K. Akita

Short-wavelength intersubband transitions down to 1.6 μm in ZnSe/BeTe type-II superlattices

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 (∼250 nm), 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.

Subpicosecond saturation of intersubband absorption in (CdS/ZnSe)/BeTe quantum-well waveguides at telecommunication wavelength

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.

Sub-picosecond electron relaxation of near-infrared intersubband transitions in n-doped (CdS/ZnSe)/BeTe quantum wells

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 doped QW 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.

Cross Phase Modulation Efficiency Enhancement in In0.8Ga0.2As/Al0.5Ga0.5As/AlAs0.56Sb0.44 Coupled Double Quantum Wells by Tailoring Interband Transition Wavelength

The cross-phase modulation (XPM) efficiency in new coupled double quantum wells (CDQWs) was examined. Since the XPM efficiency was enhanced under the resonant condition of interband transition (IBT), the new CDQWs were designed to control the IBT absorption edge wavelength (IBT-EW) while the intersubband transition (ISBT) wavelength was maintained at around the optical communication wavelength. By controlling the coupling strength between two wells and the bandgap energy of the wells, the new CDQWs can have a possibility of individual control of the IBT and ISBT wavelengths. In the new CDQWs, a high XPM efficiency close to 0.5 rad/pJ was observed; this value was three times higher than that observed in previous studies.

Short-wavelength (λ<2 μm) intersubband absorption dynamics in ZnSe/BeTe quantum wells

We report on linear and nonlinear short-wavelength (λ<2 μm) intersubband (ISB) absorption characteristics in ZnSe/BeTe quantum wells by means of an interband pump and ISB pump/probe technique. The ISB absorption saturates with a hole burning effect, indicating the absorption band is broadened inhomogeneously. The saturation intensity is as low as 4.3 MW/cm2 at λ=1.76 μm. The direct ISB energy relaxation time increases gradually from 0.20 to 0.38 ps with decreasing λ from 2.2 to 1.8 μm, while the saturation recovery is replaced by another slow relaxation process with a time constant of a few ps. The Γ(ZnSe)–X(BeTe) electron transfer is a relevant mechanism for this slow relaxation.

Structural study of (CdS/ZnSe)/BeTe superlattices for λ=1.55 μm intersubband transition

A (CdS/ZnSe)/BeTe superlattice (SL), based on wide band gap II–VI compounds, with a large band offset of 3.1 eV was grown on a GaAs (001) substrate using molecular-bean epitaxy and an intersubband transition (ISB-T) of 0.78 eV (λ=1.58 μm) with a full width at half maximum (FWHM) of 96 meV observed. We studied structural properties using high-resolution x-ray diffraction combined with dynamic simulation and found through the strain state in samples that a ZnSe/BeTe interface having a quaternary interface layer (ZnTe)0.45(BeSe)0.55 is preferentially formed despite the promotion of one molecular layer (ML) ZnTe interface formation. Be–Se bonds thus replace the Zn–Te bond in the transition region. For the CdS/ZnSe interface, an approximately 1 ML Zn0.75Cd0.25S ternary layer accompanied by ∼1 ML Zn0.85Cd0.15Se forms at the transition region due to Cd diffusion. X-ray (002) ω/2θ scan curves for (CdS/ZnSe)/BeTe SLs show sharp, intense satellite peaks exceeding ten orders, indicating high structure quality. We ob...

λ∼1.49–3.4μm intersubband absorptions in (CdS∕ZnSe)∕BeTe quantum wells grown by molecular beam epitaxy

We demonstrate that intersubband transition (ISBT) can be tuned in a wide range (far to near infrared) in (CdS∕ZnSe)∕BeTe quantum wells (QWs). The strain in the structures shifts from compressive to tensile due to formation of Be–Se and Zn–S bonds at interfacial region, which significantly affects the ISBT performance. ISBTs of λ=1.52–3.4μm with narrow linewidths (67–92meV) are observed in QWs with the nominal well width of 3.75–14 molecular layers (MLs). ISBT at 1.49μm was also observed in QWs with 3.5 ML well width by means of photoinduced absorption measurement, indicating carrier compensation limits the shortest ISBT wavelength.

Intersubband absorption with different sublevel couplings in [(CdS∕ZnSe∕BeTe)∕(ZnSe∕BeTe)] double quantum wells

The authors demonstrated the strong sublevel coupling on the intersubband absorption at telecommunication wavelengths in asymmetric II-VI double quantum wells consisting of a deep CdS well and a shallow ZnSe well coupled by a 1-monolayer-thick BeTe barrier. With increasing ZnSe well thickness, the sublevel coupling between excited states first increases to a maximum and then decreases, which results in a transition energy anticrossing and a reverse intensity evolution for e1-e2 and e1-e3 transitions. These results are in good agreement with the self-consistent Schrodinger-Poisson calculation. For optical switch applications, the sublevel-coupling-dependent relaxation time could be increased to lower the switching energy.

ZnSe interlayer effects on properties of (CdS∕ZnSe)∕BeTe superlattices grown by molecular beam epitaxy

We study the dependence of structural properties on the thickness of the ZnSe interlayer (IL) in (CdS∕ZnSe)∕BeTe superlattices (SLs); this is crucial for improving the growth mode in this heterostructure. The in situ reflection of high-energy electron diffraction oscillation and high-resolution x-ray diffraction spectra show a perfect structure that is obtained by introducing a ZnSe IL between CdS and BeTe. An intersubband transition (ISB-T) down to 1.57μm with a full width at half maximum of 90meV has been observed in (CdS∕ZnSe)∕BeTe SLs. A strong ISB-T is observed when the ZnSe IL ⩾1 monolayer (ML); however, it completely disappears with the introduction of a 0.5 ML ZnSe IL in (CdS∕ZnSe)∕BeTe SLs. High-resolution transmission electron microscopy images reveal that a sharp interface is formed in the barrier and well transition region in the structure with ZnSe IL ⩾1 ML; however, the interfaces become rough and thick in those with a 0.5 ML ZnSe IL. This indicates that the properties of the interface in (C...

Ultrafast electron dynamics of intersubband excitation concerning cross-phase modulation in an InGaAs/AlAs/AlAsSb coupled double quantum well

The ultrafast electron dynamics accompanying intersubband excitation in an InGaAs/AlAs/AlAsSb double quantum well has been investigated by femtosecond pump-probe spectroscopy. The photoinduced changes in the refractive index that causes cross-phase modulation is dominated by a decreased number as well as increased temperature of the electrons in the lower conduction subbands. The change in the number of conduction electrons decays monotonically, whereas the change in the electron temperature exhibits a maximum at around 0.9 ps after the photoexcitation. Heating of the conduction electrons is caused by their excess energy after intersubband longitudinal-optical phonon scattering.