Ground-state Optical Transition Research Articles

Увеличение эффективности тандема полупроводниковый лазер-оптический усилитель на основе самоорганизующихся 8s квантовых точек

The rate equations are used to analyze the characteristics of a tandem consisting of a laser diode and a semiconductor optical amplifier made of a single heterostructure with quantum dots. The optimal value of the current distribution coefficient the amplifier and the laser, as well as the optimal resonator length that provides the highest output power of the tandem were determined. It is shown that the use of the tandem allows, at the same total consumed current, to significantly (more than 4 times for 1 A) increase the power emitted through the ground-state optical transition in comparison with that achievable with a laser diode solely being limited by the onset of lasing through an excited-state optical transition.

Saturated layer gain in waveguides with InGaAs quantum well-dot heterostructures

Modal absorptions in laser-like heterostructures containing InAs self-assembled quantum dots (QDs) and InGaAs quantum well-dots (QWDs) have been studied. The evaluation of photoresponse as a function of waveguide length has allowed us to determine per-layer modal absorptions of 69 and 13 cm-1 for the ground state optical transitions of QWDs and QDs, respectively. The values of the modal absorption can be used as a measure of the maximal (saturated) modal gain. To compare quantum heterostructures with different dimensionality we have introduced the layer gain constant, a parameter characterizing the light transmittance through the absorbing or gaining layer. We have shown that the QWD layer gain constant significantly exceeds quantum well and quantum dot ones.

Lasing in III–V microdisk core–TiO2 shell lasers

Microdisk lasers having a III–V core coated with a TiO2 shell are experimentally studied under optical pumping. Initial core microdisk lasers with a 5–10 μm diameter comprising five layers of InAs/In0.15Ga0.85As quantum dots demonstrate room temperature lasing with excellent characteristics (threshold, quality factor) at the ground state optical transition in the 1.28–1.30 μm spectral range. Deposition of the TiO2 dielectric shell results in a decimation of the whispering gallery modes of the microdisk resonator and, for thicker (>100 nm) shells, in a blueshift of the lasing wavelength. Using numerical analysis, we demonstrate that coating a III–V core microdisk with a high-index shell causes strong penetration of the second and third radial modes into the shell, whereas the first radial mode remains nearly undisturbed. Though optical loss added by the TiO2 dielectric shell is low (it does not exceed 2 cm−1 even for a 250-nm-thick TiO2 layer), mode leakage to the TiO2 results in a decrease in the optical confinement factor and an increase in the threshold pump power. This effect is more pronounced in microlasers of the smallest diameter studied (5 μm). Thus, in addition to other applications, a TiO2 shell can be used for mode selection in microdisk lasers or for selective outcoupling of the emission to the waveguide structure, which requires proper adjustment of the TiO2 shell thickness and microdisk diameter.

The effect of p-doping on multi-state lasing in InAs/InGaAs quantum dot lasers for different cavity lengths

The effect of modulation p-doping on multi-state lasing in InAs/InGaAs quantum dot (QD) lasers is studied for different levels of acceptor concentration. It is shown that in case of the short laser cavities, p-doping results in higher output power of the ground-state optical transitions of InAs/InGaAs QDs whereas in longer samples p-doping may result in the decrease of this power component. On the basis of this observation, the optimal design of laser active region and optimal doping level are discussed in details.

Effect of the cap-layer composition on the electronic properties of InAs/GaAs quantum dots

The capping of an array of self-assembled InAs/GaAs quantum dots by an InGaAs quantum-well layer leads to an increase in their size due to indium enrichment of the region near the top of the quantum dots, which decreases the energy of the ground-state optical transition in quantum dots by 50 meV and shifts the hole wave function toward the top of the quantum dot.

Effect of modulation p-doping level on multi-state lasing in InAs/InGaAs quantum dot lasers having different external loss

The influence of the modulation p-doping level on multi-state lasing in InAs/InGaAs quantum dot (QD) lasers is studied experimentally for devices having various external losses. It is shown that in the case of short cavities (high external loss), there is an increase in the lasing power component corresponding to the ground-state optical transitions of QDs as the p-doping level grows. However, in the case of long cavities (small external loss), higher dopant concentrations may have an opposite effect on the output power. Based on these observations, an optimal design of laser geometry and an optimal doping level are discussed.

Lasing in microdisks of ultrasmall diameter

It is demonstrated by calculations and experimental results that room-temperature lasing can be obtained at the ground-state optical transition of InAs/InGaAs/GaAs quantum dots in optical microcavities with a record-small diameter of 1.5 μm. In 1-μm cavities, lasing occurs at the wavelength of one of the whispering-gallery modes within the band corresponding to the first excited-state optical transition.

The analytical approach to optimization of active region structure of quantum dot laser

Using the analytical approach introduced in our previous papers we analyse the possibilities of optimization of size and structure of active region of semiconductor quantum dot lasers emitting via ground-state optical transitions. It is shown that there are optimal length' dispersion and number of QD layers in laser active region which allow one to obtain lasing spectrum of a given width at minimum injection current. Laser efficiency corresponding to the injection current optimized by the cavity length is practically equal to its maximum value.

Wide Gain Bandwidth InAs/Ingaalas/InP Quantum-Dashes for Broadband Emitter Applications

The reduction in the dimensionality of the active region and exploiting the quantum mechanical effects from this device design has lead to the realization of quantum well (Qwell), quantum wire and eventual quantum dot (Qdots semiconductor lasers with remarkable improvements in their performance. In particular, Qdots lasers, provides control of the optical transitions due to their delta type density of states, lead to strong reduction in the threshold current density and a high temperature stability.In general, Qdots are grown by strain induced self assembled Stranski-Krastanow (S-K) growth technology on the matured GaAs platform. The intentional lattice mismatch between InAs Qdots and GaAs substrate facilitate the formation of these 3D islands spanning ~ 1.1 - 1.3 µm wavelength. The thrust to extend the emission wavelength in the C - L - U bands (1.4 - 2.0 µm) targeting principally the third communication window (1.55 µm) has set path to the development of InP material platform via a new class of self assembled grown quantum dash (Qdash) nano-structures. These are elongated Qdots grown as a result of low lattice mismatch between InAs and InP and the resulting complex strain distribution, with an interesting mixed type character, between Qwell and Qdot. Promising Qdash based laser demonstrations has already been reported with wide tunability spanning ~ 1.5 - 2.0 µm. The inherent inhomogeneous nature of these nano-structures resembling dot-like and wire-like physical features has also been exploited in the realization of ultra-broad gain bandwidth (> 300 nm) material, a highly promising in realization of broadband semiconductor devices, for instance, semiconductor optical amplifier, broadband laser diode, photo detector, etc, which will find novel applications in optical communications, particularly in wavelength division multiplexed system. In addition, realization of diode-based ultra-broadband laser source will offer a compact, high-efficiency, and cost-effective solution for many new applications in cross-disciplinary fields in optics and photonics, for instance, metrology, spectroscopy and sensing, imaging, etc. In this talk, we will give a general overview and introduction to the broadband emitters. The technology for growing these nanostructures, as well as the technologies for engineering the bandgap of the InP-based Qdash system using epitaxy growth technique and postgrowth intermixing methods will be presented. Recently, we reported an ultra-broad lasing -3dB bandwidth of ~ 21 nm from the as- grown and ~ 41 nm from the intermixed Qdash laser, as shown in Fig. 1, with superior characteristics [1-3]. We will then focus our discussion on our recent progress in extending the lasing emission linewidth via employing a novel active region device design. Instead of using a fixed AlGaInAs barrier thickness in the multi-stack Qdash active region, here we utilized a varying barrier thickness (chirp design) structure that influence the vertical strain experienced by the dashes on the subsequent stacking layer thereby creating additionally broadened dash ground state optical transitions. With this as-grown device structure we achieved an enhanced lasing bandwidth of > 50 nm from a 2×830 µm ridge laser diode, as shown in Fig. 2 [4], with exhibited superior performance characteristics. This is an enhancement by a factor of ~ 50% (20%) from our previous reports on as-grown (intermixed) fixed barrier Qdash laser structure. Moreover, we also investigated the broadband emission features of our chirped Qdash structure in the form of superluminescent diodes (SLD) by reporting high performance characteristics from this device and an extra-ordinary emission bandwidth of > 700 nm, spanning the entire O - E - S - C - L - U bands [5]. All these demonstrations show that chirped device design is promising candidate in realizing broadband semiconductor devices. [1] B. S. Ooi et al., IEEE Journal of Selected Topics in Quantum Electronics 14 (4), 1230-1238 (2008).[2] H. S. Djie et al., Applied Physics Letters 91, 111116 (2007).[3] C. L. Tan et al., Applied Physics Letters 93, 111101 (2008).[4] M. Z. M. Khan et al., Applied Physics Letters 102, 091102 (2013).[5] M. Z. M. Khan et al., Optics Letters 38 (19), 3720-3723 (2013).

Localization of Excitons in Thin Core-Multi-Shell Quantum Well Tubes

ABSTRACTRecently, Fickenscher et al. [1] have shown that, in a core-multi-shell structure where a GaAs quantum well is embedded into an AlGaAs shell wrapped around a [111] oriented GaAs nanowire, the electron and hole ground states are strongly confined to the corners of the hexagonally symmetric quantum well. Thus this confinement defines quantum wires which run along the length of the nanowires along its corners. Here we review single nanowire photoluminescence measurements which show the significant confinement energy of the excitons. For well widths larger than 5 nm, optical transitions between electron and hole excited states can be seen in excitation spectra, while for widths less than 5 nm only the ground state optical transitions are observed. For well widths smaller than 5 nm, high resolution spatially resolved photoluminescence measurements show directly the appearance of localized states. Single nanowire spectra from the 4 nm QWT sample display ultranarrow emission lines on the high energy side of the luminescence band. Spatially-resolved PL images show that these quantum dots are localized randomly along the length of the wire.

Optical features of InAs quantum dots-in-a-well structures

Electronic energy structure and features of optical interband transitions of InAs quantum dots-in-a-well structures are studied via photoreflectance (PR) and temperature-dependent photoluminescence (PL) spectroscopy. InAs dots were grown with and without InGaAs capping layer and embedded in GaAs/AlAs quantum wells. Experimental results revealed that the 5 nm thick InGaAs capping layer significantly improves PR and PL signal intensity. Moreover, a shift of the quantum dot ground-state optical transition to lower energy by about 120 meV was observed. The red-shift of the ground-state transition is associated mainly with an increase of dot size and decrease of strain within quantum dots. Furthermore, the origin of PL intensity quenching with temperature is discussed in terms of electronic energy structure revealed from PR spectra and calculations performed within effective mass approximation.

Optical features of InAs quantum dots-in-a-well structures

Electronic energy structure and features of optical interband transitions of InAs quantum dots-in-a-well structures are studied via photoreflectance (PR) and temperature-dependent photoluminescence (PL) spectroscopy. InAs dots were grown with and without InGaAs capping layer and embedded in GaAs/AlAs quantum wells. Experimental results revealed that the 5 nm thick InGaAs capping layer significantly improves PR and PL signal intensity. Moreover, a shift of the quantum dot ground-state optical transition to lower energy by about 120 meV was observed. The red-shift of the ground-state transition is associated mainly with an increase of dot size and decrease of strain within quantum dots. Furthermore, the origin of PL intensity quenching with temperature is discussed in terms of electronic energy structure revealed from PR spectra and calculations performed within effective mass approximation.

Spectral dependence of the linewidth enhancement factor in quantum dot lasers

The spectral analysis of amplified spontaneous emission is used to determine the linewidth enhancement factor (α-factor) in lasers based on InAs/InGaAs quantum dots (QDs) in a wide spectral range near the ground-state optical transition energy. The effect of the pump current and number of QDs on the spectral dependences of the α-factor is examined. The temperature dependence of the spectra of the α-factor is experimentally determined for the first time for lasers with InAs/InGaAs QDs. An explanation is suggested for the observed anomalous decrease in the α-factor with increasing temperature.

Effect of an excited-state optical transition on the linewidth enhancement factor of quantum dot lasers

An analytical expression is derived for the linewidth enhancement factor of a quantum-dot laser, which makes it possible to describe its dependence on optical loss and photon density in an explicit form. The model accounts for refractive index variations at the ground-state optical transition due to gain/absorption variations upon the first excited-state transition in quantum dots. It is shown that a decrease in optical loss, an increase in saturated gain, and an increase in the energy separation between the excited-state and ground-state transitions results in a decrease in the α factor both at and above the lasing threshold.

Conduction-band crossover induced by misfit strain inInSb∕GaSbself-assembled quantum dots

We address the occurrence of conduction-band crossover in III-V self-assembled quantum dots solely due to misfit strain. Band structure analysis in terms of standard deformation-potential theory shows that $\ensuremath{\Gamma}\text{\ensuremath{-}}X$ crossover can occur in the dot, while both $\ensuremath{\Gamma}\text{\ensuremath{-}}X$ and $\ensuremath{\Gamma}\text{\ensuremath{-}}L$ crossovers are possible in the matrix at the interface. Crossover changes the nature of the fundamental band gap in the heterostructure, which may dramatically affect the optical properties. The implications of this are studied for a realistic $\mathrm{In}\mathrm{Sb}∕\mathrm{Ga}\mathrm{Sb}$ (001) heterostructure, where $\ensuremath{\Gamma}\text{\ensuremath{-}}L$ crossover renders the ground-state optical transition indirect in $\mathbf{k}$ space. Our calculations and photoluminescence data are in remarkable agreement.

Optical properties of dilute nitrogen GaInNAs quantum dots

The authors present a theoretical study of the ground state optical transition matrix element in quantum dots (QDs) with a dilute amount of nitrogen. They have investigated the interplay between the nitrogen to the conduction band mixing and piezoelectric field on the optical matrix element. With a reduced amount of indium and an increased amount of nitrogen in the QD, the optical matrix element becomes on the average larger and less sensitive to the variation of both the QD shape and size than is the case of an InNAs QD. The optical characteristics at room temperature and 1.5μm wavelength are discussed.

Shape dependent electronic structure and exciton dynamics in small In(Ga)As quantum dots

We present a study of the primary optical transitions and recombination dynamics in InGaAs self-assembled quantum nanostructures with different shape. Starting from the same quantum dot seeding layer, and depending on the overgrowth conditions, these new nanostructures can be tailored in shape and are characterized by heights lower than 2 nm and base lengths around 100 nm. The geometrical shape strongly influences the electronic and optical properties of these nanostructuctures. We measure for them ground state optical transitions in the range 1.25–1.35 eV and varying energy splitting between their excited states. The temperature dependence of the exciton recombination dynamics is reported focusing on the intermediate temperature regime (before thermal escape begins to be important). In this range, an important increase of the effective photoluminescence decay time is observed and attributed to the state filling and exciton thermalization between excited and ground states. A rate equation model is also developed reproducing quite well the observed exciton dynamics.

Photo-induced interband absorption in group-III nitride quantum wells

We solve self-consistently Schrödinger and Poisson equations for GaN/AlGaN quantum wells, to analyze the change of optical spectra under high optical excitation. Electric fields of ∼1 MV/ cm or higher are present along the growth axis of such quantum wells. The induced separation of electron and hole wave functions reduces the oscillator strength of the ground-state optical transition, whereas those involving excited states give a much larger absorption. In presence of large electron–hole pair densities, we show that the optical density can be either reduced or enhanced, depending on the spectral region involved.

The effects of adjacent dislocations on the electronic and optical properties of GaN/AlN quantum dots

We present a theory of the electronic energy levels and wave functions within GaN quantum dots (QDs), including the effect of the strain field of a nearby dislocation and a full treatment of the built-in electrostatic potential. The QD carrier spectra and wave functions are calculated using a plane-wave expansion method within an 8-band k · p model. The GaN QDs are truncated hexagonal pyramids on a wetting layer with an edge dislocation adjacent to each dot. The built-in piezoelectric potential strongly influences the localization of the carrier wave functions. The potential pushes the electrons to the top of the dot, the holes to the bottom and, additionally, causes strong lateral confinement of the carriers near the dot center. The strong lateral confinement of the carriers indicates that the effect of the strain field due to the dislocation is relatively small. The size of the dot influences the energies and overlaps, but the presence of the dislocation has minimal effect. The dependence of the ground state optical transition energy on the size of the dot is in good agreement with experimental data.

Self-Induced Photon Absorption by Screening of the Electric Fields in Nitride-based Quantum Wells

ABSTRACTWe have calculated the change of interband absorption spectra of a quantum well based on hexagonal group-III nitride semiconductors under photo-injection of high densities of electron-hole pairs. The screening of internal electric fields by such optical excitation is known to blue-shift and reinforce the ground-state optical transition. Due to the large values of densities of states and of internal fields, we predict novel properties that rather concern optical absorption via transitions between excited states. The absorption coefficient can be strongly enhanced by the optical excitation itself, in this particular spectral region, yielding the possibility for self-induced absorption properties. In other words, if sufficiently intense, an excitation laser can increase the absorption coefficient of the system at its own wavelength, thus providing a strong nonlinear optical response. Finally, we briefly discuss the potential application of these optical phenomena.