Intersubband transitions in GaN/AlN quantum wells for Tb/s optical switching

Characteristics of the absorption recovery and the saturation of intersubband transition in GaN/AlN quantum wells are investigated for the purpose of applying these quantum wells to optical switches operating at a higher bit rate than 1 Tb/s. The pump-probe measurement verifies the absorption recovery time to be 150 fs at a wavelength of 4.5 μm. Dependence on the absorption on the input light intensity is examined at a wavelength of 1.48 μm for an optical pulse with a width of 130 fs. The characterization is performed with the Lorentzian fit of the absorption spectrum on the assumption of a two-level system. The result indicates that the recovery time is much less than 1 ps and the absorption saturation intensity is of the order of pJ/μm2. A ridge waveguide was fabricated and the onset of the intersubband absorption was confirmed. Finally, the switching performance is studied by means of the finite-difference time-domain (FDTD) simulation combined with three-level rate equations. Ridge waveguide structures with 3-QWs in the mid-layer are examined. Control and signal pulses are assumed to be the Gaussian pulses with a width of 250 fs. The results show that an extinction ratio of larger than 10 is achievable with an input control pulse energy of less than 1 pJ.

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