Resonant Spin Pumping of Resident Electrons and Nuclei in CdTe and GaAs Quantum Wells

Abstract

The main objective of this PhD dissertation was to evaluate the spin relaxation time through resonant excitation of trions in III-V and II-VI semiconductor quantum wells. However, the evaluation of spin relaxation times requires precise knowledge of g-factor of the quantum wells. Other important parameter that can affect the electron spin relaxation time is the interaction of the spin polarized electrons with the lattice nuclei, which is very hard to determine. In addition, nature of semiconductor material, i.e., magnetic or non-magnetic could also be a parameter. Hence two different kinds of semiconductor quantum wells were studied in this work. Usually, optical control of the spin states and the deeper insight into the dynamical processes were achieved by means of well-known time resolved pump-probe technique such as Faraday or Kerr rotation. Here, use of pulsed laser sources in combination with pump-probe arrangement allow one to resolve the pump induced spin dynamics in time and to evaluate the required parameters such as g-factor and spin relaxation times. Special interest represents the regime, when the spin lifetime in the studied system is longer than the pulse repetition rate of the laser source f. In this case resonant spin amplification occurs if the Larmor precession frequency is a multiple of 2πf. However, time-resolved pump-probe Faraday or Kerr rotation measurements require complex optical setup with two beams, delay line and polarization sensitive detection. This PhD dissertation presents an alternative simpler approach to evaluate the spin parameters using a single laser beam based on resonant optical pumping of excess electrons in trion state. In particular, single laser beam tuned in resonance with the charged exciton (trion) transition in a semiconductor quantum well was used for spin pumping of the ground resident electron state. Simultaneously the absorption of the same laser beam was used to monitor the spin polarization of electron ensemble. Subsequent effect of the interaction of the lattice nuclei with the spin polarized nuclei on spin relaxation time was also studied using the single beam approach with lasers of higher repetition rate. The work was performed systematically as given in the following.In the first part, electron spin relaxation time in the absence of nuclear interactions was evaluated through optical pumping of resident electron spins in resonant excitation of trions in n-type CdTe/(Cd,Mg)Te multi quantum wells (MQW) subject to a transverse magnetic field using a single beam configuration. Single beam was employed in transmission configuration to record the time integrated intensity of the excitation laser light transmitted through the quantum wells. The transmitted intensity reflected the bleaching of light absorption due to optical pumping of the resident electron spins and could also be used to evaluate the Larmor precession frequency of the optically oriented carriers and their spin relaxation time. Application of the magnetic field led to depolarization of the electron spin ensemble, so that the Hanle effect was observed. Excitation with a periodic sequence of laser pulses led to optical pumping in the rotating frame, when the Larmor precession frequency was synchronized with the pulse repetition rate. This was manifested by the appearance of Hanle curves every 3.36 or 44.2 mT at a pulse repetition rates of 75.8 or 999 MHz, respectively. From the experimental data the g-factor came out to be |g| = 1.61 and the spin relaxation time of 14 ns for the optically pumped resident electrons, in agreement with previous time-resolved pump-probe studies. In the second part, the spin relaxation time in magnetic semiconductor quantum well was determined. The effect of the interaction of the spin polarized electrons with the lattice nuclei on electron spin relaxation time was estimated using the single laser beam of higher repetition rate in reflection configuration on i-type GaAs/(Al,Ga)As MQW. Hanle curves were measured in large magnetic field range (-2.5 T to 2.5 T) both for continuous and pulsed excitation. As compared to the continuous pumping in pulsed excitation, a large set of sharp peaks (Hanle curves) were recorded as a function of the externally applied magnetic field. The observed Hanle peaks were repeating every 185.1 mT for 1 GHz laser and 1.8 T for 10 GHz laser. This allowed to determine the main parameter of spin dynamics, i.e., the g-factor of the optically pumped carriers and their spin relaxation time. From the experimental data, the spin relaxation time of 153 ns was determined, which agreed well with previous time resolved studies. The third part of the study was important as it led to the understanding of the buildup of nuclear spin polarization. The nuclear spin polarization build up times were evaluated through resonant excitation of trions in nominally un-doped GaAs/(Al, Ga)As MQW using single beam approach. Buildup of the nuclear spin polarization was recorded by measuring time-integrated intensity of the reflected laser beam, which was proportional to the optical generation rate and changed due to the pumping of the resident electrons. It was established that nuclear spin polarization time, T1 was temperature dependent and owing to the stronger electron confinement at lower temperatures becomes more rapid. It was found that the effectiveness of the dynamical nuclear polarization decreases with temperature. For the temperatures 1.8 K, 6 K and 10 K, the time T1 were 210 s, 300 s, and 450 s and the corresponding stationary value for the nuclear magnetic field 𝐵𝑁 𝑠𝑡 were 629 mT, 447 mT, and 375 mT, respectively. Also "Locking" of the nuclear field related to the anisotropy of the electron g-factor was observed, which rises from zero to the value of the external magnetic field applied at that moment, without further increase. The ratio between the in-plane 𝑔ǁ and out-ofplane 𝑔⊥ components was estimated at 𝑔ǁ/𝑔⊥= 1.3.