In this dissertation, a theory of modulation response of a semiconductor quantum dot (QD) laser is developed. The effect of the following factors on the modulation bandwidth of a QD laser is studied and the following results are obtained:<br /><br />1) Carrier capture delay from the optical confinement layer into QDs<br /><br />Closed-form analytical expressions are obtained for the modulation bandwidth omega_{-3 dB} of a QD laser in the limiting cases of fast and slow capture into QDs. omega_{-3 dB} is highest in the case of instantaneous capture into QDs, when the cross-section of carrier capture into a QD sigma_n = infinity. With reducing sigma_n, omega_{-3 dB} decreases and becomes zero at a certain non-vanishing sigma_n^{min}. This sigma_n^{min} presents the minimum tolerable capture cross-section for the lasing to occur at a given dc component j_0 of the injection current density. The higher is j_0, the smaller is sigma_n^{min} and hence the direct modulation of the output power is possible at a slower capture. The use of multiple layers with QDs is shown to considerably improve the modulation response of the laser -- the same omega_{-3 dB} is obtained in a multi-layer structure at a much lower j_0 than in a single-layer structure.<br /><br />2) Internal optical loss in the optical confinement layer<br /><br />The internal optical loss, which increases with free-carrier density in the waveguide region, considerably reduces the modulation bandwidth omega_{-3 dB} of a QD laser. With internal loss cross-section sigma_int increasing and approaching its maximum tolerable value, the modulation bandwidth decreases and becomes zero. There exists the optimum cavity length, at which omega_{-3 dB} is highest; the larger is sigma_int, the longer is the optimum cavity.<br /> <br />3) Excited states in QDs<br /><br />Direct and indirect (excited-state-mediated) mechanisms of capture of carriers from the waveguide region into the lasing ground state in QDs are considered, and the modulation response of a laser is calculated. It is shown that, when only indirect capture is involved, the excited-to-ground-state relaxation delay strongly limits the ground-state modulation bandwidth of the laser -- at the longest tolerable relaxation time, the bandwidth becomes zero. When direct capture is also involved, the effect of excited-to-ground-state relaxation is less significant and the modulation bandwidth is considerably higher.<br /> / Ph. D.

This work develops a comprehensive theoretical model for a semiconductor laser, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells (QWs). The potential of such a tunneling-injection QD laser for temperature-stable and high-power operation is studied under the realistic conditions of out-tunneling leakage of carriers from QDs (and hence parasitic recombination outside QDs) and the presence of the wetting layer (WL). The following topics are included in the dissertation: 1) Characteristic temperature of a tunneling-injection QD laser The threshold current density jth and the characteristic temperature T0 are mainly controlled by the recombination in the QWs. Even in the presence of out-tunneling from QDs and recombination outside QDs, the tunneling-injection laser shows the potential for significant improvement of temperature stability of jth — the characteristic temperature T0 remains very high (above 300 K at room temperature) and not significantly affected by the QD size fluctuations. 2) Output power of a tunneling-injection QD laser Closed-form expressions for the light-current characteristic (LCC) and carrier population across the layered structure are derived. Even in the presence of out-tunneling leakage from QDs, the intensity of parasitic recombination outside QDs is shown to remain restricted with increasing injection current. As a consequence, the LCC of a tunneling-injection QD laser exhibits a remarkable feature — it becomes increasingly linear, and the slope efficiency grows closer to unity at high injection currents. The linearity is due to the fact that the current paths connecting the opposite sides of the structure lie entirely within QDs — in view of the three-dimensional confinement in QDs, the out-tunneling fluxes of carriers from dots are limited. 3) Effect of the WL on the output power of a tunneling-injection QD laser In the Stranski-Krastanow self-assembling growth mode, a two-dimensional 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 tunneling-injection structure. Since the opposite sides of a tunneling-injection 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 (QWs 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 tunneling-injection 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. / Ph. D.

The effect of different factors on the operating characteristics of a semiconductor quantum dot (QD) laser is studied. Specifically, the following topics are included in the dissertation: 1) Effect of carrier-density-dependent internal loss in the optical confinement layer (OCL) on the characteristic temperature. Internal optical loss in a QD laser couples the confined-carrier level occupancy in QDs to the free-carrier density in the OCL. Due to this coupling, which is controlled by the threshold condition, the free-carrier density is increased and more temperature-sensitive, and also the confined-carrier level occupancy becomes temperature-dependent. As a result, the characteristic temperature of a laser is considerably reduced. Carrier-density-dependent internal loss also sets an upper limit for operating temperatures of a QD laser and constrains the shallowest potential well depth and the smallest tolerable size of a QD at which the lasing can be attained. The dependences of the characteristic temperature, maximum operating temperature, and shallowest potential well depth on the parameters of the structure are obtained. At the maximum operating temperature or when any parameter of the structure is equal to its critical tolerable value, the characteristic temperature reduces to zero. 2) Effect of excited-states in QDs on the light-current characteristic (LCC). The carrier capture from the three-dimensional reservoir (optical confinement layer – OCL) into the QD ground-state and escape from the ground-state to the OCL are assumed to occur via the QD excited-state. Such a two-step capture places a fundamental limitation on ground-state lasing—the output power saturates at high injection currents. The saturation power is controlled by the transition time between the excited- and ground-state in a QD. The longest, cut-off transition time exists, beyond which no ground-state lasing is possible. The following characteristics are analyzed versus the injection current density and the transition time: occupancies of the ground- and excited-state, free carrier density in the OCL, threshold current density, number of stimulated photons emitted, output power, internal and external differential quantum efficiencies. 3) Effect of longitudinal spatial hole burning (SHB) and multimode lasing on the LCC. The number of modes is shown to remain limited with increasing injection current. The maximum number of modes that can oscillate in a QD laser is analytically estimated. While this number increases with increasing surface density of QDs or cavity length, it remains limited (first increases and then decreases) with increasing scatter in the QD-size. The critical tolerable values of the structure parameters are derived beyond which higher-order longitudinal modes can not oscillate. It is notable that, in addition to the maximum tolerable scatter, there also exists the minimum scatter in the QD-size for each higher-order mode to start lasing. The threshold currents and output powers of modes are computed numerically. The power of the main mode is reduced due to lasing of higher-order modes and spatially nonuniform carrier distribution. As a new mode turns on, kinks appear in the LCCs of existing modes. SHB reduces the total optical power of a laser and contributes to nonlinearity of the overall LCC. The effect is more significant when any of the structure parameters is close to its critical tolerable value. The LCC becomes more linear with improving QD-size uniformity or increasing surface density of QDs or cavity length. / Ph. D.

In this thesis, our primary objective was to theoretically analyze the real world modulation bandwidth of a DTI QD laser and this was done by analyzing the effect of out-tunneling leakage of carriers from QDs, and by analyzing the effect of electron-hole asymmetry on the device characteristics. We are confronted with the following results: 1) Effect of Out-Tunneling Leakage on Modulation Bandwidth in Double Tunneling Injection Quantum Dot Lasers To purely focus on this effect, the conditions of instantaneous carrier exchange between the OCL and QW (on each side of the structure) and tunneling injection into QDs are assumed and closed-form analytical expressions for modulation bandwidth are obtained. The relative decrease in modulation bandwidth, due to this effect, in a DTI QD laser (from plots of modulation bandwidth vs j on increasing wout) is then shown to be small, and at ranges of injection currents of operational interest, nearly negligible. Consequently, it is shown that the DTI laser is a robust device in terms of sensitivity to out-tunneling leakage i.e. much effort need not be paid in suppressing this phenomenon. 2) Effect of Electron-Hole Asymmetry on Modulation Bandwidth of Double Tunneling Injection Quantum Dot Lasers On analyzing the effect of electron-hole asymmetry on the device characteristics of a DTI QD laser, it can be noted (from plots of modulation bandwidth vs injection current) that there is no reduction in the maximum modulation bandwidth i.e. electron-hole asymmetry does not indicate a reduction in the effectiveness of such a DTI design. This is shown to occur as the maximum modulation bandwidth depends on both, the effective differential non-stimulated recombination time as well the photon lifetime in the optical cavity. The photon lifetime being much smaller than the former acts as the dominating factor, and hence we see no appreciable change in the maximum modulation bandwidth. In the course of this analysis, we also see that the actual condition i.e. that of electron hole asymmetry is closer, among the cases of symmetry, to symmetry assuming hole parameters rather than electron parameters. As such, in cases where electron-hole symmetry must be used (in order to facilitate numerical simplifications), a recommendation of this study is to use hole parameters instead. / Master of Science / In this age of internet and optical communications, semiconductor lasers have a profound impact on the way we interact with our world. They act as intermediaries converting digital signals into optical pulses (in order to be transmitted) and then back into digital code. Understandably, the maximum speed at which these lasers can encode and decode information limits the speed of this entire communication network. This speed can be defined as the modulation bandwidth. A new design, the double tunneling-injection (DTI) quantum dot (QD) laser shows considerable promise, however its modulation bandwidth under real world operating conditions was yet to be analyzed. The aim of this thesis was to then theoretically analyze the real world modulation bandwidth of this new semiconductor laser design. This was done by analyzing the effect of unwanted leakage of carriers (out-tunneling) from the active region (Quantum Dots), and by analyzing the effect of electron-hole asymmetry on the device characteristics. The relative decrease in modulation bandwidth, due to leakage of carriers, in a DTI QD laser is then shown to be nearly negligible. Consequently, it is shown that the DTI QD laser is a robust device in terms of sensitivity to out-tunneling leakage, i.e., much effort need not be paid in suppressing this phenomenon. On analyzing the effect of electron-hole asymmetry on the device characteristics of a DTI QD laser, it is shown that there is no reduction in the maximum modulation bandwidth, i.e., electron-hole asymmetry does not indicate a reduction in the effectiveness of such a design. Thus, this analysis reiterates the fact that DTI QD lasers indeed show incredible potential to drastically improve modulation bandwidth and must be investigated further.

The purpose of this thesis is to design and fabricate cross-loop cavity filter. We fabricated optical filter by bended waveguide and 2x2 90-degree MMI crossing model. By this design, we get the power splitter with coupling coefficient is 0, 0.15, 0.5, 0.85. By MatFhcad and BPM simulation, we showed that the device volume was decrease to 34%. In the quantum dot lasers, we fabricated the Fabry-Perot laser by optical waveguide and cleavage surface. In the material, a 1.3£gm quantum dots InGaAs epitaxial wafer is used to fabricate the lasers. Broad area lasers and ridge waveguide lasers are fabricated and their static properties (IV, LI) are analyzed experimentally. In fabrication process, first, we defined the device pattern by using photo-lithography technique. Second, we etched ridge waveguide by using dry etching method. Finally, we used the etching solution HBr:HCl:H2O2:H2O=5:4:1:70 to smooth the sidewall and reduce the scattering loss. We showed that the waveguide loss was decrease to 27.9dB/cm. In the QD lasers characteristic, we can not observe laser characteristics, partly because of the low optical power. Through the optimization of QD growth conditions, we can increase the QD sheet density and increase the number of QD layers. We can also optimize the device processing techniques and laser structure design in order to reduce the series resistance and to increase the optical confinement factor. By using the methods mentioned above, we believe the laser signal can be further increase. In the cross-loop cavity filter characteristic, we get the FSR=300GHz (simulation value FSR=50GHz) in throughput port and drop port. We attribute this appearance induced by cross couple for 2x2 90-degree MMI. The contracts for the drop port of 10.22dB have been achieved.

In this thesis, novel multi-section laser diodes based on quantum-dot material are designed and investigated which exhibit a number of advantages such as low threshold current density; temperature-insensitivity and suppress carrier diffusion due to discrete nature of density of state of quantum-dots. The spectral versatility in the range of 1.1 µm – 1.3 µm wavelengths is demonstrated through novel mode-locking regimes such as dual-wavelength mode-locking, wavelength bistability and broad tunability. Moreover, broad pulse repetition rate tuning using an external cavity configuration is presented. A high peak power of 17.7 W was generated from the quantum-dot laser as a result of the tapered geometry of the gain section of the laser has led to successful application of such device for two-photon imaging. Dual-wavelength mode-locking is demonstrated via ground (?=1180 nm) and excited (?=1263 nm) spectral bands with optical pulses from both states simultaneously in the 5-layer quantum-dot two-section diode laser. The widest spectral separation of 83 nm between the modes was achieved in a dual-wavelength mode-locked non-vibronic laser. Power and wavelength bistability are achieved in a mode-locked multi-section laser which active region incorporates non-identical QD layers grown by molecular beam epitaxy. As a result the wavelength can be electronically controlled between 1245 nm and 1290 nm by applying different voltages to the saturable absorber. Mode-locked or continuous-wave regimes are observed for both wavelengths over a 260 mA – 330 mA current ranges with average power up to 28 mW and 31 mW, respectively. In mode-locked regime, a repetition rate of 10 GHz of optical pulses as short as 4 ps is observed. Noticeable hysteresis of average power for different bias conditions is also demonstrated. The wavelength and power bistability in QD lasers are potentially suitable for flip-flop memory application. In addition, a unique mode-locked regime at expense of the reverse bias with 50 nm wavelength tuning range from 1245 nm to 1290 nm is also presented. Broad repetition rate tunability is shown from quantum-dot external cavity mode-locked 1.27 µm laser. The repetition rate from record low of 191 MHz to 1 GHz from fundamental mode-locking was achieved. Harmonic mode-locking allows further to increase tuning up to 6.8 GHz (34th-order harmonic) from 200 MHz fundamental mode-locking. High peak power of 1.5 W can be generated directly from two-section 4 mm long laser with bent waveguide at angle of 7° at 1.14 GHz repetition rate without the use of any pulse compression and optical amplifier. Stable mode-locking with an average power up to 60 mW, corresponding to 25 pJ pulse energy is also obtained at a repetition frequency of 2.4 GHz. The minimum time-bandwidth product of 1.01 is obtained with the pulse duration of 8.4 ps. Novel tapered quantum-dot lasers with a gain-guided geometry operating in a passively mode-locked regime have been investigated, using structures that incorporated either 5 or 10 quantum dot layers. The peak power of 3.6 W is achieved with pulse duration of 3.2 ps. Furthermore, the record peak power of 17.7 W and transform limited pulses of 672 fs were achieved with optimized structure. The generation of picosecond pulses with high average power of up to 209 mW was demonstrated, corresponding to 14.2 pJ pulse energy. The improved optical parameters of the tapered laser enable to achieve nonlinear images of fluorescent beads. Thus it is for the first time that QD based compact monolithic device enables to image biological samples using two-photon microscopy imaging technique.

Here we present direct investigation of the lasing behaviour by performing gain spectroscopy of solution-based CQDs enabled via in-situ tuning of the feedback wavelength of an open-access hemispherical microcavity. The investigation is performed on two different types of CQDs, namely spherical CdSe/CdS core-shell CQDs and nanopletelets (NPs). The lasing threshold and the differential gain/slope efficiency of the fundamental cavity mode are measured as a function of their spectral position over a spectral range of &Tilde; 32 nm and of &Tilde; 42 nm for the spherical CQDs and NPs, respectively. The results of the gain spectroscopy are described using theoretical models, providing insights into the mechanism governing the observed lasing behaviour. Furthermore, the open-access cavity architecture provides a very convenient way of producing in-situ tunable lasing, and single-mode lasing of the fundamental cavity mode over a spectral range of &Tilde; 25 nm and &Tilde; 37 nm is demonstrated using spherical CQDs and NPs, respectively. In addition, the stability of laser emission is investigated, with the lasing intensity of the fundamental cavity mode remaining constant over a time period of almost 6 mins. It is hoped that the results will provide a detailed understanding of the lasing behaviour of CQDs. This information can be fed back into the design of CQDs in which the lasing threshold can be reduced to the point where useful devices can be constructed, and in the design of resonant optical feedback structures for which the appropriate wavelength must be carefully selected.

Terahertz (THz) technology is still currently a rapidly developing area of research with applications already demonstrated in the fields of biology, medicine, security, chemical/materials inspection and astrophysics to name a few. The diversity of applications which require the generation and measurement of THz or sub-millimeter (sub-mm) electromagnetic (EM) signals is the result of the vast number of chemical elements and compounds which exhibit molecular transitions and vibrational behavior that occur at frequency ranges corresponding to the so-called 'THz gap', roughly defined as 0.05-10 THz. The THz gap was named as such because of the relative difficulty in generating and analysing EM waves in this frequency band. This was due to the inherent challenges in generating either electrical signals with response periods below 1 picosecond (ps), or optical signals with wavelengths in the far-infrared (FIR) range. High absorption of THz signals in atmosphere via absorption by molecules such as H2O also impeded early developments and is a key issue in THz systems even today. There is now a wide variety of THz system solutions, each of which exhibits a different set of operational advantages and limitations. Arguably, the most well-established THz technique to date is based on the use of photoconductive antennas (PCAs) driven by ultrafast pulsed or dual-wavelength laser systems. This technique is the basis for the work presented in this thesis, which is an investigation into the potential utilisation of quantum dot (QD)-based semiconductor materials and devices in THz systems. This thesis discusses the work carried out in the development of a novel class of PCA devices which were postulated to enable efficient optical-to-THz signal conversion, whilst also overcoming several major limitations normally exhibited by PCA devices such as limited optical wavelength pumping range and thermal breakdown. To summarise briefly, these issues were addressed by considering: the additional pump absorption energy ranges enabled by the inclusion of multiple bandgap-engineered semiconductor materials and quantum-confined structures; the higher thermal conductivity and hence pump tolerance exhibited by relatively high-quality (low defect) absorption layers; and by simultaneously harnessing the ultrafast charge carrier modulation exhibited by the integrated QDs. Additionally, some work was carried out using QD-based lasers as pump sources, with the initial intention to explore the feasibility of a fully QD-based THz transceiver system and draw some conclusions as to the future potential for ultra-compact or even lab-on-chip THz systems, for example.

This thesis reports a passive method for Fermi level regulation in quantum dot assemblies through ground state transfer between QDs. Here, ZnTe/CdS, and PbSe/CdSe core/shell QDs were used as valence band electron donors, while Cu containing CdS or ZnSe acts as electron acceptor QDs. Prior to study of ground state charge transfer process, this report discusses the synthesis of ZnTe/CdS, and PbSe/CdSe core shell QDs, which are later used to study charge transfer. Since ZnTe QDs are unstable and prone to oxidation, a CdS coated ZnTe QDs were used. Growing a CdS shell on ZnTe core is difficult because high reduction potential of Te. To overcome this problem, partially reduced sulphur is used for the synthesis of ZnTe/CdS. The peculiar optical properties exhibited by ZnTe/CdS also have been discussed. Even though the synthesis of Lead chalcogenide nanoparticles has been investigated previously, certain inconsistencies between the behavior expected from known mechanisms and empirical observations. An anion exchange mechanism is proposed and demonstrated to be involved in PbSe formation. Both ZnTe and PbSe based QDs are extensively used to study hole injection and copper containing QDs were used as acceptors. The charge transfer has been studied using optical spectroscopy. The structure and composition of the assemblies was identified using powder crystallography, electron-microscopy and composition analysis. The unique physical and chemical properties of these materials are exciting both fundamentally as well as from the point of view of applications.