Long-wavelength vertical-cavity surface-emitting lasers for high-speed applications and gas sensing

Long-wavelength InGaAlAs-InP vertical-cavity surface-emitting lasers (LW-VCSELs) covering the wavelength range from 1.3 to 2.3 mum are presented. Furthermore, these lasers can be fabricated in a novel high-speed design-reducing parasitics to enable bandwidths in excess of 11 GHz at 1.55 mum. To the best of the authors' knowledge, this is the fastest 1.55 mum VCSEL ever presented. As a proof-of-concept one- and two-dimensional arrays have been fabricated with high yield. All devices use a buried tunnel junction for current confinement and a dielectric backside reflector with integrated electroplated gold-heatsink. This concept enables CW operation at room temperature with typical single-mode output powers above 1 mW. Both, wavelength range and modulation performance, together with VCSEL features such as operation voltage around IV and power consumption as low as 10-20 mW enable applications in tunable diode laser spectroscopy (TDLS) and optical data transmission. Error-free data transmission at 10 Gbit/s over 22 km which can be readily applied in uncooled coarse wavelength division multiplex passive optical networks is presented. A laser hygrometer using a 1.84 mum VCSEL demonstrates the functionality of TDLS systems with VCSELs.     

High-gain injection quantum-dot lasers operating at wavelengths above 1300 nm

We have created and studied the properties of semiconductor lasers on InAs/GaAs quantum dots with a maximum modal gain of 46 cm^?1 at a wavelength of 1325 nm achieved due to increased optical confinement factor and optimized heterostructure growth parameters. In a wavelength interval of 1315–1345 nm, the modal gain exceeded 20 cm^?1 at a current density of 500 A/cm².     

Anomalous dynamic characteristics of semiconductor quantum-dot lasers generating on two quantum states

Dynamics of the emission spectra of semiconductor quantum dot (QD) lasers generating on two quantum states has been experimentally studied. Being pumped with 30-ns current pulses, a QD laser ceased to generate 2–5 ns after switch-on and exhibited a pause up to 10 ns or longer, depending on the pumping pulse amplitude. After the subsequent switch-on, the laser generated short (200–300 ps) pulses of emission from the excited state of QDs followed by minima of comparable duration (dark pulses) corresponding to the ground-state emission. This behavior is explained in terms of the laser Q-switching due to the charge carrier density redistribution between the excited and ground states.     

Long-wavelength long-lifetime luminophores

We describe a new approach to making luminophores that display long emission wavelengths, long decay times, and high quantum yields. These luminophores are covalently linked pairs with a long-lifetime resonance-energy-transfer donor and a long-wavelength acceptor. The donor was a ruthenium (Ru) metal-ligand complex. The acceptor was the Texas Red. The donor and acceptor were covalently linked by polyproline spacers. The long-lifetime donor results in a long-lived component in the acceptor decay, which is due to RET. Importantly, the quantum yield of the luminophores approaches that of the higher quantum yield acceptor, rather than the lower quantum yield typical of metal-ligand complexes. The emission maxima and decay time of such tandem luminophores can be readily adjusted by selection of the donor, acceptor, and distance between them. Luminophores with these useful spectral properties can also be donor-acceptor pairs brought into close proximity by some biochemical association reaction. Luminophores with long-wavelength emission and long lifetimes can have numerous applications in biophysics, clinical diagnostics, DNA analysis, and drug discovery.     

Long-wavelength cutoff filters of a new type

A method for the design of cutoff filters is proposed that is based on the use of light at nonnormal angles of incidence and on the use of coating materials with a large dispersion of the optical constants. The optical constants are presented of several films made from indium tin oxide-a material that satisfies this requirement. The filtering is due to absorption, critical angle effects, or both. The theory of this type of filter is described, and the calculated performance of two long-wavelength cutoff filters based on the use of indium tin oxide films is given. The angular performance and the producibility of one of these filters are examined. Some applications for these filters are considered, and the feasibility of producing similar filters for other spectral regions is discussed.     

Polarization-selective plasmon-enhanced silicon quantum-dot luminescence

The photoluminescence intensity of silicon quantum dots is enhanced in a polarization-selective way by coupling to elongated Ag nanoparticles. The observed polarization dependence provides direct proof that the PL enhancement is due to electromagnetic coupling of the silicon quantum-dot emission dipoles with dipolar plasmon modes of the Ag nanoparticles. The polarization selectivity demonstrates the potential of engineered plasmonic nanostructures to optimize and tune the performance of light sources in a way that goes beyond solely enhancing the emission and absorption rates.     

Long-wavelength Marangoni Convection in a Thermally Modulated Field

This paper is concerned with the long-wave Marangoni instability in a horizontal liquid layer. The analysis has been carried out for the heat flux periodically oscillating at the deformable upper interface and at the rigid lower boundary. This type of instability is caused by the amplification of synchronous disturbances in response to the external forcing. Convection thresholds are determined, and the critical Marangoni number is expressed as a function of frequency. It is shown that the long-wavelength mode can give rise to the instability prior to the cellular mode within the definite range of parameters. The long-wavelength instability can have an influence on the space-qualified manufacturing, as well as the industrial process of thin films deposition on substrates.     

The engineering of quantum-dot devices

Insofar as quantum-dot structures represent the extrapolation of today's microstructures used for electronic and optoelectronic device applications, their low-cost manufacture presents many challenges which will take some time to overcome if tunnel and quantum-well devices are any guide.     

Fabrication of quantum-dot devices in graphene

Abstract

We describe our recent experimental results on the fabrication of quantum-dot devices in a graphene-based two-dimensional system. Graphene samples were prepared by micromechanical cleavage of graphite crystals on a SiO₂/Si substrate. We performed micro-Raman spectroscopy measurements to determine the number of layers of graphene flakes during the device fabrication process. By applying a nanofabrication process to the identified graphene flakes, we prepared a double-quantum-dot device structure comprising two lateral quantum dots coupled in series. Measurements of low-temperature electrical transport show the device to be a series-coupled double-dot system with varied interdot tunnel coupling, the strength of which changes continuously and non-monotonically as a function of gate voltage.     

Subdiffraction photon guidance by quantum-dot cascades

We report on a waveguide composed of a cascade of gain-enabled quantum dots with subwavelength dimensions. Fabrication is demonstrated through DNA-mediated self-assembly and a two-layer molecular self-assembly process that enables rapid prototyping. The device, which is identified with fluorescence microscopy and tested by optical near field detection, allows optical signal transfer at a well-defined wavelength in flexible routing geometry, such as straight paths and 90 degrees bends. The structure serves as a critical building block for nanophotonic systems with high integration density.     

Reappraisal of variable-range hopping in quantum-dot solids

The temperature dependence of the electrical conductivity of assemblies of ZnO nanocrystals, studied with an electrochemically gated transistor is very accurately described by the relation ln sigma=ln sigma0-(T0/T)(x) with x=2/3 over the entire temperature range from 7 to 200 K, independent of charge concentration and dielectric environment. These results cannot be explained by existing models but are supported by results on Au nanocrystals where an identical temperature dependence was observed (Zabet-Khosousi et al., Phys. Rev. Lett. 2006, 96 (15), 156403). We propose an adaptation of the Efros-Shklovskii variable-range hopping model by introducing an expression for nonresonant tunneling based on local energy fluctuations, which yields exactly the temperature dependence that is observed experimentally.     

Split current quantum-dot cellular automata-modeling and simulation

We examine a novel quantum-dot cellular automata device concept using the interaction of resonant tunneling currents through a system of four quantum wells. The interaction of resonant tunneling currents forces the total current to flow predominantly in the wells along one of the two diagonals, effectively polarizing the cell. We refer to this device concept as split current quantum cellular automata (SCQCA). A free cell will settle to a random diagonal, whereas charge interactions between adjacent cells will cause the polarization to synchronize between cells. In contrast with the standard QCA cell, this device does not require tunneling between dots. Electron tunneling occurs along the vertical direction, where highly controllable deposition techniques are able to deposit very thin films and effectively tune the device parameters. Clocking of an SCQCA cell is performed by controlling the bias across the device, and none of the potential barriers between the dots need to be controlled. We believe this device concept lends itself to fabrication using currently available fabrication technologies.     

Serial bit-stream analysis using quantum-dot cellular automata

Quasi-adiabatically switched quantum-dot cellular automata (QCA) devices present the opportunity to extend our efforts from the implementation of combinational logic devices to more useful sequential logic devices. One very important application of sequential logic is in the recognition of patterns in serial bit streams. This is important, for example, in Internet applications, where particular bit patterns are designated as "sentinel" characters that indicate a particular action should be taken. The foundation of a serial bit-stream analyzer is a shift register, which can be implemented very easily using quasi-adiabatically switched QCA devices. In addition to the shift register, the device will require a multiple-bit comparator, which has not yet been demonstrated in a QCA architecture. We will present a multiple-bit serial stream analyzer that combines the functions of the shift register and the comparator. This device will be analyzed first using behavioral and structural models developed specifically for this project, then its correct quasi-adiabatic behavior will be demonstrated using well established quantum mechanical models. We will show that the proposed device can be used to detect any arbitrary bit pattern appearing in a serial stream of data applied to its serial input. The bit pattern to be detected can be changed using device inputs, and a successful match will be indicated by asserting an output.     

Long-wavelength emission in InGaAsN/GaAs heterostructures with quantum wells

The properties of InGaAsN/GaAs heterostructures with quantum wells on GaAs substrates were studied. The GaAsN layers containing InGaAsN quantum wells with a high (exceeding 1%) nitrogen concentration were obtained. The long-wavelength emission in the InGaAsN quantum wells is obtained in the wavelength range up to 1.32 μm at room temperature. The effect of the InGaAsN quantum parameters on the optical properties of heterostructures is studied.     

Ultrafast dynamics of quantum-dot semiconductor optical amplifiers

We report on a series of experiments on the dynamical properties of quantum-dot semiconductor optical amplifiers. We show how the amplifier responds to one or several ultrafast (170 fs) pulses in rapid succession and our results demonstrate applicability and ultimate limitations to application of quantum-dot amplifiers in e.g. amplification of signals in a telecommunications system. We also review experiments on pulse propagation control and show the possibility to slow down or speed up 170 fs pulses in a quantum-dot based device.     

Long-wavelength infrared cascade laser with coherent transport of electrons

Possible ways to overcome difficulties encountered in the creation of cascade lasers with collisionless tunneling of electrons are discussed. A structure of an IR laser using a single three-barrier structure and capable of working at a frequency of 4.5 THz is proposed.     

Spontaneous long-wavelength interlevel emission in quantum-dot laser structures

Spontaneous emission has been observed for the first time as a result of interband transitions of holes and electrons between size-quantization levels in vertically coupled quantum dots and also as a result of transitions from quantum-well states to a quantum-dot level. The spectral range of the emission was in the far-infrared (λ≅10–20 μm). The long-wavelength emission was only recorded simultaneously with short-wavelength interband emission (λ≅0.94 μm) in InGaAs/AlGaAs quantum-dot laser structures at above-threshold currents. Pis’ma Zh. Tekh. Fiz. 24, 20–26 (August 12, 1998)     

Optical properties of a carbon-zirconia quantum-dot photonic crystal

We report the optical properties of a new type of photonic crystal: a transparent fused silica matrix containing quantum dots—nanoparticles of another material. In this study, nanoparticles consist of graphite zones several nanometers in size, stabilized by zirconia. The photonic crystal is prepared by high-temperature annealing (1200°C) of synthetic opal infiltrated with zirconia and a small amount of carbon. We demonstrate selective reflection of visible light from the surface of the quantum-dot crystal under broadband illumination. Such crystals are potentially attractive as narrow-band selective filters that would reflect the exciting light in Raman measurements and might be used to convert short-wavelength broadband radiation to quasi-monochromatic light in the visible range.     

Low-threshold quantum-dot injection heterolaser emitting at 1.84 μm

The use of InAs quantum dots in an InGaAs matrix lattice-matched with an InP substrate can appreciably increase the emission wavelength of quantum-dot lasers. Lasing via quantum-dot states at the 1.84 μm wavelength (77 K) was obtained for the first time at a threshold current density of 64 A/cm². Pis’ma Zh. Tekh. Fiz. 24, 49–54 (January 12, 1998)