Pulse Shaping and Optical Bistability in CuCl Due to Nonlinear Dispersion and Absorption

Abstract

CuCl is a direct gap semiconductor whose optical properties can be well modelised by a three-level system: the crystal ground state |1>, the exciton |2> with energy E_ex and the biexciton state |3> (E_Bi). One-photon transitions between |1> and |2> or |2> and |3> are allowed as well as the two-photon transition between |1> and |3> [1]. The transition between |2> and |3> becomes possible when the exciton state is populated and the oscillator strength for this transition is a function of the exciton density [2,3,4]. This two-step process is called here induced absorption. Since the number of excitons depends on the intensity of the exciting laser beam and on its wavelength, the dielectric function of the system shows a renormalization at this particular energy (E_Bi−E_ex). In addition, the dielectric function is also strongly nonlinearly modified near the two-photon resonance situated at the photon energy E_Bi/2 = 3.186 eV [5,6,7]. It has been reported earlier that the resulting nonlinear dispersion gives rise to optical bistability if samples of CuCl are placed in Fabry-Pérot resonators [8,9]. Simultaneously, the absorption is strongly nonlinear too, and is in competition with the preceding dispersive effects [10]. We have shown that this nonlinear absorption shows a memory effect which is characterized by an hysteresis if the response of the medium is analysed under pulsed excitation [11]. The scope of this publication is to give a systematic study of this memory effect and to apply it to pulse shaping and to optical bistability.