Ultrafast optical switching by instantaneous laser-induced grating formation and self-diffraction in barium fluoride

Abstract

A transient refractive index grating is formed in barium fluoride crystals under irradiation with femtosecond laser pulses from two non-collinear beams, at an intensity below the threshold for white light continuum generation. At low intensities, energy is coupled from one beam to the other, at high intensities a typical self-diffraction pattern is observed with an diffraction efficiency better than 10%. The response time for all effects is limited only by the temporal pulse shape, opening the way to an application in ultrafast optical switching. PACS: 42.65.Hw; 42.65.Sf; 42.79.Ta One of the prerequisites for optical data processing and communication is the necessity to provide appropriate switches for the respective node systems [1]. One of the most commonly used techniques in this field is the application of transient volume holograms written onto the refractive index of photorefractive crystals by two interfering laser beams [2, 3]. Such holograms lead, then, to a transient diffraction of the writing or further beams, which is the basis of optical switching and multiplexing. The great advantage of photorefractive materials is that only weak laser intensities are required because of the electro-optic effect (Pockels effect), which is based on second-order optical nonlinearity [4]. Since it relies on the displacement of charge carriers due to the intensity variation of the induced interference pattern, the reaction time is, however, limited by the carrier mobility. In transparent insulators this time is usually of the order of 10−8 to 10−6 s, which limits the switching speed [2]. Considering future optical fiber transmission speeds of 10 to 100 Gbit/s, appropriate switching times of the order of picoseconds and below appear highly desirable [1]. More recent developments [5], therefore, tend to exploit higher-order nonlinearities, such as the third-order optical Kerr effect [4], which involves transient nonlinear polarization only. Today, however, near-resonant or ∗ Present address: FIMEA GmbH, Agastr. 24, D-12489 Berlin, Germany relatively long optical waveguides are employed, yielding picosecond switching times [5] limited by the lifetime of the nearby resonances. The switching mechanism in this case is an intensity-dependent polarization rotation proportional to the length of the medium. This technique yields a switching contrast of the order of 104, limited by the extinction of the subsequent polarizer. The same switching mechanism but employing a different nonlinear optical effect, namely second-harmonic generation plus down conversion, provides the fastest switching so far reported [6]. The switching occurs instantaneously on the time scale of femtosecond laser pulses, as in the technique of frequency-resolved optical gating (FROG) for the determination of pulse characteristics [7, 8]. In this article we report on the first observation, to our knowledge, of very efficient self-diffraction from a transient grating in barium fluoride with a femtosecond time constant. The grating results from an instantaneous index variation, due to the optical Kerr effect [4], along the interference pattern between two crossed laser beams. In this case, this third-order nonlinear optical (i.e. χ(3)) process is far from any resonances, so the longitudinal relaxation time T1, i.e. the lifetime of electronic energy states, does not influence the reaction time. Consequently, the response is limited by only the transversal dephasing time T2, which is of the order of 10−15 s [9]. With zero background and an efficiency η = Ed1/E0 of better than 10%, where Ed1 is the energy of the first-order diffracted beam and E0 that of zero-order transmission, instantaneous self-diffraction provides a much better contrast in the signal than the methods described above. Further, the grating should enable not only switching but also, in principle, wavelength demultiplexing with the same reaction time. 1 Experimental set-up In our experiments, we used 3-mm-thick slides of barium fluoride at room temperature, but similar effects were observed also for calcium fluoride and crown glass (BK7).