Transcript (PbS) Quantum Dots . - Florida State University

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 3rd
order nonlinear electric susceptibility c(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
Signal
Full Signal
FWM
Reference
The full signal is the result of the bare
FWM signal along with the reference
controlling the piezoelectric devices to
adjust for environmental disturbances.
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
Absorption: Wt (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.
Emission: Wt (meV)
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] http://jila.colorado.edu/content/meet-jila-monstrs
[2] http://en.wikipedia.org/wiki/Quantum_Dot