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1. Rigorous prediction of Raman intensity from multi-layer films

   https://doi.org/10.1364/OE.403705  

   Velson, Nathan_Van; Zobeiri, Hamidreza; Wang, Xinwei (November 2020, Optics Express)

   In the Raman probing of multilayer thin film materials, the intensity of the measured Raman scattered light will be impacted by the thickness of the thin film layers. The Raman signal intensity will vary non-monotonically with thickness due to interference from the multiple reflections of both the incident laser light and the Raman scattered light of thin film interfaces. Here, a method for calculating the Raman signal intensity from a multilayer thin film system based on the transfer matrix method with a rigorous treatment of the Raman signal generation (discontinuity) is presented. This calculation methodology is valid for any thin film stack with an arbitrary number of layers with arbitrary thicknesses. This approach is applied to several thin film material systems, including silicon-on-sapphire thin films, graphene on Si with a SiO₂capping layer, and multilayer MoS₂with the presence of a gap between layers and substrate. Different applications where this method can be used in the Raman probing of thin film material properties are discussed. 

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2. Multidimensional four-wave mixing signals detected by quantum squeezed light

   https://doi.org/10.1073/pnas.2105601118  

   Dorfman, Konstantin; Liu, Shengshuai; Lou, Yanbo; Wei, Tianxiang; Jing, Jietai; Schlawin, Frank; Mukamel, Shaul (August 2021, Proceedings of the National Academy of Sciences)

   Four-wave mixing (FWM) of optical fields has been extensively used in quantum information processing, sensing, and memories. It also forms a basis for nonlinear spectroscopies such as transient grating, stimulated Raman, and photon echo where phase matching is used to select desired components of the third-order response of matter. Here we report an experimental study of the two-dimensional quantum noise intensity difference spectra of a pair of squeezed beams generated by FWM in hot Rb vapor. The measurement reveals details of the χ ( 3 ) susceptibility dressed by the strong pump field which induces an AC Stark shift, with higher spectral resolution compared to classical measurements of probe and conjugate beam intensities. We demonstrate how quantum correlations of squeezed light can be utilized as a spectroscopic tool which unlike their classical counterparts are robust to external noise. 

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3. Relative timing jitter in a counterpropagating all-normal dispersion dual-comb fiber laser

   https://doi.org/10.1364/OPTICA.458339  

   Prakash, Neeraj; Huang, Shu-Wei; Li, Bowen (June 2022, Optica)

   The counterpropagating all-normal dispersion (CANDi) fiber laser is an emerging high-energy single-cavity dual-comb laser source. Its relative timing jitter (RTJ), a critical parameter for dual-comb timing precision and spectral resolution, has not been comprehensively investigated. In this paper, we enhance the state-of-the-art CANDi fiber laser pulse energy from 1 nJ to 8 nJ. We then introduce a reference-free RTJ characterization technique that provides shot-to-shot measurement capability at femtosecond precision. The measurement noise floor reaches $1.6\times <\#comment/>{10}^{-<\#comment/>7}{\mathrm{f}\mathrm{s}}^2/\mathrm{H}\mathrm{z}$, and the corresponding integrated measurement precision is only 1.8 fs (1 kHz, 20 MHz). With this characterization tool, we are able to study the physical origin of the CANDi laser’s RTJ in detail. We first verify that the cavity length fluctuation does not contribute to the RTJ. Then we measure the integrated RTJ to be 39 fs (1 kHz, 20 MHz) and identify the pump relative intensity noise (RIN) to be the dominant factor responsible for it. In particular, pump RIN is coupled to the RTJ through the Gordon–Haus effect. Finally, solutions to reduce the free-running CANDi laser’s RTJ are discussed. This work provides a general guideline to improve the performance of compact single-cavity dual-comb systems such as the CANDi laser, benefitting various dual-comb applications. 

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4. Enhanced stimulated Raman scattering during intense laser propagation

   https://doi.org/10.1364/OE.474104  

   Ryan_Peterson, J.; Hafizi, Bahman; Jones, Theodore_G; Johnson, Luke_A; Gordon, Daniel_F; Chen, Yu-Hsin; Ting, Antonio (December 2022, Optics Express)

   Stimulated Raman scattering is ubiquitous in many high-intensity laser environments. Parametric four-wave mixing between the pump and Raman sidebands can affect the Raman gain, but stringent phase matching requirements and strongly nonlinear dynamics obscure clear understanding of its effects at high laser powers. Here we investigate four-wave mixing in the presence of strong self-focusing and weak ionization at laser powers above the Kerr critical power. Theoretical analysis shows that the plasma generated at focus naturally leads to phase matching conditions suitable for enhanced Raman gain, almost without regard to the initial phase mismatch. Multidimensional nonlinear optical simulations with multiphoton and collisional ionization confirm the enhancement and suggest that it may lead to significantly higher Raman losses in some high-intensity laser environments. 

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5. Nonreciprocal wave propagation and parametric amplification of bulk elastic waves in nonlinear anisotropic materials

   https://doi.org/10.1088/1367-2630/ab61d9  

   Zakeri, Mahsa; Keller, Scott M; Wang, Yuanxun Ethan; Lynch, Christopher S (February 2020, New Journal of Physics)

   Abstract Parametric amplification of an elastic wave and a framework for using elastic waves that could enable a new generation of high performance, low noise acoustic amplifiers, mixers and circulators are presented. Using a novel approach with nonlinear materials produces highly desirable non-reciprocal characteristics. Parametric amplification of a weak elastic signal wave is achieved by an elastic pump wave of higher intensity. By careful selection of material orientation together with precise excitation of signal and pump waves, ‘up frequency conversion’ is suppressed and selective amplification of the elastic signal wave occurs at its original frequency. In addition, a general mathematical framework is developed and used for analytical studies of coupled wave equations in nonlinear anisotropic materials. The results obtained from the analytical studies are verified using a finite element implementation. 

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