Ryan R. Cooney

State-resolved studies of biexcitons and surface trapping dynamics in semiconductor quantum dots

Biexcitons in strongly confined, colloidal CdSe quantum dots were investigated with excitonic state selectivity combined with 10 fs temporal precision. Within the first 50 fs, the first excited state of the biexciton was observed. By 100 ps, mixed character biexcitons were observed, comprised of a core exciton and a surface trapped exciton. The size dependence of the biexciton binding energies is reported for these specific biexcitons. Analysis of the spectral signatures of each biexcitonic state yields a quantitative measure of enhanced excited state trapping rates at the surface of the quantum dots. By comparing the biexcitonic signals to the state-filling signals, we show that it is primarily the holes which are trapped at the interface on the 100 ps time scale.

State-resolved manipulations of optical gain in semiconductor quantum dots: Size universality, gain tailoring, and surface effects

Optical gain in strongly confined colloidal semiconductor quantum dots is measured using state resolved pump/probe spectroscopy. Though size tunable optical amplification has been previously reported for these materials, the influence of confinement enhanced multiexcitonic interactions has limited prior demonstrations to specific particle sizes or host media. Here we show that the influence of the interfering multiexcitonic interactions, and hence the development of optical gain, is dependent on the identity of the initially prescribed excitonic state. By maintaining a constant excitonic state in the size tunable electronic structure of these materials, we recover the predicted universal development of optical gain, reflected by size-independent occupation thresholds, and differential gains. In addition, we explicitly compare the influence of surface passivation on the development and lifetime of the optical gain. Furthermore, we introduce a general, state-resolved pumping scheme which enables control over the optical gain spectrum. The capacity to manipulate the optical gain spectra of these spherically confined systems is evident in both the measured stimulated emission and amplified spontaneous emission. We anticipate that state-resolved optical excitation will be a useful method of enabling the development and manipulation of optical gain in any quantized nanostructure.

Controlling Piezoelectric Response in Semiconductor Quantum Dots via Impulsive Charge Localization

By direct observation of coherent acoustic phonons, we demonstrate a novel extrinsic piezoelectric response in colloidal CdSe semiconductor quantum dots. This response is driven by the migration of charges to the surface of the quantum dot on a vibrationally impulsive time scale. Surface- and fluence-dependent studies reveal that the observed carrier capture based piezo response is controllable and is at least an order of magnitude larger than the intrinsic piezo response of wurtzite CdSe.

Experimental tests of effective mass and atomistic approaches to quantum dot electronic structure: Ordering of electronic states

The overall symmetry of the envelope functions for the four lowest energy excitonic states in colloidal CdSe quantum dots are assigned using excitonic state-resolved pump/probe spectroscopy. These experiments yield a direct test of the two prevailing approaches to quantum dot electronic structure: the effective mass and the atomistic approaches. The experimental results are in complete agreement with the atomistic approach, whereas the effective mass approach fails to assign the correct electron symmetries for higher excitonic states.

Noise analysis and noise reduction methods in kilohertz pump-probe experiments

We analyze sources of noise in kilohertz frequency pump/probe experiments and present a method for reducing experimental noise by identifying and filtering noisy shots. The power spectrum of instrumental noise shows high frequency, small amplitude modulations which cannot be averaged out. A histogram analysis shows that low frequency, large amplitude signals pose a serious obstacle to signal averaging for improved signal to noise. In kilohertz frequency pump/probe experiments, this low frequency noise typically arises from laser scatter due to bubbles, dust, and defects. We quantify the effectiveness in analyzing and rejecting these large amplitude signals which can produce a hindrance to the effectiveness of signal averaging.

Colloidal and Self-Assembled Quantum Dots for Optical Gain

In this chapter, the influence of quantum confinement upon light amplification is reviewed. Emphasis is placed upon the two broad classes of three-dimensionally confined nanostructures: self-assembled quantum dots and colloidal quantum dots. Fabrication, carrier dynamics, and state-of-the-art-device results for these classes of quantum dots are reviewed.

Quantized Extrinsic Piezoelectricity in Quantum Dots Revealed by Coherent Acoustic Phonons

Employing real time observation of coherent acoustic phonons, we demonstrate a novel extrinsic piezoelectric response of quantum dots, that is quantized, tunable and an order of magnitude larger than their intrinsic piezo response.

Initial State Selective Femtosecond Dynamics of Semiconductor Quantum Dots

Femtosecond relaxation dynamics of colloidal CdSe quantum dots are measured for different initial excitonic states. These experiments show dramatic, previously unobserved dynamics at all probe wavelengths based upon preparation of the initial state.