Vertical-cavity Surface-emitting Laser Structure Research Articles

Photomodulated reflectance study ofInxGa1−xAs/GaAs/AlAsmicrocavity vertical-cavity surface emitting laser structures in the weak-coupling regime: The cavity/ground-state-exciton resonance

Two InGaAs/GaAs/AlAs vertical-cavity surface emitting laser (VCSEL) structures have been studied by conventional reflectance (R) and photomodulated reflectance (PR) spectroscopies at \ensuremath{\sim}300 K and \ensuremath{\sim}80 K. Growth variations across the samples (2%) give rise to smooth changes in the cavity mode energy so that it can be tuned through the position of resonance with the quantum well (QW) ground-state exciton, by varying the position of measurement. The R spectra show the cavity mode but at best only a weak excitonic feature. In contrast, the PR shows two prominent and distinct signals, and there is a strong enhancement (up to 40 times) at resonance. A theory has been developed for the PR modulation of the coupled cavity and exciton modes, based on energy dependent Seraphin coefficients. This was used to fit all the PR spectra simultaneously in each complete set of position dependent measurements, using seven parameters, only one of which, the cavity mode energy, varied significantly. The resulting cavity mode and excitonic energies do not clearly show an anticrossing behavior near resonance, implying only a weak exciton-cavity coupling. The ability of PR to detect, in a nondestructive manner, both the cavity and exciton modes, and the extent to which they are in resonance, suggests it could be extremely useful in the characterization of VCSEL structures near their operating temperature.

Quantum-Well and Cavity-Mode Resonance Effects in a Vertical-Cavity Surface-Emitting Laser Structure, Observed by Photoreflectance Using Hydrostatic Pressure and Temperature Tuning

Photomodulated reflectance (PR) and conventional reflectance (R) measurements as a function of hydrostatic pressure (up to 1.24 GPa) and temperature (150 to 310 K) were performed on an InGaAs/GaAs/AlAs vertical-cavity surface-emitting laser (VCSEL) structure. Both the cavity mode and the ground-state quantum well (QW) transition could be clearly observed in the PR spectra at all temperatures and pressures, whereas the corresponding R spectra reveal only the cavity mode clearly. Close to the resonance between cavity mode and QW exciton, achieved either by tuning the temperature at a fixed pressure or vice versa, an enhancement of the PR response by almost an order of magnitude was observed. The PR results have been analysed in terms of the lineshapes of the QW exciton and cavity mode and their importance for VCSEL characterisation and fabrication has been discussed.

High-performance small vertical-cavity lasers: a comparison of measured improvements in optical and current confinement in devices using tapered apertures

We analyze the scaling characteristics of the optical and current confinement for three different vertical-cavity surface-emitting laser (VCSEL) structures with tapered apertures. The improvements in scaling have allowed devices with apertures <3 /spl mu/m to have wall-plug efficiencies over 20% at output powers as low as 150 /spl mu/W. The combination of low threshold (<200 /spl mu/A), single modedness, and good wall-plug efficiency even at low output powers makes these devices excellent candidates for short distance (<1 m) interconnects within computers.

Analysis of Bragg reflectors by lateral photoluminescence spectroscopy

A new method for the rapid determination of layer thicknesses in semiconductor Bragg mirrors for vertical cavity surface emitting lasers (VCSEL) has been developed. The photoluminescence spectrum of a complete VCSEL structure for a wavelength of 980 nm is measured in the wafer plane at room temperature with the excitation being normal to the specimen. In the spectrum from 850 to 1000 nm three regions can be distinguished; the long wavelength part of the spectrum (λ > 950 nm) gives iriformation about the quantum well emission, the shorter part (λ > 850 nm) stems from luminescence of heavily doped p type GaAs. A pronounced structure in the range from 900 to 950 nm arises from constructive interference of light generated in the GaAs layers and leaving the specimen parallel to the surface near the angle of total reflection. From the wavelength of these peaks the layer thicknesses within the Bragg mirror can be calculated with high precision.

Analysis of Bragg reflectors by lateral photoluminescence spectroscopy

A new method for the rapid determination of layer thicknesses in semiconductor Bragg mirrors for vertical cavity surface emitting lasers (VCSEL) has been developed. The photoluminescence spectrum of a complete VCSEL structure for a wavelength of 980 nm is measured in the wafer plane at room temperature with the excitation being normal to the specimen. In the spectrum from 850 to 1000 nm three regions can be distinguished; the long wavelength part of the spectrum (λ > 950 nm) gives iriformation about the quantum well emission, the shorter part (λ > 850 nm) stems from luminescence of heavily doped p type GaAs. A pronounced structure in the range from 900 to 950 nm arises from constructive interference of light generated in the GaAs layers and leaving the specimen parallel to the surface near the angle of total reflection. From the wavelength of these peaks the layer thicknesses within the Bragg mirror can be calculated with high precision.

Superlattice AlAs/AlInAs-oxide current aperture for long wavelength InP-based vertical-cavity surface-emitting laser structure

A superlattice AlAs/AlInAs structure which can be formed on an InP substrate is proposed and demonstrated for use in oxide confinement long wavelength surface emitting lasers. The large strain of AlAs layers is relaxed by the use of a superlattice structure showing no defects and uniform oxidations. Also, an AlAs layer on an InP substrate provides us with a relatively large heterobarrier, and will be useful for improving temperature characteristics of long-wavelength vertical-cavity surface-emitting lasers. A current aperture of a few micrometers in diameter was formed by precisely controlling the oxidation time. We have demonstrated a current confinement by this oxide aperture. Low-threshold long-wavelength vertical-cavity surface-emitting lasers with this oxide confinement structure are expected.

Reflectance and Photomodulated Reflectance Studies of Cavity Mode and Excitonic Transitions in an InGaAs/GaAs/AlAs/AlGaAs VCSEL Structure

Angle dependent photomodulated reflectance (PR) and conventional reflectance (R) studies have been performed on an InGaAs/GaAs/AlAs/AlGaAs vertical-cavity surface-emitting laser (VCSEL) structure at room temperature. The PR spectra show three prominent signals: one from the ground-state excitonic transition in the InGaAs quantum well (QW) embedded deep in the structure; Franz-Keldysh oscillations (FKO) from the GaAs layers; and a PR signal attributed to the cavity mode, whereas in the R spectra only the cavity mode is clearly visible. It is found that the ground-state QW transition is not in resonance with the cavity mode. The period of the FKOs shows that the built-in electric field is about 18 kV/cm. The energy positions of the excitonic features are not dependent on the angle of incidence, in contrast to that of the cavity mode, whose angular dependence can be fitted with a very simple model. The full R spectrum of the VCSEL structure has also been calculated using a Jones matrix formalism which not only predicts the actual angular dependence of the cavity mode, but also permits a determination of the layer thicknesses in the VCSEL structure by fitting to the experimental R spectrum. Excellent fits are achieved using only two adjustable parameters; the Ga and Al growth rates, which are found to be 5.3% less, and 2.9% greater, than nominal, respectively. Modelling of the measured PR spectrum of an etched sample shows that the In composition of the QW is 5% higher than the nominal value.

Monolithic 2D-VCSEL array with &gt;2 W CWand &gt;5 W pulsed output power

CW output power in excess of 2 W and pulsed output exceeding 5 W were obtained from a monolithic two-dimensional vertical cavity surface-emitting laser (VCSEL) array. These are the highest CW and pulsed powers achieved from a monolithic VCSEL structure to date.

Current spreading and carrier diffusion in long-wavelength vertical-cavity surface-emitting lasers

Current spreading and carrier diffusion in vertical cavity surface-emitting lasers (VCSELs) critically affect the threshold current density, transverse gain profile, and optical modal characteristics of the devices. We have numerically analyzed these effects for four popular long-wavelength VCSEL structures with wafer-bonded GaAs-AlGaAs Bragg bottom mirrors. The results show that current confinement for p-mirror VCSELs is approximately twice as effective as the corresponding n-mirror VCSEL's although the confinement factor for all practical VCSEL structures studied here is considerably lower than the ideal structure. Calculations also show a strong dependence of current confinement on device aperture size and a tradeoff between the optical modal property and the current confinement efficiency for different apertured devices.

Mode selectivity study of vertical-cavity surface-emitting lasers

Mode selectivity of an air-post index-guided vertical-cavity surface-emitting laser structure operating at 1550 nm is investigated using a full-vector Maxwell-equation solver with a finite-difference time-domain method. The resonance wavelengths, quality factors, and spatial field distributions are calculated for the three lowest-order modes. Transverse-mode competition is quantitatively described as a function of the cavity size and the pillar etch depth.

Dependence of lateral oxidation rate on thickness of AlAs layer of interest as a current aperture in vertical-cavity surface-emitting laser structures

The dependence of the wet oxidation process on the AlAs layer thickness used in selectively oxidized vertical-cavity surface-emitting-laser structures is studied in detail. A theoretical model based on a diffusion-reaction process is proposed. A rapid reduction in the oxidation rate is predicted with a reduction in the layer thickness of the ultrathin AlAs layer. The theoretical predictions are verified through experiments.

A single-mode high-power vertical cavity surface emitting laser

We show how a structural element introduced into a vertical cavity surface emitting laser (VCSEL) results in the device operating in single stable high-order mode. The results from a 55 μm diameter device, λ=772 nm, are presented. The stability of the mode structure is investigated as a function of injection current. A simple theory, based on a modal loss discrimination mechanism in VCSELs with slightly perturbed mirrors, is used to explain the preferred lasing mode in VCSEL structures using a central etched well.

InAsP/InGaAsP periodic gain structure for 1.5 μm vertical cavity surface emitting laser applications

We have studied the capabilities of chemical beam epitaxy (CBE) to produce high-gain media for long-wavelength (1.5 μm) vertical cavity surface emitting lasers (VCSELs). Using a parameter pair of low growth temperature and small V/III ratio the integration of up to 15 highly strained (1.78%) InAsP quantum wells (QWs) into a periodic gain structure (PGS) is successfully demonstrated. In this work we present data of atomic force microscopy (AFM), X-ray diffraction, reflectivity and electro-luminescence measurements that prove the very good structural and optical quality of this CBE grown PGS. As an alternative to conventional multi quantum well (MQW) systems as active layers, a high-performance PGS may be used in a VCSEL structure to reduce the very strict requirements on the InP-based distributed Bragg reflectors (DBRs) or to increase the achievable output power. Due to the use of thickness-reduced InP-based DBRs in conjunction with a PGS as the active region the fabrication of fully epitaxial grown long-wavelength VCSELs might also be possible with CBE.

Reflectance and photomodulated reflectance studies of an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser structure under hydrostatic pressure

An InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser structure has been studied by conventional reflectance (R) and photomodulated reflectance (PR) spectroscopies at room temperature and at different hydrostatic pressures up to 1.24 GPa. A clear cavity-mode signal is observed in both PR and R spectra, whereas the quantum-well ground-state exciton signal is observed only in the PR spectra. The PR spectra also show a strong resonance of the two signals at the highest pressure. The pressure dependences of the energies of the exciton and the cavity mode were determined to be 92 meV/GPa and 15 meV/GPa, respectively.

Vertical-cavity surface-emitting lasers with spatially adjustable DBR reflectivity to enable free-space photonic repeaters

We have created bidirectional high-performance top- and bottom-emitting vertical-cavity surface-emitting lasers (VCSELs) from a single epitaxial wafer. Using a spatially selective oxidation of AlAs layers in hybrid distributed Bragg reflectors (DBRs) of a VCSEL structure, devices processed for top emission had threshold currents of 100-110 /spl mu/A with slope efficiencies of 18%-20%. Devices from the same wafer processed for bottom substrate emission had thresholds of 110-120 /spl mu/A and slope efficiencies of 17% and 18%. This technology will greatly simplify the use of VCSEL's in large three-dimensional (3-D) parallel interconnects, photonic repeaters, and smart pixels.

Analysis and design of vertical-cavity surface-emitting lasers for self-sustained pulsation operation

The generation mechanism of self-sustained pulsation in vertical-cavity surface-emitting lasers (VCSELs) is analyzed with the influence of lateral loss effects, such as self-focusing, lateral diffraction loss, and spatial hole burning, taken into consideration. The condition for self-sustained pulsation is also derived in an analytical form. It is shown that the lateral loss effects have significant influence on the excitation of self-sustained pulsation, especially for devices with small size. Furthermore, the possibility of using diffused quantum-well (QW) structures in VCSELs to enhance self-sustained pulsation is studied. It is found that by using diffused QW structures, the range of core area to obtain self-sustained pulsation is increased. In addition, peak power and repetition frequency of the pulses can also be improved.

Selective etching of AlGaAs/GaAs structures using the solutions of citric acid/H2O2 and de-ionized H2O/buffered oxide etch

Etching results using the solution system of citric acid/H2O2 and de-ionized H2O/buffered oxide etch are shown to provide good selective wet etching of AlGaAs/GaAs structures. For AlxGa1−xAs (x&amp;lt;0.5) layers the selective characteristics of each Al composition strongly depend on the volume ratio of the citric acid/H2O2 solution. The turning volume ratio of the solution, at which etching starts, sensitively depends on Al composition. Additionally, the etch rate of AlyGa1−yAs (y&amp;gt;0.7) quickly decreases with decreasing Al composition in de-ionized H2O/buffered oxide etch solution, providing a high degree of etching selectivity. These simple selective etching processes have been applied to define AlAs/GaAs distributed Bragg reflector mesas in a vertical-cavity surface-emitting laser structure.

Numerical simulation of vertical cavity surface emitting lasers

The semiconductor laser simulator MINILASE is being extended to simulate vertical cavity surface emitting lasers (VCSELs). The electronic system analysis for VCSELs is identical to that for edge emitting lasers. A brief discussion of the capabilities of MINILASE in this domain will be presented. In order to simulate VCSELs, the optical mode solver in MINILASE must be extended to handle the reduced index guiding and significant gain guiding typical of many VCSEL structures. A new approach to solving the optical problem which employs active cavity modes rather than the standard passive cavity modes is developed. This new approach results in an integral eigenvalue equation in required gain amplitudes and corresponding modal fields. Sample results from an early implementation of a gain eigenvalue solver are shown to clarify the possibilities of this approach.

Transform-limited, narrow-linewidth lasing action in organic semiconductor microcavities

Lasing action in organic vertical-cavity surface-emitting laser (OVCSEL) structures is demonstrated. Optically pumped OVCSELs with an active layer composed of a thin-film organic semiconductor tris-(8-hydroxyquinoline) aluminum (Alq3) doped with DCM laser dye produced very narrow linewidth (0.2 +/- 0.1 angstrom), high-power (3 watts) emission that could be varied in different devices from orange to red. The efficient energy transfer from Alq3 to DCM results in a threshold input energy of 300 microjoules per square centimeter. An operational lifetime >10(6) laser pulses was achieved for a device operated well above threshold in atmosphere. The linewidths above threshold are Fourier transform-limited and could potentially be narrowed further.

Long-wavelength (1.55-μm) vertical-cavity lasers with InGaAsP/InP-GaAs/AlAs DBR's by wafer fusion

We propose a novel design for a 1.55-/spl mu/m vertical-cavity surface-emitting laser (VCSEL) structure employing double InGaAsP/InP-GaAs/AlAs distributed Bragg reflectors. The fundamental features of InP/GaAs wafer fusion are examined as a function of load pressure. We demonstrate an exact 1.55-/spl mu/m emission wavelength in the CW mode with low threshold voltage (2.1 V) and low threshold current density (1.8 kA/cm/sup 2/).