23 research outputs found
Single quantum dot spectroscopy using a fiber taper waveguide near-field optic
Photoluminescence spectroscopy of single InAs quantum dots at cryogenic
temperatures (~14 K) is performed using a micron-scale optical fiber taper
waveguide as a near-field optic. The measured collection efficiency of quantum
dot spontaneous emission into the fundamental guided mode of the fiber taper is
estimated at 0.1%, and spatially-resolved measurements with ~600 nm resolution
are obtained by varying the taper position with respect to the sample and using
the fiber taper for both the pump and collection channels.Comment: 4 pages, 3 figure
Cavity Q, mode volume, and lasing threshold in small diameter AlGaAs microdisks with embedded quantum dots
The quality factor (Q), mode volume (Veff), and room-temperature lasing
threshold of microdisk cavities with embedded quantum dots (QDs) are
investigated. Finite element method simulations of standing wave modes within
the microdisk reveal that Veff can be as small as 2(lambda/n)^3 while
maintaining radiation-limited Qs in excess of 10^5. Microdisks of diameter D=2
microns are fabricated in an AlGaAs material containing a single layer of InAs
QDs with peak emission at lambda = 1317 nm. For devices with Veff ~2
(lambda/n)^3, Qs as high as 1.2 x 10^5 are measured passively in the 1.4 micron
band, using an optical fiber taper waveguide. Optical pumping yields laser
emission in the 1.3 micron band, with room temperature, continuous-wave
thresholds as low as 1 microWatt of absorbed pump power. Out-coupling of the
laser emission is also shown to be significantly enhanced through the use of
optical fiber tapers, with laser differential efficiency as high as xi~16% and
out-coupling efficiency in excess of 28%.Comment: 6 figure
Suppression of Edge Recombination in InAs/InGaAs DWELL Solar Cells
The InAs/InGaAs DWELL solar cell grown by MBE is a standard pin diode
structure with six layers of InAs QDs embedded in InGaAs quantum wells placed
within a 200-nm intrinsic GaAs region. The GaAs control wafer consists of the
same pin configuration but without the DWELL structure. The typical DWELL solar
cell exhibits higher short current density while maintaining nearly the same
open-circuit voltage for different scales, and the advantage of higher short
current density is more obvious in the smaller cells. In contrast, the smaller
size cells, which have a higher perimeter to area ratio, make edge
recombination current dominant in the GaAs control cells, and thus their open
circuit voltage and efficiency severely degrade. The open-circuit voltage and
efficiency under AM1.5G of the GaAs control cell decrease from 0.914V and 8.85%
to 0.834V and 7.41%, respectively, as the size shrinks from 5*5mm2 to 2*2mm2,
compared to the increase from 0.665V and 7.04% to 0.675V and 8.17%,
respectively, in the DWELL solar cells
Fiber-coupled, high-Q AlGaAs microdisk cavities for chip-based cavity QED
Fiber-coupled, high-quality-factor (Q > 10^5) AlGaAs microdisks with embedded InAs quantum dots are demonstrated. Microdisk lasers employing evanescent fiber coupling for optical pumping and collection of emission exhibit up to two orders of magnitude improvement in differential efficiency compared to devices probed through normal-incidence, free-space photoluminescence. The fiber
taper coupling is also shown to provide an efficient optical channel for studying and manipulating the microcavity-quantum dot system, with in-fiber cavity mode collection efficiences greater than 20% and cavity loading approaching critical coupling. Progress and current challenges in incorporating such fiber-coupled devices into low temperature experiments for studying coherent interaction between single quantum dot and single cavity mode excitations will be discussed
Photoluminescence measurements of quantum-dot-containing semiconductor microdisk resonators using optical fiber taper waveguides
Fiber taper waveguides are used to improve the efficiency of room temperature
photoluminescence measurements of AlGaAs microdisk resonant cavities with
embedded self-assembled InAs quantum dots. As a near-field collection optic,
the fiber taper improves the collection efficiency from microdisk lasers by a
factor of ~ 15-100 times in comparison to conventional normal incidence
free-space collection techniques. In addition, the fiber taper can serve as a
efficient means for pumping these devices, and initial measurements employing
fiber pumping and collection are presented. Implications of this work towards
chip-based cavity quantum electrodynamics experiments are discussed.Comment: 10 pages, 7 figure
Increased normal incidence photocurrent in quantum dot infrared photodetectors
We have increased the ratio of s-polarization (normal incidence) to p-polarization photocurrent to 50% in a quantum dot-in-a-well based infrared photodetector form the typical s-p polarization ratio about 20%. This improvement was achieved by engineering the dot geometry and the quantum confinement via post growth capping materials of the Stranski Krastanov growth mode quantum dots (QDs). The TEM images show that the height to base ratio of shape engineered QDs was increased to 8 nm/12 nm from the control sample\u27s ratio 4 nm/17 nm. The dot geometry correlates with the polarized photocurrent measurements of the detector. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764905
Optical loss and lasing characteristics of high-quality-factor AlGaAs microdisk resonators with embedded quantum dots
Optical characterization of AlGaAs microdisk resonant cavities with a quantum
dot active region is presented. Direct passive measurement of the optical loss
within AlGaAs microdisk resonant structures embedded with InAs/InGaAs
dots-in-a-well (DWELL) is performed using an optical-fiber-based probing
technique at a wavelength (lambda~1400 nm) that is red-detuned from the dot
emission wavelength (lambda~1200 nm). Measurements in the 1400 nm wavelength
band on microdisks of diameter D = 4.5 microns show that these structures
support modes with cold-cavity quality factors as high as 360,000.
DWELL-containing microdisks are then studied through optical pumping at room
temperature. Pulsed lasing at lambda ~ 1200 nm is seen for cavities containing
a single layer of InAs dots, with threshold values of ~ 17 microWatts,
approaching the estimated material transparency level. Room-temperature
continuous wave operation is also observed.Comment: 4 pages, 3 figure
Laterally Coupled Quantum-Dot Distributed-Feedback Lasers
InAs quantum-dot lasers that feature distributed feedback and lateral evanescent- wave coupling have been demonstrated in operation at a wavelength of 1.3 m. These lasers are prototypes of optical-communication oscillators that are required to be capable of stable single-frequency, single-spatial-mode operation. A laser of this type (see figure) includes an active layer that comprises multiple stacks of InAs quantum dots embedded within InGaAs quantum wells. Distributed feedback is provided by gratings formed on both sides of a ridge by electron lithography and reactive-ion etching on the surfaces of an AlGaAs/GaAs waveguide. The lateral evanescent-wave coupling between the gratings and the wave propagating in the waveguide is strong enough to ensure operation at a single frequency, and the waveguide is thick enough to sustain a stable single spatial mode. In tests, the lasers were found to emit continuous-wave radiation at temperatures up to about 90 C. Side modes were found to be suppressed by more than 30 dB
High-quality-factor AlGaAs optical microcavities for atomic Cs and semiconductor quantum dot cavity QED experiments
High-quality-factor (Q~10^4-10^5) AlGaAs microdisks for atomic and semiconductor quantum dot cavity QED experiments are demonstrated. Cavities embedded with InAs/InGaAs dots-in-a-well exhibit room-temperature pulsed lasing with thresholds <17uW. Continuous wave operation at room-temperature is also observed