SAINT-PETERSBURG STATE SAINT-PETERSBURG STATE UNIVERSITYUNIVERSITY

EXPERIMENTAL STUDY OF SPIN EXPERIMENTAL STUDY OF SPIN MEMORY IN NANOSTRUCTURESMEMORY IN NANOSTRUCTURES

ROMAN V. ROMAN V. CHERBUNINCHERBUNIN

CONTENTSCONTENTS

1.1. About a possibility of the “spin devices” creationAbout a possibility of the “spin devices” creation..

2.2. Optical orientation - method of creation of the non-Optical orientation - method of creation of the non-equilibrium spins in semiconductors and equilibrium spins in semiconductors and semiconductor structuressemiconductor structures..

3.3. Detection of a non-equilibrium spin in semiconductors Detection of a non-equilibrium spin in semiconductors by the study by the study

aa) ) of luminescenceof luminescence.. bb) ) ofof reflection (Faraday effect).reflection (Faraday effect).

4.4. Wavelength dependence of Kerr effect Wavelength dependence of Kerr effect . .

5.5. Spin lifetime. Possible mechanisms of spin relaxation.Spin lifetime. Possible mechanisms of spin relaxation.

6.6. Our experiments. Time-resolved observation of a spin Our experiments. Time-resolved observation of a spin dynamics. Modification of a pump-probe method. dynamics. Modification of a pump-probe method.

7.7. Future trends of a “spin devices” Future trends of a “spin devices” . . Adaptation of Adaptation of quantum dots for this applications. quantum dots for this applications.

OUR DREEMSOUR 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 MEMORYLONG-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 10-11s, and it is shorter than any other time.

““FAST” SPIN MEMORY FAST” SPIN MEMORY (DURIND ONE CLOCK)(DURIND ONE CLOCK)

If we study spin polarization using light, it isn't ease to separate If we study spin polarization using light, it isn't ease to separate electron and exciton spin polarization. electron and exciton spin polarization.

OPTICAL ORIENTATION IN GaAs QUANTUM WELLSOPTICAL ORIENTATION IN GaAs QUANTUM WELLS

LaserLaser Glan Glan prismprism

RetarderRetarder λλ/4/4 Sample in Sample in cryostatcryostat

In quantum wells, to take In quantum wells, to take a spin polarization in a a spin polarization in a case of photo inducted case of photo inducted carriers is quite simple. carriers is quite simple. Irradiation by circular Irradiation by circular polarized light produce polarized light produce 100100%% spin polarization. spin polarization.

Conduction bandConduction band

Valence bandValence band

2

1

2

1

2

1

2

12

3

2

3

GaAsGaAsAlGaAsAlGaAs AlGaAsAlGaAs

Absorption of circular polarized light in a bulk GaAs result in an 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 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 narrow quantum wells they energy levels are different, and we can excite them separately. them separately.

Conduction band:Conduction band:

I = 0; S =1/2 I = 0; S =1/2 J =1/2 ; m = J =1/2 ; m = ±1/2 ±1/2

Valence band:Valence band:

I = 1; S =1/2 I = 1; S =1/2

J =1/2 J =1/2 ;; m = m = ±1/2 ±1/2

J =3/2 ;J =3/2 ;

m =m =±1/2 (light hole)±1/2 (light hole)

m=m=±3/2 (heavy hole)±3/2 (heavy hole)

CRATION OF ELECTRON SPIN POLARIZATIONCRATION OF ELECTRON SPIN POLARIZATION

Circularly Circularly polarized lightpolarized light

Photo Photo inducted inducted electron – electron – hole pairshole pairs

Electrons Electrons from donorsfrom donors Circularly polarized light creates Circularly polarized light creates

polarized electrons and holes. polarized electrons and holes. Hole lost spin orientation in a few Hole lost spin orientation in a few picoseconds (10picoseconds (10-12-12). If there are ). If there are electrons from donors, they could electrons from donors, they could recombine with holes as well as recombine with holes as well as photo inducted ones. The photo inducted ones. The remained after recombination remained after recombination electrons would have a preferred electrons would have a preferred spin orientation. This orientation spin orientation. This orientation would not 100%, but for our study would not 100%, but for our study even 10% is still enough. even 10% is still enough.

Note1: If electron – hole pairs are being produced in a bound state (exciton), they are 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 also polarized free electrons, but this process is more complicated and is not enough studied. studied.

SPIN POLARIZATION MEASUREMENTSPIN POLARIZATION MEASUREMENT

Где

Elliptically Elliptically polarized light polarized light (luminescence)(luminescence)

Photo Photo inducted inducted electron – electron – hole pairshole pairs

Electrons Electrons from donorsfrom donors

With light cooperate electronWith light cooperate electron - hole- hole pairs, whose total angular moment pairs, whose total angular moment does not exceed unit. Electrondoes not exceed unit. Electron with with spinspin 1/2 and hole1/2 and hole with spinwith spin 3/2 can 3/2 can have the total moment 1 or 2 have the total moment 1 or 2 depending on their mutual depending on their mutual orientation. As it was mentioned orientation. As it was mentioned above, a holeabove, a hole in time about ten in time about ten picoseconds loses orientation. But picoseconds loses orientation. But the electron - hole pair can be the electron - hole pair can be highlighted only at that moment highlighted only at that moment when hole orientationwhen hole orientation will return to will return to an initial state. Thus, the an initial state. Thus, the luminescence will be polarized the luminescence will be polarized the same as an absorbed light . same as an absorbed light .

USE FARADAY EFFECT FOR SPIN POLARIZATION MEASUREMENT USE FARADAY EFFECT FOR SPIN POLARIZATION MEASUREMENT

Medium in a Medium in a magnetic fieldmagnetic field

Simple Faraday effectSimple Faraday effect

Photon energyPhoton energy

Ref

ract

ive

inde

xR

efra

ctiv

e in

dex

ΔΔE = E = μμB B g Bg Bm =+1m =+1

m =-1m =-1

αα

Nonlinear Faraday effectNonlinear Faraday effect

σσ ++ σσ -- αα ~ ~ ΔΔn ~ Bn ~ B

Pumped Pumped mediummedium αα

m =+1m =+1m =-1m =-1

σσ ++ σσ --

Different level Different level population makes population makes refractive indexes refractive indexes for for σσ + + and and σσ – – polarized light polarized light different. It results different. It results in light rotation.in light rotation.

Photon energyPhoton energy

Ref

ract

ive

inde

xR

efra

ctiv

e in

dex

SPIN RELAXAOTIONSPIN RELAXAOTION

There are many reasons for spin relaxation in semiconductor. There are many reasons for spin relaxation in semiconductor. All of them are based on particle motion. All of them are based on particle motion.

The main mechanism for the hole spin relaxation is interaction with phonons.The main mechanism for the hole spin relaxation is interaction with phonons.

The reason of highThe reason of high efficiency efficiency of this mechanism is that a hole of this mechanism is that a hole has orbit angular momentum. has orbit angular momentum.

currentcurrent

magnetic fieldmagnetic field

Magnetic momentum Magnetic momentum rotate slowly around the rotate slowly around the magnetic fieldmagnetic field

MODIFICATION OF PUMP-PROBE SETUP

Ti:S PULSEDTi:S PULSED LaserLaser

Half mirrorHalf mirror

Variable delayVariable delayStatic delayStatic delay

Pump shaperPump shaperProbe shaperProbe shaper(Monochromator & Glan)(Monochromator & Glan) (Monochromator & Glan)(Monochromator & Glan)

LensLens Retarder Retarder λ/4λ/4

LensLensSample in Sample in cryostatcryostat

Balance photo detectorBalance photo detector

PUMP-PROBE SETUPPUMP-PROBE SETUP

T = 10 K. T = 10 K. Light power: Light power: 800 W cm 800 W cm-2-2

Pulse duration: 100 fs. Pulse duration: 100 fs. Repetition rate: 80 MHz Repetition rate: 80 MHz (12,5 ns).(12,5 ns).

Sensitivity: Sensitivity: 1010-6 -6 rad rad (10(10-8-8 grad). grad).

Delay Delay length: 30 length: 30 cm (600 ps) cm (600 ps)

DETAILS. BALANSE PHOTO DETECTORDETAILS. BALANSE PHOTO DETECTOR

- U

00202

0 2)45(sin)45(cos IIi

200 EI

yE

xE

0E

yE xE+ U

Signal (i)

αα

IIyy IIxx

IIxx - I - Iyy

A B

0 40 80 0 40 80 DelayDelay ( (ps)ps)

ps 10τ1

ps 200τ2

Wavelength (nm)Wavelength (nm)804 806 808 810804 806 808 810

EXPERIMENTAL RESULTSEXPERIMENTAL RESULTS

Δα

Δα

Δα

Δα

Δα

Δα

Wavelength

Wavelength

DelayDelay

Quantum well without free electronsQuantum well without free electrons

Thank youThank you for for

youryour attentionattention

The EndThe End

© Copyright. All rights reserved. Saint-Petersburg, 2005 © Copyright. All rights reserved. Saint-Petersburg, 2005