Research
Our group is interested in coherent light-matter interactions. We use ultrafast pulses to probe coherent interactions between optical excitations in semiconductor nanostructures. Multidimensional coherent spectroscopy is the primary technique in our group. We study and engineer coherent interactions in semiconductor systems using a home-built experiment and numerical simulations.
Lab Facilities
Multidimensional Coherent Spectroscopy
Multidimensional coherent spectroscopy is an extension of the widely-used third-order nonlinear four-wave mixing (FWM) experiment. Three excitation pulses excite a sample and the FWM signal is detected as a function of the inter-pulse delays. A Fourier transform of the signal is taken along two (or more) delays to obtain a multidimensional spectrum. Untangling the signal into multiple frequency axes has several advantages such as frequency-resolved measurements in presence of inhomogeneity, clear identification of coherent coupling between multiple states, and quantification of many-body interactions. We use a collinear excitation geometry of the excitation pulses. All the excitation pulses are tagged with unique frequencies using acousto-optics modulators (AOMs). The FWM signal is then detected through a lock-in amplifier.
Pulse shaper
A consequence of using AOMs is that the excitation pulses get stretched due dispersion. Different frequency components of a laser pulse are separated in time and the pulse is said to have a "chirp". Such excitation pulses lead to unwanted artifacts in the measured signal. In order to get rid of the chirp we have designed a home-built pulse shaper. We use a spatial-light modulator based pulse shaper.
Autocorrelator
The presence of chirp in an optical pulse results in an increase in its pulse duration. Thus, measuring the pulse duration can be used to approximately quantify spectral chirp. We have also developed a home-built intensity autocorrelator to measure the pulse duration. The setup splits the optical pulse into two parts, which are then overlapped in space and time on a BBO crystal. The scanning of the delay between the pulses and the data acquisition is automated.
Ultrafast laser system
We have an amplified laser system with an optical parameteric amplifier (OPA) with pulse compressor from Light Conversion. The OPA allows us to tune the excitation frequency in the range of 660 - 940 nm and 1140 - 2500 nm. Frequency doubling can be used to obtain additional excitation frequencies.
Closed-cycle cryostat
Temperature-dependent studies can be performed using a closed-cycle cryostat from Montana systems. This system allows us to reach temperatures up to 4 K.