High speed logic gate using two-photon absorption in silicon waveguides

The switching speed of conventional silicon-based optical switching devices based on plasma dispersion effect is limited by the lifetime of free carriers which introduce either phase or absorption changes. Here we report an all-optical logic NOR gate which does not rely on free carriers but instead uses two-photon absorption. High speed operation was achieved using pump induced non-degenerate two-photon absorption inside the submicron size silicon wire waveguides. The device required low pulse energy (few pJ) for logic gate operation. 2006 Elsevier B.V. All rights reserved. All-optical digital signal processing may be needed in future high capacity optical networks to overcome the speed limitations of electronics. All-optical logic gates such as NOR gate will be needed to perform the optical digital signal processing to accommodate the massive amount of traffic in terabit optical networks [1]. In addition, NOR gate can be used in performance monitoring for error detection, address and header recognition [2], encryption and data encoding [3], etc. Optical logic gates have been demonstrated in nonlinear optical fibers [4], semiconductor optical amplifier (SOA) [5] and InGaAs/AlAsSb coupled double-quantum-well structures [6]. The latency and high optical power level for nonlinear operation make fiberbased devices unattractive for practical applications, while the long carrier lifetime in conventional SOAs may limit the speed unless some complicated differential switching scheme is employed. Submicron size silicon wire waveguides are possible because of the extremely high index contrast (n = 3.5 for 0030-4018/$ see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.03.031 * Corresponding author. Tel.: +81 423276125; fax: +81 423275328. E-mail address: [email protected] (T.K. Liang). silicon, and n = 1.45 for SiO2), which allows the dimension of waveguides to be much smaller than in conventional low index contrast silica waveguidess [7]. The strong optical confinement and small effective modal area (<0.1 lm) of such waveguides can produce high optical intensities even at input optical powers typically used in telecommunications. The high optical intensities and long interaction lengths in the waveguides allow nonlinear optical effects to be readily apparent. Apart from other nonlinear optical devices such as optical fibers and SOA, silicon wire waveguides have good potential for other nonlinear devices for ultrafast photonics signal processing [8]. We demonstrated previously an ultrafast optical switch (<3 ps) with pJ pump pulse energy in wire waveguides [9,10]. In this paper, we develop a high speed all-optical logic NOR gate based on the nonlinear transmission characteristics of two-photon absorption (TPA) in silicon. The direct use of TPA allows operation speeds which are not limited by the slow photo-generated carrier lifetime in the silicon wire waveguides. Since the sum of energies of two photons at 1.55 lm wavelength is greater than the indirect bandgap of silicon, Probe 172 T.K. Liang et al. / Optics Communications 265 (2006) 171–174 high intensity pulses will experience phonon assisted TPA when propagating along the waveguide. The amount of absorption is proportional to the square of intensity and the maximum transmitted power is limited. The absorption of photons will lead to two direct consequences – the optical power depletion (photon absorption) and the generation of excess electron–hole pairs (free carriers). The former is intrinsically an ultrafast process [11], while the latter is a slow process that will further attenuate the optical signal via free-carrier absorption and hot carrier assisted absorption. If there are two light beams with slightly different energies (or wavelengths), with one source at high peak power (pump) and the other one at low power (probe), the high power pump source will then induce absorption of the low power probe signal. A preliminary two-color time-resolved pump–probe experiment was performed to measure the nonlinear transmission of weak probe pulses in the presence of strong optical pump pulses in silicon wire waveguides. The fabrication and characterization of the waveguide was described elsewhere [12]. As shown in the inset of Fig. 1, the waveguide core was formed by a silicon stripe measuring 480 nm by 220 nm. The length of the waveguide used was 10 mm. The measured transmission as a function of peak pump pulse levels is shown in Fig. 1. Both pump and probe pulses were generated by the spectral slicing of a broadband femtosecond passive mode-locked laser. By using ultrashort pulses (around 1.6 ps FWHM pulsewidth), the additional absorption loss due to photo-generated free carriers was negligible. Since ultrashort pulses were used to achieve high peak power and low average power, the amount of free carriers generated was small. Assuming the pump pulses have the Gaussian temporal profile, the carrier density created from a single pump pulse inside the waveguide along propagation direction z is denoted by [13] NðzÞ 1⁄4 b ffiffiffi