Ultrafast Hyperspectral Transient Absorption Spectroscopy: Application to Single Layer Graphene

Felice Gesuele

doi:10.3390/photonics6030095

Abstract

We describe the basic principles and the experimental implementation of the hyperspectral transient absorption technique, based on femtosecond laser sources. In this technique the samples were optically “pumped” using the femtosecond tunable pulse delivered by an Optical Parametric Amplifier, and “probed” for changes in transmission in a broad spectral range with a “white light” laser-generated supercontinuum. The spectra were collected by a pair of multichannel detectors which allowed retrieval of the absorbance change in a wide spectral range in one time. The use of the supercontinuum probe introduced artifacts in the measured 2D data set which could be corrected with a proper calibration of the chirp. The configuration with crossed polarization for pump and probe pulse extended the spectral measured range above and below the pump energy within the same experiment. We showed the versatility of the technique by applying it to the investigation of the charge carrier dynamics in two-dimensional single layer graphene.

Highlights

IntroductionThe development of femtosecond laser solid-state sources at kHz repetition rate and at millijoule pulse-energy has made fs transient absorption (TA) spectroscopy a widely-employed tool for the visualization of ultrafast electron dynamics in condensed matter physics [1,2,3], chemistry [4,5], and biology [6,7].In the TA technique (pump-probe scheme) the change in the transmission or the reflection of a sample, excited by a pump pulse, is measured by a time-delayed probe pulse
The development of femtosecond laser solid-state sources at kHz repetition rate and at millijoule pulse-energy has made fs transient absorption (TA) spectroscopy a widely-employed tool for the visualization of ultrafast electron dynamics in condensed matter physics [1,2,3], chemistry [4,5], and biology [6,7].In the TA technique the change in the transmission or the reflection of a sample, excited by a pump pulse, is measured by a time-delayed probe pulse
Due to the temporal dispersion of the supercontinuum, a variation in the temporal onset of the kinetic traces appeared. This artifact could be corrected by means of a numerical procedure which we describe in detail

Summary

Introduction

The development of femtosecond laser solid-state sources at kHz repetition rate and at millijoule pulse-energy has made fs transient absorption (TA) spectroscopy a widely-employed tool for the visualization of ultrafast electron dynamics in condensed matter physics [1,2,3], chemistry [4,5], and biology [6,7].In the TA technique (pump-probe scheme) the change in the transmission or the reflection of a sample, excited by a pump pulse, is measured by a time-delayed probe pulse. Traditional pump-probe spectroscopy exploits narrow pulses centered at two specific wavelengths (two-color scheme) and makes use of single channel detectors to monitor the probe transmission along with lock-in demodulation to extract the differential signal. This scheme, which allows for low noise measurements, suffers from the fact that it is possible to probe the time-dependent variation of one wavelength at a time. A valid solution is to use broadband fs supercontinuum pulses which can be generated by exploiting self-phase modulation in a transparent medium [8] This allows adoption of a pump-supercontinuum probe scheme. The use of high-speed multichannel detectors and of a data acquisition system (DAQ) for fast readout permits the rapid acquisition of an entire spectrum at a chosen time delay (hyperspectral detection)

Methods

Results

Conclusion