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Time-resolved Faraday rotation (TRFR) is a pulsed laser pump/probe optical measurement used to characterize electron spin dynamics in semiconductor materials. A Mach-Zehnder type interferometer with orthogonally polarized arms is presented as a device for TRFR measurement that is superior to optical bridge detection, the traditional measuring technique, since Faraday rotation can be passively optically amplified via interference. Operation of the interferometer is analyzed under ideal conditions. Corrections to the ideal case stemming from imperfectly aligned optics, finite polarization extinction ratios, and an imperfect recombination optic are analyzed using a matrix transformation approach. The design of the interferometer is presented and chronicled. A description of the single-beam active control system utilized to stabilize the interferometer by continuous corrections to the optical path length of one arm with a piezoelectric actuator is given. Optical amplification by increasing the power in either arm of the interferometer is demonstrated and TRFR measurements taken with the interferometer at ambient temperatures are compared with measurements taken with the optical bridge. We find the interferometer to offer a detection limit on the order of 50 mrad at room temperature, which is five times more sensitive than the optical bridge. Isolation and stabilization of the interferometer were also successful in reducing signal noise to a level comparable with the optical bridge. Our results demonstrate that the interferometer is a better detection device for Faraday rotation under ambient conditions. In the immediate future, improvements to the control system should be made and experiments should be performed with high-quality samples at cryogenic temperatures to confirm that the interferometer performs as favorably under those conditions.

Orientador: Gilberto Medeiros Ribeiro / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-09T10:48:50Z (GMT). No. of bitstreams: 1 GonzalezHernandez_FelixGuillermo_D.pdf: 12837677 bytes, checksum: 70e82c96ea88ab1de4fa785d908c9af6 (MD5) Previous issue date: 2007 / Resumo: Medidas experimentais foram realizadas para determinar as escalas de tempo de relaxação e decoerência do spin eletrônico como bit quântico. A estrutura dos estados de exciton foi investigada com o objetivo de servir como estados intermediários na manipulação do spin. O sistema utilizado para o estudo de decoerência é um ensemble de pontos quânticos auto-formados semicondutores. Dois temas servem como eixos centrais dos três experimentos desenvolvidos nesta tese: a polarização de spin e o fator g de Landé. No primeiro experimento, ao incluir o efeito do reservatório térmico, foi obtido o grau de polarização do spin (populações dos níveis up e down) para as camadas s e p. O desdobramento dos níveis orbitais em subníveis de spin permitiu obter a magnitude do fator g para estes estados. Mudando a orientação do campo magnético, foram observadas as anisotropias do tensor g e a sua relação com os detalhes do potencial de confinamento. Estas características permitiram inferir o tempo de relaxação T1. A medida da polarização resolvida no tempo foi realizada através de es-pectroscopia óptica de bombeio-prova. Os pulsos de luz e o campo magnético transverso permitem que uma polarização líquida seja inicializada. A rotação de Kerr permitiu observar oscilações desta polarização em torno do campo magnético com freqüência determinada pelo fator g. A perda da coerência de fase do spin resulta no decaimento destas oscilações numa escala de tempo T2. Medidas realizadas num ensemble de spins implicam em que o tempo de decoerência encontra-se limitado pela escala de defasagem T¤2< T2. Uma técnica semelhante à refocalização por spin-eco em experimentos de ressonância magnética nuclear, foi aplicada utilizando pulsos de laser para reverter a defasagem do ensemble. Tanto a possibilidade de medir o sinal de eco como o tempo de decoerência foram medidos como função da temperatura. A estrutura de níveis de exciton e a sua distribuição no ensemble foi estudada também com espectroscopia de bombeio-prova. Foram observados batimentos quânticos entre os níveis de estrutura fina do exciton para sis-temas 0D e 2D limitados pelo tempo de recombinação / Abstract: Experimental measurements were carried out to determine the scales of the relaxation and decoherence time for the electronic spin as quantum bit. The structure of the exciton states was investigated with the objective to serve as intermediate states in the spin manipulation. The system studied for the implementation of the quantum computation is an ensemble of self-assembled semiconductor quantum dots. Two subjects serve as central axes of the three experiments developed in this thesis: the spin polarization and the Landé g-factor. In the first experiment, when including the effect of the thermal reservoir, the degree of spin polarization (populations for the up and down levels) was measured for layers s and p. The splitting of the orbital levels in spin sublevels allowed to get the magnitude of factor g for these states. Changing the orientation of the magnetic field, the g-tensor anisotropies and its relation with the details of the confinement potential had been observed. These characteristics had allowed to infer the relaxation time T1. The time resolved polarization measurement was carried out by optical pump-probe spectroscopy. The pulses of light and the transverse magnetic field allow the initialization of a net polarization. The Kerr rotation allowed to observe oscillations of this polarization around the magnetic field with frequency determined for factor g. The loss of the spin phase coherence results in the decay of these oscillations in a time scale T2. Measurements carried out in an ensemble of spins imply that the decoherence time is limited by the ensemble dephasing time T¤2 < T2. A technique similar to the spin-echo refocalization in nuclear magnetic resonance experiments using laser pulses was applied to reverse the ensemble dephasing. The possibility to measure the echo signal and the decoherence time was measured as a function of the temperature. The structure of exciton levels and its distribution in ensemble were also studied with pump-probe spectroscopy. Quantum beats were observed be-tween the fine structure exciton levels for 0D and 2D systems, yet limited by the recombination time / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Time-resolved Faraday rotation (TRFR) is a pulsed laser pump/probe optical measurement used to characterize electron spin dynamics in semiconductor materials. A Mach-Zehnder type interferometer with orthogonally polarized arms is presented as a device for TRFR measurement that is superior to optical bridge detection, the traditional measuring technique, since Faraday rotation can be passively optically amplified via interference. Operation of the interferometer is analyzed under ideal conditions. Corrections to the ideal case stemming from imperfectly aligned optics, finite polarization extinction ratios, and an imperfect recombination optic are analyzed using a matrix transformation approach. The design of the interferometer is presented and chronicled. A description of the single-beam active control system utilized to stabilize the interferometer by continuous corrections to the optical path length of one arm with a piezoelectric actuator is given. Optical amplification by increasing the power in either arm of the interferometer is demonstrated and TRFR measurements taken with the interferometer at ambient temperatures are compared with measurements taken with the optical bridge. We find the interferometer to offer a detection limit on the order of 50 mrad at room temperature, which is five times more sensitive than the optical bridge. Isolation and stabilization of the interferometer were also successful in reducing signal noise to a level comparable with the optical bridge. Our results demonstrate that the interferometer is a better detection device for Faraday rotation under ambient conditions. In the immediate future, improvements to the control system should be made and experiments should be performed with high-quality samples at cryogenic temperatures to confirm that the interferometer performs as favorably under those conditions.