While most of the studies on ultrafast switching for potential applications in all-optical integrated circuits have focused on silicon-based photonic crystals (PCs), researchers from the Institute of Physics at the Chinese Academy of Sciences (Beijing, China) report optical switching in an organic PC with a response time as short as 120 fs. The operating wavelength for the PC, a high-quality face-centered cubic structure formed from organic polystyrene spheres 240 nm in diameter, is in the visible region. Switching is effected by a shift of the photonic bandgap due to optical pumping.
The researchers fabricated the 3-D PC from polystyrene spheres with a 240‑nm diameter using a vertical-deposition method. Though this method had been used previously to fabricate a well-ordered face-centered-cubic (fcc) structure from spheres 300 to 400 nm in diameter, the smaller-diameter spheres caused defects in the cubic structure. To eliminate these defects, the researchers developed an improved method that involved matching the deposition rate of the spheres with the evaporation rate of the colloidal suspension (consisting of deionized water and the polystyrene spheres). At a temperature of 50°C, humidity of 30%, and a suspension concentration of 0.036%, a well-ordered fcc structure was obtained (see figure). The PC surface measured 0.3 mm2 with a thickness of 3 to 4 µm.
The optical nonlinearity of the polystyrene comes from the delocalization of the electrons along the polymer chain of the crystal, which leads to a subpicosecond time response. Under optical pumping, the refractive index in the polystyrene varies, causing changes in the position and width of the photonic bandgap. Using the ultrafast shift of the gap edge as the physical basis of their optical switch, the slope of the transmission spectrum of the PC needs to be very steep in the gap edge region to insure high intensity contrast between the “on” and “off” states of the device. For the PC fabricated, the central wavelength of the first bandgap is at 575 nm, and the transmittances vary steeply in both regions of the shorter (550-nm) and longer (600‑nm) wavelength gap edges, allowing highly efficient optical switching.
Demonstration
To demonstrate the optical-switching effect of the polystyrene PC, the researchers used the 800-nm output from a Ti:sapphire laser (intensity 200 kW/cm2, pulse duration 120 fs, repetition rate 10 Hz) as the pump, and the tunable output from an optical parametric amplifier (OPA) pumped by the second harmonic of the same Ti:sapphire laser as the probe light. Both pump and probe light were p-polarized, and the transmittance of the pump light was more than 80%. A delay line was used to adjust the temporal relations between the pump and the probe, which were collinearly incident upon the sample along a direction perpendicular to the PC surface. Spot sizes for the pump and probe light were 200 and 100 µm, respectively. The transmitted signals were fed to a monochromator (resolution 0.1 nm) and received by a photomultiplier, the output of which was processed by a boxcar integrator and computer.
The pump pulse excitation causes the refractive index of the polystyrene to vary. If the probe wavelength is set at the gap edge, refractive-index changes alter the stop band of the PC and the transmittance of the probe light. As the pump intensity increases, the photonic bandgap shifts toward longer wavelengths. Setting the probe wavelength at 561 nm (the short-wavelength bandgap edge) and varying the pump intensity from zero to 27.5 GW/cm2 produced transmittance values of 17% and 62%, respectively. An opposite effect was noted at the long-wavelength edge of 597 nm for the probe wavelength; namely, transmittance varied from 56% to 16%, respectively. The time period to effect this transmittance change, or the time period of the optical switch, was measured to be 120 fs, and may be limited by the experimental time resolution between the pump pulse and the probe pulse.
“The third-order nonlinear susceptibility of organic materials may be much higher than that in silicon, so that under optical pumping the bandgap shift of organic photonic crystals can be larger, leading to a higher transmission contrast between the ‘on’ and ‘off’ switching states,” said Daozhong Zhang, group head and researcher. “The operating wavelength of the organic switch varies from the visible to the near-infrared, different from that of silicon-based photonic switches and enabling potential new applications outside of optical communications-we are looking for ways in which such an organic photonic-crystal switch could be integrated into an all-optical circuit.”
REFERENCE
1. Y. Liu et al., Appl. Phys. Lett.86(15), 151102 (April 11, 2005).
