High-Resolution Second Harmonic Generation Spectroscopy with Femtosecond Laser Pulses on Excitons in Cu$_2$O

TL;DR

This paper introduces a high-resolution femtosecond laser-based second harmonic generation spectroscopy technique, applied to excitons in Cu$_2$O, achieving detailed spectral insights with minimal linewidths despite broad laser spectra.

Contribution

The paper presents a novel femtosecond laser SHG method with high spectral resolution, applied to excitons in Cu$_2$O, revealing new excitonic states and polarization dependencies.

Findings

Observation of odd parity P-excitons with 100 μeV linewidths

High spectral resolution achieved with femtosecond pulses despite broad laser spectrum

Group theoretical analysis matches experimental polarization dependence

Abstract

We present a novel spectroscopic technique for second harmonic generation (SHG) using femtosecond laser pulses at 30~kHz repetition rate, which nevertheless provides high spectral resolution limited only by the spectrometer. The potential of this method is demonstrated by applying it to the yellow exciton series of Cu$_2$O. Besides even parity states with $S-$ and $D-$ envelope, we also observe odd parity, $P-$ excitons with linewidths down to 100 $\mu$eV, despite of the broad excitation laser spectrum with a full width at half maximum of 14~meV. The underlying light-matter interaction mechanisms of SHG are elaborated by a group theoretical analysis which allows us to determine the linear and circular polarization dependences, in good agreement with experiment.