L. A. Graham

Quantum dot vertical-cavity surface-emitting laser with a dielectric aperture

Data are presented on an oxide-confined vertical-cavity surface-emitting laser that uses a quantum dot active region. The laser is grown by molecular beam epitaxy, with the quantum dot active region formed from a five monolayer deposition of In0.50Ga0.35Al0.15As. Lasing occurs at wavelengths corresponding to quantum dot transitions, with a room temperature pulsed threshold as low as 560 μA for a 7 μm diameter oxide aperture.

Spontaneous lifetime control in a native-oxide-apertured microcavity

Spontaneous lifetime control is demonstrated using very small apertured microcavities, with quantum-dot light emitters used to obtain electronic confinement within the aperture. A factor of 2.3 increase in the averaged spontaneous emission rate is achieved due to the optical confinement. The enhancement/inhibition ratio of the spontaneous emission rate tracks the optical mode size and spectral response of the apertured microcavity.

Controlled spontaneous lifetime in microcavity confined InGaAlAs/GaAs quantum dots

Controlled spontaneous lifetimes are demonstrated in InGaAlAs/GaAs quantum dots confined in planar microcavities. Due to their independent and spectrally sharp light emission, the quantum dot emitters provide an excellent means for studying the spontaneous lifetime dependence on microcavity tuning, while maintaining experimental parameters such as temperature and pump intensity constant. The measured lifetime changes are compared with calculated results and show good agreement.

Ultranarrow electroluminescence spectrum from the ground state of an ensemble of self-organized quantum dots

Data are presented on the electroluminescence from an ensemble of self-organized quantum dots excited at low current densities. The ensemble contains ∼105 dots, which produce a ground state spectral emission with a 14 K linewidth of ∼1 meV at low current density (∼5×10−2 A/cm2). While the spectra show clearly discrete energy levels, we suggest that obtaining a single ground state emission from the ensemble may be due to interdot electronic coupling. Spectral broadening decreases for decreasing current density due to electronic state filling, even for the lowest current densities studied.

SPONTANEOUS LIFETIME CONTROL OF QUANTUM DOT EMITTERS IN APERTURED MICROCAVITIES

Data are presented on modified spontaneous emission rates in quantum dot microcavity arrays. Three-dimensional optical confinement is achieved using apertured microcavities with square aperture sizes varied from 5 to 1 μm on a side. A maximum measured increase in the on-resonance spontaneous emission rate over that off resonance is ∼50%, and we identify this enhancement with apertured modes of the microcavity. Oscillations in the spontaneous emission rate versus wavelength correspond to spectral resonances in the higher order aperture modes.

Exciton spectral splitting near room temperature from high contrast semiconductor microcavities

Spectral splitting due to the exciton response of a three InGaAs quantum well active region placed in various high contrast semiconductor microcavities is observed near room temperature. The planar microcavities consist of one-wavelength thick cavity spacers surrounded by AlGaAs/GaAs along with high contrast distributed Bragg reflectors formed from AlxOy/GaAs and MgF/ZnSe. Microcavities having different loss rates are characterized over a range of temperatures using reflectance, transmission, and photoluminescence.

Effects of steam oxidation on a single In0.20Ga0.80As quantum well in a half-wave microcavity VCSEL

Time resolved photoluminescence at 295 degrees K has been used to characterize carrier recombination in a single 80 angstrom In0.20Ga0.80As quantum well before and after wet thermal oxidation of a 300 angstrom Al0.96Ga0.04As layer which is separated from the quantum well by 100 angstrom GaAs and a 225 angstrom Al0.75Ga0.25As barrier layer. Both of these layers are repeated on the other side of the quantum well and all together are typical of a half wave cavity spacer section used in low threshold microcavity VCSELs. Before oxidation the radiative lifetime is 12 ns. After steam oxidation for 5 minutes at 420 degrees Celsius the lifetime and intensity of the photoluminescence remains unchanged. An oxidation time of 10 minutes at the same temperature reduces the radiative lifetime to less than 1 ns and decreases the photoluminescence intensity by a factor of five. In addition, the lifetime and intensity of the photoluminescence remain the same as in the unoxidized case when the Al0.96Ga0.04As layer is etched off in a 1:1 HCl solution, possibly indicating that surface recombination at the Al0.75Ga0.25As barrier is not responsible for the shorter lifetimes in the oxidized samples. Furthermore, secondary ion mass spectrometry data on steam oxidized and unoxidized samples shows the presence of a significant oxygen concentration in the quantum well for oxidized samples that had sub nanosecond lifetimes and no oxygen in the quantum wells for samples that were not steam oxidized and displayed 12 ns lifetimes.

Compact surface-normal coupled optical interconnects with wavelength-division-demultiplexing capability

We present a novel surface-normal optical wavelength-division-demultiplexer (WDDM), working at 750, 770, 790, 810, 830 and 850 nm wavelengths. The device is based on an integration of a planar waveguide, a substrate waveguide and waveguide holograms. The unique optical in-plane to surface-normal conversion converts the difficult three spatial and three angular edge coupling problem into a planar surface one, resulting in a practical compact face-to-face packaging between the photodetector array and the demultiplexer. A six-channel wavelength-division-demultiplexer with equally spaced collinear surface-normal outputs are designed and demonstrated in a polymer-based planar waveguide in conjunction with holograms on a glass substrate.

Purcell effect and the bias-free pulse response of vertical-cavity surface-emitting lasers

Although microcavity confinement has been proposed for some time as a means to improve laser characteristics, it has been difficult to connect these predictions to actual laser characteristics. We believe that the reason for this is at least partly due to the lack of rate equations that accurately include the microcavity confinement in terms of straightforward design parameters such as the mode size or spontaneous linewidth. Instead, great emphasis is placed on the spontaneous coupling coefficient /spl beta/ without considering how /spl beta/ explicitly depends on the cavity and active region parameters. This has prevented the inclusion of the cavity effects in rate equations In general, /spl beta/ depends on the mode volume, the optical loss rate, the spontaneous linewidth, and the spontaneous lifetime, and the expression for /spl beta/ is known explicitly only in the limit that the cavity loss rate greatly exceeds the emitter dephasing rate (or spontaneous linewidth). We present experimental results showing the change in the spontaneous lifetime that can be achieved in apertured-microcavities such as the oxide-confined vertical cavity surface-emitting lasers. A cavity with InGaAlAs quantum dot (QD) light emitters is used to obtain electronic confinement to the aperture.

Observation of enhanced spontaneous decay in 1-μm-aperture microcavities with InGaAlAs quantum dot active regions

Enhanced spontaneous decay rates are observed in 1 μm microcavity structures with both etched and oxidized apertures. The enhanced decay rate is found to track the individual transverse aperture modes of each structure. The decay rate on the aperture modes of the oxide confined structure was ∼2.3 times greater than the free space spontaneous emission rate. The enhancement observed in the etched aperture device was only a factor of 1.37.