Quantum Dot Devices

The response of an optically injected quantum dot semiconductor laser is studied both experimentally and theoretically. Specifically, the locking boundaries are investigated, revealing features more commonly associated with Class A lasers rather than conventional Class B semiconductor lasers (SLs). Further, various dynamical regimes are observed including excitability and multistability. Of particular interest is the observation of a phase-locked bistability. We determine the stability diagram analytically using appropriate rate equations for quantum dot lasers. In particular, the saddle-node (SN) and Hopf bifurcations forming the locking boundaries are examined and are shown to reproduce the observed experimental stability features. The generation of the phase-locked bistability is also explained via a combination of these bifurcations.

Passively mode-locked semiconductor lasers with self-assembled quantum dot active regions can be operated in exotic output modes, stabilized by the complex gain and absorption dynamics inherent in these structures. One such device emits dark pulses—sharp dips on an otherwise stable continuous wave background—in an extended cavity design. We show that a dark pulse train is a solution to the master equation for mode-locked lasers and perform numerical modeling to test the stability of such a solution. A separate, monolithic design displays wavelength bistability and can be electrically switched between these two modes within just a few cavity round trips. This device can be made to switch between two stable wavelengths separated by just 7 nm up to over 40 nm with a contrast ratio of over 40 dB.

In this chapter the multi-wavelength emission capabilities of quantum dot (QD) lasers, due to splitting effects in the ground-state (GS), are analyzed. These emission sub-bands are not related to carrier transitions from different excitation levels like GS/excited-state (ES) emission, but are strongly depended on gain saturation effects. The existence of these sub-bands alongside their wavelength tuning capabilities, have enabled the identification of novel regimes of operation like pulse width narrowing in the presence of dual GS emission, and tunable dual state mode locking. The exploitation of these regimes can allow the deployment of QD mode locked lasers into newly emerging applications both in the telecomm and medical field.

We perform characterization of the pulse shape and noise properties of quantum dot passively mode-locked lasers (PMLLs). We propose a novel method to determine the RF linewidth and timing jitter, applicable to high repetition rate PMLLs, through the dependence of modal linewidth on the mode number. Complex electric field measurements show asymmetric pulses with parabolic phase close to threshold, with the appearance of waveform instabilities at higher currents. We demonstrate that the waveform instabilities can be overcome through optical injection-locking to the continues wave (CW) master laser, leading to time-bandwidth product (TBP) improvement, spectral narrowing, and spectral tunability. We discuss the benefits of single- and dual-tone master sources and demonstrate that dual-tone optical injection can additionally improve the noise properties of the slave laser with RF linewidth reduction below instrument limits (1 kHz) and integrated timing jitter values below 300 fs. Dual-tone injection allowed slave laser repetition rate control over a 25 MHz range with reduction of all modal optical linewidths to the master source linewidth, demonstrating phase-locking of all slave modes and coherence improvement.

A review of the high power performance of quantum dot (QD) lasers and one of its failure modes by catastrophic optical damage (COD) is presented. Since the first lasing action reported in 1994, a rapid advancement in the output power of QD lasers has been achieved. QD lasers with excellent optical power from a few mW to more than 11 W have been reported. As the QD laser output power continues to reach higher levels, problems such as COD which causes sudden failure of the laser inevitably become a problem that requires an immediate solution. Over the years, COD failure has been widely reported in QD lasers with emission wavelengths varying from 0.9 to 1.3 μm. In this chapter, factors contributing to the COD failure in high power QD lasers are discussed and existing methods to suppress the COD are assessed. Finally, a novel laser annealing technique with in situ monitoring and control capabilities for the formation of non-absorbing mirrors in QD laser is described.

Post-growth intermixing is a powerful technique currently applied in areas such as high power laser arrays and photonics integrated circuits. The application of this technique to quantum dot (QD) based laser materials is of significant interest offering new types of device and allows large-scale integrated devices, but brings about new challenges. In this paper, we will initially review quantum well (QW) intermixing processes and applications and move on to describe specific differences between QW and QD based materials and review the literature on various forms of QD intermixing. Structural and spectroscopic studies of intermixed QD materials will be discussed, and the importance of modulation p-doping of structures will be highlighted. We will then go on to describe active intermixed QD devices including both lasers and broadband devices such as super luminescent diodes and amplifiers, and conclude with our latest results on selective area intermixed devices.

Photonic laser sources have great potential in communication and lighting applications. Optical resonators reduce the lasing threshold by enhancing the light-matter interaction, increasing the efficiency and modulation rate. We explore the design, fabrication, and characterization of lasers based on photonic crystal (PC) cavities. We first describe the fundamentals of the PC cavity in one dimensional (1D) and two dimensional (2D) settings, and how cavity designs enable high quality factor, low mode volume resonators that facilitate high Purcell enhancements. Next, we show how such designs are implemented to fabricate low threshold lasers using quantum dot(QD) materials. Experimentally under optical injection, we are able to obtain lasing thresholds of microwatts at room temperature and cryogenic temperature, fitting the behavior of different lasers to rate equations. We also theoretically and experimentally characterize the time dynamics of the lasers at cryogenic temperature under modulated pumping, observing that the lasers can be modulated at 30 GHz. Finally, we explore novel approaches to electrically inject PC cavity devices using a lithographically defined lateral p-i-n junction, and demonstrate a lateral junction PC cavity light-emitting device.

An InGaAs submonolayer (SML) quantum-dot photonic-crystal light-emitting diode (QD PhC-LED) with for fiber-optic applications is reported. The active region of the device contains three InGaAs SML QD layers. Each of the InGaAs SML QD layers is formed by alternate depositions of InAs (<1 ML) and GaAs. A maximum CW output power of 0.34 mW at 20 mA has been obtained in the 980 nm range. The internally reflected spontaneous emission can be extracted and collimated out of the photonic-crystal etched holes. High-resolution imaging studies indicate that the device emits narrower light beams mainly through the photonic-crystal etched holes making it suitable for fiber-optic applications.

Laser and strong coupling can coexist in a single quantum dot (QD) coupled to nanostructures. This provides an important clue toward the realization of quantum optical devices, such as quantum optical transistor, slow light device, fast light device, or light storage device. In contrast to conventional electronic transistor, a quantum optical transistor uses photons as signal carriers rather than electrons, which has a faster and more powerful transfer efficiency. Under the radiation of a strong pump laser, a signal laser can be amplified or attenuated via passing through a single quantum dot coupled to a photonic crystal (PC) nanocavity system. Such a switching and amplifying behavior can really implement the quantum optical transistor. By simply turning on or off the input pump laser, the amplified or attenuated signal laser can be obtained immediately. Based on this transistor, we further propose a method to measure the vacuum Rabi splitting of exciton in all-optical domain. Besides, we study the light propagation in a coupled QD and nanomechanical resonator (NR) system. We demonstrate that it is possible to achieve the slow light, fast light, and quantum memory for light on demand, which is based on the mechanically induced coherent population oscillation (MICPO) and exciton polaritons. These QD devices offer a route toward the use of all-optical technique to investigate the coupled QD systems and will make contributions to quantum internets and quantum computers.

Photonic devices employing semiconductor quantum dots (QDs) are anticipated to play an important role within power-efficient optical networks. In this chapter, we consider the prospects for signal processing using all-optical QD switches. Vertical cavity structures have been developed to enhance the light-QD interaction and accordingly the optical nonlinearity of QDs which leads to low energy consumption. Such structures show great potential for the realization of power-efficient, polarization-insensitive and micrometer-size switching devices for future photonic signal processing systems.

In this Chapter we describe the experimental studies of ultrafast carrier dynamics and all-optical switching in semiconductor quantum dots (QDs) using ultrafast terahertz (THz) techniques. In the first part of this chapter we describe the studies of carrier capture into the QDs, and thermionic carrier release from the QDs with (sub-)picosecond time resolution, using optical pump–THz probe measurements. In the second part of this chapter we investigate the direct manipulation of the quantum confinement potential of the QDs by an electric field of a strong THz pulse. The resulting THz-driven quantum-confined Stark effect leads to a strong modulation of a ground-state optical absorption in the QDs. Dynamically, such a THz-induced electro-absorption modulation in QDs (near-)instantaneously follows the absolute value of the electric field of the THz pulse, providing the capability for Tbit/s—rate all-optical switching in QDs using THz signals. The principles of experimental techniques used in our studies: optical pump–THz probe, and THz pump–optical probe spectroscopies, and strong-field THz generation, are also described in this chapter.

The nonlinear optical response of self-assembled quantum dots (QD) is relevant to the application of QD-based devices in nonlinear optics, all-optical switching, slow light, and self-organization. Theoretical investigations are based on numerical simulations of a spatially and spectrally resolved rate equation model, which takes into account the strong coupling of the quantum dots to the carrier reservoir created by the wetting layer (WL) states. The complex dielectric susceptibility of the ground state is obtained. The saturation is shown to follow a behavior in between the one for a dominantly homogeneously and inhomogeneously broadened medium. Approaches to extract the nonlinear refractive index change by fringe shifts in a cavity or self-lensing are discussed. Experimental work on saturation characteristic of InGa/GaAs quantum dots close to the telecommunication O-band (1240–1280 nm) and of InAlAs/GaAlAs QD at 780 nm is described and the first demonstration of the cw saturation of absorption in room temperature QD samples is discussed in detail.

We present theoretical and experimental results of electron kinetics and transport in quantum dot structures with potential barriers created around dots via intentional or unintentional doping. Monte Carlo simulations demonstrate that photoelectron capture is substantially enhanced in strong fields and electron kinetics can be controlled by potential barriers. Therefore, by creating potential barriers around dots, we found that our novel quantum dots with built-in charge (Q-BIC) solar cells and infrared (IR) photodectors enhance electron intersubband transitions and suppress fast electron capture processes. These factors lead to a 60% increase in the photocurrent of the Q-BIC solar cells (without degradation of the open circuit voltage) and ~25 times increase in the photoresponse of the Q-BIC photodetectors.

CdSe quantum dot (QD)-sensitized solar cells (QDSCs) were synthesized by adsorbing CdSe QDs onto TiO₂ nanostructured electrodes with different morphologies, i.e., nanoparticles, nanotubes, and inverse opals. The optical absorption, photoelectrochemical, and photovoltaic properties of the QDSCs were characterized and the dependences of these properties on the QD deposition time and the TiO₂ nanostructure are discussed. To improve the photovoltaic performance of the CdSe QDSCs, surface passivation with a ZnS coating was introduced and Cu₂S counter electrodes were applied. All aspects of the photovoltaic performance, including the short-circuit photocurrent density, open-circuit voltage, fill factor, and efficiency, were found to be significantly improved by the surface modification with ZnS. For the counter electrode, the Cu₂S electrode was demonstrated to be more efficient than platinum against the polysulfide electrolytes usually used as redox couples in CdSe QDSCs. Moreover, CdS QD adsorption on the TiO₂ electrodes prior to CdSe QD adsorption also resulted in better solar cell performance. In addition, we found that the morphology of the TiO₂ electrodes had a great influence on the photovoltaic properties of the QDSCs. Finally, a power conversion efficiency as high as 4.9% was achieved for a combined CdS/CdSe QDSC under solar illumination of 100 mW/cm².

This chapter highlights recent optoelectronic applications of colloidal quantum dots (QDs). In recent years, many colloidal QD-based optoelectronic devices, and device concepts have been proposed and studied. Many of these device concepts build on traditional optoelectronic device concepts. Increasingly, many new optoelectronic device concepts have been based on the use of biomolecule QD complexes. In this chapter, both types of structures are discussed. Special emphasis is placed on new optoelectronic device concepts that incorporate DNA-based aptamers in biomolecule QD complexes. Not only are the extensions of traditional devices and concepts realizable, such as QD-based photo detectors, displays, photoluminescent and photovoltaic devices, light-emitting diodes (LEDs), photovoltaic devices, and solar cells, but new devices concepts such a biomolecule-based molecular sensors possible. This chapter highlights a number of such novel QD-based devices and device concepts.