Tunneling-injection Quantum Dot Laser Research Articles

Comparison between InP-based quantum dot lasers with and without tunnel injection quantum well and the impact of rapid thermal annealing

An InP based tunnel injection quantum dot (QD) laser and a reference quantum dot laser designed to emit at 1.55 μm were grown by molecular beam epitaxy. The lattice matched tunnel injection (TI) structures consist of an InGaAs quantum well (QW), an InAlGaAs barrier and multiple InAs quantum dot layers. Quantum well and quantum dot test structures as well as the actual laser structures were treated with rapid thermal annealing (RTA) at varying temperatures (680–780 °C). Photoluminescence measurements were performed at 10 K on all samples and a strong effect of this annealing process on the emission properties is observed. The laser structures were processed into broad area lasers. Power-current characteristics were measured and evaluated. A strong improvement in device performance is observed after annealing for QD lasers while for TI-QD lasers no significant improvement was obtained, which might be caused by detuning of band energy alignment and different impact of the RTA process on the material quality of QW and QD layers.

Effect of pumping delay on the modulation bandwidth in double tunneling-injection quantum dot lasers.

The modulation bandwidth of double tunneling-injection (DTI) quantum dot (QD) lasers is studied, taking into account noninstantaneous pumping of QDs. In this advanced type of semiconductor lasers, carriers are first captured from the bulk waveguide region into two-dimensional regions (quantum wells [QWs]); then they tunnel from the QWs into zero-dimensional regions (QDs). The two processes are noninstantaneous and, thus, could delay the delivery of the carriers to the QDs. Here, the modulation bandwidth of DTI QD lasers is calculated as a function of two characteristic times (the capture time from the waveguide region into the QW and the tunneling time from the QW into the QD ensemble) and is shown to increase as either of these times is reduced. The capture and tunneling times of 1 and 0.1 ps, respectively, are shown to characterize fast capture and tunneling processes; as the capture and tunneling times are brought below 1 and 0.1 ps, the bandwidth remains almost unchanged and close to its upper limit.

Effects of Homogeneous and Inhomogeneous Broadening on the Dynamics of Tunneling Injection Quantum Dot Lasers

We present a comprehensive model of a tunneling injection quantum dot laser. The spatially resolved model incorporates homogeneous and inhomogeneous gain broadenings, both of which are found to be important. The tunneling injection improves dynamical properties as a result of an efficient carrier injection into the laser state and also because of an effective lowering of the carrier temperature. The lower temperature yields lower carrier densities, which reduces scattering and hence narrows the homogeneous linewidth, improving the modulation response. The inhomogeneous broadening is naturally found to deteriorate the modulation response.

Carrier Dynamics in Tunneling Injection Quantum Dot Lasers

We describe a detailed model for carrier dynamics in tunneling injection quantum dot lasers. The model includes the spatial dependence of the mobile carriers and the coupling between mobile carriers and confined carriers. The Poisson and Schrodinger equations are solved together, which enables consideration of band structure modification due to an electron-hole interaction. The model is solved both in steady state and for temporal perturbations. Steady-state carrier distributions and the band structure are presented, as well as large and small signal modulation responses.

Effect of the Wetting Layer on the Output Power of a Double Tunneling-Injection Quantum-Dot Laser

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> To suppress bipolar population and hence electron–hole recombination outside quantum dots (QDs), tunneling-injection of electrons and holes into QDs from two separate quantum wells was proposed earlier. Close-to-ideal operating characteristics were predicted for such a double tunneling-injection (DTI) laser. In the Stranski–Krastanow growth mode, a two-dimensional wetting layer (WL) is initially grown followed by the formation of QDs. Due to thermal escape of carriers from QDs, there will be bipolar population and hence electron–hole recombination in the WL, even in a DTI structure. In this work, the light–current characteristic (LCC) of a DTI QD laser is studied in the presence of the WL. Since the opposite sides of a DTI structure are only connected by the current paths through QDs and the WL is located in the n-side of the structure, the only source of holes for the WL is provided by QDs. It is shown that, due to the zero-dimensional nature of QDs, the rate of the hole supply to the WL remains limited with increasing injection current. For this reason, as in the other parts of the structure outside QDs (quantum wells and optical confinement layer), the parasitic electron–hole recombination remains restricted in the WL. As a result, even in the presence of the WL, the LCC of a DTI QD laser becomes increasingly linear at high injection currents, which is a further demonstration of the potential of such a laser for high-power operation. </para>

Output power of a double tunneling-injection quantum dot laser

We develop a comprehensive theoretical model for a double tunneling-injection(DTI) quantum dot (QD) laser. Both electrons and holes are injected into QDsby tunneling from two separate quantum wells (QWs). Ideally, out-tunneling ofeach type of carriers from QDs into the opposite-to-injection-side QW should becompletely blocked; as a result, the parasitic electron–hole recombination outsideQDs will be suppressed and the light–current characteristic (LCC) of a laser willbe strictly linear. To scrutinize the potential of a DTI QD laser for high-poweroperation and the robustness of an actual device, our model includes out-tunnelingleakage of carriers from QDs. We complement our calculations by an analyticalmodel and derive closed-form expressions for the LCC and carrier populationacross the layered structure. We show that, even in the presence of out-tunnelingleakage, the flux of parasitic recombination outside QDs remains restricted withincreasing injection current. As a consequence, the LCC exhibits a remarkablefeature distinguishing the DTI QD laser from other types of injection lasers—itbecomes increasingly linear and the slope efficiency grows closer to unity at highinjection currents. The linearity is due to the fact that the current paths connectingthe opposite sides of the structure lie entirely within the QDs—in view of thethree-dimensional confinement in QDs, the out-tunneling fluxes of carriers from dots arelimited.

Characteristic temperature of a tunneling-injection quantum dot laser: Effect of out-tunneling from quantum dots

A laser structure is studied, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells (QWs). An extended theoretical model is developed allowing for out-tunneling leakage of carriers from QDs into the opposite-to-injection-side QWs (electrons into the p-side QW and holes into the n-side QW). Due to out-tunneling leakage, parasitic recombination of electron-hole pairs occurs outside QDs – in the QWs and optical confinement layer. The threshold current density j th and the characteristic temperature T 0 are shown to be mainly controlled by the recombination in the QWs. Even in the presence of out-tunneling from QDs and recombination outside QDs, a tunneling-injection laser shows potential for significant improvement of temperature stability of j th – the characteristic temperature T 0 remains very high (above 300 K at room temperature) and not significantly affected by the QD size fluctuations.

Experimental demonstration of the polarization-dependent photon-mediated carrier redistribution in tunneling injection InP quantum-dot lasers with external-grating feedback

Measured modal gain spectra of the tunneling injection InP quantum-dot (QD) laser with and without an external feedback are presented to experimentally demonstrate the polarization-dependent photon-mediated carrier redistribution in the tunneling injection QD laser. The peak gain wavelength in the transverse-electric gain narrowing spectra near threshold follows the external feedback wavelength. This indicates that the carrier redistribution in the QD-coupled-quantum well structure is determined by the spectral distribution of the stimulated emission, which can be controlled by external grating.

Analysis of the Linewidth-Enhancement Factor of Long-Wavelength Tunnel-Injection Quantum-Dot Lasers

We have studied the linewidth-enhancement factor of 1.3-mum tunnel-injection quantum-dot (QD) lasers utilizing a rate-equation model that takes into account the injection of electrons directly into the QDs from a coupled quantum well, the presence of wetting layer states, and nonequilibrium carrier relaxation in the QDs. In a conventional separate confinement heterostructure QD laser, plasma effects, which result from a large portion of the injected carriers preferably occupying the barrier and wetting layer states, largely determine the values of the linewidth-enhancement factor and lead to a strong dependence of the linewidth-enhancement factor on injection current. In a tunnel-injection QD laser, however, due to the injection of "cold" electrons directly into the lasing states of the QDs, both the values of linewidth-enhancement factor and the dependence on injection current are substantially reduced. The calculated linewidth-enhancement factors of conventional separate confinement heterostructure and tunnel-injection QD lasers are in excellent agreement with reported experimental values. Our analysis elucidates the role of tunnel injection in achieving near-zero alpha-parameter, which would be important in the design of chirp-free high-speed QD lasers

Design of tunneling injection quantum dot lasers

To implement high quality tunneling injection quantum dot lasers, effects of primary factors on performance of the tunneling injection quantum dot lasers were investigated. The considered factors were tunneling probability, tunneling time and carriers thermal escape time from the quantum well. The calculation results show that with increasing of the ground-state energy level in quantum well, the tunneling probability increases and the tunneling time decreases, while the thermal escape time decreases because the ground-state energy level is shallower. Longitudinal optical phonon-assisted tunneling can be an effective method to solve the problem that both the tunneling time and the thermal escape time decrease simultaneously with the ground-state energy level increasing in quantum well

Growth and characteristics of ultralow threshold 1.45μm metamorphic InAs tunnel injection quantum dot lasers on GaAs

The molecular beam epitaxial growth and characteristics of 1.45μm metamorphic InAs quantum dot tunnel injection lasers on GaAs have been studied. Under optimized growth conditions, the quantum dots exhibit photoluminescence linewidths ∼30meV and high intensity at room temperature. The lasers are characterized by ultralow threshold current (63A∕cm2), large frequency response (f−3dB=8GHz), and near-zero α parameter and chirp.

Tunneling injection quantum-dot lasers with polarization-dependent photon-mediated carrier redistribution and gain narrowing

A theoretical and experimental study of a particular transverse-electric (TE) mode lasing mechanism of a tunneling injection InP quantum-dot (QD) laser is reported. In the experiment, the TE mode lasing action takes place at the first excited state of InP biaxially compressively strained QDs. This QD state is coupled to the ground state of two tensile-strained InGaP quantum wells (QWs) although the tensile-strained QW structure favors the transverse-magnetic (TM) polarization light emission. The measured TE and TM modal gain spectra show a typical QW gain evolution behavior at low injection currents, which can be theoretically modeled by the quasi-equilibrium of carrier distribution. When the injection current is increased near threshold, a TE gain narrowing and a simultaneous TM gain pinning are observed in the measured modal gain spectra, which cannot be explained via the quasi-equilibrium model. We propose a polarization-dependent photon-mediated carrier redistribution in the QD-coupled-QW structure to explain this TE and TM gain evolution behavior. When the injection current is just below threshold, the strong carrier depletion via stimulated emission due to coupling between the InP QD and InGaP QW states plays an important role in carrier redistribution, which depends on the optical transition energy and polarization. This concept of the polarization-dependent photon-mediated carrier redistribution explains the TE gain narrowing and TM gain pinning behavior. In addition, a coupled rate equation model is established, and the calculated polarization power ratio based on the coupled rate equations explains the experimental observation.

High-speed quantum dot lasers

The modulation bandwidth of conventional 1.0–1.3 µm self-organized In(Ga)As quantum dot (QD) lasers is limited to ∼6–8 GHz due to hot carrier effects arising from the predominant occupation of wetting layer/barrier states by the electrons injected into the active region at room temperature. Thermal broadening of holes in the valence band of QDs also limits the performance of the lasers. Tunnel injection and p-doping have been proposed as solutions to these problems. In this paper, we describe high-performance In(Ga)As undoped and p-doped tunnel injection self-organized QD lasers emitting at 1.1 and 1.3 µm. Undoped 1.1 µm tunnel injection lasers have ∼22 GHz small-signal modulation bandwidth and a gain compression factor of 8.2 × 10−16 cm3. Higher modulation bandwidth (∼25 GHz) and differential gain (3 × 10−14 cm2) are measured in 1.1 µm p-doped tunnel injection lasers with a characteristic temperature, T0, of 205 K in the temperature range 5–95°C. Temperature invariant threshold current (infinite T0) in the temperature range 5–75°C and 11 GHz modulation bandwidth are observed in 1.3 µm p-doped tunnel injection QD lasers with a differential gain of 8 × 10−15 cm2. The linewidth enhancement factor of the undoped 1.1 µm tunnel injection laser is ∼0.73 at lasing peak and its dynamic chirp is <0.6 Å at various frequencies and ac biases. Both 1.1 and 1.3 µm p-doped tunnel injection QD lasers exhibit zero linewidth enhancement factor (α ∼0) and negligible chirp (< 0.2 Å). These dynamic characteristics of QD lasers surpass those of equivalent quantum well lasers.

High-speed 1.3μm tunnel injection quantum-dot lasers

1.3 μ m tunnel injection quantum-dot lasers are demonstrated. The laser heterostructures are grown by molecular-beam epitaxy. The InAs self-organized quantum dots are p doped to optimize the gain. The lasers are characterized by Jth=180A∕cm2, T0=∞, dg∕dn≈1×10−14cm2, f−3dB=11GHz, chirp of 0.1Å, and zero α parameter.

Measurement of modal gain in 1.1 [micro sign]m p-doped tunnel injection InGaAs∕GaAs quantum dot laser heterostructures

Single-pass optical gain of self-assembled InGaAs/GaAs p-doped tunnel injection quantum dot laser heterostructures emitting at 1.1 µm is measured by the multisection device technique. The heterostructures, consisting of three layers of quantum dots in the active region, demonstrate net modal gain as high as 57 cm−1.

Temperature dependent steady-state characteristics of high-performance tunnel injection quantum dot lasers

Enhanced temperature stability in tunnel injection In0.4Ga0.6As/GaAs quantum dot lasers is demonstrated. Value of T0=363 K in the temperature range 5°C<T<60°C and T0=202 K in the range 60°C<T<100°C are derived from the temperature-dependent 1ight-current characteristics. The variation of the slope efficiency of the devices against temperature confirms that reduced carrier leakage and non-radiative recombination contribute to the observed behaviour.

Tunneling-injection quantum-dot laser: ultrahigh temperature stability

We propose a genuinely temperature-insensitive quantum dot (QD) laser. Our approach is based on direct injection of carriers into the QDs, resulting in a strong depletion of minority carriers in the regions outside the QDs. Recombination in these regions, which is the dominant source of the temperature dependence, is thereby suppressed, raising the characteristic temperature T/sub 0/ above 1500 K. Still further enhancement of T/sub 0/ results from the resonant nature of tunneling injection, which reduces the inhomogeneous line broadening by selectively cutting off the nonlasing QDs.