The purpose of this thesis is to fabricate 12-layer InxGa1-xAs quantum dots grown on 2-nm In0.1Ga0.9As quantum wells (DWell) laser structures grown by molecular-beam epitaxy (MBE) on GaAs substrats. We expect to optimum the lasers performance by tune the epitaxial recipe and fabrication condition. For the carrier injection efficiency, DWell structure of quantum dots grown on quantum wells is proposed to enhance the carrier capture rate. So we analyze a series of DWell structure in this work. In the epitaxial recipe, we investigate the influences of p-type doping and change the quantum wells growth temperature for the laser structures. In the laser fabrication, to transport the light wave in smaller dispersion loss single mode waveguide, dry etching photolithography processes are adapted in this study to fabricate 2.2mm width ridge waveguide. The as-cleaved facets are used as Fabry-Perot laser mirrors in ridge waveguide lasers. The pattern can be transferred effectively with less under-cut by dry etching compare with wet etching. Finally, the P-type doping DWell laser exhibits high power/facet of 24mW, slope efficiency of 0.209W/A. The maximum power/facet of PWell580 laser reach to 24mW, slope efficiency of 0.238W/A after raising the growth temperature to 580oC.

In this thesis, we fabricate the quantum dots (QDs) micro-pillar of metal-coated by E-beam lithography, and analyze the optical and electrical properties of micro-pillar cavity devices. For the sample materials, we use S-K mode to grow 3-layer In0.75Ga0.25As QDs structures sandwiched by up and down Al0.5Ga0.5As cladding layer on GaAs substrate by molecular-beam epitaxy (MBE). 40nm GaAs spacer layers with 2nm p-modulation doping in the central barrier are adopted in this study. The micro-pillar with diameter of 2 m, metal coated on top (p-type) and down (n-type) facet are designed. The good reflectivities of metal contacts provide more energy extraction inside the cavity. We expect the device lasing while the current injection. First, we design the morphology and size of patterns by AutoCAD software. Then, we use e-beam lithography with proper exposure condition to define the patterns, and thermal evaporation to deposit metals. The superfluous metal is lifted off and the defined area metal is served as dry etching mask to transfer the pattern to the dielectric layer and epi-layer. Finally, we use SiO2 layer to prevent current leakage, and the p-n contact on each facet to complete the devices. Micro-pillar samples with/without metal coated are analyzed by micro-PL system. The emission wavelength of 1282nm and the calculated Q-value of 100 are obtained for the sample with metal coated, an increase of 500%. From the EL measurement results, the device of micro-pillar samples with metal coated generate three peaks, 1149nm, 1221nm and 1291nm. Besides, it can efficiently improve the emission intensities. The measured result matched the simulation result.

We have investigated the polarization effect of optical process in the vertically coupled InGaAs quantum dots (QDs) triple layers by varying the thickness of GaAs spacer layer. The TE/TM ratio for the ground state emission decreases from near 4 to 1.5 as the spacer thickness (d) decreases from 40 nm to 5 nm. And, the TE polarization (in-plane polarization) is anisotropic with a stronger component along [01-1] direction. P-type modulation doping further decreases the TE/TM ratio to r = 1.2 for the strong vertical coupling QDs structure of 5-nm spacer. Then, using a cross-sectional transmission electron microscopy directly reveals the InGaAs QDs of 5-nm spacer well aligned along the growth direction. From the electroluminescence (EL) and differential absorption (£G£\) experiments, the higher optical gain and absorption change for the excited state suggest that the e2-hh transition has higher oscillator strength for the vertically coupled QDs. We also investigate for the triple-layer InGaAs vertically coupled quantum dots (VCQDs) by adding modulation doping (MD) in the 5-nm GaAs spacer layers. In addition to the QDs fundamental and excited transitions, a coupled-state transition is observed for the VCQDs. For the VCQDs of p-type MD, the optical transitions at ground state and coupled state are enhanced by the improvement of hole capture for the valence subbands. For the VCQDs of n-type MD, the main absorption change occurs at the coupled state, consistent with the dominant emission peak observed in EL spectra. For GaAs-based solar cells application, in order to enhance absorption at infrared range for GaAs-based solar cells, multi-stack InGaAs VCQDs of 5-nm GaAs spacers are grown in the active region. Due to the strong vertical coupling between QDs would promote quantum efficiency. We have investigated the photovoltaic response for the solar cells by increasing the layer numbers of VCQDs. The device of nine-layer InGaAs VCQDs shows an enhanced short-circuit current density (Jsc) of 10.5 mA/cm2. The value is increased by 42% compared to GaAs reference device. However, the open-circuit voltage (Voc) is reduced from 0.88 V to 0.54 V. Then, we change the GaAs spacer thickness of coupled In0.75Ga0.25As QDs, and investigated the effects on photovoltaic response. For the sample of d =10 nm shows the best performance of current density (Jsc~24 mA/cm2) and efficiency (h~10.6%). The Jsc and h are increases by 55% and 112% more than the device without QDs, respectively.

This paper is using the Time-resolved Pump-Probe spectroscopy to study the quantum dots samples. The samples are InAs/GaAs multi-stacked quantum dots that with different spacer layer (10~30 nm). The stain between the InAs quantum dots and GaAs spacer layer that makes the valence band to split into heavy-hole and light-hole energy band. From the photoluminescence (PL), we see the heavy-hole and light-hole energy band are blue shift in InAs quantum dot, when the GaAs spacer layer decrease. We use the optic property of Pump-Probe spectroscopy of the change in the refraction index £Gn to investigate the shift of heavy-hole energy band, when the GaAs spacer layer decrease. We see the heavy-hole energy band of GaAs is blue shift when the GaAs spacer layer decrease. When we change the pump energy, the TRPP spectroscopy signal will change from positive to negative. This is the band-filling effect changes the refraction index £Gn , when the energy close to the GaAs heavy hole energy state. When the energy is above the GaAs heavy hole energy state, the TRPP signal is positive. When the excited carrier density decrease and the delay time increase, TRPP signal will change the positive value to negative value. These are band-gap renormalization and free-carrier absorption effect change the refraction index £Gn, when the carrier density decrease.

Nonlinear optical microscopy (NLOM) offers many advantages when imaging intact biological samples. By using ultrafast lasers in the near infrared and two photon excitation (TPE), signal production is limited to the focal volume and provides an excellent means for rendering thin, microscopic images from within the sample. Exogenous fluorophores/lumiphores may be used as efficient contrast agents to tag specific targets and provide enhanced signal. The efficiency of the TPE process in these contrast agents is broadly assumed to vary inversely with the laser pulsewidth, τ. In this work, we investigate the TPE efficiency of transform limited broadband (~133nm, ~10fs) and narrowband (~11nm, ~170fs) pulses in the generation of twophoton luminescence from semiconductor nanocrystals or quantum dots (QD’s) both theoretically and experimentally. Compared to standard organic dyes, QD’s possess a relatively broad, uniform spectral response that enables better use of the full bandwidth from the broadband laser. Theoretical calculations including both degenerate and non-degenerate TPE indicate a rolloff from the 1/τ behavior as the pulses’ spectral bandwidth becomes broader than the absorption spectra of the QD’s. Experimentally measured enhancement in luminescence intensity while using a broadband pulse is compared with the simulated enhancement in two-photon luminescence. A combination of increased understanding of the excitation processes in NLOM and proper selection of contrast agents will help in advancing the role of broadband ultrafast lasers in NLOM.

In this dissertation, we construct a temperature controlled sample stage that is compatible with high numerical aperture objective optical microscope, and perform single molecule experiments under the system. Mixing dilute fluorophore (CdSe/ZnS quantum dot, DiI, Rhodamine B) into the liquid crystal matrix (5CB), we monitor the fluorescence dynamics of the individual fluorophore at various temperature. Different from the thermodynamic states of conventional materials, those specific class of materials which we called ¡§liquid crystals¡¨ are attracted for their existence of unique liquid crystal phase, which exhibits a solid-state like higher orientation ordering, and a liquid-state like liquidity. Probe individual fluorophore allows us to monitor the nanometer length scale local structural and dynamic heterogeneity in the solid, liquid crystal and liquid phases. The operating temperature of the platform covers more than 20 oC to 40 oC range with stability much better than 0.1 oC. Quantum dot in PMMA exhibits a clear on-off blinking behavior, and the single exponential fluorescence lifetime relaxation. While in the solid phase of the liquid crystal matrix, quantum dot exhibits similar behavior, which indicates the quantum dot is confined in the matrix. However, there exists slightly difference in decay lifetime. On the contrary, in the liquid crystalline phase as well as the liquid phase, quantum dot exhibits bi-exponential relaxation behavior. Besides a similar time scale relaxation dynamics, there exists additional fast decay behavior, which is from the feasible rotational rotation in the non-rigid matrix. In particular, the anisotropic decay dynamics in the liquid crystalline phase indicates the orientation preference of the liquid crystal molecules. Fluorescence Correlation Spectroscopy (FCS) provides the information of local dynamics of various time scales. FCS results exhibit an unclear transition that crossovers several decades in time scale, which indicates the highly heterogeneity of the liquid crystal. The results of DiI exhibits different rising time in the fluorescence lifetime measurement, which implies the forming of aggregation due to the limited solubility of the DiI molecules in the liquid crystal matrix. Results of Rhodamine B exhibit a clear rotational diffusion dynamics at ~ microsecond scale and the corresponding translational diffusion dynamics at ~ mini-second scale. Moreover, the transition time scale of translational diffusion exhibits a temperature dependence. At higher temperature, it shifts to a shorter time scale.

Carrier dynamics of mullti-stacked quantum dots (MSQDs) have been studied by means of time-integrated and time-resolved photoluminescence (PL). The MSQD with different spacer thickness of 10, 15, 20 and 30 nm were grown by molecular beam epitaxy. Time-integrated PL exhibit red shift as spacer thickness increases. The red shift originated from the vertical coupling relaxes the strain in the MSQDs, leading to a decrease in the PL peak energy. From time-resolved PL, the MSQD with spacer thickness increased reveals the shorter lifetime of PL peak among samples studies. We attribute the maximum of lifetime to a better vertical alignment. We report on a measurement of the rise and decay of luminescence intensity in the MSQDs excited at 1.54 eV (808 nm) and 3.09 eV (404 nm). The results show a slow rise time of electrons from the L to the £F valley for high photoexcitation energies. The decay in luminescence is longest with photoexcitation at 3.09 eV, we demonstrate the importance of the penetration depth and carriers tunneling. In addition, the MSQDs strongly depends of on the carrier injection. The rise times decrease with increasing excitation density. The properties are characteristic features of Auger processes.

Nonlinear optical microscopy (NLOM) offers many advantages when imaging intact biological samples. By using ultrafast lasers in the near infrared and two photon excitation (TPE), signal production is limited to the focal volume and provides an excellent means for rendering thin, microscopic images from within the sample. Exogenous fluorophores/lumiphores may be used as efficient contrast agents to tag specific targets and provide enhanced signal. The efficiency of the TPE process in these contrast agents is broadly assumed to vary inversely with the laser pulsewidth, τ. In this work, we investigate the TPE efficiency of transform limited broadband (~133nm, ~10fs) and narrowband (~11nm, ~170fs) pulses in the generation of twophoton luminescence from semiconductor nanocrystals or quantum dots (QD's) both theoretically and experimentally. Compared to standard organic dyes, QD's possess a relatively broad, uniform spectral response that enables better use of the full bandwidth from the broadband laser. Theoretical calculations including both degenerate and non-degenerate TPE indicate a rolloff from the 1/τ behavior as the pulses' spectral bandwidth becomes broader than the absorption spectra of the QD's. Experimentally measured enhancement in luminescence intensity while using a broadband pulse is compared with the simulated enhancement in two-photon luminescence. A combination of increased understanding of the excitation processes in NLOM and proper selection of contrast agents will help in advancing the role of broadband ultrafast lasers in NLOM.

This thesis presents research on the initialization, control, and readout of electron spin states in gate defined GaAs quantum dots. The first three experiments were performed with Singlet-Triplet spin qubits in double quantum dots, while the remaining two experiments were performed with an Exchange-Only spin qubit in a triple quantum dot. / Physics

Two dimensional electronic materials provide a vibrant area for applying basic quantum mechanics and scattering theory. In quantum corrals, multiple scattering leads to resonances closely approximating eigenstates of an equivalently shaped billiard. We extend the analogy using methods from acoustics to demonstrate that the billiard conception of quantum corrals is a useful one even in wavelength regimes close to corral size. Resonance widths can be described by a simple relationship proportional to the perimeter to area ratio of the enclosure and the average reflection of a classical path. / Physics