Transient Light‐Induced Gratings Due to Excitons in CdSe

Abstract

Transient light-induced grat,ings due to free carriers and excitons are created in CdSe in a medium density range of lOI5 to 1017 by two coherent intense (ns) IR-pump beams. Detailed investigations of probe-beam diffraction due to thin excitonic gratings in the spectral region just below the A,=l-exciton line a t 85 K and its dependence on sample temperature and on the excitation level are presented. First-order diffraction efficiences up to 5% and a nonlinear dependence of diffraction on the excitation level can be observed. For the first time numerical calculations of probe-beam diffraction due to excitonic phase and amplitude gratings are presented. The comparison with the experiment shows, that the damping of a dense exciton gas increases proportional to the excitation intensity in the case of two-photon interband excitation, and that the lightinduced absorptive changes Aha due to dense excitone dominate over the dispersive changes An in the spectral region under consideration. Mittels zweier, kohirenter und intensiver (ns) IR-Pumpstrahlen werden lichtinduzierte Gitter in CdSe, hervorgerufen durch freie Triiger und Exzitonen in einem mittleren Dichtebereich vonlox5 bis IOl7 cm-s, erzeugt. Vorgestellt werden detaillierte Untersuchungen der Probenstrahlbeugung an dunnen exzitonischen Gittern bei 85 K sowie deren Temperatur-und Intensitatsabhiingigkeiten im Spektralbereich nahe unterhalb der A, = 1-Exzitonenlinie. Beugungseffizienzen in der 1. Ordnung bis zu 5% und eine nichtlineare Intensitatsabhiingigkeit der Beugung werden beobachtet. Erstmalig werden numerische Berechnungen der Lichtbeugung an exzitonischen Phasen-und Amplitudengittern vorgestellt. Der Vergleich mit dem Experiment zeigt ein proportionales Wachsen der exzitonischen Dampfung mit der Anregungsintensitat im Falle der ZweiPhotonen-Interbandanregung sowie das Dominieren der durch das erzeugte dichte Exzitonengas hervorgerufenen licht-induzierten absorptiven hderungen Acu iiber die dispersiven hderungen An im betrachteten Spektralbereich.

Many‐Body Theory for the Dense Exciton Gas of Direct Semiconductors II. Calculation of Exciton Level Shift and Damping in Dependence on Exciton Density

“…Recently, diffraction measurements on volume gratings due to excitons were carried out [20]. The results could be well explained using a density-dependent exciton damping.…”

Transient Gratings in Metrology of Semiconductor Parameters and Optoelectronic Devices

“…Here two sets of experiments were performed. Gratings were created and probed either by using a ruby laser (hw = 1.786 eV), which resonantly excited the A-exciton at 86 K [12] (hw&-,, = = 1.813 eV) or by homogeneous excitation of the crystal by two-photon absorption of an Nd:YAG laser (hw = 1.17 eV) and probing by a dye laser at hw = 1.797 eV [13]. In both cases the dependence q,(E,) showed a tendency to saturation and even a decrease of ql at an exciton density (2 to 6) x 1016 cme3.…”

Laser-Induced Probe-Beam Defocusing at the Band Edge of CdSxSe1−x Mixed Crystal at Room Temperature

“…To do this the IR-pump beam was split into two parts of equal intensities I,J2 intersecting in the sample a t an angle of 3" t o create in the sample a free-carrier grating with a period of = 40 pin via two-photon interband absorption. A detailed description of the experimental arrangement used is given in [5]. For the investigation of self-diffraction of the IR-beams the probe heam was switched off and the 3lt-filter behind the sample >?as taken out.…”

“…I n contrast t o the behaviour a t low temperatures [6] a small excitationinduced shift of the band edge to higher energiesfirstly found in [3] -is observed. This behaviour corresponds t o a decrease of the absorption coefficient a t a fixed wavelength a t the band edge and is in agreement with the increase of the probe-beam transmission T,(0)/To(O) = 1.3 observed in the defocnsing experiment a t I,, = 653 nm.By the method of IR-light-induced gratings (see experimental set-up in[5]) the spectral dependence of the first-order probe-beam diffraction intensity I,@) was measured. The efficiency q1 = Il(I)/!P0(A) is shown in curve c inFig.…”