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.

We haved studied the magneto-transport properties of two dimensional electron gas (2DEG) in Si £_-doped In0.22Ga0.78As/AlGaAs single quantum wells ( QWs ) by using Shubnikov-de Haas ( SdH ) measurements . From the SdH measurement , we can clearly observe the SdH oscillations and obtain the SdH frequencies. It indicates the 2DEG in these QWs was confirmed . We also obtain the deep level binding energies¡G 104.4 meV and 9.6 meV for sample 1 and 50.2meV for sample2 by T-dependent Van der Pauw Hall effect measure- ment at magnetic field 0.3T. The difference of these two samples was the In0.1Ga0.9As layer of sample 2 was inserted between In0.22Ga0.78As well and the GaAs spacers . So in this paper , we tried to propose a model to interpret the deep-level traps in the QWs and studied the effect of In0.1Ga0.9As inserted-layer on the In0.22Ga0.78As/GaAs Single Quantum Wells.

Abstract We have extended the spectral range of the current PL-upconversion apparatus to be operated in infrared. Using the IRPL-upconversion¡Awe study the behavior of carrier cooling of InN film and the relationship between the spacer and lifetime in InAs/GaAs stacked QDs . We excited InN film of the band gap of 0.74eV with ultrafast Ti:sapphire laser of the wavelength 404nm. We found the phonon emission time by hot carriers of InN is 14fs. The hot carriers release their excess energy to the lattice of 35K with a timescale of 100ps. We observed in InAs/GaAs QDs that the shorter life time for samples with thin spacer is due to tunneling effect.

In this thesis, a home made chirp-controlled pump-probe measurement system has been developed and is used to explain possible mechanism of THz radiation enhancement under positive chirped incident pulse. The chirp-controlled pump-probe measurement system with temporal resolution of around 100 femtosecond and chirp parameter tuning from ¡V350 fs2 to +650 fs2 is demonstrated. Meanwhile, using chirp-controlled pump-probe measurement system, ultrafast dynamics of photogenerated carrier in low-temperature growth GaAs in different chirp by is characterized. The relaxation time of low-temperature growth GaAs in positive chirp pump pulse is 461fs and shorter than one, which is 497fs, in negative chirp pump pulse. The result is explained by the Pump-Dump process in negative chirp pump pulse and similar band-filling effect in positive chirp pump pulse.

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.

In this thesis, a home made chirp-controlled pump-probe measurement system has been developed and is used compare the time-resolved photo-reflectance measurements of GaAs:O with different fabricated condition(2.5¡Ñ10^13 ions/cm^2 (500Kev & 800Kev), 4¡Ñ10^13 ions/cm^2 (1200Kev) and 6¡Ñ10^13 ions/cm^2 (500Kev & 800Kev), 1¡Ñ10^14 ions/cm^2 (1200Kev)).The lower-dose sample were annealed at 0,350,400,450,550 and 600¢J,respectively. The higher-dose sample were annealed at 550¢J. The chirp-controlled pump-probe measurement system with temporal resolution of around 100 femtosecond and chirp parameter tuning from ¡V539 fs^2 to +663 fs^2 is demonstrated. Meanwhile, using chirp-controlled pump-probe measurement system, ultrafast dynamics of photogenerated carrier in GaAs:O in different chirp by laser pulse is characterized.

<p>Long-wavelength (1.3-μm) vertical-cavity surface-emitting lasers (VCSELs) are of great interest as low-cost, high performance light sources for fiber-optic metro and access networks. During recent years the main development effort in this field has been directed towards all epitaxial GaAs-based structures by employing novel active materials. Different active region candidates for GaAs-based 1.3-μm VCSELs such as GaInNAs/GaAs QWs, GaAsSb QWs or InAs/InGaAs QDs have been investigated. However, the difficult growth and materials properties of these systems have so far hampered any real deployment of the technology. More recently, a new variety of VCSELs have been developed at KTH as based on highly strained InGaAs QWs and negative gain cavity detuning to reach the 1.3-μm wavelength window. The great benefit of this approach is that it is fully compatible with standard materials and processing methods.</p><p>The aim of this thesis is to investigate long-wavelength (1.3-μm) VCSELs using ~1.2-μm In0.4GaAs/GaAs Multiple Quantum Wells (MQWs). A series of QW structures, DBR structures and laser structures, including VCSELs and Broad Area lasers (BALs) were grown by metal-organic vapor phase epitaxy (MOVPE) and characterized by various techniques: Photoluminescence (PL), high-resolution x-ray diffraction (XRD), atomic force microscopy (AFM), high accuracy reflectance measurements as well as static and dynamic device characterization. The work can be divided into three parts. The first part is dedicated to the optimization and characterization of InGaAs/GaAs QWs growth for long wavelength and strong luminescence. A strong sensitivity to the detailed growth conditions, such as V/III ratio and substrate misorientation is noted. Dislocations in highly strained InGaAs QW structure and Sb as surfactant assisted in InGaAs QW growth are also discussed here. The second part is related to the AlGaAs/GaAs DBR structures. It is shown that the InGaAs VCSELs with doped bottom DBRs have significantly lower slope efficiency, output power and higher threshold current. By a direct study of buried AlGaAs/GaAs interfaces, this is suggested to be due to doping-enhanced Al-Ga hetero-interdiffusion. In the third part, singlemode, high-performance 1.3-μm VCSELs based on highly strained InGaAs QWs are demonstrated. Temperature stable singlemode performance, including mW-range output power and 10 Gbps data transmission, is obtained by an inverted surface relief technique.</p>

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

The nonlinear optical response of low-temperature (LT) grown GaAs were studied using four-wave mixing techniques. Through measurements of the four-wave mixing response as a function of pulse delay and photon energy, a strong optical response was identi ed associated with the fundamental band gap exciton. These experiments therefore demonstrated the importance of the exciton in understanding the ultrafast nonlinear optical response of LT-GaAs despite the absence of any evidence of the exciton in past linear absorption studies in this material. Measurement of the fourwave mixing response as a function of pulse delay and the polarization states of the two excitation pulses shows that the dominant contribution to the exciton signal is tied to excitation-induced dephasing. Four-wave mixing experiments in which the sample is exposed to an additional laser pulse indicate that the exciton signal may be strongly diminished due to a combination of screening and a reduction in the total dephasing time. The short temporal duration of the above e ect provides evidence of an ultrashort (< 100 fs) electron trapping time in this system tied to arsenic related defects introduced during low-temperature growth. These ndings are of importance to the understanding of the optical properties of LT-GaAs and will aid in the development of optoelectronic devices using this material system.

Submitted by Livia Mello (liviacmello@yahoo.com.br) on 2016-10-04T13:54:42Z No. of bitstreams: 1 DissARHC.pdf: 1491720 bytes, checksum: 5be10931f2f7a3b5e0c8b77f8b50f990 (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-10-10T14:32:17Z (GMT) No. of bitstreams: 1 DissARHC.pdf: 1491720 bytes, checksum: 5be10931f2f7a3b5e0c8b77f8b50f990 (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-10-10T14:32:27Z (GMT) No. of bitstreams: 1 DissARHC.pdf: 1491720 bytes, checksum: 5be10931f2f7a3b5e0c8b77f8b50f990 (MD5) / Made available in DSpace on 2016-10-10T14:32:33Z (GMT). No. of bitstreams: 1 DissARHC.pdf: 1491720 bytes, checksum: 5be10931f2f7a3b5e0c8b77f8b50f990 (MD5) Previous issue date: 2015-08-04 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / In order to investigate optical and spin properties of GaBiAs / GaAs quantum wells, we performed both photoluminescence and magneto-photoluminescence spectroscopy measurements in three samples: 10 nm quantum wells with Bi concentrations of 1%, 2% and 3%. The photoluminescence study was conducted as a function of Bi concentration, temperature and excitation power. The results indicate that the effects associated with carrier localization by defects are expressive, especially for the 3% sample. The defects are incorporated into the solid due to growth conditions at low temperatures (315 °C), necessary to enable Bi incorporation into the GaAs matrix.They are also due to the different properties between the atoms, such as electronegativity and size. The magneto-luminescence results exibited high spin- splitting (8.4 meV to 15 T) and high excitonic g factor (9.6) for the 3% sample. It also showed a small diamagnetic shift (approximately 3 meV) for the three samples and moreover, it decreases with increasing Bi content. This suggests higher carrier localization by defects, confirming previous results. Overall, the results show that such materials are interesting and good candidates for spintronic applications. / Este trabalho tem como objetivo estudar as propriedades ópticas e de spin de poços quânticos de GaBiAs/GaAs. Para isso, foram realizadas medidas de espectroscopia de fotoluminescência e magneto-fotoluminescência em altos campos magnéticos (B ≤ 15T). Em particular, foram estudadas três amostras de poços quânticos de 10 nm de largura e com concentrações de 1%, 2% e 3% de Bi. O estudo de fotoluminescência foi realizado em função da concentração de Bi, da temperatura e da potência de excitação. Foi evidenciado que efeitos associados à localização de portadores por defeitos são muito significativos, em especial para a amostra de 3%. Os defeitos são incorporados à rede devido às condições de crescimento em baixas temperaturas (315 oC), necessárias para que haja incorporação de Bi na matriz de GaAs, e às diferentes propriedades entre os átomos. Nas medidas de magneto- luminescência, foi encontrado um alto spin-splitting (8.4 meV para 15 T) e alto fator g excitonico (9.6) para a amostra de 3%. Verificou-se também que deslocamento diamagnético é pequeno (da ordem de 3 meV) para as três amostras e, além disso, diminui com o aumento da concentração de Bi. Isso implica em uma maior localização de portadores por defeitos, corroborando os resultados anteriormente encontrados. De forma geral, os resultados obtidos mostraram que tais materiais são interessantes para possíveis aplicações, em particular na área de spintrônica.