T. Hasama

All-optical demultiplexing of 160–10Gbit∕s signals with Mach-Zehnder interferometric switch utilizing intersubband transition in InGaAs∕AlAs∕AlAsSb quantum well

We have developed a Mach-Zehnder interferometric all-optical switch employing intersubband transition in an InGaAs∕AlAs∕AlAsSb-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 160Gbit∕s data signals to 10Gbit∕s using this switch. At the input control pulse energy of 8pJ, the demultiplexed signals showed an extinction ratio better than 10dB, and an error-free demultiplexing was achieved.

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.

Optical Bistable Operations in AlGaAs-Based Photonic Crystal Slab Microcavity at Telecommunication Wavelengths

We report on bistable operations in a microcavity formed on an AlGaAs-based photonic crystal slab at telecommunication wavelengths. We designed the cavity to achieve a high quality (Q)-factor, while maintaining a small mode volume; this was accomplished by removal of a single air hole. The fabricated microcavity exhibited a resonant peak with a Q-factor of 1900 at 1548 nm. In nonlinear transmission measurement, when the input power was increased, a bistable characteristic which was dependent on the detuned wavelength from the resonance was observed. The bistability was considered to originate from thermal nonlinearity due to two-photon absorption

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.

Energy consumption and traffic scaling of dynamic optical path networks

Dynamic path switching in lower layers such as optical or sub-wavelength layer-1 path connections is essential for future networks to provide end-to-end, bandwidth-guaranteed, large-capacity services without energy crunch. While this is almost generally agreed, the number of ports in optical switches tends to be limited by technological difficulties, severely restraining the scale of the network. However, video-related services, that occupies most of the traffic nowadays, could significantly alleviate such restraints if we utilized the nature of video usage. In most cases, video-related services are virtually provided through prior reservation scheme in which a relatively high call-blocking probabilities or long latency for a connection can be tolerated. This situation allows us to accommodate a relatively high number of subscribers with a limited number of switch ports. This paper shows that a network using optical switches with a technologically feasible number of ports, multi-granular paths, and a hierarchical network topology can be of a national scale accommodating several tens of millions of subscribers. The purpose of detailing a plausible network topology is to show that such a network offers a benefit of energy efficiency approximately three orders of magnitude compared with that extrapolated from recent router-based networks. We then discuss important technical aspects of such dynamic optical path networks including our several research activities. We emphasize the importance of vertically integrated research activities from application to device layers to develop the dynamic optical path networks.

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...

Simultaneous generation of intersubband absorption and quantum well intermixing through silicon ion implantation in undoped InGaAs/AlAsSb coupled double quantum wells

We demonstrated the intersubband absorption in undoped InGaAs/AlAsSb coupled double quantum wells through silicon ion implantation and rapid thermal annealing. For an implantation dose of 1×1014 cm−2, the actual carrier density of a sample annealed at 600 °C for 1 min was ∼7.5×1013 cm−2 (∼75% activation efficiency); the activation energy was ∼1.41 eV. The simultaneously generated quantum well intermixing (QWI) was nonuniform due to the silicon ion distribution. The effects of QWI nonuniformity on both intersubband and interband transitions were explained by eight-band k⋅p calculation. This study will open a route for monolithic integration of intersubband-transition-based high-speed all-optical switches.

Energy bottlenecks in future networks and optical signal processing

This talk will provide a high-level overview to redress the value of optical signal processing in the context of the looming energy issues, suggesting the potential of dynamic optical path switching along with some enabling technologies.