Fabry-Perot Oscillations Research Articles

Backscattering from width variations in quasi-one-dimensional strips of topological insulators

Conductance modulations in wide–narrow–wide electron waveguides constructed from a two-dimensional topological insulator are investigated numerically. The conductance exhibits the Fabry–Perot oscillation at the opening of the helical edge states in the narrow segment when the potential offset imposed in the segment is varied. The quantum multiple reflections between the two ends of the narrow segment manifested by the oscillation demonstrate that the topological states are not protected from the scattering caused by an abrupt change in the channel width. The bulk states do not affect the vulnerability against the geometry scattering but they give rise to resonant transmission in an unconventional fashion.

Asymmetric Fabry–Perot Oscillations in Metal Grating-Coupled Terahertz Quantum Well Photodetectors

Asymmetric Fabry-Perot oscillations are observed in a high-resolution photocurrent spectrum of a 1-D metal grating-coupled terahertz quantum well photodetector (THzQWP). This behavior is carefully studied through analysis of the field in the device obtained by the finite element method. It is found that such asymmetric oscillation is a pure near-field effect caused by the phase shift of the reflected wave at the grating surface. Our findings also indicate that, because of the long wavelength in the THz range, the near field properties of a microstructured surface could be extracted through the photocurrent measurement on a THzQWP.

Perpendicular transmission of acoustic waves between two substrates connected by sub-wavelength pillars

We discuss theoretically the acoustic resonant transmission and zeros of transmission between two substrates connected by sub-wavelength pillars. The features of the transmission coefficient are explained in terms of the coupling of the incident waves with the Fabry–Perot oscillations inside the pillars and with the surface waves of both substrates. We discuss the dependence of the selective and zero transmission frequencies, in particular Fano resonances resulting from the proximity of a resonance to a zero of transmission, on the geometrical and physical parameters of the materials constituting the pillars and the substrate. These phenomena are studied in both one- and two-dimensional periodicities where the substrates are connected by a series of parallel plates or by a square lattice of cylindrical pillars, respectively. Finally, the calculation is extended to a periodic stacking of slabs and pillars that constitute a type of three-dimensional phononic crystal.

Routing and photodetection in subwavelength plasmonic slot waveguides

Abstract The ability to manipulate light at deeply sub-wavelength scales opens a broad range of research possibilities and practical applications. In this paper, we go beyond recent demonstrations of active photonic devices coupled to planar plasmonic waveguides and demonstrate a photodetector linked to a two conductor metallic slot waveguide that supports a mode with a minute cross-sectional area of ∼λ2/100. We demonstrate propagation lengths of ∼10λ (at 850 nm), routing around 90° bends and integrated detection with a metal-semiconductor-metal (MSM) photodetector. We show polarization selective excitation of the slot mode and measure its propagation characteristics by studying the Fabry-Perot oscillations in the photocurrent spectra from the waveguide-coupled detector. Our results demonstrate the practicality of transferring one of the most successful microwave and RF waveguide technologies to the optical domain, opening up many opportunities in areas such as biosensing, information storage and communication.

Quantum Behavior of Graphene Transistors near the Scaling Limit

The superior intrinsic properties of graphene have been a key research focus for the past few years. However, external components, such as metallic contacts, serve not only as essential probing elements, but also give rise to an effective electron cavity, which can form the basis for new quantum devices. In previous studies, quantum interference effects were demonstrated in graphene heterojunctions formed by a top gate. Here phase coherent transport behavior is demonstrated in a simple two terminal graphene structure with clearly resolved Fabry-Perot oscillations in sub-100 nm devices. By aggressively scaling the channel length down to 50 nm, we study the evolution of the graphene transistor from the channel-dominated diffusive regime to the contact-dominated ballistic regime. Key issues such as the current asymmetry, the question of Fermi level pinning by the contacts, the graphene screening determining the heterojunction barrier width, the scaling of minimum conductivity, and of the on/off current ratio are investigated.

Fourier-transformed photoreflectance and fast differential reflectance of HgCdTe layers. The issues of spectral resolution and Fabry–Perot oscillations

Fourier-transformed photoreflectance and fast differential reflectance (FDR) spectroscopies have been used to investigate the optical properties of HgCdTe layers of various compositions that spectrally target a broad range of mid and long infrared wavelengths. For this spectral range, the extraordinary sensitivity of these modulation techniques has allowed for direct measurement of the fundamental band gap as a function of temperature and Cd-atom concentration. Additionally, employing the FDR technique has allowed us to detect the absorption-like spectra in this long-wavelength range within a collection time of less than 1 min. Finally, it is seen that, in order to obtain an unequivocal interpretation of the experimental data from Fourier-spectrometer-based measurements of differential reflectivity, a proper selection of the spectral resolution and number of scans is crucially important.

Interference of Fano–Rashba conductance dips

We study the interference of two tunable Rashba regions in a quantum wire with onepropagating mode. The transmission dips (Fano–Rashba dips) of the two regionseither cross or anti-cross, depending on the distance between the two regions.For large separations we find Fabry–Perot oscillations due to the interference offorwards and backwards propagating modes. At small separations overlappingevanescent modes play a prominent role, leading to an enhanced transmission anddestroying the conductance dip. Analytical expressions in scattering matrix theoryare given and the relevance of the interference effect in a device is discussed.

Intense terahertz emission from undoped GaAs/n-type GaAs and InAs/AlSb structures grown on Si substrates in the transmission-geometry excitation

Intense terahertz radiation was generated from femtosecond laser-irradiated InAs and GaAs layers on Si substrates. Results show that InAs/Si and GaAs/Si films can be excited in reflection and transmission geometries. The InAs/Si film exhibited weaker emission for both excitation cases but it will be more feasible as a spectroscopic THz source due to the absence of complex spectral features in its emission spectrum. The GaAs/Si emission is characterized by Fabry–Perot oscillations but it is 90% of that of p-InAs bulk crystal emission intensity in the reflection geometry. Excitation fluence measurements showed that the InAs/Si film saturates easily due to the laser’s shallow penetration depth in InAs.

Experimental determination of the principal dielectric functions in silver nanowire metamaterials

The polarization dependent optical properties of silver nanowire arrays are investigated by angular resolved transmission measurements. The corresponding spectra show clear Fabry–Perot oscillations, which exhibit an unusual shift toward longer wavelengths for the extraordinary waves. From the peak shift both principal dielectric functions of the metamaterial are determined and compared with effective medium theories. Their different signs lead to a hyperbolic wave vector surface for the extraordinary waves and cause the observed peculiar peak shift.

Surface-emitting dye-doped polymer laser coupled with stimulated resonant Raman scattering

Surface-emitting polymer laser was fabricated with 1,4-bis[2-[4-[N,N-di(p-totyl)amino]phenyl]vinyl]benzene-doped poly(vinyl-pyrrolidone) thin films sandwiched between two distributed Bragg reflector (DBR) mirrors. Under pulsed optical pumping, Fabry–Perot (FP) type resonation resulted in multi-mode laser oscillations depending upon the active film thickness. With increasing excitation wavelengths, an emission peak based on stimulated resonant Raman scattering (SRRS) was superimposed on the multimode band region. When the SRRS peak just overlapped with one of the FP modes, the emission intensity was enhanced and the line width was considerably narrowed. Such SRRS-coupled FP oscillations can be applied to realize a tunable single-mode surface-emitting polymer laser.

Improvement of crystal quality of AlN grown on sapphire substrate by MOCVD

AbstractThe crystalline quality of aluminum nitride (AlN) epilayers grown on sapphire substrates by MOCVD was improved by increasing hydrogen flow rate during the high temperature growth process. The AlN epilayer exhibited a root mean square (rms) of roughness was 1.944 nm from the 2×2 µm2 size atomic force microscopy (AFM) images. Full widths at half maximum (FWHMs) of (002) and (102) rocking curves of triple‐axis high resolution X‐ray diffraction (HRXRD) measurements were as narrow as 28.8 arc sec and 868 arc sec, respectively. The optical transmittance spectra showed a sharp absorption edge at a wavelength of 200 nm and strong Fabry‐Perot (FP) oscillations. It is proposed that the improvement in crystalline quality is due to the surface in the low‐temperature aluminum nitride (LT‐AlN) buffer layer is promoted to be stable Al‐polarity by the conditions of increasing hydrogen flow rate and ramping up the growth temperature. Addtionally, the parasitic reactions are effectively suppressed by increasing the hydrogen flow rate during the growth process of high temperature. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modeling the thermopower of ballistic graphene ribbons

We investigate the thermopower of ballistic graphene ribbons using linear response theory and the Landauer formalism. The dependence of thermopower on temperature and chemical potential is investigated and the obtained results are qualitatively in agreement with many features recently observed in thermoelectric measurements on high mobility graphene ribbons. Fabry–Perot oscillations in the thermopower of short nannoribbons as a function of chemical potential, at finite temperatures, are revealed. The validity of generalized Mott's relation between thermopower and temperature dependent conductivity, is investigated in a wide range of temperatures.

Temperature Dependence of the Conductivity of Ballistic Graphene

We investigate the temperature dependence of conductivity in ballistic graphene using Landauer transport theory. We obtain results which are qualitatively in agreement with many features recently observed in transport measurements on high mobility suspended graphene. The conductivity sigma at high temperature T and low density n grows linearly with T, while at high n we find sigma approximately square root(|n|) with negative corrections at small T due to the T dependence of the chemical potential. At moderate densities the conductivity is a nonmonotonic function of T and n, exhibiting a minimum at T=0.693 hv square root(|n|) where v is the Fermi velocity. We discuss two kinds of Fabry-Perot oscillations in short nanoribbons and their stability at finite temperatures.

On the Modeling of Losses in Short Length Photonic Crystal Waveguides

The 3-D finite difference time domain (FDTD) cut-back method is used to study losses in non-disordered photonic crystal silicon membrane waveguides. Losses above the light-line have been shown to be in good agreement with other methods. Below the light-line, however, FDTD is predicting a rapid increase in losses. This paper studies the possible causes for this effect, including meshing effects, back reflections, and finite thickness sidewalls. It is found that since below the light-line the group index becomes very high and the loss becomes very low, strong Fabry-Perot oscillations dominate the cut-back results. The paper also discusses the impact of operating near to the cut-off wavelength of the photonic crystal waveguide Bloch mode and the implications this has for loss calculation.

Fast and Accurate Thickness Determination of Unknown Materials using Terahertz Time Domain Spectroscopy

We present a fast and accurate time domain based algorithm which extracts simultaneously the thickness and refractive index of highly transparent samples from terahertz time domain spectroscopy data. The utilized transfer function considers the Fabry-Perot oscillations of the sample and enables to analyze data with multiple reflections. The algorithm can also be applied to signals corrupted by vapor absorption lines. Since the data extraction takes only fractions of a second, this computation method is well suited for real-time monitoring of industrial processes. We show that the accuracy of the new algorithm is comparable to that of sophisticated, highly accurate and time consumptive frequency domain algorithms.

Analyzing sub-100-μm samples with transmission terahertz time domain spectroscopy

The authors propose a method for the extraction of material parameter and thickness information from sub-100-μm thin samples using non-differential transmission terahertz time domain spectroscopy. The approach relies on an additional Fourier transform of the frequency dependent material parameters to a quasi space regime. In this quasi space, periodic Fabry–Perot oscillations from the frequency domain, which originate from multiple reflections inside the sample, correspond to discrete peaks. By iterative minimization of these peaks, the highly precise thickness information along with the refractive index and absorption coefficient of the sample can be determined. Experimental verification of the approach is also provided.

Time-resolved thermal crosstalk characterisation of laser diode arrays

Thermal crosstalk in a diode array has been investigated using a time-resolved technique. The transmission of a probe beam injected into any of the array diodes is measured while one of them is alternatively switched on and off. The dynamics of temperature evolution of each diode and the thermal crosstalk owing to lateral heat spreading are deduced from temperature-induced Fabry-Perot oscillations.

Vanadium dioxide thermochromic opals grown by chemical vapour deposition

The semiconductor–metal transition that occurs in vanadium dioxide at 68 °C is the object of increasing interest in modern optics due to its applications in ultrafast optical devices. In a three-dimensional photonic crystal, the fine control of the structure is important for optimizing the switching magnitude and the spatial homogeneity. We report a controlled process to fabricate large-area high quality VO2–SiO2 opals with fine control over the filling volume. The method comprises two stages. The first stage is a chemical vapour deposition synthesis whereby the vanadium pentoxide is grown. The second one is a thermal annealing that allows reducing the vanadium pentoxide to vanadium dioxide. We have carried out a steady-state study of the semiconductor–metal transition, for opals with the Bragg peak around the 1.55 µm spectral region, by means of reflectance spectroscopy. As the temperature increases approaching the phase transition the intensity of the reflectance peak decreases and a small blueshift can be observed at 65 °C. When the phase transition is achieved at 68 °C the intensity of the reflectance peak decreases drastically and the Fabry–Perot oscillations disappear.

Onset of Landau-Level Formation in Carbon-Nanotube-Based Electronic Fabry-Perot Resonators

We report on the onset of Landau-level formation in a carbon nanotube-based Fabry-Perot resonator. Supported by excellent agreement between calculated and measured magnetoconductance patterns, the applied perpendicular magnetic field is shown to modulate the Fabry-Perot conductance oscillations consistently with the formation of a Landau level in the 1D massless Dirac fermions particle excitations.

Spectral properties of photonic crystal double heterostructure resonant cavities

Spectral properties of photonic crystal double heterostructure resonant cavities are calculated numerically using the three-dimensional finite-difference time-domain method. Resonance frequencies and quality factors are reported for various bound states that form near stationary points in the photonic crystal dispersion diagram. The associated electric field spatial profiles are presented indicating potential for in-plane laser optimization. In addition, Fabry-Perot oscillations are observed in the spectra.