Cavity-Enhanced Ultrafast Transient Absorption Spectroscopy

TL;DR

This paper introduces a cavity-enhanced ultrafast transient absorption spectroscopy technique using frequency comb lasers and optical cavities, significantly improving sensitivity and enabling detection of extremely low optical densities and molecular concentrations.

Contribution

The authors develop a novel cavity-enhanced method that boosts sensitivity in ultrafast transient absorption spectroscopy using resonant enhancement of both probe and pump pulses.

Findings

Achieved sensitivity of ΔOD = 1×10^{-9}/√Hz with 30 s averaging.

Detected signals from samples with OD as low as 10^{-8}.

Enabled ultrafast spectroscopy at unprecedented low sample densities.

Abstract

We present a new technique using a frequency comb laser and optical cavities for performing ultrafast transient absorption spectroscopy with improved sensitivity. Resonantly enhancing the probe pulses, we demonstrate a sensitivity of $\Delta$OD $ = 1 \times 10^{-9}/\sqrt{\mbox{Hz}}$ for averaging times as long as 30 s per delay point ($\Delta$OD$_{min} = 2 \times 10^{-10}$). Resonantly enhancing the pump pulses allows us to produce a high excitation fraction at high repetition-rate, so that signals can be recorded from samples with optical densities as low as OD $\approx 10^{-8}$, or column densities $< 10^{10}$ molecules/cm$^2$. This high sensitivity enables new directions for ultrafast spectroscopy.