In the shadow of the laser phantom needle cross: dynamic air-plasma aperture sheds light on terahertz microscopy

Two plasma filaments crossing above the target create a subwavelength window for terahertz microscopy that excludes any subwavelength probe in vicinity

Analysis of 3D-printed metal for rapid-prototyped reflective terahertz optics

We explore the potential of 3D metal printing to realize complex conductive terahertz devices. Factors impacting performance such as printing resolution, surface roughness, oxidation, and material loss are investigated via analytical, numerical, and experimental approaches. The high degree of control offered by a 3D-printed topology is exploited to realize a zone plate operating at 530 GHz. Reflection efficiency at this frequency is found to be over 90%. The high-performance of this preliminary device suggest that 3D metal printing can play a strong role in guided-wave and general beam control devices in the terahertz range.Comment: 13 pages, 6 figures, submitted to Optics Expres

Directional excitation of surface plasmons by dielectric resonators

An important aim of current research on plasmonics is to develop compact components to manipulate surface plasmon polaritons (SPPs) and specifically to develop efficient SPP couplers. The commonly used metallic resonators are inefficient to couple free-space waves to SPPs and metallic gratings require oblique incidence for achieving unidirectional propagation. In this article, we propose to use nanoscale nonuniform arrays of dielectric resonator antennas (DRAs) to realize unidirectional launching of SPPs. DRAs are made of low-loss high-permittivity nanostructures operating on a metal surface. The applications of metallodielectric nanostructures can produce resonances mainly in the low-loss dielectric parts and hence the power dissipated through oscillating current in metal can be reduced. Similar to metallic resonators, DRAs operating near resonance can provide phase control when coupling incident waves into SPPs, adding degrees of freedom in controlling propagation direction. The theoretical analysis in this article, with numerical validation, shows efficient SPPs launching by nonuniform array of cylindrical DRAs into a predesigned direction. Furthermore, with proper patterning, optimal launching can be achieved by avoiding power leakage via deflection into free space. The SPP launching condition and the influence of propagation loss are also mathematically analyzed from the viewpoint of antenna array theory. The SPPs launchers based on DRAs have a potential for applications in highly efficient integrated optics and optical waveguides.C. Fumeaux acknowledges the Australian Research Council (ARC) Future Fellowship funding scheme for support under Grant No. FT100100585

Metamaterial-inspired multichannel thin-film sensor

A multichannel thin-film sensor is implemented from a set of microstrip-coupled split-ring resonators (SRR's) with different dimensions. Each SRR exhibits a unique high-Q resonance that is sensitive to the presence of a sample in a particular area. Hence, this SRR-based sensor can function (i) to detect different samples simultaneously to increase the throughput or (ii) to characterise nominally identical samples at multiple frequencies to increase the sensor selectivity. The design principle is validated with simulation and measurement. Owing to the optimized design, sensing a low-permittivity film with a thickness as small as one thousandth of the operating wavelength is achievable.Comment: 7 pages, 4 figure

Sub-diffraction thin-film sensing with planar terahertz metamaterials

Planar metamaterials have been recently proposed for thin dielectric film sensing in the terahertz frequency range. Although the thickness of the dielectric film can be very small compared with the wavelength, the required area of sensed material is still determined by the diffraction-limited spot size of the terahertz beam excitation. In this article, terahertz near-field sensing is utilized to reduce the spot size. By positioning the metamaterial sensing platform close to the sub-diffraction terahertz source, the number of excited resonators, and hence minimal film area, are significantly reduced. As an additional advantage, a reduction in the number of excited resonators decreases the inter-cell coupling strength, and consequently the resonance Q factor is remarkably increased. The experimental results show that the resonance Q factor is improved by 113%. Moreover, for a film with a thickness of \lambda/375 the minimal area can be as small as 0.2\lambda by 0.2\lambda. The success of this work provides a platform for future metamaterial-based sensors for biomolecular detection.Comment: 8 pages, 6 figure

Metamaterial-based microfluidic sensor for dielectric characterization

A microfluidic sensor is implemented from a single split-ring resonator (SRR), a fundamental building block of electromagnetic metamaterials. At resonance, an SRR establishes an intense electric field confined within a deeply subwavelength region. Liquid flowing in a micro-channel laid on this region can alter the local field distribution and hence affect the SRR resonance behavior. Specifically, the resonance frequency and bandwidth are influenced by the complex dielectric permittivity of the liquid sample. The empirical relation between the sensor resonance and the sample permittivity can be established, and from this relation, the complex permittivity of liquid samples can be estimated. The technique is capable of sensing liquid flowing in the channel with a cross-sectional area as small as (0.001λ0)2, where λ0 denotes the free-space wavelength of the wave excitation. This work motivates the use of SRR-based microfluidic sensors for identification, classification, and characterization of chemical and biochemical analytes. © 2012 Elsevier B.V.Withawat Withayachumnankul, Kata Jaruwongrungsee, Adisorn Tuantranont, Christophe Fumeaux, Derek Abbot

Compact electric-LC resonators for metamaterials

Alternative designs to an electric-LC (ELC) resonator, which is a type of metamaterial inclusion, are presented in this article. Fitting the resonator with an interdigital capacitor (IDC) helps to increase the total capacitance of the structure. In effect, its resonance frequency is shifted downwards. This implies a decreased overall resonator size with respect to its operating wavelength. As a result, the metamaterial, composed of an array of IDC-loaded ELC resonators with their collective electromagnetic response, possesses improved homogeneity and hence is less influenced by diffraction effects of individual cells. The impact of incorporating an IDC into ELC resonators in terms of the electrical size at resonance and other relevant properties are investigated through both simulation and experiment.Comment: 5 pages, 5 figure

Optimisation of sample thickness for THz-TDS measurements

How thick should the sample be for a transmission THz-TDS measurement? Should the sample be as thick as possible? The answer is `no'. Although more thickness allows T-rays to interact more with bulk material, SNR rolls off with thickness due to signal attenuation. Then, should the sample be extremely thin? Again, the answer is `no'. A sample that is too thin renders itself nearly invisible to T-rays, in such a way that the system can hardly sense the difference between the sample and a free space path. So, where is the optimal boundary between `too thick' and `too thin'? The trade-off is analysed and revealed in this paper, where our approach is to find the optimal thickness that results in the minimal variance of measured optical constants.Comment: 13 pages, 11 figure

Ultrasensitive terahertz sensing with high-Q Fano resonances in metasurfaces

High quality factor resonances are extremely promising for designing ultra-sensitive refractive index label-free sensors, since it allows intense interaction between electromagnetic waves and the analyte material. Metamaterial and plasmonic sensing have recently attracted a lot of attention due to subwavelength confinement of electromagnetic fields in the resonant structures. However, the excitation of high quality factor resonances in these systems has been a challenge. We excite an order of magnitude higher quality factor resonances in planar terahertz metamaterials that we exploit for ultrasensitive sensing. The low-loss quadrupole and Fano resonances with extremely narrow linewidths enable us to measure the minute spectral shift caused due to the smallest change in the refractive index of the surrounding media. We achieve sensitivity levels of 7.75 X 10^3nm/refractive index unit (RIU) with quadrupole and 5.7 X 10^4nm/RIU with the Fano resonances which could be further enhanced by using thinner substrates. These findings would facilitate the design of ultrasensitive real time chemical and biomolecular sensors in the fingerprint region of the terahertz regime.Peer reviewedElectrical and Computer Engineerin