were considered, a digitally graded, quasiparabolic well and a simple square well, and their performance was compared. Despite the higher overall emitted power from the square well system, the emission spectrum of the quasiparabolic well is generally narrower, and up to the moderate fields remains rather monochromatic, as expected from spontane- ous emission-based sources like LED devices. The total op- tical output power at 7 THz is limited due to the cubic de- pendence of the spontaneous emission on the subband energy spacing. However, the spectral power at 7 THz still exceeds, by approximately a factor of two, the ideal black body radia- tion at 300 K. The analysis presented predicts that this de- vice, in contrast to quantum cascade lasers, may operate at room temperature, or even at 400 K. A higher optical output may be achieved either by increasing the electron density or by using a stack of multilayer structures. Furthermore, by appropriate modifications of the digitally graded quantumwell, it appears possible to realize limited-bandwidth inco- herent terahertz sources, the spectral properties of which are controllable by the built-in GaAs/AlGaAs composition dis- tribution, and also dynamically 共to an extent兲 by the lateral field.
Bragg stack. A practical difficulty with this approach is that the total width of semiconductor layers is about 100 m, which makes it somewhat impractical for the whole structure to be grown by the slow molecular beam epitaxy (MBE) (the growth time might take a few days). A way out might be to em- ploy the much faster liquid phase epitaxy (LPE) for the lower Bragg stack, polish the surface to prepare it for quantum dots growth, use MBE next, and finally make the upper Bragg stack by LPE. While the Bragg mirror performance might not be af- fected by 0.05 m layer width tolerance/roughness inherent to LPE (because the layer widths are here large), it is not quite clear whether the growth speed-up, at the expense of increased complexity, would make this approach practical.
Various models for transport in QCLs exist : most commonly employing semiclassical approaches such as self- consistent rate-equation (RE) modeling, which considers non- radiative transitions of carriers due to the various scattering mechanisms, including interactions of electrons with longitu- dinal optical (LO) and acoustic phonons (AP), alloy disorder (AD), interface roughness (IFR), ionized impurities (II) and other electrons (CC). These models are semiclassical because they consider transitions of discrete electrons between energy levels and neglect coherence effects and quantum mechanical dephasing. Although RE models are usually computationally ef- ficient and provide insight into the scattering behavior, they are unable to correctly describe transport between adjacent periods of a QCL structure  because they do not take injection bar- rier thickness into account in transport calculations. This leads to the prediction of instantaneous transport between the periods, whereas the actual transport that occurs is based on the resonant tunneling.
Aside from the optical transition matrix elements, for ob- taining large inversion (and, hence, gain) it is essential to have fast relaxation of the lower laser level. This proceeds mostly via optical phonons and, to a smaller exten,t via the acoustic phonons. The scattering rate depends on both the levels spacing and the wavefunctions. Furthermore, the spacing between the laser upper and ground states has to be matched to the pump laser wavelength, and this transition has to be allowed, just as is the lasing transition; hence, the structure must be asymmetric, in that the QWs must not be of the same thickness (this applies for a three-level system, as discussed in more detail in Section II-A). In this paper, we discuss a systematic procedure for opti- mization the QW profile with respect to maximizing the gain, where the gain dependence not only on the dipole matrix ele- ments, but also on the levels relaxation rates, is accounted for. The procedure relies on supersymmetric quantum mechanics (SUSYQM) , . This formalism enables one to manip- ulate the states of a quantum system (deletion and/or insertion of a state, while leaving others intact) by changing the poten- tial and, at the same time, it introduces one or more free pa-
In order to test the modulation speed of the device, a more powerful, stable and narrow frequency THz source is required. A bound to continuum quantum cascade laser was used for all these measurements. The QCL emits at 2.05 THz in single mode around the maximum current density, which is a fundamental feature required in order to test the speed performance of these amplitude modulators. In fact, single mode operation is required in order to avoid laser mode hopping which would translate in a laser amplitude modulation. Correspondingly, this would affect the data interpretation because of the convolution of different modes, each characterized by a distinct frequency and power, with the plasmonic resonances. The emission frequency overlaps significantly with the plasmonic resonances for a = 26 m and partly also for the a = 24 m plasmonic antenna arrays. A general schematic drawing of the experimental set-up is shown in Fig. 4. QCL radiation is collimated with a parabolic mirror and after a beam splitter is partly focused onto the device with another parabolic mirror with a focal length of 2.5 cm and partly focused on a Golay cell detector in order to monitor the stability of the power source. Another parabolic mirror focuses the light reflected from the sample to a second Golay cell, which was used to measure the reflectivity modulation. The first experiment aimed to demonstrate the modulation of the THz light emitted from the QCL. A schematic of the experimental set-up is shown in Fig. 4. In this configuration the QCL laser was operating in continuous pulse mode, with a 30% duty cycle and 100 kHz repetition rate at maximum power current density. The optical modulator source and drain contacts were grounded and a voltage bias was applied to the gate. The DC bias applied to the gate spanned from -40 V
Abstract The objective of this research is design and simulation of a single quantumwell laser (SQW) to observe the effects of quantumwell number increment and temperature variation on the performance of this laser. This laser is structured of three different compounds GaN, InGaN and AlGaN with different mole fractions. It has a cavity length of 300 um and a width of 30 um. This laser has designed to be a high power laser at room temperature and continues wave condition. The optimized number of quantum wells to obtain the highest optical power is 2. The double quantumwell (DQW) structure of this an ultraviolet at room temperature with a wavelength of 326 nm and maximum optical power of 16W.
lifetime . In (110)-grown GaAs/AlGaAs QW, the DP spin relaxation mechanism is not efficient for electron spins parallel to the growth direction because the spin orientation of electrons is parallel to the direction of effective magnetic field induced by spin-orbit coupling . Spin relaxation times longer than 1 ns at RT in (110) GaAs QW have indeed been measured . Long electron spin diffusion lengths can thus be expected at high temperature in these structures. In this report, the electron spin diffusion is measured by the TSG technique with heterodyne detection in (110) GaAs/AlGaAs QWs at RT. We find that the spin diffusion length L s is about
Our study is suitable for all parameter ranges, so avoids the insufficiency in the previous work mentioned above and allows us to investigate whether or not the laser linewidth suppression occurs to the one-atom sub-Poissonian lasers for whole range of pump rates. Fig. 4.5 plots the resulting linewidths for coherently pumped A-type lasers. They are substantially different from the four-level lasers, for which the linewidth suppression are shown for a wide pump range. Meanwhile a good intensity squeezing occurs(cf. Fig. 4.3). This feature is ruled out provided the coherent driving field becomes sufficiently strong. The minimum linewidth in Fig 4.5 is about 0.002 g, which is much smaller than the Schawlow-Townes linewidth, 0.0066 g. The linewidth narrowing for coherently pumped V-type laser also occurs, but because of the laser- frequency splitting for some pumping ranges, we do not present the result (which will be discussed in a subsequent paper).
Droplet epitaxy [10,11] is a flexible growth method, based on Molecular Beam Epitaxy (MBE), which allows for the fabrication of a large variety of three dimensional nanostructures with different geometries, such as quan- tum dots , quantum dot molecules , quantum rings [8,14] and coupled disk-ring structures . The intrinsic design flexibility of the DE is permitted mainly by the splitting in time of the III-column and V-column element supply. This allows an independent choice, for each of the two elements, of specific growth conditions.
We have investigated the conditions under which conventionally pumped lasers can produce amplitude squeezed lig h t We have emphasized the relationship between our models and real systems. We have found that the squeezing is strongly degraded by losses within the pump cycle due to spontaneous emission or excited state absorption and by losses across the lasing levels due to spontaneous emission. The latter type of losses are large near threshold hence squeezing is only significant when the laser is well above threshold (more than ten times). We have found that the squeezing is only weakly degraded by depletion of the pump mode and by spatial variations in the pump and lasing mode. The effect of pump noise on the squeezing was found to be strongly dependent on the extent to which the pump was absorbed. We discussed the frequency dependence of the squeezing and found that the requirement that squeezing appears at frequencies in the tens of kHz places realistic requirements on the finesse of the cavity and the atomic density. W e have concluded that solid state systems exist in which rate matching could be achieved at moderate power levels and have identified fibre lasers as possessing the characteristics favourable to squeezed light production. However we have noted that the presence of classical noise at low frequencies can completely obscure any squeezing.
driven by square current pulses . From that we obtain that it takes approximately 140 ns to tune of the emission frequency of a DFB QCL by 30 GHz by ramping the pump current from zero to above a QCL threshold. In reality, the highest practically possible current-tuning speed for room- temperature single-mode CW devices is at least 10 times longer, because high-performance CW DFB QCLs have effi- cient thermal packaging that prevents excessive heating and the pump current must be modulated within the device operating range rather than from 0 to well above threshold. Thermal effects aside, rapid modulation of the pump current does not lead to significant change of the emission frequency in current QCL designs due to small value of the linewidth enhancement factor α < 0.5 , , which is due to the atomic-like lineshapes of the intersubband transitions. A free carrier injection (electrons and holes) is traditionally used to achieve GHz-speed frequency tuning or modulation in diode lasers . However, this technique is not easily applicable to QCLs, which are unipolar devices. A proposal for rapid FM modulation of QCLs using an intersubband transition that changes the effective refractive index of the laser mode (and results in a large α-factor) was made in 2007 by one of the co-authors in 2007 .
During droplet epitaxial QD fabrication , first liquid metallic droplets are generated on semiconductor surfaces, e.g., by Ga deposition without As flux. The growth temperature T = 100–350° typically is kept very low compared to usual MBE growth conditions. After Ga droplet formation, an As pressure is applied in order to crystallize the droplets and transform them into GaAs QDs. Interestingly, deposition of Ga droplets on GaAs at significantly higher temperatures T = 450–620° results in the formation of deep nanoholes in the substrate surface. This effect was first observed by Wang et al.  in 2007 and represents a local removal of material from semiconductor surfaces without the need of any litho- graphic steps. As an important advantage compared to conventional lithography processes, this local droplet etching (LDE) is fully compatible with usual MBE equipment and can be easily integrated into the MBE growth of heterostructure devices. LDE was demonstrated in addition on AlGaAs [19, 20] and AlAs  surfaces Ch. Heyn ( & ) A. Stemmann T. Ko¨ppen Ch. Strelow
Deep cooling of electron and nuclear spins is equivalent to achieving polarization degrees close to 100% and is a key requirement in solid state quantum information technologies [1–7]. While polarization of individual nuclear spins in dia- mond  and SiC  reaches 99% and beyond, it has been limited to 50-65% for the nuclei in quantum dots [8–10]. Theoretical models have attributed this limit to formation of coherent ”dark” nuclear spin states [11–13] but experi- mental verification is lacking, especially due to the poor accuracy of polarization degree measure- ments. Here we measure the nuclear polarization in GaAs/AlGaAsquantum dots with high accu- racy using a new approach enabled by manipula- tion of the nuclear spin states with radiofrequency pulses. Polarizations up to 80% are observed – the highest reported so far for optical cooling in quantum dots. This value is still not limited by nuclear coherence effects. Instead we find that optically cooled nuclei are well described within a classical spin temperature framework . Our findings unlock a route for further progress to- wards quantum dot electron spin qubits where deep cooling of the mesoscopic nuclear spin en- semble is used to achieve long qubit coherence [4, 5]. Moreover, GaAs hyperfine material con- stants are measured here experimentally for the first time.
well as device demonstrations have followed including, among the latter, reports of whispering gallery mode lasers [7, 8, 9], vertical cavity surface emitting lasers [10, 11] and distributed feedback (DFB) lasers [12, 13, 14]. However, except in a limited number of cases such laser demonstrations have used ultrafast (10s of fs to a few ps) optical pulse excitation. While this is appropriate for photophysical studies and acceptable for initial development of the technology, operation with such short pulses has limited applicability because it necessitates the use of bulky and expensive pump lasers. To enable practical implementation of CQD lasers, operation with nanosecond or longer pulse duration is necessary. If low-threshold operation in such a regime can be achieved, it will then be possible to envisage pumping CQD laser systems with compact Q-switched solid-state lasers and possibly, for sufficiently low threshold, with laser diodes. The result will be a reduction in the footprint and cost of the CQD laser technology, bringing it closer to applications.
Herein, we report “hybrid plastic” CdSe/ZnS CQD lasers, i.e. made from an inorganic gain material in combination with a polymer cavity, operating in the nanosecond regime when gain-switched with 5 ns-long pump pulses. One identified barrier to operation in such a temporal regime when using CQDs is the effect of Auger recombination, which basically limits the optical gain lifetime . Circumventing or minimizing this problem necessitates optimization of the gain medium (e.g. using a high-density of CQDs ) and/or of the cavity (e.g. with a low-loss optical cavity ). In our work, CQD lasers with a few nanosecond-pulse duration are made possible by utilizing a planar distributed feedback cavity that includes a high surface quality thin film made of an optimized CQD-composite. Importantly, the planar thin film architecture of DFB lasers enables low-threshold oscillation thanks to a simple and efficient optical excitation of the gain medium as well as an adequate laser mode confinement. The resulting lasers have, to our knowledge, the lowest threshold for such a pulse duration (as low as 0.5 mJ/cm 2 , 100 kW/cm 2 ), making them suitable for pumping with compact solid-state lasers. The DFB structure chosen here also enables emission wavelength versatility at the design stage through fine tuning of the cavity parameters: our lasers are shown to operate over a 11-nm spectral window in this way, while using the same CQD gain material. Furthermore, for ease-of-implementation, our DFB lasers are designed for vertical emission and, by way of example for applications, we demonstrate their potential for sensing.
In conclusion, to allow for a comparison with conventional InGaAs material, the lattice-matched conditions for InGaNAs quantumwell materials are determined from experimental results published in the literature. The optical gain, LWEF and their temperature dependence for InGaNAs QWs are calculated with microscopic theory and a 10x10 effective-mass Hamiltonian and a comparison is made with those of InGaAs QWs. It is shown that an InGaNAs/GaAs QW has better characteristics in optical gain and LWEF than InGaAs QWs. The enhanced differential gain and the smaller LWEF of InGaNAs show its potential for application in optical communications. The temperature insensitivity of the LWEF and its clamping with increasing carrier injection in InGaNAs/GaAs QW promise more stable semiconductor lasers which have spectrally and spatially more stable modal properties under high-speed modulation conditions with narrow laser linewidth, reduced filamentation, and low frequency chirp. Also, the larger electron effective mass and conduction band offset of InGaNAs/GaAs QWs, the greater refractive index contrast achievable in GaAs-based systems than conventional InP-based materials make the application of the material for long-wavelength VCSELs possible.
However, in Fig. 11 it can be seen that ;10% of the probability distribution of 7 is localized in the central well of the active region. The Fermi energy of 7 is just 44 meV below the top of the central well, which has an absolute energy of 259 meV. About 0.3% of the carriers in 7 are in the central well and at or above this energy ~Fig. 11! so is possible that carriers could undergo thermionic emission into the continuum from the upper laser level. As mentioned ear- lier, an elevated electron temperature, while not affecting the gain directly, would give an increased loss in this manner, thus lowering the gain. This could be one reason why the measured gain is below that calculated here.