here, we design an infrared photodetector structure based on Ti2O3, enhanced by a resonant metasurface, to improve its light absorption in mid-wave and long-wave infrared windows. We show that light absorption in the mid-wave infrared window (wavelength 3–5 μm) in the active Ti2O3 layer can be significantly enhanced from 30%–40% to more than 80% utilizing a thin resonant metasurface made of low-loss silicon, facilitating efficient scattering in the active layer. Furthermore, we compare the absorptance of the Ti2O3 layer with that of conventional semiconductors, such as InSb, InAs, and HgCdTe, operating in the infrared range with a wavelength of 2–10 μm and demonstrate that the absorption in the Ti2O3 film is significantly higher than in these conventional semiconductors due to the narrow-bandgap characteristics of Ti2O3. The proposed designs can be used to tailor the wavelengths of photodetection across the near- and mid-infrared ranges. 

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4. Quasi-continuous wavelength tuning of single-mode interband cascade lasers based on V-coupled cavity

   https://doi.org/10.1117/12.2575270  

   Jingli, Gong; Yang, Hanting; Yang, Rui Q.; He, Jian-Jun (October 2020, Semiconductor Lasers and Applications X)

   Zhu, Ning Hua; Hofmann, Werner H.; He, Jian-Jun (Ed.)

   Mid-infrared semiconductor lasers have a wide range of applications in gas sensing, environmental monitoring, medical diagnosis and other fields. The V-coupled cavity laser (VCCL) approach has been successfully applied in the communication band to achieve single-mode operation with a wide tuning range because of its advantages of no grating, compact structure and simple wavelength control. In this paper, the concept of V-coupled cavity is introduced to the interband cascaded lasers, and a monolithically integrated mid-infrared widely tunable single-mode laser is developed. In addition, we experimentally demonstrated a simple and general algorithm for wavelength tuning controlled by two electrodes synchronously, and realized quasi-continuous tuning of single-mode wavelength in mid-infrared interband cascade laser based on the V-coupled cavity configuration for the first time. In the tuning process, the injection current of the short cavity remains unchanged, and the stepped increase of the long cavity current is equivalent to the realization of discrete tuning with the channel spacing of 1.1 nm determined by the short cavity. With the increase of the injection current of the coupler electrode while fixing the long cavity current, the thermo-optic effect caused by the coupler current will cause the refractive index of the two FP cavities to change together, thus realizing the fine tuning of the laser wavelength. A total tuning range of 53.2 nm has been achieved, from 2.8244 μm to 2.8776 μm, with the temperature adjusted from 110K to 120K. 

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5. Wavelength scaling of the high-intensity laser pulse compression dynamics in gas-filled capillaries

   Nagar, G.; Shim, B. (November 2021, 63rd Annual Meeting of the APS Division of Plasma Physics)

   The multimodal carrier-resolved unidirectional pulse propagation equation is solved to study the wavelength-dependent (λ = 1, 2, 3 and 4 μm) spatio-temporal dynamics, particularly pulse self-compression during high-intensity laser pulse propagation in gas-filled capillaries. We find that pulse self-compression in gas-filled capillaries due to plasma is more efficient for short wavelengths in contrast to wavelength-dependent pulse self-compression in laser filamentation [1]. To explain our finding, a detailed analysis is performed by quantifying the contributions of higher-order modes and calculating the temporal delay among modes, which reveals that pulse self-compression at longer wavelengths does not occur due to larger group velocity mismatch between the fundamental and higher-order modes for longer wavelengths [2]. Our study has important implications for the various fields of high-intensity nonlinear optics in gas-filled capillaries such as supercontinuum generation and high-order harmonic generation [3]. [1] L. Bergé et al., Phys. Rev. A 88, 023816 (2013). [2] G. Nagar and B. Shim, submitted. [3] T. Popmitchev et al. Science 336, 1287 (2012). 

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