Experimental Tools
Frontier ultrafast spectroscopy for quantum materials
The Kambhampati Group develops and applies advanced ultrafast optical spectroscopies to study quantum materials in real time. Our instrumentation is designed to resolve the coupled dynamics of excitons, multiexcitons, polarons, coherences, and collective optical response on femtosecond to nanosecond timescales.
A defining feature of the group is that we do not simply use standard commercial spectroscopy platforms. We build, modify, and operate highly specialized instruments optimized for difficult quantum-materials problems, especially lead-halide perovskite quantum dots and other nanomaterials whose key optical transitions occur in the blue and visible spectral range.
Our experimental tools allow us to ask not only how fast a process occurs, but what quantum states are involved, how they are coupled, and how those couplings evolve in time.
Coherent Multidimensional Spectroscopy
Coherent multidimensional spectroscopy is the group’s most powerful tool for resolving quantum dynamics in semiconductor nanomaterials. CMDS extends ultrafast spectroscopy beyond ordinary pump–probe measurements by correlating excitation and detection frequencies, separating homogeneous and inhomogeneous broadening, resolving coherent couplings, and distinguishing population dynamics from quantum coherences.
Our CMDS instrument is unique in its ability to operate with broad bandwidth in the blue spectral region required for perovskite quantum dots. This is a technically demanding regime where conventional multidimensional spectroscopy platforms are often limited by pulse bandwidth, phase stability, spectral tunability, and pulse-shaping constraints.
The group’s instrument combines hollow-core-fiber spectral broadening, optical-parametric-amplifier pumping, and fully programmable acousto-optic pulse shaping using Dazzlers. This architecture provides broadband, phase-stable, programmable pulse sequences suitable for coherent nonlinear spectroscopy of quantum-confined materials.
The instrument allows us to measure excitonic couplings, homogeneous linewidths, anti-diagonal spectral dynamics, spectral diffusion, inter-excitonic coherence, exciton–phonon coupling, polaron formation dynamics, biexciton and multiexciton structure, and collective optical response.
This capability is central to our work on the real-time birth of Landau polarons, emergent electronic coherence, decoherence-protected subspaces, and Dicke physics in perovskite quantum dots.
In practical terms, CMDS lets us observe how a quantum material builds its optical response after excitation. It gives access to information that is hidden in one-dimensional transient absorption spectra, including whether spectral features arise from static disorder, dynamical broadening, coherent coupling, population relaxation, or many-body state formation.
Transient Absorption and Multipump Spectroscopy
The group also operates advanced transient absorption spectroscopy platforms for studying population dynamics, gain, excited-state absorption, multiexciton formation, and nonlinear optical response.
Transient absorption remains one of the most versatile probes of semiconductor quantum-dot dynamics, but our implementation goes beyond standard pump–probe spectroscopy. We use programmable pulse shaping to generate multipump pulse sequences in which the timing, phase, amplitude, and spectral content of the excitation pulses can be controlled.
This allows us to perform both coherent and incoherent multipump experiments, depending on the physical question. Coherent pulse sequences can be used to prepare and manipulate quantum superpositions, test phase-sensitive response, and connect transient absorption to nonlinear wave-mixing observables. Incoherent multipump sequences can be used to prepare controlled excited-state populations, isolate state-filling effects, and distinguish sequential population dynamics from coherent optical response.
Our multipump transient absorption tools allow us to study stimulated emission, ground-state bleach, excited-state absorption, optical gain, state-resolved optical pumping, biexciton formation, Auger and nonradiative dynamics, superabsorption-like nonlinear response, gain without conventional inversion, and pump-sequence-dependent optical function.
This platform is central to our work on optical gain in quantum dots. By preparing specific excitonic states and following their subsequent optical response, we can separate true stimulated emission from competing excited-state absorption into the biexciton manifold. This provides a microscopic view of gain that is not accessible from conventional pump–probe spectroscopy alone.
The broader purpose of our multipump TA platform is to turn transient absorption into a programmable nonlinear spectroscopy: a way to prepare, perturb, and interrogate quantum materials with controlled optical histories.
Time-Resolved Photoluminescence
Time-resolved photoluminescence provides the complementary radiative perspective on quantum-materials dynamics. Whereas transient absorption measures changes in optical transmission and nonlinear response, TRPL measures the photons emitted by the material as excited states decay.
The group operates an ultrafast streak-camera-based TRPL system with an exceptionally fast instrument response function measured directly in situ. This is critical because many of the radiative and relaxation processes in quantum dots occur on timescales comparable to or faster than the nominal response of standard TRPL instruments.
By measuring the instrument response under the same experimental conditions used for the sample, we can accurately deconvolve early-time emission dynamics and avoid the ambiguity that often limits ultrafast PL measurements.
Our TRPL system allows us to study radiative recombination rates, exciton lifetimes, superradiant emission, size-dependent radiative scaling, temperature-dependent emission dynamics, delayed emission and relaxation pathways, and coupling between ultrafast state formation and photon emission.
This capability is especially important for connecting coherent spectroscopy to measurable optical function. CMDS can reveal how quantum states form and couple; TRPL shows how those states radiate.
In our work on perovskite quantum dots, TRPL is used to test whether emergent excitonic states and collective oscillator strength observed spectroscopically lead to corresponding changes in radiative recombination. This connection between microscopic quantum dynamics and emitted light is essential for understanding superradiance, optical gain, and device-relevant quantum-optical response.
Integrated Measurement Philosophy
The strength of the group’s experimental program is not any single instrument alone. It is the combination of complementary spectroscopies.
CMDS resolves couplings, coherences, homogeneous linewidths, and emergent quantum-state structure. Multipump transient absorption controls and probes nonlinear population and gain dynamics. Time-resolved photoluminescence measures the radiative consequences of the states created by the material.
Together, these tools provide a complete view of quantum-materials dynamics: how states are prepared, how they evolve, how they couple, how they decohere or persist, and how they emit light.
This integrated platform is designed for discovery-grade spectroscopy. It allows the group to observe phenomena that are invisible to standard measurements and to connect those observations to deep analytic theory. The result is a direct path from experimental signal to physical mechanism.
