Development Of Transient Grating Laser Spectroscopy

Spintronics or spin-based electronics,which has been paid intense attentions in the field of Condensed Matter Physics and Information Sciences,aims to exploit the spin degree of freedom of the electron to develop a new generation of electronic devices.Two of the important issues on realizing the spintronics device are how to prolong the spin relaxation time and the spin diffusion length.So it is important to understand the mechanism of spin relaxation and processes of spin transport.In this thesis,we first give a brief review of Spintronics on the mechanism of spin relaxation,spin detection and spin generation.And then,we present our work on spin transport in(110)-oriented GaAs/AlGaAs quantum well(QW).Usually,the properties of spin transport is assumed to be the same as that of charge transport,but recent results demonstrate that spin diffusion constant D_s is reduced due to Coulomb interaction.So the properties of spin transport need to be exploited in detail experimentally.Transient spin grating(TSG) is a very important technique to determine D_s and characterize the processes of spin and charge transport.When two coherent beams of pulse laser with orthogonal linear(collinear) polarization interfere, the helicity(intensity) of light will be modulated periodically across the excitation spot.As a consequence,spatially-modulated spin(charge) density,which is so called transient spin(concentration) grating,will be generated in the sample according to the optical selection rules.In our system,the heterodyne scheme,which can provides 4-5 orders of magnitude of improvement in the signal,is used to detect the signal combining the lock-in technique.In addition,it is easy to convert the spin grating to concentration,create the different period of grating by a special configurationWe measure the spin lifetime in a single(110)-oriented GaAs/AlGaAs QW at room temperature(RT).Maximum of spin lifetime is about 1.22ns under low excitation power.We found that spin lifetime decreases with increasing excitation power.In an intrinsic(110)-oriented QW,spin lifetime is longer due to suppression of D'yakonov-Perel' mechanism of spin relaxation.The exchange interaction between hole and electron will result in the spin relaxation.Increasing hole concentration with high excitation power is the main reason of the decrease of spin lifetime.The signals of transient grating are measured by TSG at RT.Both signals of transient spin and concentration gratings are described by a mono-exponential decay.We found the decay rate of a fixed-period spin grating is accelerated by decreasing pump power. Power-dependence D_s presents a complex process:fast decrease-slow decrease-constant.We also measure the ambipolar D_c by transient concentration grating.D_c keeps in a constant and is independent on pump power.We contribute it to that D_c is determined by the diffusion processes of holes due to larger effective mass of hole comparing to that of electron.