SAINT-PETERSBURG STATE UNIVERSITY EXPERIMENTAL STUDY OF SPIN MEMORY IN NANOSTRUCTURES ROMAN V. CHERBUNIN

CONTENTS 1.About a possibility of the “spin devices” creation. 2.Optical orientation - method of creation of the non- equilibrium spins in semiconductors and semiconductor structures. 3.Detection of a non-equilibrium spin in semiconductors by the study a) of luminescence. b) of reflection (Faraday effect). 4.Wavelength dependence of Kerr effect. 5.Spin lifetime. Possible mechanisms of spin relaxation. 6.Our experiments. Time-resolved observation of a spin dynamics. Modification of a pump-probe method. 7.Future trends of a “spin devices”. Adaptation of quantum dots for this applications.

OUR DREEMS SUBSTRATE QW GAP We have a two dimensional array of electrons in a quantum well. This electrons content our information in it’s polarization. To save information electrons have do not move and do not change polarization. “LONG-TIME” SPIN MEMORY SUBSTRATE QW GAP We have a two dimensional array of excitons in a quantum well. This excitons also can content information in it’s polarization. But their lifetime is about s, and it is shorter than any other time. “FAST” SPIN MEMORY (DURIND ONE CLOCK) If we study spin polarization using light, it isn't ease to separate electron and exciton spin polarization.

OPTICAL ORIENTATION IN GaAs QUANTUM WELLS Laser Glan prism Retarder λ/4 Sample in cryostat In quantum wells, to take a spin polarization in a case of photo inducted carriers is quite simple. Irradiation by circular polarized light produce 100% spin polarization. Conduction band Valence band GaAs AlGaAsAlGaAs Absorption of circular polarized light in a bulk GaAs result in an appearance of two kinds of excitons: (-3/2,+1/2) and (+1/2,-1/2). In a narrow quantum wells they energy levels are different, and we can excite them separately. Conduction band: I = 0; S =1/2 J =1/2 ; m = ±1/2 Valence band: I = 1; S =1/2 J =1/2 ; m = ±1/2 J =3/2 ; m =±1/2 (light hole) m =±1/2 (light hole) m=±3/2 (heavy hole) m=±3/2 (heavy hole)

CRATION OF ELECTRON SPIN POLARIZATION Circularly polarized light Photo inducted electron – hole pairs Electrons from donors Circularly polarized light creates polarized electrons and holes. Hole lost spin orientation in a few picoseconds ( ). If there are electrons from donors, they could recombine with holes as well as photo inducted ones. The remained after recombination electrons would have a preferred spin orientation. This orientation would not 100%, but for our study even 10% is still enough. Note1: If electron – hole pairs are being produced in a bound state (exciton), they are also polarized free electrons, but this process is more complicated and is not enough studied.

SPIN POLARIZATION MEASUREMENT Где Elliptically polarized light (luminescence) Photo inducted electron – hole pairs Electrons from donors With light cooperate electron - hole pairs, whose total angular moment does not exceed unit. Electron with spin 1/2 and hole with spin 3/2 can have the total moment 1 or 2 depending on their mutual orientation. As it was mentioned above, a hole in time about ten picoseconds loses orientation. But the electron - hole pair can be highlighted only at that moment when hole orientation will return to an initial state. Thus, the luminescence will be polarized the same as an absorbed light.

USE FARADAY EFFECT FOR SPIN POLARIZATION MEASUREMENT Medium in a magnetic field Simple Faraday effect Photon energy Refractive index ΔE = μ B g B m =+1 m =-1 α Nonlinear Faraday effect σ +σ +σ +σ + σ -σ -σ -σ - α ~ Δn ~ B Pumped medium α m =+1 m =-1 σ +σ +σ +σ + σ -σ -σ -σ - Different level population makes refractive indexes for σ + and σ – polarized light different. It results in light rotation. Photon energy Refractive index

SPIN RELAXAOTION  There are many reasons for spin relaxation in semiconductor. All of them are based on particle motion.  The main mechanism for the hole spin relaxation is interaction with phonons.  The reason of high efficiency of this mechanism is that a hole has orbit angular momentum. current magnetic field  Magnetic momentum rotate slowly around the magnetic field

MODIFICATION OF PUMP-PROBE SETUP Ti:S PULSED Laser Half mirror Variable delay Static delay Pump shaper Probe shaper (Monochromator & Glan) Lens Retarder λ/4 Lens Sample in cryostat Balance photo detector PUMP-PROBE SETUP T = 10 K. Light power: 800 W cm -2 Pulse duration: 100 fs. Repetition rate: 80 MHz (12,5 ns). Sensitivity: rad (10 -8 grad). Delay length: 30 cm (600 ps)

DETAILS. BALANSE PHOTO DETECTOR - U + U Signal (i) α IyIyIyIy IxIxIxIx I x - I y

Delay (ps) Wavelength (nm) EXPERIMENTAL RESULTS Δα Δα Δα Wavelength Delay Quantum well without free electrons

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