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

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.