Time-Resolved Speckle Analysis: A New Approach to Coherence and Dephasing of Optical Excitations in Solids

Langbein, Wolfgang Werner; Hvam, Jørn Märcher; Zimmermann, R.

doi:10.1103/physrevlett.82.1040

Cited by 130 publications

(125 citation statements)

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“…." In this Comment we show that the SE calculated by Kira et al is incoherent and nonlinear, and therefore inadequate for SE experiments performed under low-excitation conditions [2][3][4][5][6][7][8]. Such experiments are in agreement with an explanation in terms of disorder-induced resonant Rayleigh scattering (RRS) and have indeed given clear evidence for the following: (i) The temporal coherence of the SE at early times.…”

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confidence: 71%

“…The coherent part of SE produces interferences with a replica of the exciting laser pulse [5,8] and present speckles [7]; because the quantum average of the electric field operator in the SE direction is nonzero, ͗E q ͘ ϵ ͗b q 1 b y q ͘ fi 0. Experiments show that this average electric field makes up 25% [5] or up to 50% [8] of the total intensity of the SE.…”

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Comment on “Quantum Theory of Secondary Emission in Optically Excited Semiconductor Quantum Wells”

2000

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In a recent Letter, Kira et al. [1] propose a fully quantum mechanical theory for the secondary emission (SE) of a quantum well (QW), assuming that the QW is free of disorder, and that SE originates from the interaction between excitons in the system. The authors ask if "disorder is necessary to explain coherent signatures in the SE." They claim that their "microscopic theory reproduces many of the experimentally observed effects, showing that the SE exhibits intrinsic coherent characteristics . . . ." In this Comment we show that the SE calculated by Kira et al. is incoherent and nonlinear, and therefore inadequate for SE experiments performed under low-excitation conditions [2][3][4][5][6][7][8]. Such experiments are in agreement with an explanation in terms of disorder-induced resonant Rayleigh scattering (RRS) and have indeed given clear evidence for the following: (i) The temporal coherence of the SE at early times. (ii) A fully linear relation between emission intensity and excitation density over 3 orders of magnitude. (iii) The temporal profile depends only on statistical properties of the disordered potential.On the other hand, the new theoretical formalism by Kira et al. combining semiconductor Bloch equations and field quantization is of largest importance for the dynamics of temporally incoherent resonant QW luminescence [3,9].The term "temporal coherence" of SE has received very lately a clear definition that is experimentally verifiable. The coherent part of SE produces interferences with a replica of the exciting laser pulse [5,8] and present speckles [7]; because the quantum average of the electric field operator in the SE direction is nonzero, ͗E q ͘ ϵ ͗b q 1 b y q ͘ fi 0. Experiments show that this average electric field makes up 25% [5] or up to 50% [8] of the total intensity of the SE. Fluctuations of the electric field operator are much too weak to explain these experimental findings. The calculations by Kira et al. [1] use explicitly the fact that only in the specular directions is ͗E q ͘ fi 0. For the scattered directions only the correlation functions ͗b y q z ,q k h 2k e k1q k ͘ are nonzero. The calculated SE is therefore mainly incoherent as it does not carry a phase that could produce strong enough interferences, which would be quantitatively in agreement with the experiments [5,8].Another important property of the coherent SE is the linear dependence on the excitation density, which has been explicitly reported [5,6,10]. The early SE calculated by Kira et al. shows a nonlinear dependence of the peak intensity and density-dependent rise time (see Fig. 1 in [1]).Nonlinearity is also explicit in their two pulse calculations in which "the two pulse excitation for F 0 leads to an enhancement of the emission intensity by more than an order of magnitude" in comparison to the single-pulse excitation. Experiments on SE excited by two pulses show that the enhancement factor is never larger than 4 [4,10], as predicted by linear theories [11].Finally, the physical origin of the "pulse replica" f...

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confidence: 99%

Comment on “Quantum Theory of Secondary Emission in Optically Excited Semiconductor Quantum Wells”

2000

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“…[4] to the interference between light waves scattered by different localized exciton states in a QW plane. Experiments on single QWs [3] do not show such features and are well explained using a correlation length shorter than the exciton radius. Reference [10] has demonstrated a possibility of the secondary emission (SE) from QWs due to the quantum fluctuations of the light field.…”

Section: (Received 25 October 1999)mentioning

confidence: 99%

“…The essential difference between the resonant Rayleigh scattering [1] (RRS) and the photoluminescence (PL) is that the optical coherence is kept in the former case and broken in the latter case, which is not easily distinguished experimentally. Recently, RRS by excitons in semiconductor quantum wells (QWs) has been reported [2,3]. The spectroscopic data have been interpreted in terms of scattering of excitons by a disorder potential in the plane of the QWs [4][5][6].…”

Section: (Received 25 October 1999)mentioning

confidence: 99%

“…The excited exciton density of around 10 8 cm 22 per well was close to the low density limit. Using the speckle analysis technique [3], we deduce from the temporally and directionally resolved emission intensity I͑t, q͒ the average emission intensity I͑t͒ and the average coherence c I coh ͞I, where the average is taken over the scattering directions q at fixed time, and I coh is the SE intensity which is coherent to the excitation.…”

Section: (Received 25 October 1999)mentioning

confidence: 99%

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