High-Field Ultrafast Nonlinearity at Epsilon-Near-Zero – Yan Li (Attosecond Optical Science)
Epsilon-near-zero (ENZ) materials, particularly transparent conductive oxides (TCOs), exhibit extraordinary linear and nonlinear optical properties that enable time-varying modulations of light-matter interactions, offering promising avenues for ultrafast optical applications, such as all-optical signal control and novel time-varying optical designs. Among them, Indium Tin Oxide (ITO) has drawn significant attention recently due to its ability to undergo refractive index changes of one order upon optical excitation. This study presents a comprehensive investigation of the ultrafast nonlinear response of a 310 nm ITO thin film using a time-resolved pump-probe spectroscopy setup. A near-infrared pump excites the electron dynamics, while a probe at the ENZ resonance wavelength records the optical modulation. By varying pump intensities, we identify three nonlinear regimes: a low-intensity regime where modulations are governed by the traditional two temperature model, a saturation regime with constant amplitude modulation and spectral shifts, and a high-intensity regime where a new, rapid relaxation process emerges alongside new unexpected spectral shifts in the transient response. Our results suggest that, at extreme intensities, non-equilibrium carrier dynamics play a role on top of the saturation of Fermi electrons, deviating from traditional two-temperature models. Theoretical modelling with a phenomenological three-temperature approach provides insights into the emergence of ultrafast relaxation times and spectral shifts, bridging the gap between strong-field laser physics and time-varying optical materials. These findings offer new perspectives for applications in ultrafast photonics, optical modulators, and photonic time crystals.
Recent developments on diagrammatic Monte Carlo algorithms – Boyuan Shi (Quantum Science and Technology)
Diagrammatic Monte Carlo methods are powerful quantum many-body algorithms to obtain controlled results in the thermodynamical limit. I will talk about new series and sampling methods, with abundant of examples and benchmarks from SU(N_f) Hubbard model in square and honeycomb lattices. As cross benchmarks, we also implement the state-of-art symmetry breaking connected determinant algorithm to the 2D Hubbard model in the honeycomb lattices and established its applicability from weak to intermediate couplings. This work extends the current scope of diagrammatic Monte Carlo simulations and paves the way for future developments.
Brillouin-Mandelstam scattering in telecommunications optical fiber at millikelvin temperatures – Harsh Rathee (Quantum Science and Technology)
Brillouin-Mandelstam scattering is a strong and readily accessible optical nonlinearity with diverse applications and research directions. For instance, the three-wave mixing process has been employed for narrow-linewidth lasers, distributed sensing, microscopy, and signal processing. While most studies focus on room-temperature operation, there is growing interest in cryogenic operation due to its significant potential for applications and fundamental physics at low temperatures. We measure the Brillouin scattering spectrum in standard single-mode telecommunications fiber at millikelvin (mK) temperatures using a closed-cycle dilution refrigerator and optical heterodyne detection. Our experiments, ranging from 50 mK to 27 K, extend beyond previous studies that used liquid helium-4 cryostats at temperatures above 1 K. At millikelvin temperatures, we observe coherent acoustic interactions with two-level systems (TLS)—microscopic defects in the amorphous material—previously unobserved in optical fiber. The temperature-dependent linewidth aligns with established models of ultrasonic attenuation in amorphous materials, involving intrinsic and thermally-activated scattering as well as incoherent and coherent TLS interactions.