Evgenya I. Smirnova

Thin-film sensing with planar terahertz metamaterials: sensitivity and limitations.

The limiting effects of varying the thickness of a dielectric overlayer on planar double split-ring resonator (SRR) arrays are studied by terahertz time-domain spectroscopy. Uniform dielectric overlayers from 100 nm to 16 mum thick are deposited onto fixed SRR arrays in order to shift the resonance frequency of the electric response. We discuss the bounds of resonance shifting and emphasize the resulting limitations for SRR-based sensing. These results are presented in the context of typical biosensing situations and are compared to previous work and other existing sensing platforms.

Optically thin terahertz metamaterials

Resonant properties of optically thin metamaterials are studied by terahertz time-domain spectroscopy. Both the lower energy inductor-capacitor (LC) and the higher energy dipole resonances of the planar double split-ring resonators (SRRs) exhibit characteristic evolution with various sub-skin-depth thicknesses of the constituent Pb film. The signature of the LC resonance begins to emerge at a critical thickness near 0.15 skin depth. The resonances reveal a characteristic enhancement; they are strengthened remarkably with increasing SRR thicknesses at sub-skin-depth level and then gradually saturate beyond the skin depth.

Characterization and analysis of terahertz metamaterials based on rectangular split-ring resonators

We present the experimental characterization of planar terahertz metamaterials based on rectangular electric split-ring resonator designs. Comparisons to square-ring designs reveal that rectangular shapes greatly affect the overall metamaterial response by altering the spectral separation and coupling between multiple ring resonances. A simple model is used to help us understand this coupling behavior and the extent of its effects. Advantages and disadvantages of these unconventional ring designs are discussed in terms of possible applications.

Metamaterials for THz polarimetric devices

We present experimental and numerical investigations of planar terahertz metamaterial structures designed to interact with the state of polarization. The dependence of metamaterial resonances on polarization results in unique amplitude and phase characteristics of the terahertz transmission, providing the basis for polarimetric terahertz devices. We highlight some potential applications for polarimetric devices and present simulations of a terahertz quarter-wave plate and a polarizing terahertz beam splitter. Although this work was performed at terahertz frequencies, it may find applications in other frequency ranges as well.

Effects of Microstructure Variations on Macroscopic Terahertz Metafilm Properties

The properties of planar, single-layer metamaterials, or metafilms, are studied by varying the structural components of the split-ring resonators used to comprise the overall medium. Measurements and simulations reveal how minor design variations in split-ring resonator structures can result in significant changes in the macroscopic properties of the metafilm. A transmission-line/circuit model is also used to clarify some of the behavior and design limitations of the metafilms. Though our results are illustrated in the terahertz frequency range, the work has broader implications, particularly with respect to filtering, modulation, and switching devices.

Large-area metamaterials on thin membranes for multilayer and curved applications at terahertz and higher frequencies

A possible path for fabricating three-dimensional metamaterials with curved geometries at optical and infrared frequencies is to stack flexible metamaterial layers. We have fabricated highly uniform metamaterials at terahertz frequencies on large-area, low-stress, free-standing 1 μm thick silicon nitride membranes. Their response remains comparable to that of similar structures on thick substrates as measured by the quality factor of the resonances. Transmission measurements with a Fourier transform infrared spectrometer highlight the advantage of fabricating high frequency metamaterials on thin membranes as etalon effects are eliminated. Releasing the membranes enables layering schemes and placement onto curved surfaces in order to create three-dimensional structures.

Beam Line Design, Beam Alignment Procedure, and Initial Results for the

A gain experiment was performed at Los Alamos using a 120-keV 2-A cylindrical electron beam with a ridged waveguide slow-wave structure at 94 GHz, demonstrating 22 dB of amplification through a traveling-wave interaction. The structure was planar with a gap of 0.75 mm and a length of 5 cm. The 2-A electron beam was confined in a 3.2-kG axial magnetic field, with roughly a 0.5-mm diameter. The electron beam was aligned along the magnetic axis of the solenoid by scribing out its cyclotron motion on a novel optical diagnostic using a procedure that depends on varying the solenoidal field strength. The transport through the structure was verified by letting the beam drill holes in a series of thin metallic foils before insertion of the structure

W

We have designed, fabricated, and tested with low power a novel W-band traveling-wave tube (TWT) structure based on a slow-wave cylindrically symmetric photonic band gap (PBG) structure, or an ldquoomniguide.rdquo The development of wideband millimeter-wave power amplifiers is underway at Los Alamos National Laboratory. PBG TWT structures have great potential for very large bandwidth and linear dispersion. In addition, being cheap to fabricate, the PBG structures enhance commercial transferability of W-band TWT technology. The omniguide structure was designed and fabricated with silica dielectric with a copper harness. Cold-test results were found to be in excellent agreement with the design. A bandwidth of more than 10% was demonstrated. This structure is designed to generate millimeter-wave RF when driven by a 2-A 120-keV electron beam.

-Band Gain Experiment at Los Alamos

Summary form only given. A sheet-beam traveling-wave amplifier has been proposed as a high-power generator for RF from 95 to 300 GHz, using a microfabricated RF slow-wave structure. The planar geometry of microfabrication technologies matches well with the nearly planar geometry of a sheet beam, and the greater allowable beam current leads to high-peak power (up to 500 kW at 95 GHz), high-average power (up to 5 kW), and wide bandwidths (up to 10%). Simulations have indicated gains in excess of 1 dB/mm, with extraction efficiencies greater than 20%.

Design, Fabrication, and Low-Power Tests of a W-band Omniguide Traveling-Wave Tube Structure

We have recently proposed an experiment on verification of the Reverse Cherenkov Radiation (RCR) effect in a Left-Handed-Material-loaded waveguide [1]. Applications of the RCR effect may range from novel higher-order-mode suppressors in microwave and millimeter-wave sources to improved particle detectors for satellite non-proliferation missions. The experimental configuration includes a circular waveguide filled with an artificial metamaterial with simultaneously negative permittivity and permeability, in which the electromagnetic wave with a frequency of 95 GHz will interact with an electron beam. We have demonstrated that for certain values of effective permittivity and permeability only the backward-propagating mode can be exited by the electron beam. At the conference we will present some newly developed metamaterial designs, which we plan to employ for producing the proper effective medium parameters for this experiment.