Demonstration of a terahertz coplanar-strip spoof-surface-plasmon-polariton low-pass filter.

There is a growing interest in spoof surface plasmon polariton (SSPP) structures at terahertz (THz) frequencies for applications such as filtering, sensing, and communications. However, to date, there are limited experiments that confirm SSPP characteristics at THz frequencies. The majority of literature focuses on simulation or verification by device scaling to Gigahertz (GHz) frequencies where standard vector network analyzers are readily available. This paper presents the first experimental verification of SSPP characteristics at THz frequencies in a guided wave system using coplanar strip (CPS) feedlines. Specifically, we design three SSPP structures with varying band-edge frequencies (1.04 THz, 0.63 THz, and 0.53 THz), then fabricate and verify the low-pass transmission characteristics using a modified THz-time-domain spectrometer (THz-TDS) system. We find strong agreement between simulation, theory, and experiment.

Demonstration of an Integrated terahertz band-stop filter using an apodized Bragg grating.

This paper presents the demonstration of an on-chip integrated terahertz (THz) apodized Bragg grating (TABG) which functions as band-stop filter with a center frequency of 0.8 THz and a bandwidth of 200 GHz. For experimentation, we integrate the TABG into our THz system-on-chip to enable wideband (DC - 1.5 THz) device characterization. Using this methodology, we measure the signal transmission through the TABG and find the experimental results align with simulation and theory provides a rejection of approximately 20 dB across the stop-band.

Characterization of a split-ring-resonator-loaded transmission line at terahertz frequencies.

Recently we demonstrated the fabrication and testing of a variety of RF-engineered passive transmission-line-based components designed for operation at terahertz frequencies and fabricated on thin (1 µm) silicon-nitride membranes. In this work we measure the transmission response of a coplanar-strip transmission line loaded with split-ring resonators up to 2.5 THz. We observe three dominate modes within the measured frequency range; the predicted LC resonator mode at 0.510 THz, a higher-order LC resonator mode at 1.03 THz, and a higher-order dipole mode at 1.85 THz. The LC resonator mode is investigated using a modified version of the standard lumped element model which incorporates the transmission line between adjacent meta-atoms using ABCD matrices.

Tapered transmission lines for terahertz systems.

Complex terahertz (THz) System-on-Chip (TSoC) circuits require ultra-wideband low-loss low-dispersion interconnections between building-block components of various dimensions and characteristics. Tapered transmission lines, which enable the gradual transformation of both physical dimensions and characteristic impedance, are a convenient basis for these interconnections. In this paper, we quantify both experimentally and through simulation, the efficacy of transmission-line tapers connecting two different coplanar-strip transmission-line configurations, for frequencies up to 2.0 THz and with 25 GHz spectral resolution. We demonstrate tapers that enable transitioning from a small device-constrained transmission-line dimension (10 µm line width) to a lower-loss (20-40 µm line width) dimension, as a method to reduce the overall attenuation, and outline design constraints for tapered sections that have minimal detrimental impact on THz pulse propagation.

Terahertz low-pass filter based on cascaded resonators formed by CPS bending on a thin membrane.

A membrane-based coplanar-stripline (CPS) transmission-line platform has recently enabled implementation of diverse THz system-on-chip (TSoC) components. In this paper, we demonstrate an elliptic-function THz low-pass filter (TLPF) using cascaded λ/4 resonators between the right-angle bending of a CPS transmission line defined on a 1 µm-thin membrane. We investigated the effect of bending the CPS transmission line with different angles that introduces a frequency response similar to a simple LC low-pass filter (LPF) and facilitates the design of a desired roll-off performance using traditional methods. ANSYS HFSS was used to provide a full-wave analysis and characterize the effective parameters of the TLPF with a designed cutoff-frequency around 0.6 THz. Using 7 sections of right-angle CPS bending with total length 1.4 mm, we demonstrate experimentally an elliptic-function TLPF characterized by a low-ripple at passband, a roll-off transition with zero transmission near the cutoff frequency and a wide stopband with -60 dB rejection.

Demonstration of a low-distortion terahertz system-on-chip using a CPS waveguide on a thin membrane substrate.

Distortion-free transmission of THz-bandwidth pulses over centimeter-scale distances is desirable for future THz system-on-chip (TSoC) applications. In this work we achieve this by utilizing a coplanar strip (CPS) transmission line fabricated on a thin (1 µm) silicon nitride membrane. To generate and detect the THz-bandwidth pulses we use a well-known lift-off technique to construct thousands of small (20 µm × 40 µm) thin-film LTG-GaAs photoconductive devices from a small (approx. 4 mm × 4 mm) substrate. The devices are then bonded to the CPS transmission line on the thin silicon nitride membrane, DC biased and optically pumped by a sub-picosecond laser. We demonstrate the generation and detection of a pulses containing frequencies up to 1.5 THz after propagating for 10 mm.

Photoconductive generation and detection of THz-bandwidth pulses using near-field coupling to a free-space metallic slit waveguide.

THz-bandwidth pulses are generated, transmitted along a gold-plated stainless steel metallic slit waveguide, and detected with 1.5 THz bandwidth and 60 dB dynamic range. The source and detector were edge-pumped slotlines on LT-GaAs placed within the near-field region of the waveguide entrance and exit aperture. The motivation for this work was to develop a complete dispersion-free THz system which was simple to manufacture and could be utilized for free-space waveguide experimentation.

Plasmon-enhanced LT-GaAs/AlAs heterostructure photoconductive antennas for sub-bandgap terahertz generation.

Photocurrent generation in low-temperature-grown GaAs (LT-GaAs) has been significantly improved by growing a thin AlAs isolation layer between the LT-GaAs layer and semi-insulating (SI)-GaAs substrate. The AlAs layer allows greater arsenic incorporation into the LT-GaAs layer, prevents current diffusion into the GaAs substrate, and provides optical back-reflection that enhances below bandgap terahertz generation. Our plasmon-enhanced LT-GaAs/AlAs photoconductive antennas provide 4.5 THz bandwidth and 75 dB signal-to-noise ratio (SNR) under 50 mW of 1570 nm excitation, whereas the structure without the AlAs layer gives 3 THz bandwidth, 65 dB SNR for the same conditions.

THz-TDS using a photoconductive free-space linear tapered slot antenna transmitter.

A near-field edge-coupled photoconductive free-space linear tapered slot antenna has been constructed as a planar alternative to the standard photoconductive switch coupled to a silicon substrate lens. The temporal response along the optical axis is investigated to ensure the structure itself does not introduce pulse distortion which would fundamentally limit the usefulness of the structure. Experimental results show that a 1.6 THz bandwidth with a ≈50dB dynamic range is achievable with the new structure which is comparable to our reference experiment with a standard silicon substrate lens. The investigated structure has the added benefit of a potential substantial physical size reduction and can also be used to excite waveguides in the near-field.

Plasmon-Enhanced below Bandgap Photoconductive Terahertz Generation and Detection.

We use plasmon enhancement to achieve terahertz (THz) photoconductive switches that combine the benefits of low-temperature grown GaAs with mature 1.5 µm femtosecond lasers operating below the bandgap. These below bandgap plasmon-enhanced photoconductive receivers and sources significantly outperform commercial devices based on InGaAs, both in terms of bandwidth and power, even though they operate well below saturation. This paves the way for high-performance low-cost portable systems to enable emerging THz applications in spectroscopy, security, medical imaging, and communication.

Nanoplasmonics enhanced terahertz sources.

Arrayed hexagonal metal nanostructures are used to maximize the local current density while providing effective thermal management at the nanoscale, thereby allowing for increased emission from photoconductive terahertz (THz) sources. The THz emission field amplitude was increased by 60% above that of a commercial THz photoconductive antenna, even though the hexagonal nanostructured device had 75% of the bias voltage. The arrayed hexagonal outperforms our previously investigated strip array nanoplasmonic structure by providing stronger localization of the current density near the metal surface with an operating bandwidth of 2.6 THz. This approach is promising to achieve efficient THz sources.

Nanoplasmonic terahertz photoconductive switch on GaAs.

Low-temperature (LT) grown GaAs has a subpicosecond carrier response time that makes it favorable for terahertz photoconductive (PC) switching. However, this is obtained at the price of lower mobility and lower thermal conductivity than GaAs. Here we demonstrate subpicosecond carrier sweep-out and over an order of magnitude higher sensitivity in detection from a GaAs-based PC switch by using a nanoplasmonic structure. As compared to a conventional GaAs PC switch, we observe 40 times the peak-to-peak response from the nanoplasmonic structure on GaAs. The response is double that of a commercial, antireflection coated LT-GaAs PC switch.

Single-walled carbon nanotubes as base material for THz photoconductive switching: a theoretical study from input power to output THz emission.

This paper studies the relation between photoexcitation of a single-walled carbon nanotube (SWNT) based device, and its THz output power in the context of THz photoconductive (PC) switching and THz photomixing. A detailed approach of calculating output THz power for such a device describes the effect of each parameter on the performance of the THz PC switch and highlights the design dependent achievable limits. A numerical assessment, with typical values for each parameter, shows that-subject to thermal stability of the device-SWNT based PC switch can improve the output power by almost two orders of magnitudes compared to conventional materials such as LT-GaAs.

Probability based modeling of cross phase modulation in 16-QAM dispersion compensated coherent systems.

A probability-based model is developed to describe cross phase modulation in multichannel multilevel amplitude/phase modulated coherent systems. Standard deviation of nonlinear phase-shift is evaluated in 16-QAM coherent systems accordingly and by numerical simulation for different values of chromatic dispersion and symbol rate. Furthermore, an error analysis is provided to evaluate the accuracy of the model which demonstrates maximum relative error of 12% in the field of interest.

Tuning plasmonic resonances of an annular aperture in metal plate.

We present theory to describe the plasmonic resonances of a subwavelength annular aperture in a real metal plate. The theory provides the reflection, including the amplitude and phase, of radially polarized surface plasmon waves from the end faces of the aperture with a significant departure from the perfect electric conductor case due to plasmonic effects. Oscillations in the reflection amplitude and phase are observed. These oscillations arise from transverse resonances and depend on the geometry of the annulus. The theory is applied to the design of various aperture structures operating at the same resonance wavelength, and it is confirmed by comprehensive electromagnetic simulations. The results are contrasted to the perfect electric conductor case and they will be of significant interest to emerging applications in metamaterials, plasmonic sensors, and near-field optics.

Efficient terahertz slot-line waveguides.

We present two solutions to the challenge of radiation loss of slot-lines at terahertz frequencies: using a slot-line in a homogeneous medium, and using a slot-line on a layered substrate. A theoretical analysis of the slot-line in a homogeneous medium as a terahertz transmission line is presented. The absorption coefficient is obtained in terms of the waveguide dimensions using the field distribution of the slot-line. Results show that the slot-line in a homogeneous medium and the slot-line on a layered substrate can be effective transmission lines for terahertz waves with 2 cm(-1) and 3 cm(-1) absorption due to conductor loss. Full-wave numerical simulations using the Finite Element Method (FEM) are applied to validate the theory.

Coupling of terahertz waves to a two-wire waveguide.

We calculate theoretically the coupling of a terahertz wave from a dipole into a two-wire waveguide. The field transmission and reflection are obtained using a Single Mode Matching (SMM) technique at the input port of the two-wire waveguide. The results show more than 70 percent coupling efficiency for the waveguide using 500 µm radii wires with 2mm center-to-center separation and the exciting field cross section of 1mm × 1mm. The results also show good agreement with the full-wave numerical simulations using the Finite Element Method (FEM).

Two-wire waveguide for terahertz.

We present a rigorous theoretical analysis of the two-wire waveguide. Obtaining the attenuation constant in terms of the dimensions of the waveguide analytically, we show that the absorption coefficient can be less than 0.01 cm(-1), with the appropriate values of the dimensions.

Expansion of field of view in digital in-line holography with a programmable point source.

We present a technique for programming the source of the spherical reference illumination in digital in-line holography using digital micromirror devices. The programmable point source is achieved by individually addressing the elements of a digital micromirror device to spatially control the illumination of the object located at some distance from the source of the spherical reference field. By moving the location of the "ON" element on the digital micromirror device, translation of both the source of the spherical reference beam and the captured holograms is achieved. Results obtained through numerical reconstruction of these translated holograms shows the possibility of expanding the field of view by about 263%.

Applications of digital micro-mirror devices to digital optical microscope dynamic range enhancement.

In this paper, we present a method of using digital micro-mirror devices to dynamic range enhancement of a digital optical microscope images. Our adaptive feedback illumination control generates a high dynamic range image through an algorithm that combines the DMD-to-camera pixel geometrical mapping and a feedback operation. The feedback process automatically generates an illumination pattern in an iterative fashion that spatially modulates the DMD array elements on pixel-by-pixel level. Via experiment, we demonstrate a system that uses precise DMD control of the projector to enhance the dynamic range ideally by a factor of 573. Results are presented showing approximately 5 times the camera dynamic range, enabling visualization over a wide range of specimen characteristics.