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Investigation of spin polarization in semiconductors by laser spectroscopy
Butkovičová, Dagmar ; Němec, Petr (advisor) ; Kadlec, Filip (referee)
This work is devoted to the investigation of a spin polarization in ferromagnetic semiconductor Ga1-xMnxAs with a broad nominal concentration of manganese ions using one method of the ultrafast laser spectroscopy - the time-resolved Kerr rotation. At first, the experimental setup was optimized for the investigation of the dynamics of spin polarized charge carriers in semiconductors which were photo-generated by circularly polarized laser pulses. It was observed that the measured signal is induced by spin-polarized electrons. Due to a small thickness of the investigated ferromagnetic films the measured signal probably monitored the dynamics of fotogenerated electrons in GaAs substrate. Nevertheless, the measured data show that the electron spin dynamics in the substrate is significantly influenced by proximity effect due to the deposited ferromagnetic layer.
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Investigation of new materials for second harmonic generation
Brajer, Martin ; Němec, Petr (advisor) ; Žídek, Karel (referee)
In these bachelor thesis optical properties of materials are studied. This materials are purposefully prepared for effective second harmonic generation (SHG) and for its usage in optoelectronics. Specificely we will measure the first hyperpolarizibility. Non-coherent phenomenon of hyper-Rayleigh scattering will be used. Considering its incoherence, the effect is very weak. It's necessary to use a strong source of light, pulse laser and a sensitive detection device, photomultiplier. Samples are prepared in cooperating chemical laboratories of Faculty of Science of Charles University in Prague. Additionally, we will discuss participation of two-photon fluorescence in output intensity because of its interchangeability with SHG.
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Investigation of parameters of ultrafast laser pulses
Chlouba, Tomáš ; Němec, Petr (advisor) ; Preclíková, Jana (referee)
In this work we investigate parameters of laser radiation coming from continual titan-sapphire laser 3900S, pulsed titan-sapphire laser Mai-Tai HP and from optical parametric oscillator Inspire made by Spectra Physics. We are particulary insterested in dependence of power, width of the beam, noise and beam pointing stability as a function of wavelength. We have also measured beam pointing stability in time and compared it with beam stability during change of wavelength. With pulse laser we are also interested in the spectra and time width of the pulses. We aim to compare the properties of both titan-sapphire lasers and to verify specifications of Mai-Tai and Inspire given by the manufacturer.
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Spin dynamics in GaAs-based semiconductor structures
Schmoranzerová, Eva ; Němec, Petr (advisor) ; Postava, Kamil (referee) ; Kužel, Petr (referee)
This work is dedicated to the study of spin dynamics in systems based on the semiconductor gallium arsenide (GaAs) that are suitable for use in spintronic devices. We explored two types of model structures using experimental methods of ultrafast laser spectroscopy and transport measurements. In the ferromagnetic semiconductor (Ga,Mn)As, we investigated laser-induced magnetization precession. We found out that transfer of both energy and angular momentum from the circularly polarized laser light can trigger magnetization precession, the latter one being identified as a new phenomenon, the "optical spin transfer torque". Furthermore, we demonstrate the possibility to control the energy-transfer-induced magnetization dynamics both optically and electrically using piezo-stressing. When dealing with purely non-magnetic structures for spintronics, we studied the Spin-Injection Hall Effect (SIHE) in GaAs/AlGaAs heterostructures with a special type of spin- orbit (SO) coupling that are lithographically patterned to create nanodevices. We managed to observe precession of the electron spin in the SO field directly in the space domain by extending the original detection method. This finding, together with the direct detection of a pure spin current, helped to propose a working spin Hall effect transistor.
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Dynamics of spin polarization in semiconductors
Janda, Tomáš ; Němec, Petr (advisor) ; Olejník, Kamil (referee)
In this work we study ultrafast laser-induced magnetization dynamics in samples of ferromagnetic semiconductor Ga1−xMnxAs with a nominal concentration of Mn within the range of x = 0,015-0,14. To get information about magnetization movement we use magneto-optic phenomena PKE and MLD in a time-resolved pump & probe experiment. Thorough analysis of the measured magneto-optical signal allows us to disentangle contributions due to angular movement of magnetization and due to demagnetization and to reconstruct 3D motion of magnetization vector without any numerical modeling. First we explain the basis of this experimental method and we demonstrate its utilization on the measured data. After that we study angular movement of magnetization vector and its dependence on the external magnetic field, excitation intensity and Mn concentration. The pump pulse helicity dependent and independent dynamics were treated separately. In the case of demagnetization we have been able to observe not only its intensity and Mn doping dependence but also the magnetic field dependence, which has not been reported so far in the literature.
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Investigation of Fresnel rhomb
Jesenská, Eva ; Němec, Petr (advisor) ; Jakubisová, Eva (referee)
This bachelor thesis is devoted to the investigation of the polarization propertis of the Fresnel rhomb. First of all, we was interested in the quality of the polarized light which can be created using the Fresnel rhomb. In particular, we were checking the accuracy of the circular and linear polarization which is produced by Fresnel rhomb. Secondly, we tested the wavelength dependence of the quality of polarized light. We also investigated the dependence of the polarization state on the light angle of incidence. Finally, we compared the obtained results with those for the wave plates.
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Optical spectroscopy of magnetic semiconductors suitable for spintronic applications
Saidl, Vít ; Němec, Petr (advisor) ; Mics, Zoltán (referee)
In this work we investigate optical properties of new antiferromagnetic semiconductor - LiMnAs. This semiconductor oxidizes easily in air. This fact complicates significantly the investigation of its properties by the optical spectroscopy. This problem was solved by a construction of the optical setup that enables measurements of the reflectivity spectra directly in the chamber where this semiconductor is prepared by a molecular beam epitaxy (MBE). Firstly, the reliability of this optical setup was tested by the measurements of the reflectivity spectra of the well known semiconductors (GaAs and InAs). Secondly, the first measurements of LiMnAs reflectivity directly in the MBE chamber was performed.
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Ultrafast laser spectroscopy of semiconductors
Reichlová, Helena ; Němec, Petr (advisor) ; Orlita, Milan (referee)
This work is devoted to the study of the ferromagnetic semiconductor GaMnAs by magneto-optical methods, namely we used the magnetic linear dichroism method (MLD). First, we found a suitable method for the measurement of the magneto-optical coefficient, which describes MLD. Then we used MLD method to study the hysteresis loops in the sample with almost cubic magnetic anisotropy and in the sample with both cubic and uniaxial anisotropy - we measured the polarisation, temperature and intensity dependence of the hysteresis loops and we derived a formula for the polarisation dependence of the amplitude of the hysteresis loops. Using timeresolved MLD method we investigated the ultrafast demagnetization process in the sample on various time-scales after the laser-pulse excitation. We observed that the demagnetisation dynamics depends on the pump fluence and the temperature.
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