Kwan H. Lee

Cooling and Control of a Cavity Optoelectromechanical System

We implement a cavity optoelectromechanical system integrating electrical actuation capabilities of nanoelectromechanical devices with ultrasensitive mechanical transduction achieved via intracavity optomechanical coupling. Electrical gradient forces as large as 0.40 microN are realized, with simultaneous mechanical transduction sensitivity of 1.5x10{-18} m Hz{-1/2} representing a 3 orders of magnitude improvement over any nanoelectromechanical system to date. Optoelectromechanical feedback cooling is demonstrated, exhibiting strong squashing of the in-loop transduction signal. Out-of-loop transduction provides accurate temperature calibration even in the critical paradigm where measurement backaction induces optomechanical correlations.

Interferometric detection of mode splitting for whispering gallery mode biosensors

Sensors based on whispering gallery mode resonators can detect single nanoparticles and even single molecules. Particles attaching to the resonator induce a doublet in the transmission spectrum which provides a self-referenced detection signal. However, in practice this spectral feature is often obscured by the width of the resonance line which hides the doublet structure. This happens particularly in liquid environments that reduce the effective Q factor of the resonator. In this paper we demonstrate an interferometric setup that allows the direct detection of the hidden doublet and thus provides a pathway for developing practical sensor applications.

Wavelength- and material-dependent absorption in GaAs and AlGaAs microcavities

The quality factors of modes in nearly identical GaAs and Al0.18Ga0.82As microdisks are tracked over three wavelength ranges centered at 980, 1460, and 1600 nm, with quality factors measured as high as 6.62×10^5 in the 1600 nm band. After accounting for surface scattering, the remaining loss is due to sub-band-gap absorption in the bulk and on the surfaces. The observed absorption is, on average, 80% greater in AlGaAs than in GaAs and is 540% higher in both materials at 980 nm than at 1600 nm.

Quantum-confined Stark effect in a single InGaN quantum dot under a lateral electric field

The effect of an externally applied lateral electric field upon an exciton confined in a single InGaN quantum dot is studied using microphotoluminescence spectroscopy. The quantum-confined Stark effect causes a shift in the exciton energy of more than 5 meV, accompanied by a reduction in the exciton oscillator strength. The shift has both linear and quadratic terms as a function of the applied field.

Depletion edge translation waveguide crossing optical switch

A waveguide crossing optical switch using the depletion edge translation concept is described. By using a single AlGaAs/GaAs material growth and ion implantation technology, an impedance discontinuity is formed at the intersection of two waveguides. Switching operation has been observed, and a high-speed, small-device-size, and high-extinction-ratio optical switch are expected.<<ETX>>

Registration of single quantum dots using cryogenic laser photolithography

We have registered the position of single InGaAs quantum dots using a cryogenic laser photolithography technique. This is an important advance towards the reproducible fabrication of solid-state cavity quantum electrodynamic devices, a key requirement for commercial exploitation of quantum information processing. The quantum dot positions were registered with an estimated accuracy of 50 nm by fabricating metal alignment markers around them. Photoluminescence spectra from quantum dots before and after marker fabrication were identical except for a small redshift (~1 nm), probably introduced during the reactive ion etching.

Biexciton and exciton dynamics in single InGaN quantum dots

Time-resolved and time-integrated microphotoluminescence spectrometry of exciton and biexciton transitions in a single self-assembled InGaN quantum dot gives sharp peaks, with the biexciton 41 meV higher in energy. Theoretical modelling in the Hartree approximation (using a self-consistent finite difference method) predicts a splitting of up to 51 meV. Time-resolved microphotoluminescence measurements yield a radiative recombination lifetime of 1.0 ± 0.1 ns for the exciton and 1.4 ± 0.1 ns for the biexciton. The data can be fitted to a coupled DE rate equation model, confirming that the exciton state is refilled as biexcitons undergo radiative decay.

Time-resolved and time-integrated photoluminescence studies of coupled asymmetric GaN quantum discs embedded in AlGaN barriers

We have investigated exciton dynamics in asymmetric GaN quantum discs embedded in AlGaN barriers with an Al content of 50% using time-integrated and time-resolved micro-photoluminescence measurements. Emission from the quantum discs emerges at lower energy than that from the GaN nanocolumns, which suggests that GaN quantum discs are strongly affected by the built-in electric field. The lifetimes of localized excitons in quantum discs were obtained. Nonlinear emission from quantum discs under high excitation power was attributed to tunneling of carriers to larger discs from smaller discs.

Determination of Fullerene Scattering Length Density: A Critical Parameter for Understanding the Fullerene Distribution in Bulk Heterojunction Organic Photovoltaic Devices

Fullerene derivatives are commonly used as electron acceptors in combination with (macro)molecular electron donors in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. Understanding the BHJ structure at different electron donor/acceptor ratios is critical to the continued improvement and development of OPVs. The high neutron scattering length densities (SLDs) of the fullerenes provide effective contrast for probing the distribution of the fullerene within the blend in a nondestructive way. However, recent neutron scattering studies on BHJ films have reported a wide range of SLDs ((3.6-4.4) × 10(-6) Å(-2)) for the fullerenes 60-PCBM and 70-PCBM, leading to differing interpretations of their distribution in thin films. In this article, we describe an approach for determining more precisely the scattering length densities of the fullerenes within a polymer matrix in order to accurately quantify their distribution within the active layers of OPV devices by neutron scattering techniques.

Photoluminescence properties of a single GaN nanorod with GaN/AlGaN multilayer quantum disks

Photoluminescence (PL) properties of a single nanorod containing multiple GaN quantum disks separated by AlGaN potential barriers are investigated using micro-PL spectroscopy. Previous studies reported ensemble spectra from many nanorods. The PL spectra show different features depending on the region of the nanorod excited by the laser, including a sharp feature originating from the quantum disk region. The distinct differences between the PL from the different regions are discussed. The results imply that excitons are strongly confined in the quantum disks, and the authors suggest that small quantum disks can be regarded as quantum dots having a discrete density of states.Photoluminescence (PL) properties of a single nanorod containing multiple GaN quantum disks separated by AlGaN potential barriers are investigated using micro-PL spectroscopy. Previous studies reported ensemble spectra from many nanorods. The PL spectra show different features depending on the region of the nanorod excited by the laser, including a sharp feature originating from the quantum disk region. The distinct differences between the PL from the different regions are discussed. The results imply that excitons are strongly confined in the quantum disks, and the authors suggest that small quantum disks can be regarded as quantum dots having a discrete density of states.