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arXiv:cond-mat/0410221 · Physical Review B

Inducing electron spin coherence in GaAs quantum well waveguides: Spin coherence without spin precession

Susanta Sarkar, Phedon Palinginis, C. C. Kuo, Connie J. Chang-Hasnain, N. H. Kwong, Rudolf H. Binder, Hailin Wang

Published Jul 20, 2005• 21 citations
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Abstract

We demonstrate the generation and detection of electron spin coherence in a quantum well waveguide without either an external or internal dc magnetic field. In the absence of spin precession, the induced spin coherence is detected through effects of quantum interference in the spectral domain coherent nonlinear optical response. We interpret the experimental results qualitatively using a few-level model with optical selection rules as its basic ingredient.

DOI
10.1103/physrevb.72.035343

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Wepresent atheory foradifferential transmission experiment that generated electron spin coherence inquantum wellwaveguides without DCmagnetic fields. We showhowoptical selection rules explain thespin coherence's creation anddetection. 02004Optical Society ofAmerica OCIScodes: (190.5970) Semiconductor nonlinear optics including MQW;(300.6470) Spectroscopy, semiconductors Thedemonstration oflong-lived electron spin coherence atroomtemperature insemiconductors (1)hasenhanced theprospects ofusing coherent spin manipulation inoptoelectronics andhence hasdrawnmuchrecent research effort tothis area. Inmostexperiments, theelectron spin coherence wasdemonstrated bydetecting thespin's precession ineither anexternally applied DCmagnetic field oraninternal magnetic field induced bymagnetic ion (Mn)doping orstrain innonmagnetic materials (2). Recently theexperimental group inthis collaboration (Oregon) reported theobservation ofelectron spin coherence inGaAsquantum wells without thehelp ofmagnetic fields, external orinternal (3). Wepresent here atheoretical analysis ofthephysical processes contributing tothecreation anddetection ofthis spin coherence. A simple model, based mainly onoptical selection rules, gives aresonance structure atthespectral position ofanexperimentally observed peak inthedifferential transmission (DT)spectrum. Itsupports theinterpretation oftheobserved DTpeak asbeing duetoelectron spin coherence intheabsence ofany DCmagnetic field. Wealso calculate theDTinthepresence ofanexternal DCmagnetic field. Inthis case, wefind theconventional Zeemansplitting (inagreement withtheexperiment). Thisrules outtheinterpretation ofthe observed peak interms ofpopulation pulsations (4). Ourexperiment useslight traveling along thequantum well's plane (waveguide geometry, seeFig. 1)toexcite a light-hole exciton theelectron ofwhich isinasuperposition ofbothspin states. Thesimplest version ofourtheory is afour-level model: twoconduction bandstates andtwolight-hole valence bandstates. Fig. 2shows theusual level schematic ofaGaAsquantum well. An electron inalight-hole orbital, e.g. jz=-1/2,canbeexcited byC+ polarized light tooneconduction bandspin orbital (sz=1/2)orbyz-polarized light toanother conduction band spin orbital (s5=-1/2). Underexcitation bylight ofbothpolarizations anelectron canbeexcited into the conduction bandinasuperposition ofthetwospin orbitals. Thisspin coherence cannotbecreated intheusual, normal incidence, geometry where thelight lacks thez-polarized component. Itcanbecreated inthewaveguide geometry adopted inourexperiment, where thelight's TM component isz-polarized andtheTEcomponent contains both circular polarizations. TM

•Jan 1, 2005•0 citations
We demonstrate the generation and detection of electron spin coherence in a quantum well waveguide without either an external or internal dc magnetic field. In the absence of spin precession, the induced spin coherence is detected through effects of quantum interference in the spectral domain coherent nonlinear optical response. We interpret the experimental results qualitatively using a few-level model with optical selection rules as its basic ingredient.
Electron spin coherence isinduced vialight-hole transitions inaquantum well waveguide without aDC magnetic field. Thespincoherence isdetected through effects ofquantum interference inspectral domain nonlinear optical response. 02005 Optical Society ofAmerica OCIScodes: (190.5970) Semiconductor nonlinear optics including MQW;(300.6470) Spectroscopy, semiconductors Coherent spin manipulation plays animportant role inspin-based electronics andphotonics andinquantum information technologies (1). Inmostexperiments, coherent spin manipulation wascarried outinthepresence ofan external DC magnetic field that induces aZeemansplitting between theconduction bandelectron spin states. Coherent superposition ofthese spin states leads toaLarmor spin precession, which canbedetected with transient optical techniques. Coherent spin manipulation without external magnetic fields innon-magnetic semiconductors hasalso beendemonstrated inarecent study (2), inwhichaneffective internal magnetic field induced bystrain results inaspin splitting intheconduction band. Here, wereport experimental demonstration ofinducing anddetecting electron spincoherence inaGaAs quantum well(QW)without theuseofeither anexternal oreffective internal magnetic field. We havetaken advantage ofthespin-orbit coupling inthevalence bandandhaveusedlight-hole (LH)transitions inwaveguide geometry toinduce electron spincoherence. Intheabsence ofspin precession, theinduced spincoherence is detected through quantum interference inthespectral domain, instead oftimedomain, coherent nonlinear optical response. (a)
We report the experimental demonstration of coherent Raman resonance from electron spin coherence in GaAs\mathrm{GaAs}GaAs quantum wells. Coherent Raman resonance, which is a precursor for electromagnetically induced transparency, appears in the differential transmission response from localized excitons in an external magnetic field applied in Voigt geometry. The spectral domain spin study reveals that the optical processes involved differ from those in the transient domain, leading to distinct polarization and intensity dependences.
We report experimental studies of absorption quantum beats induced by electron spin coherence in GaAs quantum wells. Absorption quantum beats occur for strongly localized excitons, but nearly vanish for mobile excitons in the third order nonlinear optical response. Pronounced quantum beats for mobile excitons emerge in an unusual fifth order process. These results, along with a qualitative analysis based on the use of N-exciton eigenstates, elucidate how the manifestation of electron spin coherence in the excitonic nonlinear optical response can differ fundamentally from that in an atomic system.
Wepresent atheory foradifferential transmission experiment that generated electron spin coherence inquantum wellwaveguides without DCmagnetic fields. We showhowoptical selection rules explain thespin coherence's creation anddetection. 02004Optical Society ofAmerica OCIScodes: (190.5970) Semiconductor nonlinear optics including MQW;(300.6470) Spectroscopy, semiconductors Thedemonstration oflong-lived electron spin coherence atroomtemperature insemiconductors (1)hasenhanced theprospects ofusing coherent spin manipulation inoptoelectronics andhence hasdrawnmuchrecent research effort tothis area. Inmostexperiments, theelectron spin coherence wasdemonstrated bydetecting thespin's precession ineither anexternally applied DCmagnetic field oraninternal magnetic field induced bymagnetic ion (Mn)doping orstrain innonmagnetic materials (2). Recently theexperimental group inthis collaboration (Oregon) reported theobservation ofelectron spin coherence inGaAsquantum wells without thehelp ofmagnetic fields, external orinternal (3). Wepresent here atheoretical analysis ofthephysical processes contributing tothecreation anddetection ofthis spin coherence. A simple model, based mainly onoptical selection rules, gives aresonance structure atthespectral position ofanexperimentally observed peak inthedifferential transmission (DT)spectrum. Itsupports theinterpretation oftheobserved DTpeak asbeing duetoelectron spin coherence intheabsence ofany DCmagnetic field. Wealso calculate theDTinthepresence ofanexternal DCmagnetic field. Inthis case, wefind theconventional Zeemansplitting (inagreement withtheexperiment). Thisrules outtheinterpretation ofthe observed peak interms ofpopulation pulsations (4). Ourexperiment useslight traveling along thequantum well's plane (waveguide geometry, seeFig. 1)toexcite a light-hole exciton theelectron ofwhich isinasuperposition ofbothspin states. Thesimplest version ofourtheory is afour-level model: twoconduction bandstates andtwolight-hole valence bandstates. Fig. 2shows theusual level schematic ofaGaAsquantum well. An electron inalight-hole orbital, e.g. jz=-1/2,canbeexcited byC+ polarized light tooneconduction bandspin orbital (sz=1/2)orbyz-polarized light toanother conduction band spin orbital (s5=-1/2). Underexcitation bylight ofbothpolarizations anelectron canbeexcited into the conduction bandinasuperposition ofthetwospin orbitals. Thisspin coherence cannotbecreated intheusual, normal incidence, geometry where thelight lacks thez-polarized component. Itcanbecreated inthewaveguide geometry adopted inourexperiment, where thelight's TM component isz-polarized andtheTEcomponent contains both circular polarizations. TM