Memory effect in the excitonic transmission of CdS

Abstract

In recent years the excitonic absorption of CdS at high excitation levels has been studied intensively /l/. Significant non-linear variations of the absorption-and refractive index were found in the spectral range of the AnZl exciton. In spite of differences existing in the explanation of the microscopic origins, it seems to be interesting to take advantage of these variations for the realization of non-linear optical phenomena. A prominent example is the optical bistability demonstrated recently for the exciton absorption of GaAs /2/. In the present note we report on preliminary experimental results on this line for CdS.Vapour-phase-grown CdS platelets (thicknesses some pm) were investigated by three different experimental methods:(i) Probe beam transmission with additional pumping. Two pulsed dye lasers (pulse duration about 4 ns) were used as sources for a broad-band probing beam of low intensity and a high-power pumping beam with frequency at the Bn,l exciton, respectively. Spectra were detected by an OMA system with a spectral resolution of 0.2 meV,(ii) Transmission of a single tunable narrow-band (spectral halfwidth 5 0.2 meV) dye laser beam yielding excitation intensities up to 1 MW/cm at a spot diameter of 0 . 3 mm. The transmitted pulses were detected by a multi-2 plier and measured time-integrated by a boxcar integrator.(iii) Time-resolved detection of the transmission described in (ii) by a fast photodiode and boxcar integrator (time resolution 600 ps).The crystals were cooled only to 77 K. Thus peculiaritiesof the exciton absorption (spatial dispersion, polariton effect, etc. ) appearing at lower temperatures were avoided.Typical spectra of the "pumped" transmission (i) below the A resonance a r e shown in Fig. 1. The overall feature i s an enlargement of the absorption.

On the Multispike Structure of a Transmitted Pulse in Monocrystalline Films with Absorption Bistability

“…To verify the obtained analytical expressions and take into account the real dependence a(N), system (l), (2) was integrated numerically. The absorption coefficient CI in these calculations was chosen in a form close to the solid curve in Fig.…”

“…For this purpose, we analyse the steady excitation density distribution, as done particularly in [lo]. Then the light intensity at the kink position (obviously equal to I , exp (-~~4 ) in the low-diffusion limit is described by the relation It is seen from this relation and the condition Neff = I , following from (2) that N,,, = 2/(aH + 1) everywhere, except for the narrow wall regions with thickness 1, where Neff z 1 and l/aH on the front and rear walls, respectively. In the limit 1, + 0 the switching between the equilibrium condition Neff = 1 corresponding to the kink generation and Neff = 2/(uH + 1) corresponding to its motion deep into the sample occurs in an infinitely narrow region adjoining the front wall.…”

“…I n ( l ) , (2). a system of units is used in which the recombination time T and the absorption length I , = 2;' (q = r(Ri < Nc).…”

On kink dynamics in media with increasing absorption optical bistability

“…An independent communication on its first observation was made in [2], the modern state of this problem was reviewed in [3]. The ever increasing interest in bistable materials is inspired by the hopes of using them for the creation of new optical computer elements.…”

“…Now we consider different limiting cases of the solution of the system (I), (2) which are realized depending on the correlation of the three characteristic lengths : the crystal thickness L, the absorption length I, = a-l, and the carrier diffusion length which has three roots in the intensity range I+ < I < I, (Fig. 2).…”