Transient Four-Wave Mixing Spectroscopy on PbS Quantum Dots Kevin Blondino (Florida State University) Advisors: Dr. Denis Karaiskaj, USF (faculty) Jason Byslma (graduate student) 10 September 2012

Introduction  Four-Wave Mixing – the mixing of three electromagnetic waves resulting in a fourth wave in the phase-matched direction  It is a weak process governed by the 3 rd order nonlinear electric susceptibility  (3)  The JILA Multidimensional Optical Nonlinear Spectrometer (MONSTR) produces the FWM signal

Objective  To study the light-matter interactions in PbS Quantum Dots for use in solar energy harvesting  Measure exciton dephasing time  Find causes for beating in signal  Determine homogeneous/inhomogeneous linewidth in spectrum

Excitons and Quantum Dots  Exciton – the bound state of an electron and a hole; a quasi-particle, exists in some insulator and semiconductors.  Quantum Dot – a portion of a semiconductor whose excitons are confined in all dimensions. Properties of in between semiconductors and discrete molecules [2].  Researched for use in qubits, transistors, and photovoltaics.

Setup  The experiment was done at low temperature (< 5 K) and with a near-infrared laser wavelength (833 nm).  3 nm is average size of quantum dot.  Laser pulses (labeled a, b, c) in box formation.  a is the excitation pulse, phase conjugated.  b creates a grating with a on the sample.  c diffracts off the sample, creating the FWM signal.  Ultra-fast (~femtoseconds)  HeNe laser is used in conjunction to act as a reference pulse that is routed around the sample. This is referred to as heterodyne detection.

JILA MONSTR Multidimensional Optical Nonlinear SpecTRometers [1]

Setup

SignalFWM The full signal is the result of the bare FWM signal along with the reference controlling the piezoelectric devices to adjust for environmental disturbances. Reference Full Signal

Beating Beats occur from the periodic change of pulse a and b being in and out of phase. We want to find out if this is caused by quantum beating or by polarization beating.

Linewidth  Characterized as the width of a spectral line, or the coherence of the source of light.  In terms of photovoltaics, the wider the material’s natural linewidth, the higher efficiency, in general.  The exciton dephasing time is inversely proportional to the homogeneous linewidth.

2D Fourier-Transform Scan Emission:  t (meV) Absorption:   (meV) The diagonal represents where the frequencies are absorbed and then immediately emitted. This means that the electrons jump between two energy levels to absorb and emit photons, rather than having a third intermediate level. Having only two energy levels is more efficient. The extra signals represent scattered light or phonon resonances.

Next Steps  Find true cause of beating  Find a new T to minimize beating  Do polarization-dependent scans of excitation beams  Experiment on InAs quantum dots and GaAs quantum wells References [1] [2]