Spin sensitive bleaching of the absorption of far-infrared radiation has been
observed in $p$-type GaAs/AlGaAs quantum well structures. The absorption of
circularly polarized radiation saturates at lower intensities than that of
linearly polarized light due to monopolar spin orientation in the first heavy
hole subband. Spin relaxation times of holes in $p$-type material in the range
of tens of ps were derived from the intensity dependence of the absorption.Comment: Figures have been updated due to technical printing problems
  (Postscript mismatch

B. Baylac

B. E. Kane

C. R. Pidgeon

D. Hägele

D. Loss

D. P. DiVincenzo

D. Weiss

E. L. Ivchenko

E. V. Beregulin

G. A. Prinz

G. Bastard

I. Malajovich

J. Fabian

J. M. Kikkawa

J. Shah

L. E. Vorobjev

L. J. Sham

M. Altarelli

M. Bichler

M. D. Martin

M. Helm

R. Ferreira

R. Fiederling

S. Adachi

S. Bar-Ad

S. D. Ganichev

S. Datta

S. N. Danilov

T. C. Damen

V. V. Bel'kov

W. J. Li

W. Prettl

W. Wegscheider

Y. Ohno

English

arXiv

Crossref

Physical Review Letters

Spin-Sensitive Bleaching and Monopolar Spin Orientation in Quantum Wells

Spin-sensitive bleaching of the absorption of far-infrared radiation has been observed in p-type GaAs/AlGaAs quantum well structures. The absorption of circularly polarized radiation saturates at lower intensities than that of linearly polarized light due to monopolar spin orientation in the first heavy-hole subband. Spin relaxation times of holes in p-type material in the range of tens of ps were derived from the intensity dependence of the absorption

Ganichev, Sergey

Danilov, Sergey

Belkov, Vassilij

Ivchenko, Eougenious

Wegscheider, Werner

Weiss, Dieter

Bichler, Max

University of Regensburg Publication Server

Spin-sensitive bleaching of the absorption of far-infrared radiation has been observed in p-type GaAs AlGaAs quantum well structures. The absorption of circularly polarized radiation saturates at lower intensities than that of linearly polarized light due to monopolar spin orientation in the first heavy-hole subband. Spin relaxation times of holes in p-type material in the range of tens of ps were derived from the intensity dependence of the absorption

Prettl, Wilhelm

Spin sensitive bleaching and monopolar spin orientation in quantum wells

VOLUME 88, NUMBER 5 P H Y S I C A L R E V I E W L E T T E R S 4 FEBRUARY 2002057401-1Spin-Sensitive Bleaching and Monopolar Spin Orientation in Quantum WellsS. D. Ganichev,1,2 S. N. Danilov,1 V. V. Bel’kov,2 E. L. Ivchenko,2 M. Bichler,3 W. Wegscheider,1,3D. Weiss,1 and W. Prettl11Institut für Experimentelle und Angewandte Physik, Universität Regensburg, D-93040 Regensburg, Germany2Ioffe Physico-Technical Institute, RAS, 194021, St. Petersburg, Russia3Walter Schottky Institut, TU München, 85748, Garching, Germany(Received 13 July 2001; published 15 January 2002)Spin-sensitive bleaching of the absorption of far-infrared radiation has been observed in p-typeGaAsAlGaAs quantum well structures. The absorption of circularly polarized radiation saturates atlower intensities than that of linearly polarized light due to monopolar spin orientation in the firstheavy-hole subband. Spin relaxation times of holes in p-type material in the range of tens of ps werederived from the intensity dependence of the absorption.DOI: 10.1103/PhysRevLett.88.057401 PACS numbers: 78.30.Fs, 68.65.–k, 73.50.Mx, 73.50.PzThe spin of electrons and holes in solid state systems isthe decisive ingredient for active spintronic devices [1,2]and several schemes of quantum computation [3–5]. Es-pecially the combination of ferromagnetic materials withsemiconductors seems to be a promising combination fornovel functional concepts. Open problems which haveto be addressed in this respect involve spin injection intosemiconductors, spin relaxation in low-dimensional semi-conductor structures, as well as spin detection. Significantprogress was made recently: it was shown that spin po-larized electrons (or holes) can be injected from magneticsemiconductor materials into semiconductors [6,7]. Thepresence of spin polarized electrons can be probed by an-alyzing the Kerr effect [8] or by analyzing the degree ofcircular polarization of light which gets emitted when po-larized electrons recombine with holes. The inverse pro-cess, exciting free carriers by circularly polarized light [9],is frequently used to prepare an ensemble of spin polarizedcarriers. In low-dimensional systems with band splitting ink space due to k-linear terms in the Hamiltonian opticalexcitation not only leads to a spin polarized ensemble ofelectrons but also to a current whose sign and magnitudedepend on the degree of circular polarization of the inci-dent light (circular photogalvanic effect [10]).For the realization of spintronic devices long spindephasing times in quantum well (QW) structures are cru-cially needed. Spin transport must occur without destroy-ing the relevant spin information. Current investigationsof the spin lifetime in semiconductors [11–16] are basedon optical spin orientation by interband excitation. Studiesof such bipolar spin orientation, where both electrons andholes got excited, gave important insights into the mecha-nisms of spin relaxation. We show below that by com-bining the circular photogalvanic effect (CPGE) [10] withsaturation (bleaching of absorption) spectroscopy [17–21]we are able to probe spin relaxation for monopolar spinorientation. In contrast to the conventional methods of op-tical spin orientation, in our measurements only one typeof charge carriers (electrons or holes) gets spin oriented0031-90070288(5)057401(4)$20.00and is involved in relaxation processes. This is achievedby using terahertz radiation which excites intraband orintersubband, but no interband, transitions. Monopolarspin orientation allows us to study spin relaxation withoutelectron-hole interaction and exciton formation. The mostimportant advantage of monopolar spin orientation is thatrelaxation processes can be investigated for electrons inn-type material and for holes in p-type material. Theseconditions have not been met previously in quantumwells where, due to interband excitation, only the spinrelaxation times of optically generated minority carrierswere accessible (for reviews, see [14–16]).Here we report the first observation of spin-sensitivebleaching of the heavy-hole hh1–light-hole lh1 absorp-tion in p-type QW structures which allows us to investigatespin relaxation for a monopolar spin orientation. The ba-sic physics is sketched in Fig. 1. Exciting with circularlypolarized light results in direct intersubband transitions atk fi 0 (solid arrow) which depopulate and populate selec-tively spin states in the valence subbands (hh1 and lh1)and cause a monopolar spin polarization. Spin relaxationinside the hh1 subband is characterized by the relaxationtime ts. Relaxation from the lh1 subband back to hh1 ismediated by phonons and characterized by a spin indepen-dent energy relaxation time te (broken arrows in Fig. 1).The absorption coefficient a is proportional to the differ-ence of the populations (occupation numbers) of the initialand final states involved in the direct transitions. At highintensities the absorption coefficient decreases since thephotoexcitation rate becomes comparable to the nonradia-tive relaxation rate into the initial state. Thus absorptionbleaching of circularly polarized radiation is characterizedby the spin relaxation time ts and the energy relaxationtime te, respectively. In contrast to circularly polarizedlight, optical transitions induced by linearly polarized lightare not spin selective and saturation is controlled by energyrelaxation of photoexcited carriers only. The difference inabsorption bleaching for circularly and linearly polarizedradiation can be observed experimentally.© 2002 The American Physical Society 057401-1VOLUME 88, NUMBER 5 P H Y S I C A L R E V I E W L E T T E R S 4 FEBRUARY 2002FIG. 1. Sketch of direct optical transitions (full line) be-tween the first heavy-hole and the first light-hole subbandin p-GaAsAlGaAs QWs. While the splitting of the bandsin k space is necessary for an understanding of the circularphotogalvanic effect [10], it is unimportant for the saturationprocess and ignored in the sketch. The absorption of far-infraredradiation with photon energy h¯v  8.3 meV occurs very closeto k  0. Hence, initial and final states are characterized byquantum numbers m  632 and 612, respectively. Dashedlines show the energy relaxation of photoexcited carriers. ´Fis the Fermi energy.The experiments have been carried out on modulateddoped p-GaAsAlGaAs (311)-molecular-beam-epitaxy(MBE)-grown samples with a single QW or 20 QWsof width LW  15 nm and with period d  130 nmfor multiple QW structures. Samples with free carrierdensities ps of about 2 3 1011 cm22 and mobilitiesaround 5 3 105 cm2V s were studied in the temperaturerange from 4.2 to 300 K. A pair of Ohmic contacts wascentered on opposite sample edges along the directionx k 11¯0 (see Fig. 2 inset). A high power far-infraredmolecular laser, optically pumped by a transverselyexcited atmospheric pressure CO2 laser, has been used todeliver 100-ns pulses with intensities up to 1 MWcm2at wavelength l  148 mm. The radiation induces directoptical transitions between the first heavy-hole and thefirst light-hole valence subband (see Fig. 1). A l4plate has been used to obtain from the initially linearlypolarized laser light circularly polarized radiation. Thedegree of circular polarization Pcirc was 61 for right- andleft-handed circularly polarized light, respectively.The absorption of terahertz radiation by free carriers inQWs is weak and difficult to determine in transmissionmeasurements. This is even worse in the case of bleach-ing at high power levels. Therefore the nonlinear behaviorof the absorption has been investigated employing the re-cently observed circular and linear photogalvanic effects[10,22]. Both CPGE and the linear photogalvanic effect(LPGE) yield an easily measurable electric current in thex direction [23]. The absorption coefficient is proportionalto the photogalvanic current jx normalized by the radiation057401-2       FIG. 2. Photogalvanic current jx normalized by the intensity Ias a function of I for circularly and linearly polarized radiationat T  20 K. The inset shows the geometry of the experiment;eˆ indicates the direction of the incoming light. The current jxflows along the 11¯0 direction at normal incidence of radiationon p-type 113A-grown GaAsAlGaAs QWs. In order to ob-tain the CPGE right or left circularly polarized light has beenapplied. To obtain the LPGE linearly polarized radiation withthe electric field vector E oriented at 45± to the x direction wasused. The measurements are fitted to jxI ~ 1I 1 IIs withone parameter Is for each state of polarization (full line: circu-lar; broken line: linear).intensity I [24]. Thus by choosing polarization we obtain aphotoresponse corresponding to the absorption coefficientof circularly or linearly polarized radiation, respectively.Our measurements (Fig. 2) indicate that the photocur-rent jx at a low power level depends linearly on the lightintensity and gradually saturates with increasing intensity,jx ~ I1 1 IIs, where Is is the saturation intensity.This corresponds to a constant absorption coefficient atlow power levels and decreasing absorption with rising in-tensity. Saturation intensities Is have been measured fortemperatures between 4.2 and 200 K. The key result isplotted in Fig. 3 and shows that the saturation intensityIs is generally smaller for circularly polarized radiation.The experimental values increase from about 10 kWcm2at 4.2 K to 300 kWcm2 at 200 K. At room temperatureIs get nonmeasurably large.Over the whole temperature range the holes occupy, inequilibrium, the lowest heavy-hole subband hh1. Absorp-tion of l  148-mm radiation (h¯v  8.3 meV) occurs bydirect optical transitions from hh1 to the first light-holesubband lh1 close to k  0 (but fi 0) [25], as is sketchedin Fig. 1. Thus, the selection rules for the absorptionare Dm  61 with angular momentum quantum num-ber m  632 for the initial and m  612 for the fi-nal state [26]. The insets in Fig. 3 show the correspondingtransitions for linear (top left) and circular (bottom right)polarization by full arrows. Linearly polarized radiationhas been decomposed in right- and left-handed circularly057401-2VOLUME 88, NUMBER 5 P H Y S I C A L R E V I E W L E T T E R S 4 FEBRUARY 2002  FIG. 3. Temperature dependence of the saturation intensity Isfor linearly and circularly polarized radiation. The dependence isshown for one p-GaAsAlGaAs (311)-MBE-grown sample witha single QW of LW  15-nm width. The free carrier density is1.66 3 1011 cm22 and the mobility is 6.5 3 105 cm2V s. Theinsets show a microscopic picture explaining the origin of thedifference in saturation intensities.polarized light of identical amplitudes. Broken lines inthese insets indicate nonradiative energy and spin relax-ation transitions.Linearly polarized radiation (top left inset in Fig. 3)equally depopulates spin-up and spin-down states of hh1and populates lh1. With rising intensity these nonequi-librium populations approach each other causing thebleaching of absorption, controlled by the energy re-laxation time te. In contrast to linear polarization theabsorption of circularly polarized light is spin selectivebecause only one type of spin is involved in absorption(right bottom inset of Fig. 3). During energy relaxation tothe initial state in hh1 the holes loose their photoinducedorientation due to rapid relaxation [26]. Thus, spin orien-tation occurs in the initial subband hh1, only. Bleachingof absorption is hence controlled by two time constants,the te and ts. Note that te is the same for circular andlinear polarization. If ts is of the order of te or larger,bleaching of absorption becomes spin sensitive and thesaturation intensity of circularly polarized radiation dropsbelow the value of linear polarization.057401-3Spin-sensitive bleaching can be analyzed in terms ofexcitation-relaxation kinetics taking into account both op-tical excitation and nonradiative relaxation processes. Theprobability rates for direct optical transitions from the hh1states with m  632 to higher subbands are denotedas W6. For linearly polarized light, W1 and W2 areequal. For the circular polarization, right-handed, s1, orleft-handed, s2, the rates W6 are different but, due totime inversion symmetry, satisfy the condition W1s6 W2s7. If p1 and p2 are the 2D densities of heavy holeswith spin132 and232, respectively, then the rate equa-tion for p1 can be written as≠p1≠t1p1 2 p22ts 2W1 112W1 1 W2 . (1)The corresponding equation for p2 is obtained byexchange of indices 6 ! 7. Since the laser pulseduration was longer than any relaxation time we considerthe steady-state solution and omit the time derivativein Eq. (1). The second term on the left-hand side ofEq. (1) describes the spin relaxation trying to equalizethe polarization of the 632 states. The first term on theright-hand side describes the removal of holes from thehh1 subband due to photoexcitation while the second termcharacterizes the relaxation of holes which come down tothe 132 and 232 states with equal rates (see Fig. 3insets). The right side of Eq. (1) is proportional to W1 2W2 adIh¯v r0Pcirc 2 hr, where r  p1 2 p2psis the hole spin polarization degree, r0 is the excitationinduced spin polarization, and h  fi fi 2 ff   1describes the difference between the population of theinitial state, fi , and the final state, ff , respectively.Bleaching of absorption with increasing intensity is de-scribed by the function a  a01 1 IIse21, where a0is the absorption coefficient at low intensities and Ise isthe characteristic saturation intensity controlled by energyrelaxation of the hole gas. Since the photocurrent jLPGEinduced by the linearly polarized radiation is proportionalto aI, one hasjLPGEI~11 1 IIse. (2)The photocurrent jCPGE induced by the circularly polarizedradiation is proportional to W1 2 W2. Solving Eqs. (1)and (2) in the steady-state regime, we obtainjCPGEI~11 1 I≥1Ise 11Iss¥ , (3)where Iss  h¯vpsa0dts is the saturation intensity con-trolled by the hole spin relaxation. The saturation intensi-ties Iss and Ise were extracted from the measured saturationintensities Is of linear and circular photogalvanic current(Fig. 3). The low intensity absorption coefficient was cal-culated for l  148 mm and takes into account the non-parabolic behavior of the in-plane hole dispersion whichdiffers for different subbands [27]. Using Iss together with057401-3VOLUME 88, NUMBER 5 P H Y S I C A L R E V I E W L E T T E R S 4 FEBRUARY 2002 FIG. 4. Experimentally determined spin relaxation times tsof holes in p-type GaAsAlGaAs QWs as a function of T .Open triangles and full dots correspond to (113)-MBE-grown15-nm single and multiple (20) QWs, respectively. Free carrierdensities of all samples were about 2 3 1011 cm22 for each QW.a0, spin relaxation times ts have been derived. The resultsare plotted in Fig. 4 as a function of temperature. At lowtemperatures the relaxation times vary like T212.The magnitude of the observed hole spin relaxationtime ts in hh1 is in agreement with published photolu-minescence data [14–16,29–33]. There the relation be-tween spin relaxation time and free carrier density hasbeen discussed in terms of the D’yakonov-Perel (DP) andthe Bir-Aronov-Pikus (BAP) mechanisms. In our case ofmonopolar spin orientation no electron-hole was created.Thus, in contrast to all previous experiments using inter-band excitation, the BAP mechanism is absent and the holespin relaxation is not affected by high density photocre-ated carriers, exciton formation, and interband recombi-nation. The DP mechanism was investigated theoreticallyin [26,34] for spin relaxation of hh1 holes in GaAs basedQWs. The values of ts on the order of 10 ps as well as theobserved temperature dependence ts ~ T212 are in accor-dance with these calculations for samples with parametersas in our experiment.In conclusion, our experimental results demonstratethat absorption of terahertz radiation by intervalenceband transitions in p-type QWs becomes spin sensitive athigh power levels. The saturation of circularly polarizedradiation yields the spin relaxation times of majority car-riers, in our case holes. We emphasize that spin-sensitivebleaching is also expected for intersubband transitions inn-type QWs.We thank L. Golub for helpful discussions. Financialsupport by the DFG, the RFBR, the INTAS, and the NATOlinkage program is gratefully acknowledged.057401-4[1] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990).[2] G. A. Prinz, Phys. Today 48, No. 4, 58 (1995).[3] B. E. Kane, Nature (London) 393, 133 (1998).[4] D. Loss and D. P. DiVincenzo, Phys. Rev. A 57, 120 (1998).[5] D. P. DiVincenzo et al., Nature (London) 408, 339 (2000).[6] R. Fiederling et al., Nature (London) 402, 787 (1999).[7] Y. Ohno et al., Nature (London) 402, 790 (1999).[8] I. Malajovich et al., Nature (London) 411, 770 (2001).[9] Optical Orientation, edited by F. Meier and B. P. Za-kharchenya (Elsevier Science, Amsterdam, 1984).[10] S. D. Ganichev et al., Phys. Rev. Lett. 86, 4358 (2001).[11] J. M. Kikkawa et al., Science 227, 1284 (1997).[12] D. Hägele et al., Appl. Phys. Lett. 73, 1580 (1998).[13] J. M. Kikkawa and D. D. Awschalom, Nature (London)397, 139 (1999).[14] J. Shah, Ultrafast Spectroscopy of Semiconductor Nano-structures (Springer-Verlag, Berlin, New York, 1999),pp. 243–261.[15] L. J. Sham, J. Phys. Condens. Matter 5, A51 (1993).[16] J. Fabian and S. Das Sarma, J. Vac. Sci. Technol. B 17,1708 (1999).[17] E. V. Beregulin et al., Sov. Phys. Semicond. 16, 179 (1982).[18] C. R. Pidgeon et al., Phys. Rev. Lett. 50, 1309 (1983).[19] E. V. Beregulin et al., Sov. Phys. Semicond. 21, 615 (1987).[20] M. Helm et al., Appl. Phys. Lett. 63, 3315 (1993).[21] W. J. Li et al., Semicond. Sci. Technol. 9, 630 (1994).[22] S. D. Ganichev et al., Appl. Phys. Lett. 77, 3146 (2000).[23] Current jx generated by polarized radiation at normal inci-dent was measured as a function of intensity via the voltagedrop across a 50 V load resistor in a closed circuit. ForLPGE linearly polarized radiation with the electric fieldvector E oriented at 45± to the x direction was used.[24] E. L. Ivchenko and G. E. Pikus, Superlattices and OtherHeterostructures. Symmetry and Optical Phenomena(Springer, Berlin, 1997).[25] In the G8-band model based on the isotropic LuttingerHamiltonian, direct optical transitions between hh1 andlh1 are forbidden at k  0. For k fi 0 direct transitionsare possible. Band calculations taking into account non-parabolic hh and lh band structure and spectral broadeningshow that the incident photon energy of 8.3 meV fits thelh1-hh1 energy separation [26–28]. At the temperatures ofthis investigation the absorption due to direct optical tran-sitions is much stronger than free carrier absorption causedby indirect transitions [27].[26] R. Ferreira and G. Bastard, Phys. Rev. B 43, 9687 (1991).[27] L. E. Vorobjev, D. V. Donetskii, and L. E. Golub, JETP Lett.63, 977 (1996).[28] M. Altarelli, U. Ekenberg, and A. Fasolino, Phys. Rev. B32, 5138 (1985).[29] T. C. Damen et al., Phys. Rev. Lett. 67, 3432 (1991).[30] S. Bar-Ad and I. Bar-Joseph, Phys. Rev. Lett. 68, 349(1992).[31] B. Baylac et al., Surf. Sci. 326, 161 (1995).[32] S. Adachi et al., J. Lumin. 72–74, 307 (1997).[33] M. D. Martin et al., Physica (Amsterdam) 2E, 186 (1998).[34] G. Bastard and R. Ferreira, Europhys. Lett. 23, 439 (1993).057401-4

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Spin sensitive bleaching and monopolar spin orientation in quantum wells

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