We have developed a Mach-Zehnder interferometric all-optical switch employing intersubband transition in an $\mathrm{In}\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{As}∕\mathrm{Al}\mathrm{As}\mathrm{Sb}$ -coupled double quantum well waveguide. The recently discovered cross-phase modulation phenomenon was utilized as the switching mechanism; the nonlinear index of refraction for transverse electric polarized light is induced by intersubband optical excitation using transverse magnetic pump light. We demonstrate the demultiplexing operation of $160\mathrm{Gbit}∕\mathrm{s}$ data signals to $10\mathrm{Gbit}∕\mathrm{s}$ using this switch. At the input control pulse energy of $8\mathrm{pJ}$ , the demultiplexed signals showed an extinction ratio better than $10\mathrm{dB}$ , and an error-free demultiplexing was achieved.

We observed dynamical processes of the magnetic moments of photoinjected carriers and resultant perturbed magnetic moment of Mn${}^{2+}$ ions in CdTe/Cd${}_{1-x}$Mn${}_x$Te quantum wells by means of the femtosecond time-resolved magneto-optical Kerr rotation measurement in transverse magnetic fields. The time-resolved Kerr rotation signal shows that the Larmor precession of Mn${}^{2+}$ moment is initiated by photoinjected carriers. In particular, we observed a time delay in terms of the phase and an amplitude of Mn${}^{2+}$ beats as a function of the external field to confirm the mechanism of perturbing the Mn${}^{2+}$ moment, i.e., a transient $p-d$ exchange field, proposed in earlier studies. The motion of the Mn${}^{2+}$ moment is also simulated by the Bloch model including the impulsive transverse exchange fields produced by the conduction electron and heavy-hole spins. From this it is found that the exchange field of the heavy-hole spin contributes dominantly to tipping the Mn${}^{2+}$ moment away from the external field axis. The exchange field estimated by the Bloch model so as to fit experimental data is in reasonable agreement with the exchange field estimated from $p-d$ exchange interaction, which also supports the above mechanism.

We report photoinduced electron intersubband absorption in ZnSe/BeTe type-II superlattices. The wavelength of the intersubband transition as short as 1.6 μm, covering the 1.55 μm optical communication wavelengths within its absorption band width $\text{(∼250\hspace{0.05em}}\mathrm{nm}\mathrm{),}$ is achieved in the ZnSe/BeTe SLs with 4.5 ML-thick ZnSe layers. The intensity in photoinduced intersubband absorption increases sublinearly with pump intensity, reflecting the characteristic recombination processes of electron-hole pairs in a heterostructure with type-II band alignment.

A 3-D polarizing beam splitter based on a silicon nitride (Si₃N₄) vertical directional coupler is experimentally demonstrated. A new planarization technique by incorporating conventional chemical-mechanical lapping with a dry-etching process is developed, in order to obtain a flat film surface for the second Si₃N₄ core deposition after the first-layer waveguide is formed. Both the Si₃N₄ layer thicknesses are 200 nm. As there is a material refractive index mismatch between the vertically separated waveguides, the polarization splitter can be realized with a bottom waveguide width of 1.55 μm and a top core width of 1.35 μm. The transverse electric (TE) polarized light can be transmitted completely to the cross-layer output-port, whereas the transverse magnetic (TM) polarized wave outputs mostly from the port at the input layer. A high-extinction ratio and a wide operation bandwidth can be achieved. An extinction ratio of 26 dB for the cross-layer output-port at 1550-nm wavelength and that of 16 dB for the input-layer output-port are obtained. There is an excess coupling loss of for the TE light, but a 1-dB loss for the TM wave.

Ultrafast all-optical switching at an optical communication wavelength has been investigated by utilizing an intersubband transition (ISBT) of II–VI-based multiple quantum wells (MQWs) fabricated in high-mesa waveguide devices. The waveguide structure consists of a CdS∕ZnSe∕BeTe MQW core layer and two top and bottom ZnMgBeSe quaternary cladding layers grown by molecular beam epitaxy on a (001) GaAs substrate. A marked increase in waveguide transmittance was observed only for transverse-magnetic-polarized subpicosecond pulse with increasing incident pulse energy at λ=1.57μm, indicative of the ISBT absorption saturation. The pulse energy necessary for a 10dB transmittance increase is as low as 13.3pJ for a waveguide device with 2.7μm mesa, and the saturation pulse energy can be even further reduced by employing a narrower mesa structure. Ultrafast gate switching within a time window of 0.56ps was also demonstrated with pump pulse at λ=1.57μm and probe pulse at λ=1.63μm in this waveguide device.

We report on intersubband (ISB) absorption and ultrafast ISB energy relaxation of carriers in n-type doped (CdS/ZnSe)/BeTe quantum wells(QWs),grown by molecular-beam epitaxy. The highly n-type dopedQW samples were obtained by introducing a few monolayer CdS into a ZnSe/BeTe QW, and ISB absorption with a peak wavelength as short as 1.62 μm, covering 1.55 μm within its absorption bandwidth, was achieved. The ISB carrier relaxation was investigated by means of femtosecond (∼150 fs ) one-color pump and probe technique at the ISB absorption peak. The ISB carrier relaxation time of 270 fs was observed in the sample with the absorption peak at 1.82 μm. The slow decay component with a time constant of a few ps, which has been observed in ZnSe/BeTe QWs, was not observed in the (CdS/ZnSe)/BeTe QWs, indicating that the Γ( ZnSe )–X( BeTe ) electron transfer is suppressed, as expected from the band alignment.