Quantum Dot Gain Research Articles

A QUANTUM DOT MICROCAVITY TERAHERTZ LASER

We discuss a THz laser device based on a semiconductor quantum dot (QD) gain medium, where the lasing occurs through discrete conduction states. An ensemble of QDs is selectively placed in a high quality cavity, called a microdisk, which is resonant with a terahertz intersublevel QD transition. We simulate the rate equations governing lasing and discuss a variety of processes affecting lasing including nonradiative recombination and the ground state decay rate.

Self-Tuned Quantum Dot Gain in Photonic Crystal Lasers

We demonstrate that very few (2-4) quantum dots as a gain medium are sufficient to realize a photonic-crystal laser based on a high-quality nanocavity. Photon correlation measurements show a transition from a thermal to a coherent light state proving that lasing action occurs at ultralow thresholds. Observation of lasing is unexpected since the cavity mode is in general not resonant with the discrete quantum dot states and emission at those frequencies is suppressed. In this situation, the quasicontinuous quantum dot states become crucial since they provide an energy-transfer channel into the lasing mode, effectively leading to a self-tuned resonance for the gain medium.

Ultralow noise optical pulse generation in an actively mode-locked quantum-dot semiconductor laser

We report excellent noise performance of an external-cavity actively mode-locked laser based on quantum-dot gain medium. Optical pulse trains with less than 7.5fs residual timing jitter (1Hzto10MHz) for a 12.8GHz harmonically mode-locked ring laser were obtained. This result represents, to our knowledge, the lowest residual jitter reported from actively mode-locked semiconductor lasers, and shows that quantum-dot mode-locked lasers are promising as sources of ultralow noise optical pulse trains.

Optical gain in CdSe nanocrystals

Optical gain measurements with femtosecond excitation were performed in CdSe colloidal quantum dots (QDs) dissolved in toluene using the variable stripe length method. Instead of ZnS-shell overcoating, we found that the spectral narrowing and intensity enhancement of the fluorescence can be achieved alternatively by excluding octadecene (ODE) at the preparation of QDs. The maximum gain (∼1.5 mm −1) in the ODE-excluded CdSe QDs is comparable to that of CdSe/ZnS core–shell structures. Despite the uniform size distribution ( σ = 0.25 nm ), a broad gain spectrum appears in both cases due to the recombination of many excited levels between biexciton and exciton states. The remaining surface states in the ODE-excluded CdSe QDs also contribute to gain (1.0 mm −1), which is involved with a three-level process.

A solution-processed 1.53 μm quantum dot laser with temperature-invariant emission wavelength

Sources of coherent, monochromatic short-wavelength infrared (1-2 mum) light are essential in telecommunications, biomedical diagnosis, and optical sensing. Today's semiconductor lasers are made by epitaxial growth on a lattice-matched single-crystal substrate. This strategy is incompatible with integration on silicon. Colloidal quantum dots grown in solution can, in contrast, be coated onto any surface. Here we show a 1.53 mum laser fabricated using a remarkably simple process: dipping a glass capillary into a colloidal suspension of semiconductor quantum dots. We developed the procedures to produce a smooth, low-scattering-loss film inside the capillary, resulting in a whispering gallery mode laser with a well-defined threshold. While there exist three prior reports of optical gain in infrared-emitting colloidal quantum dots [1,2,3], this work represents the first report of an infrared laser made using solution processing. We also report dlambda(max)/dT, the temperature-sensitivity of lasing wavelength, of 0.03 nm/K, the lowest ever reported in a colloidal quantum dot system and 10 times lower than in traditional semiconductor quantum wells.

Ultrashort, high-power pulse generation from a master oscillator power amplifier based on external cavity mode locking of a quantum-dot two-section diode laser

We investigate an external cavity curved two-section mode-locked diode laser system based on quantum-dot (QD) gain media near 1.3μm. Pulses are generated from the external laser cavity at a 5-GHz repetition rate and amplified using a multilayer quantum-dot semiconductor optical amplifier. The pulses are compressed with a dual-grating dispersion compensator. The shortest, near transform-limited pulses are obtained when the mode-locked pulses are positive (up) chirped. The compressed pulses are 1.2ps in duration, with a pulse energy of 1.46pJ, implying a peak power of 1.22W.

Near-infrared gain in GaSb quantum dots in Si grown by MBE

Light amplifying characteristics of GaSb quantum dots embedded in Si were studied. Under visible pulsed laser excitation, a single-pass gain coefficient more than 10 dB cm −1 was obtained for planar waveguide geometry. The gain was found to decrease as the temperature increases, and eventually loss dominates beyond 25 K. All electrical operations of optical amplifier were successful with a gain value of 3 dB cm −1.

The impact of energy band diagram and inhomogeneous broadening on the optical differential gain in nanostructure lasers

We present a general theoretical model for the optical differential gain in semiconductor lasers. The model describes self assembly quantum dots (QDs), self assembly quantum wires (QWRs) and single quantum-well lasers. We have introduced the inhomogeneous broadening due to size fluctuations in the assembly cases. At each dimensionality, we have considered the carrier populations in the excited states and in the reservoirs, where conduction and valence bands are treated separately. We show that for room temperature operation the differential gain reduction due to increased size inhomogeneity is more pronounced in QDs than in QWRs. We show this reduction to be smaller than the one-order reduction attributed to state filling in conventional dot and wire assemblies operating at room temperature. The integration prefactor coefficient of the differential gain in zero-dimensional cases exceed one- and two-dimensional coefficients only for low temperatures where the homogenous broadening is considerably smaller than the thermal energy. The differential gain of QDs, QWRs, and compressively strained single quantum-well lasers operating at room temperature and close to equilibrium is nearly the same.

On the noise properties of linear and nonlinear quantum-dot semiconductor optical amplifiers: the impact of inhomogeneously broadened gain and fast carrier dynamics

We present a detailed analytical model describing the noise properties of quantum-dot (QD) optical amplifiers operating in the linear and saturated regimes. We describe the dependence of the optical noise on the main physical parameters characterizing the QD gain medium as well as on operating conditions. The optical noise at the amplifier output shows a broad-band spectrum with an incoherent spectral hole due to the gain inhomogeneity. A coherent spectral dip stemming from noise-signal nonlinear interactions is superimposed on that broad-band spectrum. The broad-band incoherent component is also calculated using an approximate model which makes use of an equivalent inhomogeneous population inversion factor. The validity of the approximation is examined in detail. We also calculate the electrical relative intensity noise and observe a spectral hole corresponding to the spectral shape of the optical noise. The most important characteristics of the optical and electrical noise spectra are determined by the degree of inhomogeneous broadening and by the fast carrier dynamics of QD amplifiers. The fast dynamics causes a very wide noise spectral hole which has important potential consequences for detection of fast data and for all optical signal processing.

Lasing emission of InGaAs quantum dot microdisk diodes

An improved three-arm airbridge contacted microdisk diode structure is presented. Continuous-wave lasing from InGaAs quantum dot (QD) in a /spl sim/4-μm-diameter microdisks is reported with the threshold current /spl sim/40 μA at T=5 K. With the increase of injection current, the QD's emission blueshifts due to the band-filling effect, while the laser mode peak redshifts by thermal effect. When the QD's gain spectra shift out of alignment with the lasing mode, the next available whispering gallery mode starts lasing from QD wetting layer. The thermal heating effect is discussed by investigating the modes redshift with respect to injection current.

TERAHERTZ EMISSION USING QUANTUM DOTS AND MICROCAVITIES

Semiconductor solid-state lasers based on conduction-valence band recombination are now commonplace for low-power red emission, and are available commercially at nearly continuous wavelengths throughout the near-UV and near-IR communications bands. Furthermore, new lasers and broadband spontaneous emission sources are available through a wide wavelength range, including 3-8μm based on both conduction-valence band and intersubband transitions, and up to 70μm using cascaded intersubband transitions. Competing processes make the design of these semiconductor lasers extremely difficult when extended to the very long, 300μm wavelength regime corresponding to low terahertz frequencies. We discuss material and device-design considerations for extending semiconductor lasers to this regime. We suggest a new set of device structures based on a semiconductor quantum dot (QD) gain medium, where the lasing occurs through discrete conduction states. In one implementation, two QDs are coupled to make a coupled-asymmetric quantum dot (CAD) laser. In another implementation, an ensemble of non-coupled QDs is selectively placed in a high quality cavity, called a microdisk, which is resonant with an intersublevel QD transition. We demonstrate the initial fabrication of these structures.

Enhanced thermal stability of laser diodes with shape-engineered quantum dot medium

Optical properties of the quantum dots (QDs) were optimized by shape engineering through the adjustment of the thickness of the GaAs overlayer prior to an additional heating step leading to QD truncation. QDs with a 6-nm-thick overlayer with the subsequent heating step were found to have the highest photoluminescence intensity at room temperature and the lowest luminescence bandwidth, 29 meV. 1.22 μm edge-emitting laser with a triple-layer truncated QD gain medium demonstrated room temperature threshold current density, 56 A/cm2, and saturated modal gain, 16 cm−1. An extremely high characteristic temperature for lasing threshold, T0=380 K up to 55 °C, and a maximum ground state lasing temperature of 219 °C were measured for these laser diodes.

High power lasers based on submonolayer InAs–GaAs quantum dots and InGaAs quantum wells

Broad area lasers based on InAs-GaAs quantum dots formed by submonolayer deposition were fabricated. High modal gain of submonolayer quantum dots permits the use of broad-waveguide and highly doped design. Continuous wave output power of 6W limited by mirror damage and conversion efficiency of 58% were demonstrated at 20 °C. The characteristic temperature of 150 K was achieved.

Nanoengineered Quantum Dot Active Medium for Thermally-Stable Laser Diodes

ABSTRACTThe influence of a nanoengineering procedure on the properties of single- and multi-layer self-assembled InAs quantum dot (QDs) has been studied using photoluminescence, transmission electron microscopy (TEM), and electroluminescence. Optical properties of QDs were optimized by shape engineering via adjustment of a GaAs overlayer thickness prior to a heating (truncation) step. TEM micrographs have confirmed that the employed growth procedure results in truncated pyramidal QDs covered entirely by AlAs with smooth top interfaces. Truncated QDs with two-monolayer-thick AlAs capping have demonstrated a strong blue shift of ground state (GS) energies and up to 10 meV larger separation between GS and first excited state energy levels as compared to non-capped QDs. We believe that AlAs capping, in combination with truncation procedure, results in significant suppression of carrier transport between QDs within the same layer as well as between QD layers. A record high characteristic temperature for GS lasing threshold, T0 = 380 K up to 55 °C, as well as a maximum saturated gain of 16 (5.3 per layer) cm-1, were measured for a 1.22 μm edge-emitting lasers with this truncated triple layer QD gain medium. A maximum saturated gain, 27 (3.9 per layer) cm-1, and T0 = 110 K have been demonstrated in a 1.19 μm lasers with truncated seven layer QD medium.

Nanoengineered Quantum Dot Active Medium for Thermally-Stable Laser

ABSTRACTThe influence of a nanoengineering procedure on the properties of single- and multi-layer self-assembled InAs quantum dot (QDs) has been studied using photoluminescence, transmission electron microscopy (TEM), and electroluminescence. Optical properties of QDs were optimized by shape engineering via adjustment of a GaAs overlayer thickness prior to a heating (truncation) step. TEM micrographs have confirmed that the employed growth procedure results in truncated pyramidal QDs covered entirely by AlAs with smooth top interfaces. Truncated QDs with two-monolayer-thick AlAs capping have demonstrated a strong blue shift of ground state (GS) energies and up to 10 meV larger separation between GS and first excited state energy levels as compared to non-capped QDs. We believe that AlAs capping, in combination with truncation procedure, results in significant suppression of carrier transport between QDs within the same layer as well as between QD layers. A record high characteristic temperature for GS lasing threshold, T0 = 380 K up to 55 °C, as well as a maximum saturated gain of 16 (5.3 per layer) cm−1, were measured for a 1.22 μm edge-emitting lasers with this truncated triple layer QD gain medium. A maximum saturated gain, 27 (3.9 per layer) cm−1, and T0 = 110 K have been demonstrated in a 1.19 μm lasers with truncated seven layer QD medium.

Theory of laser gain in InGaN quantum dots

A theory for gain is developed for wide-bandgap nitride-based quantum-dot laser structures. A semiclassical laser theory is used to describe the optical susceptibility in the presence of strong many-body Coulomb interaction and quantum-confined Stark effect. Application of this theory shows distinctly different gain behavior depending on quantum dot dimensions, because of the interplay of these effects.

Single-mode distributed feedback and microlasers based on quantum-dot gain material

Quantum-dot gain material fabricated by self-organized epitaxial growth on GaAs substrates is used for the realization of 980-nm and 1.3-/spl mu/m single-mode distributed feedback (DFB) lasers and edge-emitting microlasers. Quantum-dot specific properties such as low-threshold current, broad gain spectrum, and low-temperature sensitivity could be demonstrated on ridge waveguide and DFB lasers in comparison to quantum-well-based devices. 980-nm DFB lasers exhibit stable single-mode behavior from 20/spl deg/C up to 214/spl deg/C with threshold currents 1 mW was obtained for 30-/spl mu/m cavity length. Low-threshold currents of 4.4 mA could be obtained for 1.3-/spl mu/m emitting 400-/spl mu/m-long high-reflection coated ridge waveguide lasers. DFB lasers made from this material by laterally complex coupled feedback gratings show stable CW single-mode emission up to 80/spl deg/C with sidemode suppression ratios exceeding 40 dB.

Local Field Effects in an Isolated Quantum Dot: Self-Consistent Microscopic Approach

(a) Institute for Nuclear Problems, Belarus State University, Bobruiskaya 11,220050 Minsk, Belarus(b) Institut fu¨r Festko¨rperphysik, Technische Universita¨t Berlin, Hardenbergstr. 36,10623 Berlin, Germany(Received September 4, 2001; in revised form October 5, 2001; accepted October 17, 2001)Subject classification: 42.25.–p; 78.66.Sq; 85.35.BeA self-consistent microscopic local-field theory for an isolated arbitrary shaped quantum dot(QD) has been presented. In our approach we introduce into the Schro¨dinger equation a depo-larization field induced by an external electromagnetic field, and combine this idea with thesecond quantization technique for electron–hole pairs. As a result, we succeeded in deriving thegeneral self-consistent nonlinear equations for the system “QD + classical electromagnetic field”.In the linear approximation, our approach reproduces microscopically the depolarization shift ofthe QD gain band and, in anisotropically shaped QDs, polarization dependent splitting of thisband.

Steady state optical gain in small semiconductor quantum dots

The optical gain phenomena in a single semiconductor quantum dot as well as in an inhomogeneous distribution of quantum dots have been studied. The analytical results obtained using the density matrix technique and numerical analysis for CdS quantum dots show an enhancement of gain in the presence of biexcitons. The numerical results show larger gain in bigger quantum dots. A redshift of gain maxima occurs when the distribution width is large. The realization of high gain quantum dot lasers and photodetectors appears to be a distinct possibility.

Application of spectral-hole burning in the inhomogeneously broadened gain of self-assembled quantum dots to a multiwavelength-channel nonlinear optical device

A nonlinear device capable of handling multiple wavelength channels simultaneously by using spectral-hole burning in the inhomogeneous broadening of the gain in self-assembled quantum dots is proposed. The multichannel property of this device stems from the localized absorption or gain change at the pump wavelength within the inhomogeneously broadened gain spectrum. The spectral width of the modulation and its time evolution was also evaluated. The result indicates the applicability of this device to multichannel operation.