Chiral and degenerate perfect absorption on exceptional surfaces.

Engineering light-matter interactions using non-Hermiticity, particularly through spectral degeneracies known as exceptional points (EPs), is an emerging field with potential applications in areas such as cavity quantum electrodynamics, spectral filtering, sensing, and thermal imaging. However, tuning and stabilizing a system to a discrete EP in parameter space is a challenging task. Here, we circumvent this challenge by operating a waveguide-coupled resonator on a surface of EPs, known as an exceptional surface (ES). We achieve this by terminating only one end of the waveguide with a tuneable symmetric reflector to induce a nonreciprocal coupling between the frequency-degenerate clockwise and counterclockwise resonator modes. By operating the system at critical coupling on the ES, we demonstrate chiral and degenerate perfect absorption with squared-Lorentzian lineshape. We expect our approach to be useful for studying quantum processes at EPs and to serve as a bridge between non-Hermitian physics and other fields that rely on radiation engineering.

High spectral resolution second harmonic generation microspectroscopy at thin layer interfaces with broadband continuum pulses.

We demonstrate an effective microspectroscopy technique by tracing the dispersion of second order nonlinear optical susceptibility χ (2) in single atomic layer materials. The experimental method relies on the detection of single-shot second harmonic (SH) spectra from the materials and the subsequent data normalization. The key point in our study is that we used a broadband (˜350 nm) near-infrared femtosecond continuum pulses generated at high repetition rates in a photonic crystal fiber with superior spatial quality and stable spectral power density. This is opposite to the point-by-point laser tuning method in SH generation spectroscopy that was applied extensively in the past and has shown limited precision in obtaining χ (2) dispersion. The continuum pulse technique produces spectral resolution better than 2 meV (<0.3 nm at 450 nm) and shows low (<5-6% rms) signal detection noise allowing the detection of subtle features in the χ (2) spectrum at room temperatures. Fine sub-structure features within the main peak of χ (2) spectra indicate the impact of broadened resonances due to exciton transitions in the single layer materials. • Tailored continuum pulses are used to generate second harmonic signal in non-centrosymmetric semiconductors. • SHG spectrum carries fingerprints of the bandstructure around the direct gap states. • The technique produces fine spectral resolution and much better signal-to-noise ratio compared to point-by-point wavelength tuning methods.