Stefan Strauf | Stevens Institute of Technology (original) (raw)

Papers by Stefan Strauf

Physical Review B, 2001

Spin-flip Raman scattering has been used to provide direct experimental evidence that shallow pho... more Spin-flip Raman scattering has been used to provide direct experimental evidence that shallow phosphorus acceptors in relaxed ZnSe epitaxial layers grown on GaAs lie at sites of predominantly trigonal local symmetry. As a consequence, the energy of the heavy-hole bound exciton is about 0.45 meV less than that of the light exciton. This splitting is significantly smaller than ͑and of opposite sign and different symmetry to͒ the valence-band splitting of 3 meV observed due to the macroscopic biaxial tensile strain in the same specimens. The behavior is in complete contrast to that of nitrogen acceptors, for which no trigonal field is observed. The experiments provide confirmation of the behavior predicted by previous pseudopotential total-energy calculations for the shallow acceptor states formed by these two different group-V dopants when substituting at selenium sites.

Postconference Digest Quantum Electronics and Laser Science, 2003. QELS., 2003

We demonstrate the generation of triggered single photons at a predetermined and well defined ene... more We demonstrate the generation of triggered single photons at a predetermined and well defined energy and two-photon emission with slightly different emission energies using the radiative recombination of individual nitrogen-bound excitons in ZnSe:N.

physica status solidi (b), 2003

By combined pulsed auto-and cross-correlation measurements (4 K) on individual self-assembled CdS... more By combined pulsed auto-and cross-correlation measurements (4 K) on individual self-assembled CdSe quantum dots, besides triggered single photon emission promising for high repetition frequencies, in particular the generation of time-correlated photon pairs from the sequential radiative biexciton-exciton decay is demonstrated. For the photon pairs a strong collinear polarization degree up to 84.5% was observed which is related to an exciton fine-structure splitting. Together with the measured exciton lifetime an estimate for the relaxation time (! 480 ps) between the fine-structure levels is given.

SPIE Newsroom, 2009

Progress in the fabrication, assembly, and manipulation of nanotubes and graphene-based materials... more Progress in the fabrication, assembly, and manipulation of nanotubes and graphene-based materials promises many technological advances.

The Journal of Physical Chemistry C

Due to their unique electrical properties, graphene nanoribbons (GNRs) show great promise as the ... more Due to their unique electrical properties, graphene nanoribbons (GNRs) show great promise as the building blocks of novel electronic devices. However, these properties are strongly dependent on the geometry of the edges of the graphene devices. Thus far only zigzag and armchair edges have been extensively studied. However, several other self passivating edge reconstructions are possible, and were experimentally observed. Here we utilize the Nonequilibrium Green's Function (NEGF) technique in conjunction with tight binding methods to model quantum transport through armchair, zigzag, and several other self-passivated edge reconstructions. In addition we consider the experimentally relevant cases of mixed edges, where random combinations of possible terminations exist on a given GNR boundary. We find that transport through GNR's with self-passivating edge reconstructions is governed by the sublattice structure of the edges, in a manner similar to their parent zigzag or armchair...

Quantum Dots, Particles, and Nanoclusters IV, 2007

Cavity quantum electrodynamic (QED) effects are studied in semiconductor microcavities embedded w... more Cavity quantum electrodynamic (QED) effects are studied in semiconductor microcavities embedded with InGaAs quantum dots. Evidence of weak coupling in the form of lifetime enhancement (the Purcell effect) and inhibition is found in both oxide-apertured micropillars and photonic crystals. In addition, high-efficiency, low-threshold lasing is observed in the photonic crystal cavities where only 2-4 quantum dots exist within the cavity mode volume and are not in general spectrally resonant. The transition to lasing in these soft turn-on devices is explored in a series of nanocavities by observing the change in photon statistics of the cavity mode with increasing pump power near the threshold.

2006 Conference on Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference, 2006

ABSTRACT We demonstrate characteristics of InAs/GaAs quantum-dot photonic-crystal lasers with hig... more ABSTRACT We demonstrate characteristics of InAs/GaAs quantum-dot photonic-crystal lasers with high spontaneous emission coupling efficiencies and soft-turn-on behaviors. Pronounced bunching signals subside with the increase of pumping, revealing the onset of lasing operations.

2007 Quantum Electronics and Laser Science Conference, 2007

A single photon source is demonstrated using a novel oxide-apertured micropillar cavity embedded ... more A single photon source is demonstrated using a novel oxide-apertured micropillar cavity embedded with InGaAs quantum dots. A bright 80 MHz count rate is enabled by the Purcell effect and charge-tuning of the quantum dots.

MRS Proceedings, 1998

ABSTRACT GaN layers were grown by molecular beam epitaxy and doped with carbon of nominal concent... more ABSTRACT GaN layers were grown by molecular beam epitaxy and doped with carbon of nominal concentrations ranging from 1016 cm-1 to 10 20 cm-1. The incorporation of carbon leads to a reduction of the background electron concentration by one order of magnitude but the material remains n-type. For high carbon concentrations a re-increase of the carrier concentration is observed which is related to selfcompensation. Investigations of the donor-acceptor-pair luminescence show that doping with carbon is accompanied by the generation of a new donor exhibiting a thermal activation energy of about 55 meV. Layers grown by atomic layer epitaxy are marked by an increased intensity of the donor-acceptor-pair band luminescence which is attributed to the enforced incorporation of carbon onto the nitrogen sublattice. The yellow luminescence is found to be a typical feature of all carbon doped layers in contrast to nominally undoped samples.

2005 Quantum Electronics and Laser Science Conference, 2005

We have fabricated L3-type photonic crystal microcavities with embedded InAs/GaAs quantum dots as... more We have fabricated L3-type photonic crystal microcavities with embedded InAs/GaAs quantum dots as active material. Single mode lasing has been found for devices emitting between 910-975 nm showing ultra-low lasing thresholds down to 160 nanoWatt.

LEOS 2008 - 21st Annual Meeting of the IEEE Lasers and Electro-Optics Society, 2008

Recent years have shown tremendous progress in the fabrication of single photon sources (SPS) bas... more Recent years have shown tremendous progress in the fabrication of single photon sources (SPS) based on quantum dots (QDs) embedded in semiconductor micro and nanocavities which are of interest for applications in quantum information science. Here we will review our recent work on high-frequency single photon sources operating at single photon emission rates above 100MHz. To effectively harvest the radiative recombination of QD excitons and to overcome the total internal reflection losses of high index semiconductor materials the emission can be coupled into a cavity mode.

Scientific Reports, 2013

Graphene's unique optoelectronic properties are promising to realize photodetectors with ultrafas... more Graphene's unique optoelectronic properties are promising to realize photodetectors with ultrafast photoresponse over a wide spectral range from far-infrared to ultraviolet radiation. The underlying mechanism of the photoresponse has been a particular focus of recent work and was found to be either photoelectric or photo-thermoelectric in nature and enhanced by hot carrier effects. Graphene supported by a substrate was found to be dominated by the photo-thermoelectric effect, which is known to be an order of magnitude slower than the photoelectric effect. Here we demonstrate fully-suspended chemical vapor deposition grown graphene microribbon arrays that are dominated by the faster photoelectric effect. Substrate removal was found to enhance the photoresponse by four-fold compared to substrate-supported microribbons. Furthermore, we show that the light-current input/output curves give valuable information about the underlying photophysical process responsible for the generated photocurrent. These findings are promising towards wafer-scale fabrication of graphene photodetectors approaching THz cut-off frequencies.

Nature communications, 2013

The bright exciton emission of carbon nanotubes is appealing for optoelectronic devices and funda... more The bright exciton emission of carbon nanotubes is appealing for optoelectronic devices and fundamental studies of light-matter interaction in one-dimensional nanostructures. However, to date, the photophysics of excitons in carbon nanotubes is largely affected by extrinsic effects. Here we perform time-resolved photoluminescence measurements over 14 orders of magnitude for ultra-clean carbon nanotubes bridging an air gap over pillar posts. Our measurements demonstrate a new regime of intrinsic exciton photophysics with prolonged spontaneous emission times up to T1=18 ns, about two orders of magnitude better than prior measurements and in agreement with values hypothesized by theorists about a decade ago. Furthermore, we establish for the first time exciton decoherence times of individual nanotubes in the time domain and find fourfold prolonged values up to T2=2.1 ps compared with ensemble measurements. These first observations motivate new discussions about the magnitude of the int...

Laser Beam Control and Applications, 2006

An oxide aperture is used to confine optical modes in a micropillar structure. This method overco... more An oxide aperture is used to confine optical modes in a micropillar structure. This method overcomes the limitations due to sidewall scattering loss typical in semiconductor etched micropillars. High cavity quality factors (Q) up to 48 000 are determined by external Fabry-Perot cavity scanning measurements, a significantly higher value than prior work in III-V etched micropillars. Measured Q values and estimated mode volumes correspond to a maximum Purcell factor figure of merit value of 72. A Purcell Factor of 2.5 is experimentally observed from a single quantum dot emitter coupled to a high Q cavity mode.