Spin Relaxation by Transient Monopolar and Bipolar Optical Orientation

Abstract

We have used two-color time-resolved spectroscopy to measure the relaxation of electron spin polarizations in a bulk semiconductor. The circularly polarized pump beam induces a polarization either by direct excitation from the valence band, or by free-carrier (Drude) absorption when tuned to an energy below the band gap. We find that the spin relaxation time, measured with picosecond time resolution by resonant induced Faraday rotation in both cases, increases in the presence of photogenerated holes. In the case of the material chosen, n-InSb, the increase was from 14 to 38 ps. DOI: 10.1103/PhysRevLett.96.096603 PACS numbers: 72.25.Rb, 72.25.Fe, 78.47.+p Spin polarizations have become a focus of interest in semiconductor physics in recent years, not least for their potential application in ''spintronic'' devices [1]. It has been shown that spin polarized electrons (or holes) can be injected from magnetic semiconductor materials into semiconductors [2,3], that they can be coherently transported through a device [4], and that they can be controlled (or modulated) with an external electric field [5]. It remains challenging to improve these breakthroughs to the extent that they can be combined in a practical device [1]. A sufficiently long spin lifetime s is important in the design of structures which confine and/or transfer spin. It is therefore necessary to understand the establishment of a polarized spin population and subsequent spin relaxation mechanisms in both bulk and low dimensional semiconductors. A large amount of literature has been established to determine spin lifetimes by one or another form of timeresolved optical orientation techniques based on spin injection by pulsed interband pumping with circularly polarized light [4 -13]. In this Letter we investigate and contrast the spin relaxation lifetimes as measured in a bipolar (interband) experiment with spin lifetimes measured in a monopolar regime. In the former, significant numbers of excess electrons and holes are photopumped, whereas most device proposals involve electrical injection and manipulation of one type of charge carrier only, ideally at room temperature and often zero magnetic field. To this end we compare in such conditions the results of interband versus intraband pumping of spins with picosecond pulses, in the same sample. We find that the difference is a factor of between 2 to 3.Only one previous experiment measured the lifetime of spins pumped with monopolar excitation in zero magnetic field. In that case hole lifetimes in p-type GaAs=AlGaAs quantum wells were measured at low temperature by steady state bleaching of the circularly polarized photocurrent response [14]. The spin lifetime estimated from the saturation intensity was found to be 20 ps at liquid helium temperature. Our picosecond time-resolved measurements do not rely on theoretically derived absorption cross sections, were made on material at room temperature, and directly compare the bipolar and monopolar spin excitation and relaxation in the same sample. Our results...

Spin dynamics in semiconductors

“…Let us start with the temperature dependence of spin lifetime. Spin lifetime was measured as function of temperature in GaAs [4,371,520,543], GaSb [544], GaN [545][546][547][548], InAs [549][550][551][552], InSb [551,553], InP [554], ZnSe [555], ZnO [556], CdTe [524] and HgCdTe [557,558]. It was found to decrease with increasing temperature at high temperature (nondegenerate regime) in all these works (e.g., see Fig.…”

“…It was measured in GaAs [543,573], with τ s in the range of 15-110 ps, in InAs [552,574,575], with τ s in the range of 2-24 ps and in InSb [551,553,558], with τ s in the range of 2-300 ps, depending on the doping density.…”

Strong dependence of spin dynamics on the orientation of an external magnetic field for InSb and InAs

“…6,7 The DP process, which is the dominant spin relaxation mechanism at zero magnetic fields, is quickly suppressed with external magnetic field in either configuration. In the Faraday configuration ͓see Fig.…”

“…[5][6][7] A perpendicular magnetic field is sometimes applied with the time-resolved technique in order to observe the Larmor precession 2,8 ͑as in this work͒. In most other techniques, such as spin resonance experiments, where the line-width for microwave absorption is used to infer the spin relaxation time, 9 a field is necessarily applied.…”

Far-infrared free-electron lasers and their applications

“…As for the above intersubband spin polarised experiment the intraband (free-carrier absorption) spin selection rules are weak, requiring a probe with better sensitivity. For this reason a non-degenerate pump-probe spectroscopy with far-IR pulses from the FEL and a synchronised solid state laser system has been developed to probe interband transmission while pumping with circularly polarised intraband transitions [44]. The results show (Fig 8), counter-intuitively, that photo-holes slow down the electron spin relaxation in n-type materials.…”