GaAs Ring Research Articles

A photonic crystal waveguide intersection using phase change material for optical neuromorphic synapses

This paper introduces an innovative photonic crystal device that amalgamates a low cross-talk GaAs waveguide intersection with a ring employing phase change material (PCM) for application in all-optical neuromorphic synapses. The device uses a degenerate optical cavity housing Germanium–Antimony-Telluride (GST) phase-change material whose optical properties can be manipulated via a control signal. The proposed design facilitates the development of a non-volatile synapse for photonic neural networks and optical neuromorphic circuits. Numerical simulations using the finite difference time domain method (FDTD) exhibit a notably high-quality factor of 900 and a minimal cross-talk level of −60 dB at the intersection of two waveguides. To validate the FDTD results, the structure undergoes further simulations using the finite element method (FEM), confirming the accuracy of the initial calculations. The resultant structure achieves transmissions of 81 % and 13 % in the amorphous and fully crystalline states of PCM, respectively, enabling low output power in the crystalline state and high output power in the amorphous state. Moreover, modulation of the transmission coefficient between 13 % and 81 % is feasible by manipulating the crystallization coefficient of GST materials. To reduce the effect of the imaginary part of the refractive index of the GST material, the evaluation of the output transmission results for the GSST material (Ge2Sb2Se4Te1) has been performed. The results revealed that, despite having a lower imaginary part of the refractive index compared to GST, GSST exhibits lower absorption in both amorphous and crystalline states. However, the use of GSST materials shows a 27 % reduction in the figure of merit (FOM), indicating a lower effective range and fewer degrees of freedom in the different crystallization levels of the PCM ring. To investigate the effect of fabrication defects and structural sensitivity, the radius of photonic crystal rods and the rings of GaAs and GST have been changed, and the results indicate that the radius of the photonic crystal rods and the GST ring have negligible effects on the output transmission and resonant wavelength. However, the thickness of the GaAs ring shows a high sensitivity, which leads to a change in the resonance wavelength that is utilized as an effective tool for tuning the pass wavelength through the structure within the 171 nm bandgap range. The transmission loss in the amorphous state is −0.6 dB and in the fully crystalline state −7.5 dB, which is attributed to the absorption of GST material. This proposed photonic crystal synapse structure with an area of less than 30 μm2 significantly minimizes the required space compared to similar silicon photonic structures and increases the prospects for integration and scalability. The compatibility of the materials employed with optical fibre communications at the 1310 nm wavelength further bolsters the practical feasibility of this design. In summary, this paper presents a significant advancement in the domain of neuromorphic photonics by introducing a promising structure with substantial potential for neural network synapses.

Simulation on a non-axisymmetric ring resonator with a nano-antenna for heat-assisted magnetic recording

Heat-assisted magnetic recording is a technology to improve recording density of hard disk drives. The authors’ group previously proposed a device, in which a Au nano-antenna as a near-field transducer was attached to a GaAs ring resonator as a light source. The ring resonator should stably oscillate with a single mode to generate near-field light at the tip of the nano-antenna with sufficient energy density. A non-axisymmetric ring resonator structure was proposed this time to improve the energy density and laser oscillation stability. When the decenter between the inner and outer circles of the ring resonator increased, the resonance frequency increased and eigenmodes with higher radial mode order disappeared, which leads to stable laser oscillation. Moreover, when the decenter was 200 or 250 nm, the normalized energy density at the tip of nano-antenna increased 1.2–2.4 times compared with an axisymmetric ring resonator structure while keeping the quality factor to more than 5000.

Strong frequency conversion in heterogeneously integrated GaAs resonators

In this contribution, we demonstrate the first integrated gallium arsenide (GaAs) ring resonator for second harmonic generation (SHG) on a GaAs-on-insulator platform. Such resonators exhibit high nonlinear optical coefficients, a strong optical confinement, and intrinsic quality factors exceeding 2.6 × 105, which makes them very attractive for nonlinear optical applications. The fabricated resonators exhibit a great potential for frequency conversion: when 61 μW of pump power at 2 μm wavelength is coupled into the cavity, the absolute internal conversion efficiency is 4%. We predict an external SHG efficiency beyond 1 000 000%/W based on the GaAs resonance devices. Such nonlinear resonant devices of GaAs and its aluminum GaAs alloy can be directly integrated with active components in nonlinear photonic integrated circuits (PICs). This work paves a way for ultra-high efficient and compact frequency conversion elements in PICs.

UNIQUE PROPERTIES OF ZnO QUANTUM RINGS

Electronic states and optical transitions of a ZnO quantum ring containing few interacting electrons in an applied magnetic field are found to be very different from those in a conventional semiconductor system, such as a GaAs ring. Strong Zeeman and Coulomb interactions of the ZnO system, exert a profound influence on the electron states and on the optical properties of the ring. In particular, our results indicate that the Aharonov– Bohm effect in a ZnO quantum ring strongly depends on the electron number.

Spin and orbital angular momentum transfer into GanAsn nanocage: The change in induced magnetic field by tuning the light parameters and size of the molecule

In this study, we demonstrate that the direction of a photoinduced magnetic field in semiconductor nano-scale quantum dots (nanocages) can be changed by tuning the frequency and the light parameters. In the case of the spin angular momentum (SAM) transfer to ring-shaped GanAsn molecule, some electrons of occupied states are excited into the unoccupied states for a special laser frequency. As a result of the transition due to the circular polarized light, the excited electron which has a nonzero magnetic quantum number start to circulate around the molecular GaAs ring. The transfer of orbital angular momentum (OAM) due to the twisted light can induce similar effect on the molecular ring. In the case of twisted light, the electron can be excited different unoccupied states with higher magnetic quantum numbers. In the study, the optimization and electronic structure calculation have been performed by using density functional theory (DFT). The induced current and corresponding induced magnetic field can be controlled by the change in the frequency. Also, the type of the angular momentum of the light (SAM or OAM) can be used as a tool to manipulate the magnitude and direction of the magnetic moment.

Irregular Aharonov–Bohm effect for interacting electrons in a ZnO quantum ring

The electronic states and optical transitions of a ZnO quantum ring containing few interacting electrons in an applied magnetic field are found to be very different from those in a conventional semiconductor system, such as a GaAs ring. The strong Zeeman interaction and the Coulomb interaction of the ZnO system, two important characteristics of the electron system in ZnO, exert a profound influence on the electron states and on the optical properties of the ring. In particular, our results indicate that the Aharonov–Bohm (AB) effect in a ZnO quantum ring strongly depends on the electron number. In fact, for two electrons in the ZnO ring, the AB oscillations become aperiodic, while for three electrons (interacting) the AB oscillations completely disappear. Therefore, unlike in conventional quantum ring topology, here the AB effect (and the resulting persistent current) can be controlled by varying the electron number.

Optical Sensing of Axons in GaAs Ring Resonators

Microtubes herald a new approach to how semiconductors can be applied as actuators and sensors. This is mainly due to their three-dimensional nature, which enables to wrap the microtubes around an axon. This yields an increase in coupling, e.g. of action potentials, by orders of magnitude. While actuation can be established in a straightforward fashion, signal readout for sensing purposes is more intricate. In the current work, we present our approach of realizing microtubes as GaAs ring resonators for remote optical sensing. In practise the microtubes are first fabricated, while axons are grown at a later stage employing the tubes as growth guides. The microtubes are fabricated based on self-organized rolling-up of strained semiconductor layers. The diameter of the tubes is dialed in to be on the order of the diameter of an axon (2µm), while the length varies between 50 and 100 µm. To overcome the problem of toxicity of GaAs, the tubes are parylene-coated. Finally, transparent substrates are employed to measure optical modes in the bottle resonators without radiative losses [1]. We demonstrate our first results of optical sensing of axons in semiconductor microtubes on a transparent substrate. [1] Strelow et al. Phys Rev. Lett. 2008, 101, 127403

Theoretical study of persistent current in a nanoring made of a band insulator

AbstractWe study theoretically persistent currents in rings made of band insulators with a fully occupied valence band and empty conduction band. We derive an expression for the persistent current in a one‐dimensional (1D) insulating ring with an arbitrary valence band ε(k). Setting for ε(k) the realistic (numerically calculated) valence bands, we estimate persistent currents in the nanorings made of real band insulators like GaAs, Ge, and InAs. Further, we express the persistent current in the insulating 1D ring through the Wannier functions of the constituting infinite 1D crystal. We find these Wannier functions analytically in the LCAO approximation. We obtain fully analytically the persistent current in the insulating ring and we apply the result to a 1D GaAs ring with conduction electrons subjected to a periodic potential emulating the insulating lattice. In general, persistent currents in rings made of band insulators decay with the ring length exponentially due to the exponential decay of the Wannier function tails. In spite of that, the currents of measurable size are found for a suitably chosen nanorings similar to those fabricated recently for coherent nanoelectronics. Finally, to provide insight, our major results are rederived from the so‐called ring Wannier functions.

Spectroscopic observation of developing InAs quantum dots on GaAs ringlike-nanostructured templates

Spectroscopic study of the InAs quantum dot (QD) formation in GaAs ringlike nanostructures is carried out. Ga droplet epitaxy is used to form GaAs ringlike nanostructures. Subsequently InAs is deposited to obtain InAs QDs by self-assembly inside the holes of the nanostructures. Regularly spaced bands in the photoluminescence spectra exhibit state filling properties under increased excitation power. However, it is demonstrated that these bands do not represent excited states of a single ensemble of dots, but are separate ensembles with individual ground state energies, which are coupled through the GaAs ring structure on which they form. The most likely cause of these uniformly spaced ensemble energies is monolayer differences in the effective height of the dots. Temperature, excitation power density, and time dependent photoluminescence measurements are used to demonstrate the significance of the interdot coupling. The photoluminescent properties of these novel nanostructures make them candidates for optoelectronic applications.

Theory of nonlinear Sagnac effect

AbstractA nonlinear wave interaction is considered as a technique to increase the rotation sensitivity of the ring laser. The gyroscopic scale factor K is calculated in an active laser gyro with a dispersive medium. K is in reverse proportion to the group index n* = n + vdn/dv of the medium. In a monolithically-integrated GaAs ring laser, the value K ∼5000 is obtainable (radius ∼1 cm) in a linear case. In the presence of a strong wave, the dynamic nonlinear anomalous dispersion can provide an increase of K by 10–100 times in the vicinity of critical points where n* passes zero. An expression of K is derived for the nonlinear Sagnac effect. The nonlinear dispersion is discussed in terms of “slow/fast” light.

One-dimensional ring with Kronig–Penney periodic potential: Persistent currents at full filling

We study the persistent electron current in a one-dimensional conducting GaAs ring with the Kronig–Penney periodic potential applied along the ring circumference ( L). The periodic potential disturbs the free electron motion and changes the parabolic energy dispersion to a series of cosine-like energy bands separated by gaps. However, these cosine-like bands differ from the perfect cosine-shaped bands encountered in the lattice model with nearest-neighbor hopping or in the tight-binding model. Owing to this difference, in our model a non-zero persistent current exists when the energy band is completely filled by electrons. This current decays with increasing L exponentially, but we see measurable values for the properly chosen sample parameters. In contrast, in the tight-binding model the persistent current at full filling is zero which is a known result.

Self-assembled InAs quantum dot formation on GaAs ring-like nanostructure templates

The evolution of InAs quantum dot (QD) formation is studied on GaAs ring-like nanostructures fabricated by droplet homo-epitaxy. This growth mode, exclusively performed by a hybrid approach of droplet homo-epitaxy and Stransky-Krastanor (S-K) based QD self-assembly, enables one to form new QD morphologies that may find use in optoelectronic applications. Increased deposition of InAs on the GaAs ring first produced a QD in the hole followed by QDs around the GaAs ring and on the GaAs (100) surface. This behavior indicates that the QDs prefer to nucleate at locations of high monolayer (ML) step density.

Nanometer-scale GaAs ring structure grown by droplet epitaxy

Nanometer-scale GaAs ring structure is self-assembly realized by droplet epitaxy in a lattice-matched system. By changing the As 4 flux intensity during the crystallization of Ga droplets into GaAs, balance between the crystallization inside and at the edge of the droplets is changed, resulting in the shape control from dot to ring. The ring structure exhibits clear photoluminescence emission up to room temperature. Droplet Epitaxy is a promising growth method not only for quantum dots but also for quantum rings, with high structural and optical qualities in lattice-matched systems.

Nanoring structure and optical properties of Ga 8As 8

After several initial geometric configurations are relaxed, a ring structure of Ga 8As 8 cluster is found by first-principles calculations. It is shown that GaAs bond lengths are somewhat longer or shorter than that in bulk GaAs. The calculated vibrational spectrum implies that the optimized geometry is located at the minimum point of the potential surface. There are only two strong peaks of IR absorption, one is at about 260 cm −1 and another is at 349 cm −1. The vibrations of Ga–As bonds in plane of ring are the primary IR vibration at about 349 cm −1. Raman spectrum shows that the vibrations of two layers of the GaAs ring are the primary Raman vibration at about 222 cm −1. The dipole polarizability anisotropy invariant and hyperpolarizability also are discussed.

Spin features in the Raman spectrum of nanoscopic rings

We have studied physical aspects of the charge and spin density responses of nanoscopic rings that might be detected by Raman spectroscopy. We have taken an InAs–GaAs ring as a case of study with N=10 electrons and several values of the on-plane transferred momentum q. Distinct monopole to quadrupole modes appear in the spin density, and especially in the charge density channel. We have found that both channels are coupled if the ground-state is ferromagnetic.

Period‐doubling phenomena and chaos generated in the GaAs ring‐oscillator

AbstractGaAs MESFET has the Schottky gate structure, in contrast to Si MOSFET. Consequently, the gate current increases rapidly when the gate voltage exceeds the Schottky voltage. In the direct‐coupled FET logic circuit (DCFL) using GaAs, when the input voltage is increased, a minimum is produced in the input‐output transfer characteristic in the low‐output voltage region. Because of this property, when a ring‐oscillator is constructed by cascading DCFL, a perioddoubling phenomenon is produced, generating a long‐period or irregular chaotic oscillation. This paper discusses these phenomena. the limiting set is determined by iteratively applying Poincaré mapping to the inputoutput characteristic. the stability is discussed using the Lyapunov exponent. It is shown as a conclusion that the number of invertor stages composing the GaAs ring oscillator should be as large a prime number as possible, as long as the oscillating condition is satisfied.

Accurate characterization and modeling of transmission lines for GaAs MMICs

The authors discuss computer-aided design (CAD) tools together with high-accuracy microwave measurements to realize improved design data for GaAs monolithic microwave integrated circuits (MMICs). In particular, a combined theoretical and experimental approach to the generation of an accurate design database for transmission lines on GaAs MMICs is presented. The theoretical approach is based on an improved transmission-line theory which is part of the spectral-domain hybrid-mode computer program MCLINE. The benefit of this approach in the design of multidielectric-media transmission lines is described. The program was designed to include loss mechanisms in all dielectric layers and to include conductor and surface roughness loss contributions. As an example, using GaAs ring resonator techniques covering 2 to 24 GHz, accuracies in effective dielectric constant and loss of 1% and 15% respectively, are presented. By combining theoretical and experimental techniques, a generalized MMIC microstrip design database is outlined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

GaAs IC fabrication using a novel open-tube sulphur diffusion technique

High-transconductance (gm) GaAs FETs and ring oscillators were fabricated using a novel open-tube sulphur diffusion method to form the active layer in a 2 in (51 mm) LEC wafer. Extrinsic transconductances as high as 200 mS/mm were obtained on 2 μm-gate FETs. A DCFL ring oscillator, fabricated with 2 μm-gate FETs, had 54 ps gate delay at room temperature. This diffusion technique was found to be suitable for GaAs IC fabrication.

VIA-3 GaAs MESFET's and ring oscillators on MOCVD grown GaAs/Si

VIA-3 GaAs MESFET's and ring oscillators on MOCVD grown GaAs/Si

Direct-coupled GaAs ring oscillators with self-aligned gates

The performance of direct-coupled GaAs MESFET ring oscillators having a 1µm self-aligned recessed-gate structure defined by optical contact lithography on CVD epitaxial material is reported. Propagation delays as low as 20.9 and 16.1 ps/stage have been achieved at 300 and 77 K, respectively, representing the fastest results reported to date for GaAs ring oscillators having conventional 1-µm gate technology.