We describe the development of hybrid quantum well (QW)/quantum dot (QD) active elements to achieve broad spectral bandwidth spontaneous emission and gain. We have previously reported that the placement of the QW within the active element is a critical factor in obtaining broad spectral bandwidth emission. We now present new designs to further broaden the spontaneous emission from hybrid structures by increasing the number of QD layers and dot density, and by using QDs with wider state-separation. Introducing chirped QD layers reduced the modulation in the spontaneous emission spectra, and by utilising self-heating effects and state-filling, a spontaneous emission with 3dB line-width of 350nm is obtained.
This paper describes a technique to control the polarization property in quantum dot (QD)-semiconductor optical
amplifiers (SOAs) using vertical stacking of self-assembled InAs QDs. QD-SOAs have been expected to realize high
saturation power, multi-channel processing, and high-speed response. However, in conventional QDs, the significant
polarization dependence in the optical gain caused by the flattened QD shape has been a serious problem. One of the
well-known approaches to realize the polarization-independent gain relies on columnar QDs, in which InAs QDs layers
are closely stacked with very thin (several monolayers) intermediate layers. The isotropic shape of columnar QDs
realizes a polarization-independent gain. On the other hand, in this paper, we propose a different approach, where QDs
are vertically stacked with moderately thick intermediate layers. Therefore each QDs layer is well separated
geometrically and high precision control of overall QD shape is expected. Vertically aligned InAs QDs are known to
create the electronically coupled states, where we expect the enhancement of the optical transition probability along the
vertical direction. We have achieved such vertical stacking of QDs up to 9 layers by optimizing the amount of GaAs and
InAs deposition. The 9-stacked QDs have shown transverse-magnetic-mode dominant emission in edge
photoluminescence in the 1.3 μm telecommunication wavelength region. Our results have suggested that the
electronically coupled QDs can be a powerful tool to realize the polarization-independent QD-SOAs
An all-optical switching device has been proposed by using self-assembled InAs/GaAs quantum dots (QDs) within a
vertical cavity structure for ultrafast optical communications. This device has several desirable properties, such as the
ultra-low power consumption, the micrometre size, and the polarization insensitive operation. Due to the threedimensional
confined carrier state and the broad size distribution of self-assembled InAs/GaAs QDs, it is crucial to
enhance the interaction between QDs and the cavity with appropriately designed 1D periodic structure. Significant
QD/cavity nonlinearity is theoretically observed by increasing the GaAs/AlAs pair number of the bottom mirror. By this
consideration, we have fabricated vertical-reflection type QD switches with 12 periods of GaAs/Al0.8Ga0.2As for the top mirror and 25 periods for the bottom mirror to give an asymmetric vertical cavity. Optical switching via the QD excited
state exhibits a fast switching process with a time constant down to 23 ps, confirming that the fast intersubband relaxation of carriers inside QDs is an effective means to speed up the switching process. A technique by changing the light incident angle realizes wavelength tunability over 30 nm for the QD/cavity switch.
Based on our line shape analysis of temperature dependent absorption spectra on InGaAs/AlAsSb single quantum wells, we expect a fast carrier redistribution with in the broad inhomogeneous intersubband absorption spectrum from a wavelength as short as 1.72 micrometers . In addition, due to large resonant 3rd order susceptibility but weak absorption, we expect small saturation intensity (Is) at this short wavelength. We present wavelength dependent saturation measurements to show that the Is is, indeed, lower by more than an order of magnitude compared to that at the main peak (1.88 micrometers ). We also show from the figure of merit estimates that the carrier relaxation time at 1.72 micrometers is expected to be faster at 1.72 micron, consistent with the line shape analysis predictions.
We fabricated spincoated films of squarylium dye (SQ) J- aggregates exhibiting femtosecond optical response at room temperature. Optical dynamics measurements revealed that the saturable absorption of the SQ J-aggregates film exhibited a decay time of less than 100 fs at a pump energy of 80 fJ/micrometer2. With this ultrafast SQ optical film, four- output demultiplex operation for T bps pulses was demonstrated. A series of 4 optical pulses with 100 fs duration and 1 ps interval (corresponding to 1 T bps signals) were irradiated onto the SQ film synchronized with a 100 fs gate pulse at a finite angle. Four demultiplexed signals were clearly observed at different areas on the CCD camera. Multi- output serial-to-parallel demultiplexer for T bps optical signals can be formed using the SQ J-aggregates film.
We report the near-infrared intersubband absorption characteristics in In (formula available in paper) heterostructures lattice matched in InP substrate. We have investigated for the first time the excitation power dependence of the intersubband transitions in Sb based quantum wells using a femtosecond optical parametric amplifier tuned over wavelength ranging from 1.8 micrometers - 2.4 micrometers . The bandgap of the InGaAs material system in the regime of 1.0 eV facilitates nonlinear interband optical absorption effects in the presence of strong near-infrared intersubband resonant optical excitation. We have observed a novel nonlinear optical phenomenon,- an intersubband transition induced interband absorption due to the two- photon interband excitation after the onset of the intersubband absorption saturation. The excitation wavelength-dependence of the absorption saturation characteristics has also been studied. The absorption saturation measurements have been performed in quantum wells with various well widths.
Modulators based on interband (IB) light absorption by intersubband (ISB) excitations in undoped quantum well structures (QWs) has the inherent advantage of ultrafast response without thermal dissipation at high bit rates. In this report we present an efficient scheme to achieve ultrafast modulation in the femtosecond regime using IB and ISB light pulses in a step-like type II semiconductor QW. The threshold control-light intensity for 100% modulation in the proposed structure is less than 1 pJ, which is at least an order of magnitude lower than in any excitonic optical switch proposed until now. The peak modulation efficiency in asymmetric QW's at 1 MW/cm2 is 40% which is twice than that estimated in symmetric QW's and can be enhanced to 100% at 10 MW/cm2. A modulation speed of 500 fs can be achieved without any serious degradation of the IB signal due to thermal dissipation. This is an important step towards the development of novel ultrafast optoelectronic devices based on the pulse shaping techniques.
Electron-spin relaxation has been investigated in a variety of quantum confined structures including GaAs-based quantum wells and quantum wires as well as InP-based quantum wells, and it has been shown to exhibit ultrafast relaxation in the picosecond, and even in the femtosecond, range. This ultrafast relaxation has been used together with the exciton absorption nonlinearity as an novel principle of all-optical switching device which can avoid the speed limit due to the slow carrier lifetime in conventional switching devices. Experiments on GaAs-based quantum-well spin switches have shown 4-ps gate switch operation. Also, experimental analyses have shown the feasibility of these devices at a high contrast ratio (13 dB) and high repetition rate (40 GHz). The application of this device to a demultiplexer has been proposed.
Ultrafast optoelectronic devices are crucial for fulfilling the future requirement of network throughput exceeding 1 Tb/s. A variety of ultrafast phenomena in semiconductors are attractive for developing such new optoelectronic devices. This paper discusses requirements of ultrafast optical communication and signal processing systems and devices necessary for them. Recent advances in the development of ultrafast semiconductor-based optoelectronic devices including lasers and optical switches are described with an emphasis on new types of ultrafast all-optical switches.
Integrated optoelectronics using 111-V compound semiconductor technology is expected to offer new techniques of interconnection and signal processing within electronic digital systems solving the wiring limit in data communication and processing using the advantages of optical technique such as high data transmission rate and high parallelism. Optical interconnection devices are the key to this and the development is being carried out not only to multiplex a vast amount of data but also to provide flexible interconnection functions. This paper summarizes the role and current status of integrated optoelectronics technology for optical interconnections and signal processing systems. 1.
Recent advances in and prospects for III-V compound semiconductor integrated optoelectronics in its application to optical computing systems are examined. Attention is given to developments concerning OEICs, photonic ICs, and optical functional devices. It is concluded that this field, due to its robustness in applications, contributes greatly to the development of optical computing systems.
This paper discusses basic component technologies for the development of digital optical computing systems. The establishment of integration technologies, including optoelectronic and photonic integrations, is important for the application of optoelectronic devices in computing systems, especially for use in optical interconnections. The development of optical bistable devices is a prerequisite for improving computing performance. Both of these technologies are a significant basis for optoelectronic computing systems, and can be built up by using III-V semiconductor materials. Recent advances in these fields will be presented.
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