Weng W. Chow

Coupled resonator vertical-cavity laser diode

We report the operation of an electrically injected monolithic coupled resonator vertical cavity laser which consists of an active cavity containing In{sub x}Ga{sub 1{minus}x}As quantum wells optically coupled to a passive GaAs cavity. This device demonstrates novel modulation characteristics arising from dynamic changes in the coupling between the active and passive cavities. A composite mode theory is used to model the output modulation of the coupled resonator vertical cavity laser. It is shown that the laser intensity can be modulated by either forward or reverse biasing the passive cavity. Under forward biasing, the modulation is due to carrier induced changes in the refractive index, while for reverse bias operation the modulation is caused by field dependent cavity enhanced absorption.

Feedback and injection locking instabilities in quantum-dot lasers: a microscopically based bifurcation analysis

We employ a nonequilibrium energy balance and carrier rate equation model based on microscopic semiconductor theory to describe the quantum-dot (QD) laser dynamics under optical injection and time-delayed feedback. The model goes beyond typical phenomenological approximations of rate equations, such as the ?-factor, yet allows for a thorough numerical bifurcation analysis, which would not be possible with the computationally demanding microscopic equations. We find that with QD lasers, independent amplitude and phase dynamics may lead to less complicated scenarios under optical perturbations than predicted by conventional models using the ?-factor to describe the carrier-induced refractive index change. For instance, in the short external cavity feedback regime, higher critical feedback strength is actually required to induce instabilities. Generally, the ?-factor should only be used when the carrier distribution can follow the QD laser dynamics adiabatically.

Q-switched operation of a coupled-resonator vertical-cavity laser diode

We report Q-switched operation from an electrically injected monolithic coupled-resonator structure which consists of an active cavity with InGaAs quantum wells optically coupled to a passive cavity. The passive cavity contains a bulk GaAs region which is reverse biased to provide variable absorption at the lasing wavelength of 990 nm. Cavity coupling is utilized to effect large changes in output intensity with only very small changes in passive cavity absorption. The device is shown to produce pulses as short as 150 ps at repetition rates as high as 4 GHz. A rate equation approach is used to model the Q-switched operation yielding good agreement between the experimental and theoretical pulse shape. Small-signal frequency response measurements also show a transition from a slower (∼300 MHz) forward-biased modulation regime to a faster (∼2 GHz) modulation regime under reverse-bias operation.

Sub-wavelength InAs quantum dot micro-disk lasers epitaxially grown on exact Si (001) substrates

Subwavelength micro-disk lasers (MDLs) as small as 1u2009μm in diameter on exact (001) silicon were fabricated using colloidal lithography. The micro-cavity gain medium incorporating five-stacked InAs quantum dot layers was grown on a high crystalline quality GaAs-on-V-grooved-Si template with no absorptive intermediate buffers. Under continuous-wave optical pumping, the MDLs on silicon exhibit lasing in the 1.2-μm wavelength range with low thresholds down to 35u2009μW at 10u2009K. The MDLs compare favorably with devices fabricated on native GaAs substrates and state-of-the-art work reported elsewhere. Feasibility of device miniaturization can be projected by size-dependent lasing characteristics. The results show a promising path towards dense integration of photonic components on the mainstream complementary metal–oxide–semiconductor platform.

Many-body and nonequilibrium effects on relaxation oscillations in a quantum-dot microcavity laser

We investigate many-body and nonequilibrium effects on the dynamical behavior of a quantum-dot laser diode. Simulations, based on the Maxwell-semiconductor-Bloch equations, show strong dependence of the turn-on delay on initial cavity detuning, because of a dynamical shift in the quantum-dot distribution caused by band gap renormalization. Gain switch behavior is found to be insensitive to inhomogeneous broadening, because the balancing between many-body and free-carrier effects inhibits a cavity resonance walk-off. Both the relaxation oscillation damping and frequency are found to increase with decreasing inhomogeneous broadening widths. However, in contrast to bulk and quantum-well lasers, oscillation damping increases less than the frequency.

Bistable output from a coupled-resonator vertical-cavity laser diode

We report a monolithic coupled-resonator vertical-cavity laser with an ion-implanted top cavity and a selectively oxidized bottom cavity which exhibits bistable behavior in the light output versus injection current. Large bistability regions over current ranges as wide as 18 mA have been observed with on/off contrast ratios of greater than 20 dB. The position and width of the bistability region can be varied by changing the bias to the top cavity. Switching between on and off states can be accomplished with changes as small as 250 μW to the electrical power applied to the top cavity. The bistable behavior is the response of the nonlinear susceptibility in the top cavity to the changes in the bottom intracavity laser intensity as the bottom cavity reaches the thermal rollover point.

Many-body effects and self-contained phase dynamics in an optically injected quantum-dot laser

Quantum-dot (QD) lasers exhibit unique properties when subjected to optical injection, e.g. lower sensitivity and less complex dynamics when compared to conventional quantum-well (QW) lasers. These features can be explained by a lower phase-amplitude coupling and a higher damping of relaxation oscillations in QD laser devices. In this work, we investigate an optically injected QD laser and clarify the role of many-body effects. We model the QD laser device using a semi-classical approach based on the semiconductor-Bloch and Maxwells equations. The QD optical transition is modeled with a finite spectral width, accounting for inhomogeneous broadening due to QD imperfections. Furthermore, many-body Coulomb interactions, leading to renormalizations of the single-particle energies, are taken into account assuming the screened Hartree-Fock approximation. Throughout the literature the phase dynamics of the electric field inside the laser cavity is implemented by assuming a constant α-factor. Our model accounts for the effects of α via a more rigorous treatment of light-semiconductor interaction. As a result the model allows to extract a value for the α-factor from the intrinsic phase dynamics of the system. The extracted α-factor is not a constant, but rather changes on the one hand dynamically throughout the simulations, and on the other hand with all operation conditions. Furthermore, the dynamical shift of the band-gap energy due to the Coulomb interactions gives rise to modifications in the locking behavior of the laser, that can not be explained with the simpler free-carrier models.

Extraction of inhomogeneous broadening and nonradiative losses in InAs quantum-dot lasers

We present a method to quantify inhomogeneous broadening and nonradiative losses in quantum dot lasers by comparing the gain and spontaneous emission results of a microscopic laser theory with measurements made on 1.3u2009μm InAs quantum-dot lasers. Calculated spontaneous-emission spectra are first matched to those measured experimentally to determine the inhomogeneous broadening in the experimental samples. This is possible because treatment of carrier scattering at the level of quantum kinetic equations provides the homogeneously broadened spectra without use of free parameters, such as the dephasing rate. We then extract the nonradiative recombination current associated with the quantum-dot active region from a comparison of measured and calculated gain versus current relations.

Coupled resonator vertical cavity laser diodes

We report on electrically injected coupled resonator vertical-cavity laser (CRVCL) diodes, including two novel modulation approaches. We show a side sketch of the CRVCL which consists of a lower active resonator containing 3 InGaAs quantum wells and a passive upper resonator composed of undoped GaAs. In the bottom active cavity we employ selective oxidation of AlGaAs to form buried oxide layers for efficient electrical and optical confinement. Electrical contacts to each cavity provide independent current injection into the resonators.

Coupled resonator vertical cavity laser

The monolithic integration of coupled resonators within a vertical cavity laser opens up new possibilities due to the unique ability to tailor the interaction between the cavities. We report the first electrically injected coupled resonator vertical-cavity laser diode and demonstrate novel characteristics arising form the cavity coupling, including methods for external modulation of the laser. A coupled mode theory is used model the output modulation of the coupled resonator vertical cavity laser.