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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 $\mathrm{R}\mathrm{e}\left\{{\mathrm{\epsilon <\#comment/>}}_r\right\}=0$. 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. Temporal reflection and refraction of optical pulses inside a dispersive medium: an analytic approach

   https://doi.org/10.1364/JOSAB.416058  

   Zhang, Junchi; Donaldson, W_R; Agrawal, G_P (February 2021, Journal of the Optical Society of America B)

   We develop an analytic approach for reflection of light at a temporal boundary inside a dispersive medium and derive frequency-dependent expressions for the reflection and transmission coefficients. Using the analytic results, we study the temporal reflection of an optical pulse and show that our results agree fully with a numerical approach used earlier. Our approach provides approximate analytic expressions for the electric fields of the reflected and transmitted pulses. Whereas the width of the transmitted pulse is modified, the reflected pulse is a mirrored version of the incident pulse. When a part of the incident spectrum lies in the region of total internal reflection, both the reflected and transmitted pulses are distorted considerably. 

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3. Spatiospectral characterization of ultrafast pulse-beams by multiplexed broadband ptychography

   https://doi.org/10.1364/OE.433752  

   Goldberger, David; Schmidt, David; Barolak, Jonathan; Ivanic, Bojana; Durfee, Charles_G; Adams, Daniel_E (September 2021, Optics Express)

   Ultrafast pulse-beam characterization is critical for diverse scientific and industrial applications from micromachining to generating the highest intensity laser pulses. The four-dimensional structure of a pulse-beam, $\stackrel{~<\#comment/>}{E}(x,y,z,\mathrm{\omega <\#comment/>})$, can be fully characterized by coupling spatiospectral metrology with spectral phase measurement. When temporal pulse dynamics are not of primary interest, spatiospectral characterization of a pulse-beam provides crucial information even without spectral phase. Here we demonstrate spatiospectral characterization of pulse-beams via multiplexed broadband ptychography. The complex spatial profiles of multiple spectral components, $\stackrel{~<\#comment/>}{E}(x,y,\mathrm{\omega <\#comment/>})$, from modelocked Ti:sapphire and from extreme ultra-violet pulse-beams are reconstructed with minimum intervening optics and no refocusing. Critically, our technique does not require spectral filters, interferometers, or reference pulses. 

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4. 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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5. Ultrabroadband, few-cycle pulses directly from a Mamyshev fiber oscillator

   https://doi.org/10.1364/PRJ.8.000065  

   Ma, Chunyang; Khanolkar, Ankita; Zang, Yimin; Chong, Andy (December 2019, Photonics Research)

   While the performance of mode-locked fiber lasers has been improved significantly, the limited gain bandwidth restricts them from generating ultrashort pulses approaching a few cycles or even shorter. Here we present a novel method to achieve few-cycle pulses ( $\sim 5\text{cycles}$) with an ultrabroad spectrum ( $\sim 400\mathrm{nm}$at $-20\mathrm{dB}$) from a Mamyshev oscillator configuration by inserting a highly nonlinear photonic crystal fiber and a dispersion delay line into the cavity. A dramatic intracavity spectral broadening can be stabilized by the unique nonlinear processes of a self-similar evolution as a nonlinear attractor in the gain fiber and a “perfect” saturable absorber action of the Mamyshev oscillator. To the best of our knowledge, this is the shortest pulse width and broadest spectrum directly generated from a fiber laser. 

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