Pure circular polarization electroluminescence at roomtemperature with spin-polarized light-emitting diodesNozomi Nishizawaa, Kazuhiro Nishibayashia, and Hiro Munekataa,1

aLaboratory for Future Interdisciplinary Research of Science and Technology, Tokyo Institute of Technology, 4259-J3-15 Nagatsuta, Midori-ku, Yokohama226-8503, Japan

Edited by J. M. D. Coey, Trinity College Dublin, Dublin, Ireland, and approved January 3, 2017 (received for review June 17, 2016)

We report the room-temperature electroluminescence (EL) withnearly pure circular polarization (CP) from GaAs-based spin-polarized light-emitting diodes (spin-LEDs). External magneticfields are not used during device operation. There are two smallschemes in the tested spin-LEDs: first, the stripe-laser-like structurethat helps intensify the EL light at the cleaved side walls below thespin injector Fe slab, and second, the crystalline AlOx spin-tunnelbarrier that ensures electrically stable device operation. The purityof CP is depressively low in the low current density (J) region,whereas it increases steeply and reaches close to the pure CP whenJ > 100 A/cm2. There, either right- or left-handed CP component issignificantly suppressed depending on the direction of magnetizationof the spin injector. Spin-dependent reabsorption, spin-induced bire-fringence, and optical spin-axis conversion are suggested to accountfor the observed experimental results.

spintronics | circular polarization | semiconductors | nonlinear effect |spin injection

As well represented by the giant magnetoresistance, tunnelingmagnetoresistance, and spin-transfer-torque magnetic ran-

dom access memory, spintronics research based on spin trans-port in magnetic metals has been contributing significantly in theprogress of electronics through the advancement in recording bitdensity and low-power memory retention (1, 2). Proposal of thespin-current modulation with an electric field (3) and inventionof diluted magnetic III–V semiconductors (4) have opened theopportunity of introducing spin degree of freedom in semi-conductor technology (5). After those works, light-inducedmagnetism (6), electric-field-controlled magnetism (7), spin qubitsin semiconductors (8, 9), spin-polarized light-emitting diodes (spin-LEDs) (10, 11), and spin-metal–oxide–semiconductor field-effecttransistor (12) were either demonstrated or proposed, which havecaused an impact on the metal-based spintronics and appliedphysics that is not small. However, works that assure the room-temperature (RT) operation of those semiconductor-based deviceshave not been accomplished to date.Concerning spin-LEDs, studies on a spin injector consisting of

a ferromagnetic metal (FmM) and a tunnel barrier (TB) (13–15)succeeded those using semiconductor-based spin injectors (10,11). The idea of the FmM-TB injector is to take advantage ofspin-polarized carriers at RT with FmM and to simultaneouslysuppress with the TB the backward flow of unpolarized carriersthat is unavoidable in the diffusive transport (16, 17). Manyworks have been carried out with the FmM-TB injector sincethen. Among those, the highest circular polarization (CP) value,PCP ≡ {I(σ+) − I(σ−)}/{I(σ+) + I(σ−)}, with I(σ+) and I(σ−) theintensity of right- and left-handed EL component, respectively,was PCP ∼ 0.3 ∼ 0.35 at RT in the external magnetic flux of B =0.8 T, which was achieved in the context of studying the spin-filtering effect of the MgO TB (18, 19). Most of the past worksregarding spin-LED were carried out under the vertical ar-rangement with low J ranging from 0.1 to 1 A/cm2 and forcingspins aligned vertically by applying out-of-plane external mag-netic fields (20). With vertical-cavity surface-emitting laserstructure incorporating the quantum wells (QWs) (21–23) orthick active layer (24) together with a means of a vertical optical

resonator, pure-CP lasing was demonstrated by the optical pump-ing up to RT (21–24). On the other hand, the CP lasing achieved bythe electrical pumping has been PCP ∼ 0.23 at 50 K with a directcurrent of J ∼ 2.8 kA/cm2 and B ∼ 2.0 T in InGaAs-based QWs(25), PCP ∼ 0.55 at 230 K with a pulsed current of presumablyJ ∼ 18 kA/cm2 and B ∼ 2.0 T in InGaAs-based quantum dots (26),and PCP ∼ 0.28 at RT with pump energy of 87 μJ and B ∼ 0.35 T inGaN nanorods filled with Fe3O4 nanoparticles (27).Here, we report the unforeseen appearance of nearly pure CP

electroluminescence, PCP = 0.95 at RT from the edge-emission-type spin-LED structure without the application of an externalfield, which was less investigated in the past (28, 29). Our resultssuggest the appearance of some nonlinear effect in the regime ofmoderately high current density (J > 100 A/cm2), and lead us to theopportunity of studying the RT operation of, at least, semiconductor-based spin-photonic devices.Shown in Fig. 1A is a schematic cross-section, the cleaved GaAs

(110) side wall, of the tested LED chips. They consist of a poly-crystalline Fe in-plane spin injector, a crystalline γ-like AlOx tunnelbarrier (30), and an epitaxial AlGaAs/GaAs/AlGaAs double het-erostructure (DH) (31). A magnetization vector of the spin injectoris controlled either parallel or antiparallel to the GaAs [110] axis bythe technical magnetization. Spin polarization of electrons, Pe =(n+− n−)/(n++ n−), with n the electron density at the Fermi level, isassumed to be Pe ∼ 0.4 in Fe (32). At the stage of initial injection,70% of electron spins point toward the GaAs [110] axis that issuperposition of the two primary crystal axes [100] and [010],whereas the remaining 30% have the opposite [1ð−Þ 10] spin axis.Spin/charge transport takes place vertically toward the [001] axis.The DH wafer was designed by the authors in view of (i) not

severely reducing the spin polarization during carrier transportacross the upper n-AlGaAs–clad layer and (ii) avoiding largeoptical loss due to the top spin-injector metal. Taking account of

Significance

Most of the experiments on the spin manipulation in semicon-ductors, the principal materials in modern electronic and photonicdevices, were carried out at cryogenic temperatures and highmagnetic fields because thermal energy tends to randomize spininformation in the semiconductor that is nonmagnetic. Here, wereport very surprising experimental results of pure circular po-larization electroluminescence at room temperature with no ex-ternal magnetic fields. The results are obtained by electricallyinjecting moderately high density of spins into semiconductordouble heterostructures, the structures that were invented inconnection with semiconductor lasers one-half century ago. Theresults suggest the appearance of some spin-dependent nonlinearprocesses that lead to recovering and even enhancing the in-jected, initial spin information in semiconductors.

Author contributions: N.N. and H.M. designed research; N.N. performed research; N.N.,K.N., and H.M. analyzed data; and N.N. and H.M. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.1To whom correspondence should be addressed. Email: munekata.h.aa@m.titech.ac.jp.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1609839114/-/DCSupplemental.

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these two contradictory requirements, the thickness and alloycomposition of the n-AlxGa1-xAs layer was chosen to be L = 500 nmand x = 0.3, respectively. With this layer, 60% of the injectedx-axis spins are supposed to preserve their spin axis at thep-GaAs active layer (33, 34), whereas ∼4% of the electrolumi-nescence (EL) energy is absorbed in the Fe injector, assumingthe extinction coefficient κ = 3.89 at the wavelength 909 nm (35)and the confinement efficiency of DH Γ = 0.91 (36). A thickGaAs active layer, in which heavy- and light-hole bands aredegenerated, is used to accommodate spins of in-plane axes in

the active layer. Moreover, the active layer is intentionally dopedp-type to reduce the radiative recombination lifetime (37) andsuppress the contribution of nonradiative recombination. Notethat initially injected [110] (or [1ð−Þ 10]) spins are dispersedinto the spins of other orthogonal orientations ([11ð−Þ 0] and[001]) as a result of spin relaxation. Radiative recombination ofthese spins is observed as the linearly polarized emission: namely,the helicity-independent background, in the CP-resolved EL de-tection system whose optical axis is along the [110] axis (Fig. 2A).The DH part was grown on a p-GaAs (001) wafer using a

metal-organic vapor phase epitaxy reactor at Optowell Co., Ltd.to ensure high optical quality that the radiative recombinationdominates at RT. A 1-nm-thick crystalline γ-like AlOx tunnelbarrier was then grown by the authors using a molecular beamepitaxy chamber (30) The density of interface states at the AlOx/GaAs interface has been found to be Dit ∼ 3 × 1011 cm−2·eV−1

(38), which is far less than that at the amorphous AlOx/GaAsinterface. This was followed by the fabrication of 100-nm-thick,40-μm-wide Au (20 nm)/Ti (5 nm)/Fe(100 nm) spin-injectorstripes on top of the tunnel barrier using a separate e-beamevaporator and standard photolithography. Finally, the waferwas thermally annealed at 230 °C for 60 min in the nitrogen gasatmosphere, and then cleaved into 1.1 × 2.0-mm rectangle chips.The resultant aspect ratio of the spin injector is 1:25. The longside of the injector, the easy axis, is parallel to the short side ofthe chip, which is the GaAs [110] axis. They were mounted on acopper block for the EL experiments.EL spectra obtained from the control experiment using the

chip without an Fe layer are shown in Fig. 1B, together withphotoluminescence (PL) spectrum obtained by the surface

Fig. 1. (A) A schematic cross section of spin-LEDs. (B) EL and PL spectraobtained from the control chip consisting of Au/Ti/γ-like AlOx/DHs. Solid linesrepresent EL spectra with three different current densities, J = 100 (σ+, red;and σ−, black), 75 (green) and 50 (blue) A/cm2. The range of applied voltageis between 2–4 V. A black, dotted line shows PL spectrum obtained by thesurface excitation (hν = 1.58 eV, λ = 785 nm, 40 mW). The extinction coef-ficient of a top Au electrode is κ = 6.06 at 1.36 eV (λ = 909 nm) (39).

Fig. 2. (A) A schematic experimental setup for EL measurements, showing,from upper left to lower right, a wire-bonded rectangle spin-LED chip on acopper block, a (λ/4) QWP, an LP, and an MCS. A pair of lenses, one betweenthe chip and QWP and another between LP and MCS, is omitted forgraphical clarity. Orange waves represent EL from the chip with right-handed (σ+, red circle) and left-handed (σ−, blue circle) EL components.Straight orange arrows accompanied by double-headed arrows representlight waves converted into linear polarization by QWP. Thin dotted, bluearrows on polarizers represent optical axes. (Inset) Pictures of chip A with ELfrom the cleaved edge; current density J = 22 (Left) and 110 A/cm2 (Right). (B) Acouple of helicity-specific EL spectra obtained when direction of remnant mag-netization points toward QWP (+M, Upper) and against QWP (−M, Lower).

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excitation of the DH wafer. The peak energy at 1.43 eV in the PLspectrum indicates the domination of near-band-edge emission,whereas in the EL spectrum, the near-band-edge emission is weakand the peak appears at the photon energy that is ∼100 meV lessthan that in the PL spectrum. That is to say, reflecting a relativelylong, lateral optical path, the near-band-edge emission is sig-nificantly reabsorbed due to a large absorption coefficient (α ≥104 cm−1), whereas the low-energy emission (hν≤ 1.38 eV),which is attributed to the transition from the conduction band tothe valence-band tail states caused by the mixing with acceptorstates (40), is less reabsorbed owing to a small absorption coef-ficient (α < 102 cm−1). The right- and left-handed EL spectra areidentical to each other, indicating PCP = 0.Before EL measurements, the Fe spin injector was magnetized

along the long side of the stripe ([110]) by an external magneticfield of H = 5 kOe. The EL emitted from the cleaved (110) sidewalls was transmitted through a quarter-wave plate (QWP) and alinear polarizer (LP), and was collected into a multichannelspectrometer (MCS) that was placed 30 cm away from thecleaved edge (Fig. 2A). Setting the optical axis of QWP either at0° or 90°, while fixing the optical axis of LP at 45°, either a right-handed (σ+) or left-handed (σ−) EL component was selected,respectively. This method excludes confusion with the linearlypolarized EL composed of coradiation of transverse-magneticand transverse-electronic modes. In fact, as shown in Fig. 2B, theintensity relation between the right-handed (QWP at 0°) and theleft-handed (QWP at 90°) components is reversed when the di-rection of remnant magnetization is reversed: e.g., for the emissionband around hν = 1.35 eV, the PCP values (PCP ≡ {I(σ+)−I(σ−)}/{I(σ+)+I(σ−)}) are 0.83 and −0.80 for the magnetization pointingtoward (+M) or against (−M) the QWP, respectively. The I valuehereafter is the emission intensity integrated over the photon ener-gies within the full width at half maximum of the EL emission band.EL occurs when the p-GaAs substrate side is positively biased

above ∼1 V. Increasing the bias results in an increase of a currentas well as the emission intensity especially from the side-wallarea underneath the spin-injector stripe. A representative J(current density)−V (voltage) curve from the chip A is shown inFig. 3 A and B in linear and semilogarithmic scales, respectively.Here, J is estimated using the spin-injector area, 40 μm ×1.1 mm, assuming the short current spreading length (∼2 μm) inthe lateral direction (41). The slope of the J−V curve, eV/nkT, atthe relatively high bias region (V ≥ 1.3 V) shows the diode factorn  ∼ 2, indicating the recombination-dominated carrier transportaround the p-n junction region (42).Fig. 3C shows EL spectra obtained at three different J values:

22, 55, and 110 A/cm2. Two relatively broad emission bands are

present: the one with the peak around 1.42 eV (band A) is due tothe band-to-band emission, and the other peaking around1.36 eV (band B) is attributed to the emission via the valence bandtails that propagates laterally through the GaAs active layer (43).Comparing these spectra with that of the control experiment(Fig. 1B), the appearance of band A and the blue shift of band Bare noticeable. These facts can be understood in terms of thedifference in the extinction coefficient between Fe and Au lay-ers: κ = 3.89 and 6.06 at the wavelength of 909 nm for Fe and Au,respectively (35, 39). The band A (B) shifts toward higher(lower) energies when J is increased; the peak photon energies ofband A are 1.406, 1.4065, and 1.414 eV, whereas those of band Bare 1.365, 1.360, and 1.351 eV, at J = 22, 55, and 110 A/cm2,respectively. Using the observed blue and red shifts at J = 110 A/cm2,electron density and the temperature in the p-GaAs active layer areestimated to be ∼6 × 1017 cm−3 and ∼319 K, respectively (44). Notethat this electron density is around ∼1/5 of the thresholddensity for CP lasing that has been estimated by the opticalpumping experiments using the vertical-cavity surface-emittinglaser incorporating a bulk, undoped GaAs active layer (d =485 nm) (24).It is clear that the difference in intensity between the σ+ and

σ− EL components becomes larger with increasing the currentdensity J. The spectral shape, however, is nearly identical be-tween the two components (Fig. 3C, Inset). As for the CP value,which was defined previously by the form PCP ≡ {I(σ+)−I(σ−)}/{I(σ+)+I(σ−)}, PCP = 0.14, 0.42, and 0.95 for band A, whereasPCP = 0.11, 0.44, and 0.94 for band B, at J = 22, 55, and 110 A/cm2,respectively. Bias dependence on the PCP value for band B issummarized in Fig. 3D. The observed increase in the PCP value atrelatively low bias region (V < 6 V) verifies the theoretically pro-posed spin-injection–impeding effect that is supposed to appear inthe depletion region in semiconductor junctions (45). Spectral-dependent PCP data are presented in Spectral-Resolved CP DegreePlots and Fig. S1, including the data from other chips. Those resultssuggest that there is no essential difference between majority andminority spins in terms of the electronic states associated with theelectron–hole recombination. In terms of the relation in emissionintensity between bands A and B, the ratio IB/IA gradually increaseswith increasing J irrespective of the σ+ and σ− EL components:IB/IA = 1.1 and 2.2 at J = 20 and 100 A/cm2, respectively. Namely,band B becomes dominant at high J region.Reduction in the intensity of the σ− (minority) component

I(σ−) with increasing J is the common trend for the tested chipsthat have survived in the region J ≥ 50 A/cm2. Representativeintensity data obtained from three different chips are shown inFig. 4A for emission band B. The value of I(σ−) tends to saturate

Fig. 3. J−V curves of chip A in (A) linear scale and(B) semilogarithmic scale, together with (D) a plot CPvalue (PCP) against J. Somewhat larger bias voltagecompared with the control sample suggests theformation of interface resistance in the Au/Ti/Fe/γ-AlOx electrode. (C) Helicity-specific EL spectraobtained at RT from a cleaved side wall of the chipat three different current densities: J = 22 (green),55 (blue), and 110 (red) A/cm2, respectively. Solid anddotted lines show right-handed (σ+) and left-handed(σ−) components, respectively. Three vertical arrowsand dotted lines in A, B, and D represent the J valuesat which EL spectra are measured. The EL spectrameasured at J = 110 A/cm2 are replotted in semi-logarithmic scale in C (Inset).

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in the region J ∼ 40–80 A/cm2 beyond which it decreases,whereas the σ+ (majority) component I(σ+) increases nearlylinearly throughout the entire J region within the limit of thepresent work. Consequently, nearly pure CP is realized when Jreaches ∼100 A/cm2 or higher (Fig. 4B). Moderate narrowing ofthe EL band B has also been observed for these chips in theregion J > 50 A/cm2. (Fig. 4C).At the point of writing this report, EL intensity and CP value

both tend to degrade in around half a day of operation when J >100 A/cm2. The origin of the observed degradation has not beenelucidated by metallurgical evaluations with cross-sectionaltransmission electron microscope and secondary ion mass spec-trometry. We suppose that it is originated from some slow deg-radation of a crystalline γ-AlOx tunnel barrier, which should beimproved in future study. At least it is clear that without theAlOx barrier, chips only show poor EL performance as disclosedin EL Spectra from the Spin-LED Chip Without a Crystalline AlOxTunnel Barrier and Fig. S2.Fig. 4D and E shows the horizontal line profiles of the PCP

value and integrated EL intensity; the point y = 0 represents theposition of the cleaved side wall right under the Fe strip center.Measurements were carried out by laterally moving the 0.1-mm-wide, 10-mm-long, vertical optical slit that was placed 0.1 mmaway from the cleaved side wall. At J = 75 A/cm2, the PCP value,which maximizes at y = 0 with PCP = 0.18, drops abruptly andbecomes nearly zero at y ∼ 0.1 mm, whereas the EL intensityprofile extends out to larger y values. At J = 125 A/cm2, the PCPvalue increases significantly at y = 0 and decreases to nearly zeroat y ∼ 0.2 mm. Interestingly, the intensity profile is narrowedwithin y ∼ 0.2 mm, suggesting the concentration of EL emissionenergy in the region under the stripe electrode. We infer that thisphenomenon is relevant to the observed significant increase inthe PCP value.Suppression of the spin-injection–impeding effect (45) by a

large forward bias will enhance the electron spin polarization in

the active region up to Pe ∼ 0.24, and will result in the EL withthe PCP values ranging between 0.12 and 0.24. The lower andupper bounds of PCP correspond to the case in which the spin-dependent optical selection rule (46) (Fig. 5B) fully applies andrelaxes, respectively. On the other hand, as discussed previously,the density of nonequilibrium electrons in the active layer isestimated to be in the range of middle 1017 cm−3 at J ∼ 100 A/cm2,which is ∼1/5 of the threshold value for the CP lasing in anundoped GaAs layer (24). The threshold value in our chips ispresumably even larger, referring to the fact that the lifetime ofradiative recombination in p-GaAs (5 × 10−9 s at p ∼ 1018 cm−3) istwo orders of magnitude shorter than that in lightly-doped p-GaAs(>1 × 10−6 s with P < 5 × 1015 cm−3) (37). These comparisons leadus to the inference that the mechanism of the observed suppres-sion of the minority CP EL component at J > 100 A/cm2 does notoriginate from effects associated with the spin-injection–impedingeffect (45) or the spin-polarized semiconductor lasers (47, 48); inthe latter, nearly pure CP appears when majority spins enter theregime of stimulated emission whereas minority spins stay in thespontaneous emission regime.In our experiments, the variation in the EL intensity as a

function of J is higher for emission band B than that for emissionband A. We examine this fact in detail on the basis of micro-scopic process as depicted in Fig. 5A. A part of electrons injectedin the conduction band of the active layer undergoes the tran-sitions to the empty states in the valence band that includes theband-tail states, as represented by process A in the figure. Here,the Fermi level is positioned around the valence band edge.During this process, electrons in the valence band are expelled fromthe active layer as the consequence of ultrafast dielectric relaxation;its time constant is estimated to be τD = e·e0/σ ∼ 60 fs in which e0 isthe dielectric constant of the vacuum, e the relative dielectricconstant (e = 13.2), and σ the conductivity (σ ∼20 Ω−1·cm−1). Thisprocess, as denoted by process B in the figure, is alternativelycalled hole injection in the valence band. In parallel with thisprocess, electrons that are captured by the tail states relax to-ward the empty states near the Fermi level EF with the timeconstant in the range of subpicoseconds (49) (process C). In themeantime, a part of photons generated by process A are reab-sorbed by the electrons near EF (process D) within the residualtime of photons that is ∼10 ps for the lateral optical pass lengthof 1 mm. The rate of reabsorption is higher for photons whoseenergy is comparable to or higher than the intrinsic band gap(Eg = 1.44 eV, α > 5 × 103 cm−1) than those of hν < 1.44 eV (α <102 cm−1), which results in the J-dependent IB/IA. The dynamicsof the entire EL process is completed by adding the nonradiativerecombination process E. Rate equations are shown in RateEquations on the Basis of Fig. 5A and Fig. S3.We next introduce spins on the carrier dynamics. We then

notice that spins expelled by the dielectric relaxation from theactive layer still preserve their own spin polarization, because thelifetime of spins in the valence band is longer, being τS ∼ 900 fs(50, 51), than the τD value. This naturally means that carriersnear the Fermi level are spin-polarized. The magnitude of spinpolarization (Ph) is negligibly small when the number of non-equilibrium electrons (N*) reaching the empty states in the va-lence band via process A is small, as is the case for the low-Jregion. Ph becomes noticeably high when N* reaches the value thatis comparable to the background hole concentration, 1018 cm−3 inthe present case, as is the case for the high-J region. Note that thepolarization of Ph is opposite from that of Pe, because the sameamount of spins entering the conduction band is expelled from thevalence band. Turning to reabsorption, the σ− EL component thatconsists of minority-spin photons is more absorbed than the σ+ ELcomponent, reflecting the reversed spin population of carriers in thevalence band. Thus, the observed reduction in the σ− EL compo-nent at high-J region can be explained qualitatively in terms of thedielectric relaxation followed by the reabsorption.For an undoped active layer, the contribution of nonradiative

recombination is presumably still significant at J ∼100 A/cm2

because the lifetime of radiative recombination is around two

Fig. 4. Plots of (A) the integrated intensities of right-handed (σ+, closedsymbols) and left-handed (σ−, open symbols) components, (B) PCP values,(C) spectral width of the band B represented by full width at half maximumvalues, as a function of current density J for three different spin-LED chips,and horizontal line profiles of the PCP value and integrated EL intensityobtained from chip C at J = 75 (D) and 125 A/cm2 (E). The point y = 0 re-presents the position of a cleaved side right under the center of the Fe stripelectrode (Inset). Measurements were carried out by laterally moving the0.1-mm-wide, 10-mm-long, vertical optical slit that was placed 0.1 mm awayfrom the cleaved edge.

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orders of magnitude longer (37). We suppose that the effectassociated with the reversed spin population is not observedunless J reaches near the stimulated emission condition. It islikely that p-type doping in the active layer has led us to the ob-servation of pure CP at J ∼ 100 A/cm2. This inference also raisesinteresting questions concerning the blueprint of spin-LED; e.g.,how much is the appropriate p-type doping level and how much isthe lowest bound of the Pe value for the electrons injected in theactive layer? To find quantitative answers, it is desired to carry outthe model calculation based on couples of spin-polarized rateequations that describe dynamics of electrons in the conduction

band, in the tail states, and around the Fermi level, as well asphotons, incorporating the spin-dependent optical selection rule,in addition to further experimental investigations. A starting pointis suggested in Fig. S3.Let us next infer the possible nonlinear effects in the optical

process. Referring to the optical selection rule mentioned pre-viously (Fig. 5B), radiative recombination results in the ellipticpolarization (EP), the superposition of σ+ and σ− EL compo-nents. Because GaAs does not show birefringence under thenormal condition, the relative phase difference between the twoorthogonal light waves having the electric fields Ey and Ez in therectangular coordinate system shown in Fig. 1,

�Ey

Ez

�=�A1expfiðkx−ωt+ϕ1ÞgA2expfiðkx−ωt+ϕ2Þg

�,

is unchanged during the light propagation. If one assumes the

inducement of birefringence�

1 +δ− δ 1

�due to spin-polarized

carriers, the relative phase delay occurs between the Ey and Ezwaves when EL light propagates along the GaAs active layerunder the Fe slab electrode in which spin-polarized carriers withthe density of middle 1017−1018 cm−3 are injected. Consequently,the ratio between σ+ and σ− components of the light comingoutside the active layer along the x axis differs from the originalEP condition. Referring to the experimental results shown inFigs. 3 and 4, the relative phase delay between Ey and Ezis supposed to be spin dependent: the ± sign of δ in the off-diagonal term in the dielectric tensor is swapped by alteringthe spin axis, whereas its magnitude varies with injected spindensity. This scenario explains well the almost identical EL spec-tra of the two different helicity (Fig. 3C, Inset and Fig. S1). Onthe other hand, this scenario requires the derandomization ofoptical path length from the point of electron–hole recombina-tion radiation to the cleaved side wall. An increase in the effec-tive optical path length would occur when stimulated emissiontakes place in part in the active region. The observed moderatespectral and spatial narrowing at J > 100 A/cm2 (Fig. 4 C–E) maybe a faint indication of the precursory stage to the stimulatedemission. Introduction of biaxial tensile strain in the active layerwill shed light on this issue because the strain is expected tocontrol the valence-band mixing and result in the reduction inthe threshold current for the spin-polarized stimulated emission(52). The difference in refractive indices is estimated to be Δn =2 × 10−4 assuming the phase delay of π/2 in the optical path of1 mm, which is rather small in view of the birefringence materialsstandard (53).It is worth addressing another interesting view that is associ-

ated with the radiative recombination of the [11ð−Þ 0] and [001]electron spins with valence-band holes. Recall that the lightemission due to such recombination is detected as the linearlypolarized background of equal intensity in the present mea-surement setup (Fig. 2A). If some sort of nonlinear processconverts the linearly polarized background into CP emissionat moderately high J, the observed decrease (increase) inminority- (majority-) CP component may occur (Fig. 4A).This scenario would rather be likely to take place in the bulk-type active medium than in the QW-type medium in whichspin axis is quantized in one particular direction (the GaAs[001] axis).Although the present work serves as a solid footstep toward

the development of semiconductor-based, spin-photonic devices,it is just the beginning of our journey. Investigation at higher Jregion is extremely interesting not only in view of elucidating themechanism of nonlinear effects, but also to clarify whetherstimulated emission with CP is possible in the lateral-waveguide–type geometry in which stimulated light emission with linear po-larization dominates. Exploitation toward both shorter and longerwavelengths with different materials combinations, together with

Fig. 5. (A) A schematic illustration of transport–reabsorption scenario. Thelabels “Rad. rec.” and “Non-rad. rec.” represent radiative and nonradiativerecombination processes, respectively. A red curve denotes the Fermi dis-tribution function around the Fermi level (EF) that is shown by a dashed–dotted line. Hatched areas depict the states occupied by electrons (red) andholes (blue) above and below the Fermi level, respectively. Inside the areasurrounded by a dotted line represents an active layer, whereas the outsidedenotes cladding layers (upper and lower spaces) and a free space (right andleft spaces). (B) A schematic illustration that shows nonlinear effects in theoptical process; electrical spin injection from an Fe spin injector into a 3DGaAs active layer through a 3D, n-AlGaAs clad layer (Upper), radiative re-combination of [110], [11ð−Þ 0] and [001] spins in conduction band (C.B.) withdegenerated heavy- and light-holes in valence band (V.B.) (Lower Left), andconversion from elliptic polarization into pure CP through hypotheticalnonlinear optical process (Lower Right).

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electrical CP switching (20, 54, 55) and high-frequency modulation(56, 57), is another important direction in view of investigating theusefulness of spin-LEDs in the existing and new applications, e.g.,the chiral resolution in synthetic chemistry (58), diagnosis of can-cerous tissues (59), circularly polarized ellipsometry (60), the opti-cally enhanced nuclei imaging (61), LP-polarization-insensitive 3Ddisplay (62), quantum eraser technique (63), optical secure com-munications (64), and beyond.

ACKNOWLEDGMENTS. The authors are grateful to M. Aoyama for his technicalassistance in various experiments, and F. Minami, F. Koyama, T. Miyamoto, andS. Arai for various discussions concerning light-emitting devices. We acknowl-edge the Hellium Recovery Facility in the campus for the steady supply of liquidHe, and financial support from Advanced Photon Science Alliance Project fromthe Ministry of Education, Culture, Sports, Science, and Technology, Grant-in-Aidfor Scientific Research (22226002) from the Japan Society for the Promotion ofScience, and Strategic Information and Communication R&D PromotionProgramme (SCOPE, 162103004) from the Ministry of Internal Affairs andCommunications.

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