Eindhoven, The Netherlands, June 11-13, 2008

Ultrafast all optical switching in AlGaAs photonic crystal waveguides

D.M. Szymanski1,2, B.D. Jones1, David O’Brien2, M.S. Skolnick1, A.M. Fox1, T.F.Krauss2

1Department of Physics and Astronomy, University of Sheffield, UK S3 7RH d.szymanski@sheffield.ac.uk

2School of Physics and Astronomy, University of St Andrews, Fife, UK KY16 9SS

Abstract. We have demonstrated all optical switching using photonic crystals integrated into an AlGaAs Mach Zehnder interferometer (MZI). Enhanced phase modulation efficiency together with fast carrier recovery times of 4 ps and high nonlinearities of the AlGaAs material make this design suitable for ultra fast all optical switching applications.

Ultrafast all optical switching Nowadays telecommunication networks rely on electro – optic components that limit the network’s performance. Only full optical control of light by light can meet the demands of increased bandwidth. Optical components are not limited by the RC constants like their electronic counterparts and allow generation of much shorter pulses which allow higher network speeds. However still some work must be done to fulfil all the requirements for optical devices. The most important are low power consumption, high S/N ratio, and high repetition speed. It is very hard to meet all of these criteria at once so the device should be carefully chosen. All optical elements rely on the refractive index change. This optically induced change is limited by the material nonlinearity and the required phase change entails that the size of the device be in the range of millimetres. Proper device design, material choice and the optical nonlinearity used can significantly increase the device performance. High repetition rates can be achieved using nonresonant optical nonlinearities. The main disadvantage of that approach is the high power required to utilize the full material nonlinearity. Power consumption and the speed can be enhanced by utilizing resonant coherent effects. Coherent effects are not limited by real carrier relaxation. The pulse width of the laser should be shorter than the carriers’ relaxation times, if not, real carrier generation occurs and limits device speed. Even if the laser pulse width is shorter real carrier generation can still mask coherent effects. Carriers are generated then via multiple photon absorption. From a practical point of view it is easier to work with incoherent effects because the device can be operated at low powers. In this experiment we show a way to decrease carrier relaxation time utilizing TPA (two photon absorption).

Experimental methods We have investigated all optical switching using photonic crystals integrated into a GaAs/AlGaAs Mach Zehnder interferometer (MZI). The MZI employs multimode interference (MMI) optical power splitters and photonic crystal waveguides in the functional area of each arm. Optical switching can be achieved by optically pumping the photonic crystal sections of one of the arms causing a carrier induced refractive index change in this arm of the MZI. A compact 3dB 1x2 power splitter operating at 950nm centre wavelength was designed by selecting correct position along MMI’s and waveguides configurations.

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time and normalized frequency for self-collimator based MZ switch. Decay time was measured for different periods at 920nm wavelength and average pump power 2.25 uW.

The carrier decay time is highly dependent on the geometry of the photonic crystal waveguides used in the experiments. A non patterned MZ switch shows a decay time of 61 ps which is about a factor of 2 faster than in bulk AlGaAs material [3]. Introduction of a photonic crystal in the active area of the switch changes the mean distance of the carriers from the surface hence increasing surface recombination. The self-collimator based MZ switch has a larger fraction of etched holes in comparison to W1 and W3 waveguides, so the recombination of optically excited carriers is further enhanced. A time decay of 4 ps was measured for a self-collimator based MZ switch with a period of 280nm. A decay time has been measured for a range of periods close to the self collimation region. Figure 3 shows a decrease of the decay time with an increase of normalized frequency. This implies that the probe beam is more collimated for normalized frequency 0.304 and undergoes higher modulation by the pump beam. Modulation depth was calculated using: (1)

where Am is a peak-peak value of the modulated beam and A is peak-peak value of the unmodulated beam. The self-collimator based device showed a modulation depth of 34%. The estimated switching energy is 2.82nJ (for average absorbed power 2.25 W).

References [1] P.A. Besse, M. Bachmann, H. Melchior, L. B. Soldano, and M. K. Smit, "Optical Bandwidth and

Fabrication Tolerances of Multimode Interference Couplers", Journal of Lightwave Technology, vol.12, no.6, June 1994

[2] J. Witzens, M. Loncar, and A. Scherer “Self-Collimation in Planar Photonic Crystals”. IEEE Journal of Selected Topics in Quantum Electronics, Vol. 8, no 6, November/December 2002

[3] M. J. LaGasse, K. K. Anderson, C.A. Wang, H.A Haus and J.G. Fujimoto, “Femtosecond measurements of the nonresonant nonlinear index in AlGaAs,” Appl. Phys. Lett. 56, 417 (1990).

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