Short-wavelength intersubband traditions (ISBT) in semiconductors quantum wells (QWs) with ultra fast carrier relaxation (~1ps) has attracted much attention, especially by aiming at potential applications for all-optical signal processing for high-bit-rate (above 160 Gbit/s) optical fiber communication system at λ~1.55 μm. Recently, sub-ps all-optical modulation at optical communication wavelength have been demonstrated in ISBT-waveguide devices based on several material systems, such as InGaAs/AlAsSb[1], GaN/AlGaN[2], (CdS/ZnSe)/BeTe[3].

In this presentation, we discuss the fabrication and characterization of ISBT-waveguide devices for telecom wavelength applications in wide-gap II-VI semiconductor based (CdS/ZnSe)/BeTe multiple quantum wells (MQWs).

The waveguide has a separate confinement heterostructure (SCH) structure which is consisted by ZnMgBeSe cladding layer (CL, n=2.35), a ZnBeSe optical confinement layer (OCL, n=2.45), (CdS/ZnSe)/BeTe MQW (n=2.54) active layer. The active layer has 40 period of QWs which is designed such that 15-ML(mono layer)-thick BeTe barriers with ZnSe/CdS/ZnSe (1/~2/1 ML) well layers exhibit　intersubband (ISB) absorption at a wavelength of around 1.55 μm. For the fabrication of sub-μm mesa patterns, we employed reactive ion etching in an inductive-coupled-plasma using Ar and BCl₃ gases.

For the switching characteristics, we estimated ISB absorption saturation and recovery time which correspond to extinction ratio and gate time window for the ISBT switching device. A 10 dB extinction ratio is achieved by pump pulse energy of 7pJ (1565 nm). As the gate open time, the modulation band width of 0.36 (FMHW) ps is estimated. These results indicate that optical modulation up to 1 Tbit/s is possible in present II-VI-based ISBT waveguide.

[1] S. Sekiguchi., et al, OFC 2005 [2 ] N.Iizuka., et al, Opt. Express, 13 (2005), 3835 [3] R.Akimoto., et al, APL. 87 (2005), 181104

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