The ability of a single monolithic semiconductor structure to emit or lase in a broad spectrum range is of great importance for many applications such as solid-state lighting and multi-spectrum detection. But spectral range of a laser or light-emitting diode made of a given semiconductor is typically limited by its emission or gain bandwidth. Due to lattice mismatch, it is typically difficult to grow thin film or bulk materials with very different bandgaps in a monolithic fashion. But nanomaterials such as nanowires, nanobelts, nanosheets provide a unique opportunity. Here we report our experimental results demonstrating simultaneous lasing in two visible colors at 526 and 623 nm from a single CdSSe heterostructure nanosheet at room temperature. The 97 nm wavelength separation of the two colors is significantly larger than the gain bandwidth of a typical single II-VI semiconductor material. Such lasing and light emission in a wide spectrum range from a single monolithic structure will have important applications mentioned above.
This dissertation aims to study and understand the effect of nonlinear dynamics and quantum chaos in graphene, optomechanics, photonics and spintronics systems.
First, in graphene quantum dot systems, conductance fluctuations are investigated from the respects of Fano resonances and quantum chaos. The conventional semi-classical theory of quantum chaotic scattering used in this field depends on an invariant classical phase-space structure. I show that for systems without an invariant classical phase-space structure, the quantum pointer states can still be used to explain the conductance fluctuations. Another finding is that the chaotic geometry is demonstrated to have similar effects as the disorders in transportations.
Second, in optomechanics systems, I find rich nonlinear dynamics. Using the semi-classical Langevin equations, I demonstrate a quasi-periodic motion is favorable for the quantum entanglement between the optical mode and mechanical mode. Then I use the quantum trajectory theory to provide a new resolution for the breakdown of the classical-quantum correspondences in the chaotic regions.
Third, I investigate the analogs of the electrical band structures and effects in the non-electrical systems. In the photonic systems, I use an array of waveguides to simulate the transport of the massive relativistic particle in a non-Hermitian scenario. A new form of Zitterbewegung is discovered as well as its analytical explanation. In mechanical systems, I use springs and mass points systems to achieve a three band degenerate band structure with a new pair of spatially separated edge states in the Dice lattice. A new semi-metal phase with the intrinsic valley-Hall effect is found.
At last, I investigate the nonlinear dynamics in the spintronics systems, in which the topological insulator couples with a magnetization. Rich nonlinear dynamics are discovered in this systems, especially the multi-stability states.
Semiconductor nanowires (NWs) are one dimensional materials and have size quantization effect when the diameter is sufficiently small. They can serve as optical wave guides along the length direction and contain optically active gain at the same time. Due to these unique properties, NWs are now very promising and extensively studied for nanoscale optoelectronic applications. A systematic and comprehensive optical and microstructural study of several important infrared semiconductor NWs is presented in this thesis, which includes InAs, PbS, InGaAs, erbium chloride silicate and erbium silicate. Micro-photoluminescence (PL) and transmission electron microscope (TEM) were utilized in conjunction to characterize the optical and microstructure of these wires. The focus of this thesis is on optical study of semiconductor NWs in the mid-infrared wavelengths. First, differently structured InAs NWs grown using various methods were characterized and compared. Three main PL peaks which are below, near and above InAs bandgap, respectively, were observed. The octadecylthiol self-assembled monolayer was employed to passivate the surface of InAs NWs to eliminate or reduce the effects of the surface states. The band-edge emission from wurtzite-structured NWs was completely recovered after passivatoin. The passivated NWs showed very good stability in air and under heat. In the second part, mid-infrared optical study was conducted on PbS wires of subwavelength diameter and lasing was demonstrated under optical pumping. The PbS wires were grown on Si substrate using chemical vapor deposition and have a rock-salt cubic structure. Single-mode lasing at the wavelength of ~3000-4000 nm was obtained from single as-grown PbS wire up to the temperature of 115 K. PL characterization was also utilized to demonstrate the highest crystallinity of the vertical arrays of InP and InGaAs/InP composition-graded heterostructure NWs made by a top-down fabrication method. TEM-related measurements were performed to study the crystal structures and elemental compositions of the Er-compound core-shell NWs. The core-shell NWs consist of an orthorhombic-structured erbium chloride silicate shell and a cubic-structured silicon core. These NWs provide unique Si-compatible materials with emission at 1530 nm for optical communications and solid state lasers.
Nanowires are 1D rod-like structures which are regarded as the basis for future technologies. III-V nanowires have attracted immense attention because of their stability, crystal quality and wide use. In this work, I focus on the growth and characterization of III-V semiconductor nanowires, in particular GaP, InP and InGaP alloys. These nanowires were grown using a hot wall CVD(Chemical Vapor Deposition) setup and are characterized using SEM (Scanning Electron Microscope), EDX (Energy Dispersive X-ray Spectroscopy) and PL (Photoluminescence) techniques.
In the first chapter, Indium Phosphide nanowires were grown using elemental sources (In and P powders). I consider the various kinds of InP morphologies grown using this method. The effect of source temperature on the stoichiometry and optical properties of nanowires is studied. Lasing behavior has been seen in InP nanostructures, showing superior material quality of InP.
InGaP alloy nanowires were grown using compound and elemental sources. Nanowires grown using compound sources have significant oxide incorporation and showed kinky morphology. Nanowires grown using elemental sources had no oxide and showed better optical quality. Also, these samples showed a tunable alloy composition across the entire substrate covering more than 50% of the InGaP alloy system. Integrated intensity showed that the bandgap of the nanowires changed from indirect to direct bandgap with increasing Indium composition. InGaP alloy nanowires were compared with Gallium Phosphide nanowires in terms of PL emission, using InGaP nanowires it is possible to grow nanowires free of defects and oxygen impurities, which are commonly encountered in GaP nanowires.
This dissertation aims to demonstrate a new approach to fabricating solar cells for spectrum-splitting photovoltaic systems with the potential to reduce their cost and complexity of manufacturing, called Monolithically Integrated Laterally Arrayed Multiple Band gap (MILAMB) solar cells. Single crystal semiconductor alloy nanowire (NW) ensembles are grown with the alloy composition and band gap changing continuously across a broad range over the surface of a single substrate in a single, inexpensive growth step by the Dual-Gradient Method. The nanowire ensembles then serve as the absorbing materials in a set of solar cells for spectrum-splitting photovoltaic systems.
Preliminary design and simulation studies based on Anderson's model band line-ups were undertaken for CdPbS and InGaN alloys. Systems of six subcells obtained efficiencies in the 32-38% range for CdPbS and 34-40% for InGaN at 1-240 suns, though both materials systems require significant development before these results could be achieved experimentally. For an experimental demonstration, CdSSe was selected due to its availability. Proof-of-concept CdSSe nanowire ensemble solar cells with two subcells were fabricated simultaneously on one substrate. I-V characterization under 1 sun AM1.5G conditions yielded open-circuit voltages (Voc) up to 307 and 173 mV and short-circuit current densities (Jsc) up to 0.091 and 0.974 mA/cm2 for the CdS- and CdSe-rich cells, respectively. Similar thin film cells were also fabricated for comparison. The nanowire cells showed substantially higher Voc than the film cells, which was attributed to higher material quality in the CdSSe absorber. I-V measurements were also conducted with optical filters to simulate a simple form of spectrum-splitting. The CdS-rich cells showed uniformly higher Voc and fill factor (FF) than the CdSe-rich cells, as expected due to their larger band gaps. This suggested higher power density was produced by the CdS-rich cells on the single-nanowire level, which is the principal benefit of spectrum-splitting. These results constitute a proof-of-concept experimental demonstration of the MILAMB approach to fabricating multiple cells for spectrum-splitting photovoltaics. Future systems based on this approach could help to reduce the cost and complexity of manufacturing spectrum-splitting photovoltaic systems and offer a low cost alternative to multi-junction tandems for achieving high efficiencies.
Optical metasurfaces, i.e. artificially engineered arrays of subwavelength building blocks supporting abrupt and substantial light confinement, was employed to demonstrate a novel generation of devices for circularly polarized detection, full-Stokes polarimetry and all-optical modulation with ultra-compact footprint and chip-integrability.
Optical chirality is essential for generation, manipulation and detection of circularly polarized light (CPL), thus finds many applications in quantum computing, communication, spectroscopy, biomedical diagnosis, imaging and sensing. Compared to natural chiral materials, chiral metamaterials and metasurfaces enable much stronger chirality on subwavelength scale; therefore, they are ideal for device miniaturization and system integration. However, they are usually associated with low performance due to limited fabrication tolerance and high dissipation mainly caused by plasmonic materials. Here, a bio-inspired submicron-thick chiral metamaterial structure was designed and demonstrated experimentally with high contrast (extinction ratio >35) detection of CPL with different handedness and high efficiency (>80%) of the overall device. Furthermore, integration of left- and right-handed CPL detection units with nanograting linear polarization filters enabled full-Stokes polarimetry of arbitrarily input polarization states with high accuracy and very low insertion loss, all on a submillimeter single chip. These unprecedented highly efficient and high extinction ratio devices pave the way for on-chip polarimetric measurements.
All-optical modulation is widely used for optical interconnects, communication, information processing, and ultrafast spectroscopy. Yet, there’s deficiency of ultrafast, compact and energy-efficient solutions all in one device. Here, all-optical modulation of light in the near- and mid-infrared regimes were experimentally demonstrated based on a graphene-integrated plasmonic nanoantenna array. The remarkable feature of the device design is its simultaneous near-field enhancement for pump and probe (signal) beams, owing to the localized surface plasmon resonance excitation, while preserving the ultrafast photocarrier relaxation in graphene. Hence, a distinct modulation at 1560nm with record-low pump fluence (<8μJ/cm^2) was reported with ~1ps response time. Besides, relying on broadband interaction of graphene with incident light, a first-time demonstration of graphene-based all-optical modulation in mid-infrared spectral region (6-7μm) was reported based on the above double-enhancement design concept. Relying on the tunability of metasurface design, the proposed device can be used for ultrafast optical modulation from near-infrared to terahertz regime.
Semiconductor nanolasers, as a frontier subject has drawn a great deal of attention over the past decade. Semiconductor nanolasers are compatible with on-chip integrations towards the ultimate realization of photonic integrated circuits. However, innovative approaches are strongly required to overcome the limitation of lattice-mismatch issues. In this dissertation, two alternative approaches are employed to overcome the lattice-mismatch issues. i) By taking advantage of nanowires or nanobelts techniques, flexibility in bandgap engineering has been greatly expanded, resulting in the nanolasers with wide wavelength coverage and tunability. Simultaneous two-color lasing in green and red is firstly achieved from monolithic cadmium sulfide selenide nanosheets. The wavelength separation is up to 97 nm at room temperature, larger than the gain bandwidth of a single semiconductor material in the visible wavelength range. The strategies adopted for two-color lasers eventually leads to the realization of simultaneous red, green and blue lasing and white lasing from a single zinc cadmium sulfide selenide nanosheet with color tunability in the full visible range, making a major milestone in the ultimate solution of laser illumination and laser display. In addition, with the help of nanowire techniques, material emission has been extended to mid-infrared range, enabling lasing at ~3µm from single lead sulfide subwavelength wires at 180 K. The cavity volume of the subwavelength laser is down to 0.44 λ3 and the wavelength tuning range is over 270 nm through the thermo-optic mechanism, exhibiting considerable potentials for on-chip applications in mid-infrared wavelength ranges. ii) By taking advantage of membrane transfer techniques, heterogeneous integration of compound semiconductor and waveguide material becomes possible, enabling the successful fabrication of membrane based nano-ring lasers on a dielectric substrate. Thin membranes with total thickness of ~200nm are first released from the original growth substrate and then transferred onto a receiving substrate through a generally applicable membrane transfer method. Nano-ring arrays are then defined by photolithography with an individual radius of 750 nm and a radial thickness of 400-500 nm. As a result, single mode lasing is achieved on individual nano-ring lasers at ~980 nm with cavity volumes down to 0.24 λ3, providing a general avenue for future heterogeneous integration of nanolasers on silicon substrates.
Light Emitting Diodes even with their longer life, robust build and low power consumption, they are still plagued by some problems the most significant of which are the current droop and thermal droop. Current droop causes a lowering in the Internal Quantum Efficiency with increased current injection while thermal droop lowers the whole Internal Quantum Efficiency curve with increase in temperature. The focus here was understanding effects of thermal droop and develop a method to control it.
Shockley Read Hall recombination plays a dominant role in the thermal droop effect when the current injection is low. Since the blue light emitting diode is based on Gallium Nitride, we need to take into consideration the effect of piezoelectric polarization in the quantum wells. The effects of the piezoelectric fields were studied based on the Gallium Nitride plane orientations. It was found in a Gallium Nitride light emitting diodes simulation study that more the number of quantum wells, lower would be the Radiative recombination rate. The problem of exacerbated spatial separation of electron hole wavefunctions in a thick single quantum well structure lead to the development of a dual well structure where one well assisted the other during high temperature operations. The Electron Blocking Layer was reduced in thickness and was made only 10 nm thick with a 5 nm Gallium Nitride buffer between it and the active region wells. The main reason for reducing the electron blocking layer thickness was to reduce the valance band offset and improve hole transport into the active region. Three different dual well designs were simulated of 3nm, 6nm and 9nm wide wells. The output parameters like the Power Spectral Density, Electron bound density, Light Output Power and Electron-Hole wavefunction overlaps were calculated. It was found that one of the wells acted as an assisting well where it had very little radiative recombination activity in it at room temperature.
As the temperature increased, it was observed that the electrons in the main well started to overflow out of it and into the assisting well where the radiative recombination rate increased significantly. This lead to a boost in Internal Quantum Efficiency.
Photonic integrated circuit (PIC) in the visible spectrum opens up new opportunities for frequency metrology, neurophotonics, and quantum technologies. Group III nitride (III-N) compound semiconductor is a new emerging material platform for PIC in visible spectrum. The ultra-wide bandgap of aluminum nitride (AlN) allows broadband transparency. The high quantum efficiency of indium gallium nitride (InGaN) quantum well is the major enabler for solid-state lighting and provides the opportunities for active photonic integration. Additionally, the two-dimensional electron gas induced by spontaneous and polarization charges within III-N materials exhibit large electron mobility, which is promising for the development of high frequency transistors. Moreover, the noncentrosymmetric crystalline structure gives nonzero second order susceptibility, beneficial for the application of second harmonic generation and entangled photon generation in nonlinear and quantum optical technologies. Despite the promising features of III-N materials, the investigations on the III-N based PICs are still primitive, mainly due to the difficulties in material growth and the lack of knowledge on fundamental material parameters. In this work, firstly, the fundamental nonlinear optical properties of III-N materials will be characterized. Then, the fabrication process flow of III-N materials will be established. Thirdly, the waveguide performance will be theoretically and experimentally evaluated. At last, the supercontinuum generation from visible to infrared will be demonstrated by utilizing soliton dynamics in high order guided modes. The outcome from this work paves the way towards fully integrated optical comb in UV and visible spectrum.