R. P. Stanley

Confined Electrons and Photons

The scientific fields of confined electrons and photons have become areas of major efforts worldwide. Their appeal originates in the many facets they offer in fundamental and applied science, in technology and device development, and to high technology, large-scale industries.

Coupled semiconductor microcavities

The coupled semiconductor microcavity is a system in which there are three oscillators, two photonic and one electronic (quantum well excitons). It develops three strongly coupled modes which allow a wide design range for a variety of optoelectronic applications. The MBE grown structure is comprised of two λ sized GaAs cavities containing InxGa1−xAs quantum wells, separated by a common mirror. Reflectivity measurements show both two coupled photon mode behavior and three coupled mode behavior, i.e., two photon and one exciton, depending on the relative position of the exciton resonance.

DUAL-WAVELENGTH LASER EMISSION FROM A COUPLED SEMICONDUCTOR MICROCAVITY

We report photopumped operation of a monolithic coupled semiconductor microcavity laser. The structure consists of two λ-sized GaAs vertical cavities, one on top of the other, coupled together through a common mirror. Due to a wedge induced into each cavity, the detuning between the cavities can be continuously varied when moving across the sample. Depending on the detuning, laser action is simultaneously achieved at two different wavelengths or occurs only at one wavelength. At resonance, we observe coupled dual-wavelength laser emission at two widely spaced wavelengths (13 nm) with the same threshold and same dependence on pump power.We report photopumped operation of a monolithic coupled semiconductor microcavity laser. The structure consists of two λ-sized GaAs vertical cavities, one on top of the other, coupled together through a common mirror. Due to a wedge induced into each cavity, the detuning between the cavities can be continuously varied when moving across the sample. Depending on the detuning, laser action is simultaneously achieved at two different wavelengths or occurs only at one wavelength. At resonance, we observe coupled dual-wavelength laser emission at two widely spaced wavelengths (13 nm) with the same threshold and same dependence on pump power.

Ultrahigh finesse microcavity with distributed Bragg reflectors

We have grown a very high finesse microcavity using distributed Bragg reflectors of AlxGa1−xAs and AlAs. The measured Fabry–Perot mode has a linewidth of 0.84 A at 930 nm. This implies a finesse in excess of 5500 and an effective (mirror corrected) finesse greater than 1450. Comparison with theoretical calculations for such a structure shows that (i) the growth rates are stable to 0.25% over 14 h and (ii) the internal losses are less than 1 cm−1.

Tuning InAs/GaAs quantum dot properties under Stranski-Krastanov growth mode for 1.3 μm applications

In this paper, we present a systematic study of the effect of growth parameters on the structural and optical properties of InAs quantum dot (QD) grown under Stranski-Krastanov mode by molecular beam epitaxy. The dot density is significantly reduced from 1.9x10(10) to 0.6x10(10) cm(-2) as the growth rate decreases from 0.075 to 0.019 ML/s, while the island size becomes larger. Correspondingly, the emission wavelength shifts to the longer side. By increasing the indium fraction in the InGaAs capping layer, the emission wavelength increases further. At indium fraction of 0.3, a ground state transition wavelength as long as 1.4 mum with the excited state transition wavelength of around 1.3 mum has been achieved in our dots. The optical properties of QDs with a ground state transition wavelength of 1.3 mum but with different growth techniques were compared. The QDs grown with higher rate and embedded by InGaAs have a higher intensity saturation level from excitation dependent photoluminescence measurements and a smaller intensity decrease from temperature dependent measurements. Finally, single mirror light emitting diodes with a QD embedded in InGaAs have been fabricated. The quantum efficiency at room temperature is 1.3%, corresponding to a radiative efficiency of 21.5%

Time-resolved optical characterization of InAs/InGaAs quantum dots emitting at 1.3 μm

We present the time-resolved optical characterization of InAs/InGaAs self-assembledquantum dots emitting at 1.3 μm at room temperature. The photoluminescence decay time varies from 1.2 (5 K) to 1.8 ns (293 K). Evidence of thermalization among dots is seen in both continuous-wave and time-resolved spectra around 150 K. A short rise time of 10±2 ps is measured, indicating a fast capture and relaxation of carriers inside the dots.

Matrix effects on the structural and optical properties of InAs quantum dots

InAs quantum dots (QDs) have been grown by molecular-beam epitaxy on different InGaAs or GaAs surface layers to investigate the effect of the matrix on the structural and optical properties of the QDs. The density of QDs directly grown on GaAs is 1.1×1010 cm−2, and increases to 2.3×1010 cm−2 for dots grown on a 1 nm InGaAs layer. Single-mirror light-emitting-diode (SMLED) structures with InAs QDs capped by InGaAs and grown on GaAs and InGaAs layers were fabricated to compare the electroluminescence efficiency between the two structures. The maximum external quantum efficiency for QDs on a GaAs structure is 1.1% while that for QDs on InGaAs is 1.3%. The corresponding radiative efficiency could be deduced to be 17.5% for QDs on GaAs and 21.5% for QDs on InGaAs, respectively.

Recent results and latest views on microcavity LEDs

We are progressively approaching the physical limits of microcavity LEDs (MC-LEDs) for high brightness, high efficiency LEDs. They are promising high efficiency devices and they offer the very attractive prospect of full planar fabrication process. However, to compete with other high efficiency LED schemes, they need to approach or surpass the 50 % efficiency mark. We first explore the limits of planar MC-LEDs in both the GaAlInAsP and GaInAlN materials systems, and show that the single-step extraction limit is in the 40 % range at best, depending on the materials system used, with the largest part of the non-extracted light being emitted into guided modes. The waveguided light can itself be extracted by photon recycling, when the internal quantum efficiency is high. Otherwise, another extraction scheme for that light is provided by various photonic-crystal-assisted extraction schemes. Simple photonic crystals (PCs) appear to lack the omnidirectional extraction properties required. However, more rotation-invariant PCs like Archimedean tilings allow to obtain such extraction with added efficiencies already in the 10% range. We discuss the further improvements to such structures.

The dual wavelength Bi-vertical cavity surface-emitting laser

We present a monolithically integrated vertical coupled cavity surface-emitting laser diode which exhibits stable laser emission at two design wavelengths simultaneously. The device consists of two slightly asymmetric coupled vertical cavities containing strained InGaAs quantum wells as the gain media. The shorter cavity is pumped electrically. Lasing starts on the short wavelength mode at 927 nm. The laser emission then acts as an optical pump for the quantum wells in the longer cavity and provides additional gain for the long wavelength mode, resulting in a subsequent laser emission at 955 nm. With increasing injection current, the device maintains stable emission at the two wavelengths. The threshold for dual lasing is 4 kA/cm2 and dual lasing is stable over six times the threshold.

Lasing properties of disk microcavity based on a circular Bragg reflector

The lasing properties of quantum well structures, where the cavity is defined in the plane of the wells by circular Bragg reflectors are investigated. Diffraction of the in-plane lasing modes into the vertical direction by the circular distributed Bragg reflector (DBR) allows the simultaneous measurement of near-field emission patterns and emission spectra, allowing unambiguous assignment of azimuthal quantum numbers to the lasing modes. The radial quantum number is determined by fitting the lasing spectrum to theory. Lasing is shown to occur in modes whose wave vector is mainly radial, confined by the circular DBR structure, rather than in whispering gallery type modes which are mainly azimuthal.