Resonant interband tunneling FET

A lateral three-terminal Resonant Interband Tunneling Field Effect Transistor (RITFET) has been fabricated. It consists of a resonant interband tunnel diode (RITD) built using the GaSb-AlSb-InAs material system and a pseudomorphic InGaAs channel FET in which the current is controlled by a Schottky gate. The three terminal device has current-voltage characteristics that exhibit negative differential conductance with a peak-to-valley current ratio of 8 at room temperature     

High extinction guided-wave optical polarization splitter

Conventional guided-wave optical polarization splitters exhibit poor polarization extinction characteristics because of the necessity of controlling waveguide parameters. In order to improve the characteristics, an electrical control scheme was investigated in an alternating Δβ directional coupler. Polarization-dependent electrooptic as well as polarization-independent plasma effects were employed to produce high extinction for two orthogonal polarizations independently. As a result, an extinction ratio as high as 32 dB was obtained with a fabricated GaAs-GaAlAs device. This is the highest value ever reported with a channel waveguide device     

Resonant tunneling injection quantum-well lasers

We report, for the first time, the observation of resonant tunneling induced negative differential characteristics in the light-current (L-I) and light-voltage (L-V) curves of a single-quantum-well semiconductor laser. Broad-stripe lasers have exhibited CW threshold current density of 50 A/cm² at 77 K, with a sharp decrease in output light power at 68 A/cm², which corresponds to a resonant point in the current-voltage (I-V) characteristics of the device. Peak to valley ratios of more than 1.5:1 were observed in the L-I and L-V curves     

Resonant tunneling in graphene microstructures

We present results for the resonant electronic transmission through graphene-based single and double barriers as a function of the incident wave vector, the widths and heights of the barriers, and the separation between them. Resonant features in the transmission result from resonant electron states in the wells or hole states in the barriers and strongly influence the ballistic conductance of the structures.     

Tunable Mach-Zehnder polarization splitter using height-tapered Y-branches

A tunable TE/TM polarization splitter, based on a Mach-Zehnder interferometer with an electrooptic switch, is demonstrated in GaAs-AlGaAs. Symmetric and asymmetric Y-branches employing height-tapered waveguides are used to achieve power splitting and mode sorting, respectively, in the interferometer. The device has an extinction ratio of /spl sim/20 dB and an excess loss less than 1.5 dB for both TE and TM polarized light. The device can be reconfigured by tuning the switching voltage for operation at both 1.3- and 1.55-pm wavelengths.     

Surface micromachined integrated optic polarization beam splitter

An integrated polarization beam splitting system composed of one binary phase Fresnel collimating lens and one polarization beam splitter (PBS) has been realized on a single Si chip using a commercially available foundry polysilicon surface micromachining process [Multi-User Mems Process, or (MUMPS)] at Mems Center at North Carolina (MCNC). The polarization extinction ratios of 10 dB for the transmitted light and over 20 dB for the reflected light have been achieved. The whole system is prealigned through computer-aided design on a Si substrate and is lifted up perpendicular to the substrate after structure release. This system opens opportunities for various applications where the light polarization states could be utilized     

Resonant tunneling in quantum cascade lasers

Experimental evidence that in quantum cascade lasers electron injection into the active region is controlled by resonant tunneling between two-dimensional subbands is discussed. A quantitative analysis is carried out using an equation for the current density based on a tight-binding approximation. Electron injection into the active region is optimized when the current density is limited by the lifetime of the excited state of the laser transition. In this regime, quasi-equilibrium is reached between the population of the injector ground state and that of the excited state of the laser transition characterized by a common quasi-Fermi level. The design of the injector depends on the selected laser active region; in particular, the choice of physical parameters, such as doping concentration and injection barrier thicknesses, is in general different for vertical or diagonal transition lasers. The paper concludes with an investigation of the transport properties at threshold and its dependence on stimulated emission; a relationship between the differential resistance above threshold and the value of the slope efficiency is deduced     

Resonant tunneling through magnetic edge states

Resonant magneto-tunneling of electrons through AlGaAs/GaAs/AlGaAs double barrier structures is investigated for samples with different quantum well widths (300–800Å) and barriers (130–200Å). In a strong transverse magnetic field, resonant tunneling is quenched, and a new set of resonances appear at low bias. These resonances are interpreted as due to tunneling between the edge states on either side of the first barrier. The edge states are the quantum mechanical analog of the classical skipping orbits.     

Polymeric waveguide polarization splitter with a buriedbirefringent polymer

A waveguide polarization splitter is demonstrated based on a low-loss polymer waveguide and a birefringent polyimide. Crosslinkable fluorinated polymers with an excellent stability and a low absorption loss are utilized for the device. The polyimide is buried under one branch of the Y-branch waveguide to enhance the birefringence between the TE and TM modes. By the adiabatic mode evolution, the TE mode is coupled to the branch with the polyimide strip, while the TM mode propagates through the other branch without the polyimide. For the device with a branch angle of 1/400 rad, we obtained a crosstalk less than -20 dB and a fiber-to-fiber insertion loss of 3.8 dB     

Resonant tunneling diodes: models and properties

The resonant tunneling diode (RTD) has been widely studied because of its importance in the field of nanoelectronic science and technology and its potential applications in very high speed/functionality devices and circuits. Even though much progress has been made in this regard, additional work is needed to realize the full potential of RTD's. As research on RTD's continues, we will try in this tutorial review to provide the reader with an overall and succinct picture of where we stand in this exciting field or research and to address the following questions: What makes RTD's so attractive? To what extent can RTD's be modeled for design purposes? What are the required and achievable device properties in terms of digital logic applications? To address these issues, we review the device operational principles, various modeling approaches, and major device properties. Comparisons among the various RTD physical models and major features of RTD's, resonant interband tunneling diodes, and Esaki tunnel diodes are presented. The tutorial and analysis provided in this paper may help the reader in becoming familiar with current research efforts, as well as to examine the important aspects in further RTD developments and their circuit applications     

Mode evolution type polarization splitter on InGaAsP/InP

A new type of mode-evolution polarization-splitter based on InGaAsP/lnP has been designed and realized. The component uses the large waveguide birefringence of the first-order TE and TM modes in a ridge waveguide made in a heterostructure, In the input section an asymmetric Y-junction acts as a mode converter in order to inject first-order TE- and TM-modes in a polarization-splitting section, which consists of a Y-junction formed by a bimodal and a monomodal waveguide. In the output section a third Y-junction is connected to the bimodal waveguide to couple the first-order mode to a monomodal output waveguide. Components that are 6-mm long and show polarization splitting at a wavelength of 1.55 μm have been realized. The best splitting ratios are close to -20 dB, which is in agreement with BPM simulations. Excess losses are below 1 dB     

A short polarization splitter without metal overlays on InGaAsP-InP

A new and very short polarization splitter on InGaAsP-InP is designed and realized for the first time. The component contains a ridge waveguide directional coupler of 0.4 mm length and an output section of 0.7 mm. It uses the large waveguide birefringence of the first-order TE and TM modes to obtain polarization selective directional coupling. In this way, additional metal layers on the waveguides to create birefringence are avoided and fabrication becomes very simple. Components are realized, which show splitting ratios close to -20 dB. Excess losses are below 1 dB. The polarization splitting is investigated in the wavelength region of 1525-1560 nm and found to be better than -9 dB.     

Resonant tunneling permeable base transistors with high transconductance

A GaAs-based resonant tunneling permeable base transistor has been developed and evaluated at room temperature. The transistor is fabricated by overgrowing a tungsten gate placed next to an AlGaAs-GaAs-InGaAs resonant tunneling heterostructure. By changing the gate voltage, the effective conducting area of the tunnel diode can be modulated and the collector-emitter current thus controlled. Peak currents above 300 mA/mm and a maximum transconductance of 270 mS/mm have been obtained.     

Resonant tunneling transistors with controllable negative differential resistances

Three-terminal devices based on resonant tunneling through two quantum barriers separated by a quantum well are presented and analyzed theoretically. Each proposed device consists of a resonant tunneling double barrier heterostructure integrated with a Schottky barrier field-effect transistor configuration. The essential feature of these devices is the presence, in their output current-voltage (I_{D} - V_{D}) curves, of negative differential resistances controlled by a gate voltage. Because of the high-speed characteristics associated with tunnel structures, these devices could find applications in tunable millimeter-wave oscillators, negative resistance amplifiers, and high-speed digital circuits.     

A compact two-dimensional grating coupler used as a polarization splitter

We demonstrate a novel polarization splitter based on a two-dimensional grating etched in a silicon-on-insulator waveguide. The device couples orthogonal modes from a single-mode optical fiber into identical modes of two planar ridge waveguides. The extinction ratio is better than 18 dB in the wavelength range of 1530-1560 nm and the coupling efficiency is approximately 20%. The device is very compact and couples light only to transverse-electric modes of the planar waveguides. Therefore, it may be used in a polarization diversity configuration to implement a polarization insensitive photonic integrated circuit based on photonic crystal waveguides.     

A small-size polarization splitter based on a planar optical phasedarray

A device is proposed in which the dispersive properties of an array of bent optical waveguides are utilized for splitting the two polarizations present in the input waveguide. An experimental device for operation at a 633-μm wavelength, with dimensions of 0.6×2.5 mm ², was designed and fabricated using conventional (high-quality) optical lithography. Insertion losses as low as 0.5 dB and far-end crosstalk values of 17-21 dB have been achieved     

Polarization stabilizer using a polarization splitter and athermooptic polymer waveguide device

For the stabilization of the time-varying light polarization in the optical fiber, we demonstrates a polarization stabilizer by using a fiber-optic polarization splitter, a passive polarization converter, and a power equalizing waveguide device. Because the ratio of the output light power from the polarization splitter is time-varying depending on the input polarization, we adopt a power equalizer consisting of a polymeric thermooptic (TO) phase modulator and an asymmetric X-junction waveguide. A feedback signal is obtained from the difference of the two outputs of the power equalizer. Then it is applied to the TO phase modulator. The output power is stabilized with the TE polarization independent of the time-varying polarization state of the input light     

Resonant interband tunneling device with multiple negativedifferential resistance regions

The first observations of multiple negative differential resistance (NDR) regions in resonant interband tunneling devices are reported. In vertically integrated poly-type heterostructures of InAs/AlSb/GaSb, the peak voltages are reduced by a factor of 2 compared to the AlInAs/GaInAs material system, while high peak-to-valley ratios of 4:1 (17:1) at 300 K (77 K) are maintained. The InAs/AlSb/GaSb material system offers advantages for circuit applications of such devices, particularly operation at lower voltages