Transient Four-Wave Transient Four-Wave Mixing Spectroscopy Mixing Spectroscopy on PbS Quantum Dotson PbS Quantum Dots

Kevin Blondino (Florida State University)Kevin Blondino (Florida State University)

Advisors: Dr. Denis Karaiskaj, USF (faculty)Advisors: Dr. Denis Karaiskaj, USF (faculty)

Jason Byslma (graduate student)Jason Byslma (graduate student)

10 September 201210 September 2012

IntroductionIntroduction

Four-Wave Mixing – the mixing of three Four-Wave Mixing – the mixing of three electromagnetic waves resulting in a fourth electromagnetic waves resulting in a fourth wave in the phase-matched directionwave in the phase-matched direction

It is a weak process governed by the 3It is a weak process governed by the 3rdrd order nonlinear electric susceptibility order nonlinear electric susceptibility (3)(3)

The JILA Multidimensional Optical The JILA Multidimensional Optical Nonlinear Spectrometer (MONSTR) Nonlinear Spectrometer (MONSTR) produces the FWM signalproduces the FWM signal

ObjectiveObjective

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

linewidth in spectrumlinewidth in spectrum

Excitons and Quantum DotsExcitons and Quantum Dots

Exciton – the bound state of an electron and a Exciton – the bound state of an electron and a hole; a quasi-particle, exists in some insulator hole; a quasi-particle, exists in some insulator and semiconductors.and semiconductors.

Quantum Dot – a portion of a semiconductor Quantum Dot – a portion of a semiconductor whose excitons are confined in all dimensions. whose excitons are confined in all dimensions. Properties of in between semiconductors and Properties of in between semiconductors and discrete molecules [2].discrete molecules [2].

Researched for use in qubits, transistors, and Researched for use in qubits, transistors, and photovoltaics.photovoltaics.

SetupSetup The experiment was done at low temperature (< 5 The experiment was done at low temperature (< 5

K) and with a near-infrared laser wavelength (833 K) and with a near-infrared laser wavelength (833 nm).nm).

3 nm is average size of quantum dot.3 nm is average size of quantum dot. Laser pulses (labeled Laser pulses (labeled aa, , bb, , cc) in box formation.) in box formation.

aa is the excitation pulse, phase conjugated. is the excitation pulse, phase conjugated. b b creates a grating with creates a grating with aa on the sample. on the sample. c c diffracts off the sample, creating the FWM diffracts off the sample, creating the FWM

signal.signal. Ultra-fast (~femtoseconds)Ultra-fast (~femtoseconds)

HeNe laser is used in conjunction to act as a HeNe laser is used in conjunction to act as a reference pulse that is routed around the sample. reference pulse that is routed around the sample. This is referred to as heterodyne detection.This is referred to as heterodyne detection.

JILA MONSTRJILA MONSTRMultidimensional Optical Nonlinear SpecTRometers [1]

SetupSetup

SignalSignal

FWMFWM

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

Reference

Full Signal

BeatingBeating

Beats occur from Beats occur from the periodic the periodic change of pulse change of pulse aa and and bb being in being in and out of phase.and out of phase.

We want to find We want to find out if this is out if this is caused by caused by quantum beating quantum beating or by polarization or by polarization beating.beating.

LinewidthLinewidth

Characterized as the width of a spectral line, or Characterized as the width of a spectral line, or the coherence of the source of light.the coherence of the source of light.

In terms of photovoltaics, the wider the In terms of photovoltaics, the wider the material’s natural linewidth, the higher material’s natural linewidth, the higher efficiency, in general.efficiency, in general.

The exciton dephasing time is inversely The exciton dephasing time is inversely proportional to the homogeneous linewidth.proportional to the homogeneous linewidth.

2D Fourier-Transform 2D Fourier-Transform ScanScan

Emission: Emission: tt (meV) (meV)

Abs

orpt

ion:

A

bsor

ptio

n:

(m

eV)

(m

eV)

The diagonal represents where The diagonal represents where the frequencies are absorbed the frequencies are absorbed and then immediately emitted. and then immediately emitted. This means that the electrons This means that the electrons jump between two energy jump between two energy levels to absorb and emit levels to absorb and emit photons, rather than having a photons, rather than having a third intermediate level. Having third intermediate level. Having only two energy levels is more only two energy levels is more efficient.efficient.

The extra signals represent The extra signals represent scattered light or phonon scattered light or phonon resonances.resonances.

Next StepsNext Steps Find true cause of beatingFind true cause of beating Find a new Find a new TT to minimize beating to minimize beating Do polarization-dependent scans of Do polarization-dependent scans of

excitation beamsexcitation beams Experiment on InAs quantum dots and Experiment on InAs quantum dots and

GaAs quantum wellsGaAs quantum wells

References[1] http://jila.colorado.edu/content/meet-jila-monstrs

[2] http://en.wikipedia.org/wiki/Quantum_Dot