We report on the use of standing surface acoustic waves, formed on a single-crystal piezoelectric substrate, to organize micron-scale latex particles into an array comprising a series of lines in an adjacent microfluidic system. The lines of particles are formed parallel to the substrate surface and perpendicular to the surface acoustic wave vector. They extend across the width of the acoustic beam aperture, with a periodicity of one-half the surface acoustic wavelength. The position and spacing of the particle arrays can be altered by adjusting the acoustic wave frequency within the device passband. We discuss the mechanism responsible for the formation of the lines, which could be widely applicable to the alignment of microscopic objects held in suspension.

The two-dimensional concentration and manipulation of micron-scale particles by orthogonal, surface acoustic, standing waves is demonstrated. The particles are organized by liquid pressure waves in a microfluidic system over a piezoelectric substrate and form a uniform two-dimensional array with a spacing governed by the mechanical nodes of the two orthogonal, surface acoustic, standing waves. The nodal spacing can be controlled in each orthogonal direction independently by adjustment of the radio frequency applied to the separate acoustic wave transducers. This technique could be used to enhance the particle concentrations at sensing locations in DNA or protein array detectors.

We report the operation of band-stop filters in the terahertz (THz) frequency range, working at a center frequency of $600\mathrm{GHz}$ . The filters were characterized by embedding them in a microstrip line attached to photoconductive switches which act as THz emitters and detectors. The filters have applications in sensing and detection. The chosen filter design allows cascading of several filters along the same microstrip line, which is of particular importance for a proposed molecular sensing array.

We demonstrate that terahertz microstrip-line waveguides can be used to measure absorption spectra of polycrystalline materials with a high frequency resolution $\left(\sim 2\text{GHz}\right)$ and with a spatial resolution that is determined by the microstrip-line dimensions, rather than the free-space wavelength. The evanescent terahertz-bandwidth electric field extending above the microstrip line interacts with, and is modified by, overlaid dielectric samples, thus enabling the characteristic vibrational absorption resonances in the sample to be probed. As an example, the terahertz absorption spectrum of polycrystalline lactose monohydrate was investigated; the lowest lying mode was observed at $534\left(\pm 2\right)\text{GHz}$ , in excellent agreement with free-space measurements. This microstrip technique offers both a higher spatial and frequency resolution than free-space terahertz time-domain spectroscopy and requires no contact between the waveguide and sample.

We demonstrate the generation of broadband terahertz (THz) frequency radiation from photoconductive emitters formed from Fe-doped InGaAs (Fe:InGaAs), grown by metal-organic chemical vapor deposition, following pulsed (femtosecond) laser excitation at wavelengths ranging from 830 nm to $1.55\mu \text{m}$ . The Fe is incorporated epitaxially during growth, giving precise control over the doping level. Using both single-crystal ZnTe and GaP electro-optic detectors over the same wavelength range, the emission spectra from several Fe:InGaAs wafers with different Fe content were measured, with THz emission from all wafers showing bandwidths in excess of 2.0 THz. The THz output power was found to be strongly dependant on the Fe content, the thickness of the Fe:InGaAs layer, and the excitation wavelength.

Terahertz frequency time-domain spectroscopy employing free-space radiation has frequently been used to probe the elementary excitations of low-dimensional systems. The diffraction limit, however, prevents its use for the in-plane study of individual laterally-defined nanostructures. Here, we demonstrate a planar terahertz frequency plasmonic circuit in which photoconductive material is monolithically integrated with a two-dimensional electron system. Plasmons with a broad spectral range (up to ~ 400 GHz) are excited by injecting picosecond-duration pulses, generated and detected by a photoconductive semiconductor, into a high mobility two-dimensional electron system. Using voltage modulation of a Schottky gate overlying the two-dimensional electron system, we form a tuneable plasmonic cavity and observe electrostatic manipulation of the plasmon resonances. Our technique offers a direct route to access the picosecond dynamics of confined electron transport in a broad range of lateral nanostructures.