Brenna R. Walsh

Ultrafast Electron Trapping at the Surface of Semiconductor Nanocrystals: Excitonic and Biexcitonic Processes

Aging of semiconductor nanocrystals (NCs) is well-known to attenuate the spontaneous photoluminescence from the band edge excitonic state by introduction of nonradiative trap states formed at the NC surface. In order to explore charge carrier dynamics dictated by the surface of the NC, femtosecond pump/probe spectroscopic experiments are performed on freshly synthesized and aged CdTe NCs. These experiments reveal fast electron trapping for aged CdTe NCs from the single excitonic state (X). Pump fluence dependence with excitonic state-resolved optical pumping enables directly populating the biexcitonic state (XX), which produces further accelerated electron trapping rates. This increase in electron trapping rate triggers coherent acoustic phonons by virtue of the ultrafast impulsive time scale of the surface trapping process. The observed trapping rates are discussed in terms of electron transfer theory.

Terahertz Bandwidth All-Optical Modulation and Logic Using Multiexcitons in Semiconductor Nanocrystals

Optical pumping of semiconductor nanocrystals with femtosecond pulse sequences was performed in order to modulate multiexciton populations. We show for the first time that control of multiexciton populations produces high speed modulation of stimulated emission. Upon the basis of the speed of multiexcitonic processes in nanocrystals, we show modulation rates approaching 1 THz by virtue of strong quantum confinement effects. Employing femtosecond optical pulse sequences, we demonstrate all-optical logic using these nanocrystals in two forms: an AND gate, and an inverter, a key step toward all optical signal processing.

Two-Color Two-Dimensional Electronic Spectroscopy Using Dual Acousto-Optic Pulse Shapers for Complete Amplitude, Phase, and Polarization Control of Femtosecond Laser Pulses

We demonstrate a dual pulse-shaper setup capable of independent polarization, phase, and amplitude control over each pulse. By using active phase stabilization, we achieve a phase stability of ~λ/314 between the two pulse shapers, making the dual-shaper setup suitable for both two-quantum and one-quantum measurements. The setup is compact and easily switchable between pump-probe and collinear geometries. We further illustrate the functionality of the dual-shaper setup by performing two-color 2D visible spectroscopy on colloidal CdSe quantum dots in pump-probe geometry.

Control of phonons in semiconductor nanocrystals via femtosecond pulse chirp-influenced wavepacket dynamics and polarization.

The realistic electronic structure of semiconductor nanocrystals is characterized by excitonic fine structure and atomistic symmetry breakings that are challenging to resolve experimentally. Exciton-phonon coupling is one of the most sensitive measures of the excitonic wave functions of the nanocrystals. Here, we exploit this sensitivity via chirped pulse and polarization resolved femtosecond pump/probe spectroscopy of colloidal CdSe nanocrystals. Pulse chirp measurements and simulations are used to explore the contributions of excited- and ground-state vibrational wavepackets to the observed coherent phonons in the pump/probe signals. Polarization resolved pump/probe spectroscopy is used to explore electronic and vibrational polarization anisotropies. We find no electronic polarization anisotropy, whereas vibrational anisotropy is preserved.

Spectral and spatial contributions to white light generation from InGaN/GaN dot-in-a-wire nanostructures

We analyze epitaxially grown InGaN/GaN dot-in-a-wire heterostructures to relate growth and design properties to trends seen in photoluminescence (PL) and resonance Raman spectra. Temperature-dependent PL measurement of these dot-in-a-wire heterostructures illustrate an expected decrease in integrated PL emission and an unusual narrowing of peak width with increasing temperature. Information extracted from Resonance Raman spectra was utilized in a time-dependent model to analyze and to simulate PL spectra. These spectra were found to be in good agreement with the experimental PL data and provided insight into the broadening mechanisms affecting the samples. PL measurements were taken as a function of position on the sample and radial variation of peak energies was observed. This variation was attributed to the radial temperature gradient present during nanowire growth. These additional characteristics of the nanowire heterostructures will allow for increased understanding of these systems potentials for ap...

Kilohertz generation of high contrast polarization states for visible femtosecond pulses via phase-locked acousto-optic pulse shapers

We present a detailed analysis of a setup capable of arbitrary amplitude, phase, and polarization shaping of broadband visible femtosecond pulses at 1 kHz via a pair of actively phase stabilized acousto-optic programmable dispersive filters arranged in a Mach-Zehnder interferometer geometry. The setup features phase stability values around λ/225 at 580 nm as well as degrees of polarization of at least 0.9 for any polarization state. Both numbers are important metrics to evaluate a setups potential for applications based on polarization-shaped femtosecond pulses, such as fully coherent multi-dimensional electronic spectroscopy.