All-optical femtosecond switch using two-photon absorption

Over the last few decades, there has been a growing interest in all-optical approaches to computing and information processing 1 . All-optical approaches do not suffer from the intrinsic slow response of electronic components and have the potential for signal routing, processing, multiplexing, and demultiplexing with speeds exceeding terahertz THz rates. As is the case for electronic logic gates, all-optical information processing relies on nonlinear computing elements. The most commonly investigated nonlinear optical element is an all-optical switch, where a laser beam turns another laser beam on and off. Over the last few decades several approaches for all-optical switches have been suggested and demonstrated. These include optical Kerr shutters in nonlinear crystals 2–5 , approaches that utilize semiconductor waveguides and quantum well structures 6–12 , interferometric approaches 13 , and approaches that utilize photonic crystals and organic materials 14–17 . These approaches have successfully demonstrated picosecond-timescale all-optical switches. However, it still remains a challenge to construct all-optical switches with switching time in the femtosecond domain. In this paper, as an alternative to these solid state approaches, we suggest the use of an atomic vapor gas and propose a femtosecond-time-scale all-optical switch. Our proposal utilizes two-photon absorption in a three level atomic system. Noting Fig. 1, the electronic ground state of the atom, state a , is coupled to state c with two laser beams, Es and Ep. The femtosecond switching beam, Es, is strong, and its frequency is selected such that the detuning from the intermediate state, b, is large when compared with the switching bandwidth. A weak probe beam, Ep, then couples the intermediate state to state c . Due to the large detuning, the excitation to state b is adiabatic and this state acquires significant population only for the duration of the switching laser beam. The nonadiabatic excitation of the atom to state c then causes loss on the probe beam. Since this excitation only happens when the switching laser beam has high intensity, the probe beam, Ep, is attenuated only for the duration of the switching laser beam, Es. The scheme of Fig. 1 is similar to two-photon absorption experiments performed in nonlinear crystals 18 . However as will be demonstrated later, our scheme requires about two orders of magnitude lower switching intensities and also allows switching speeds well into the femtosecond domain. We proceed with the analysis of our scheme. Noting Fig. 1, we consider the interaction of two laser beams with a three level atomic system. We expand the total wave function for the atomic system as = ca t exp − j at a + cb t exp − j bt b + cc t exp − j ct c . 1