Intersubband transitions (ISBT) in quantum wells are attractive for high-speed all-optical switches. We previously reported that low absorption saturation intensities (Is) of ISBTs were achieved in an InGaAs/AlAsSb system on InP. The Is of a multiple quantum well sample finally reduced to 3 fJ/μ m² when AlAs diffusion-stopping layers were inserted into the well-barrier interfaces. Our most recent study indicates that intersubband-absorption response time was greatly improved, decreasing it to 690 fs, by using coupled double quantum wells (C-DQWs). This accomplishment has led to the expectation that the application of diffusion-stopping layers to C-DQWs will improve the ISBT properties in both Is and response time. We therefore tried to grow C-DQWs with AlAs diffusion-stopping layers by molecular beam epitaxy. The InGaAs wells were doped to 1x10¹⁹ cm^-3 with Si. The Is and response time in the present sample were measured using an optical parametric amplifier. The results showed that the Is was extremely low, down to 34 fJ/μ m² at the optical communication wavelength of 1.62 μ m, while ultrafast response times of 600 fs were maintained. This demonstrates that AlAs stopping layers can also be effectively used with C-DQWs. Although the optical measurements of the sample indicated its high quality, there are some problems that need to be solved. An optical microscope observation indicated that the sample had many cross-hatched lines, suggesting that the sample may not be of the highest quality. Preliminary results to optimize the thickness of AlAs layers, however, indicate that cross-hatched lines can be reduced. Further optimization of the growth parameters promises to enhance the quality of C-DQW structures, thereby reducing the switching energy of ultrafast ISBT switches. This work was performed under the management of Femtosecond Technology Research Association supported by the New Energy and Industrial Technology Development Organization.

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1. Raman scattering study of InGaAs/AlAsSb and InGaAs/AlAs/AlAsSb heterostructures