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1. Adiabatic frequency shifting in epsilon-near-zero materials: the role of group velocity

   https://doi.org/10.1364/OPTICA.374788  

   Khurgin, Jacob B. ; Clerici, Matteo ; Bruno, Vincenzo ; Caspani, Lucia ; DeVault, Clayton ; Kim, Jongbum ; Shaltout, Amr ; Boltasseva, Alexandra ; Shalaev, Vladimir M. ; Ferrera, Marcello ; et al ( March 2020 , Optica)

   The conversion of a photon’s frequency has long been a key application area of nonlinear optics. It has been discussed how a slow temporal variation of a material’s refractive index can lead to the adiabatic frequency shift (AFS) of a pulse spectrum. Such a rigid spectral change has relevant technological implications, for example, in ultrafast signal processing. Here, we investigate the AFS process in epsilon-near-zero (ENZ) materials and show that the frequency shift can be achieved in a shorter length if operating in the vicinity of$\mathrm{R}\mathrm{e}\{{\mathrm{\epsilon <\#comment/>}}_r\}=0$. We also predict that, if the refractive index is induced by an intense optical pulse, the frequency shift is more efficient for a pump at the ENZ wavelength. Remarkably, we show that these effects are a consequence of the slow propagation speed of pulses at the ENZ wavelength. Our theoretical predictions are validated by experiments obtained for the AFS of optical pulses incident upon aluminum zinc oxide thin films at ENZ. Our results indicate that transparent metal oxides operating near the ENZ point are good candidates for future frequency conversion schemes.

    

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2. Quasi-static optical parametric amplification

   https://doi.org/10.1364/OPTICA.442550  

   Jankowski, Marc ; Jornod, Nayara ; Langrock, Carsten ; Desiatov, Boris ; Marandi, Alireza ; Lončar, Marko ; Fejer, Martin M. ( March 2022 , Optica)

   High-gain optical parametric amplification is an important nonlinear process used both as a source of coherent infrared light and as a source of nonclassical light. In this work, we experimentally demonstrate an approach to optical parametric amplification that enables extremely large parametric gains with low energy requirements. In conventional nonlinear media driven by femtosecond pulses, multiple dispersion orders limit the effective interaction length available for parametric amplification. Here, we use the dispersion engineering available in periodically poled thin-film lithium niobate nanowaveguides to eliminate several dispersion orders at once. The result is a quasi-static process; the large peak intensity associated with a short pump pulse can provide gain to signal photons without undergoing pulse distortion or temporal walk-off. We characterize the parametric gain available in these waveguides using optical parametric generation, where vacuum fluctuations are amplified to macroscopic intensities. In the unsaturated regime, we observe parametric gains as large as 71 dB (118 dB/cm) spanning 1700–2700 nm with pump energies of only 4 pJ. When driven with pulse energies$><\#comment/>10\mathrm{p}\mathrm{J}$, we observe saturated parametric gains as large as 88 dB ($><\#comment/>146\mathrm{d}\mathrm{B}/\mathrm{c}\mathrm{m}$). The devices shown here achieve saturated optical parametric generation with orders of magnitude less pulse energy than previous techniques.

    

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3. Spectrally resolved wedged reversal shearing interferometer

   https://doi.org/10.1364/OL.417997  

   Lam, Billy ; Guo, Chunlei ( April 2021 , Optics Letters)

   In this Letter, we introduce a technique to fully determine the spatio-temporal electric field$E(x,y,t)$of an arbitrary ultrashort pulse. By passing the beam through a wedged reversal shearing interferometer followed by a scanning Michelson interferometer, the field autocorrelation of the shearing interferograms is measured. The spectrum of the shearing interferograms is obtained after a Fourier transform by the Whittaker–Shannon sampling theorem, yielding the amplitude and wavefront information at every wavelength. With the addition of the phase information of a single point, we are able to directly reconstruct the spatio-temporal electric field$E(x,y,t)$of an arbitrary ultrashort pulse.

    

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4. Resonance tuning for dynamic Huygens metasurfaces

   https://doi.org/10.1364/JOSAB.427848  

   Ollanik, Adam J. ; Oguntoye, Isaac ; Ji, Yaping ; Escarra, Matthew D. ( July 2021 , Journal of the Optical Society of America B)

   Metasurfaces with dynamic optical performance have the potential to enable a broad range of applications. We computationally investigate the potential of dielectric Huygens metasurfaces, supporting both electric and magnetic dipole resonances, as a candidate platform for dynamic tuning. The asymmetric response of the two dipole resonances to changes in geometric and material parameters, and the potential for separate control of amplitude and phase, is analyzed. A review of dynamic materials, and their promise and limitations for use in dynamic Huygens metasurfaces, is discussed. Vanadium dioxide (${\mathrm{V}\mathrm{O}}_2$) is recognized as a singularly interesting material, due to its variable refractive index and optical absorption in response to several stimuli. Transmitted phase modulation of$\pm <\#comment/>\mathrm{\pi <\#comment/>}$is computationally demonstrated as a function of decaying resonance utilizing only the first 5% of the insulator-metal transition, corresponding to a temperature change of$<<\#comment/>{10}^{\circ <\#comment/>}\mathrm{C}$. As another case study utilizing asymmetric resonance tuning in response to changing incidence angle, phase modulation ($2\mathrm{\pi <\#comment/>}$range for reflected light and$><\#comment/>1.5\mathrm{\pi <\#comment/>}$for transmitted light) and amplitude modulation (from$\mathrm{R}=1$to$\mathrm{T}=1$) are demonstrated using a simple silicon metasurface with varying incident angle within a range of$\sim <\#comment/>{15}^{\circ <\#comment/>}$on two axes. A promising implementation within a micro-electromechanical system (MEMS)-based spatial light modulator, similar to conventional digital micromirror devices, is discussed.

    

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5. High energy yield bifacial spectrum-splitting photovoltaic system

   https://doi.org/10.1364/AO.391821  

   Chrysler, Benjamin D. ; Kostuk, Raymond K. ( April 2020 , Applied Optics)

   In this paper a photovoltaic system is proposed that achieves high energy yield by integrating bifacial silicon cells into a spectrum-splitting module. Spectrum splitting is accomplished using volume holographic optical elements to spectrally divide sunlight onto an array of photovoltaic cells with different bandgap energies. Light that is reflected from the ground surface onto the rear side of the module is converted by the bifacial silicon cells. The energy yield of the system is optimized by tuning the volume holographic element parameters, such as film thickness, index modulation, and construction point source positions. An example is presented for utility-scale illumination parameters in Tucson, Arizona, that attains an energy yield of$1010\frac{kw\cdot <\#comment/>hr}{yr\cdot <\#comment/>m^2}$, which is 32.8% of the incident solar insolation.

    

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