B. S. Li

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.

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 transition based on a novel II-VI quantum well structure for ultrafast all-optical switching

We have achieved intersubband absorption as short as 1.58 µm in (CdS/ZnSe)/BeTe quantum wells (QWs) by employing relatively thicker BeTe barriers of 10 monolayers, covering an optical communication wavelength of 1.55 µm within its absorption band. The ISB carrier relaxation was investigated by means of femtosecond one-color pump and probe measurement at the ISB absorption peak position for samples with various well widths. 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 2–4 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. For applications toward ultrafast all-optical switching devices, a ridge-waveguide structure composed of (CdS/ZnSe)/BeTe QWs active and ZnMgBeSe cladding layers is proposed. Refractive indices of these layers have been examined and these data are fed to design ridge-waveguide structures. As a first step, a slab waveguide structure has been successfully grown.

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...

Composition Profile of ZnSe/BeTe Multiple Quantum Well Structures Studied by Cross-Sectional Scanning Tunneling Microscopy

The compositional distribution of ZnSe/BeTe interfaces in ZnSe/BeTe multiple quantum wells with a type-II band alignment has been investigated using cross-sectional scanning tunneling microscopy and X-ray diffraction measurements. The filled- and empty-state images revealed that Zn–Te and Be–Se bonds exist at the ZnSe/BeTe interface within the range of 2–4 monolayers along the growth direction. The transition layer between the ZnSe layer and the BeTe layer is composed of the BeZnSeTe quaternary alloy. X-ray diffraction analysis confirmed the existence of a (ZnTe)0.58(BeSe)0.42 transition layer with a width of 0.40 nm at the ZnSe/BeTe interface.

Fabrication of high-mesa waveguides based on wide-band-gap II-VI semiconductors for telecom wavelength applications

We fabricated high-mesa waveguides for application to inter-subband transition (ISBT) all-optical switches based on (CdS/ZnSe)/BeTe multiple-quantum well (MQW). These waveguides comprised a (CdS/ZnSe)/BeTe MQW core layer sandwiched between two ZnMgBeSe cladding layers and were fabricated by dry etching in an inductively coupled plasma (ICP). As a result of the optimization of etching conditions using BCl3, Cl2, and Ar as process gases, we found that BCl3/Ar is effective for etching these II–VI waveguide materials. We studied transmissivity at an optical communication wavelength of 1.55 µm in the fabricated waveguides. The insertion losses of the waveguides at transverse electric (TE) polarization were as low as 2.8 to 3.0 dB, which did not depend on waveguide length up to 0.95 mm. This indicates that a coupling loss between a waveguide and a fiber is a major contributor of TE insertion loss. On the other hand, transverse magnetic (TM) polarization insertion loss due to inter-subband absorption was observed, in which the extinction ratio of TM to TE polarization was 19 dB/mm.

Ultrafast intersubband optical switches in II–VI-based quantum-well waveguide with separate confinement heterostructure

Sub-picosecond all-optical switching at telecommunication wavelengths has been demonstrated by utilizing an intersubband transition of II-VI-based MQW fabricated in high-mesa waveguide devices with separate confinement heterostructure, where (CdS/ZnSe)/BeMgTe QW is employed as active layer.

Effects of ZnSe Interlayer on Properties of (CdS/ZnSe)/BeTe Type‐II Super‐lattices Grown by Molecular Beam Epitaxy

An intersubband transition (ISB‐T) down to 1.57 μm is realized in (CdS/ZnSe)/BeTe super‐lattices for the first time. We studied the dependence of properties of super‐lattices on the ZnSe interlayer by using in situ reflection of high energy electron diffraction, high‐resolution X‐ray diffraction, ISB‐T, and high‐resolution transmission electron microscopy. It is crucial to improve the growth mode, the structural and optical properties by inserting ZnSe interlayer between BeTe and CdS layers.