Browsing by Author "Astley, Victoria"

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    A study of background signals in terahertz apertureless near-field microscopy and their use for scattering-probe imaging
    (2009) Astley, Victoria; Mittleman, Daniel M.
    Apertureless near-field microscopy is an imaging technique in which a small metal tip is held close to a surface, converting evanescent waves to propagating waves and permitting sub-wavelength spatial resolution. In the terahertz region of the spectrum, the interpretation of measured signals and the suppression of background scattering can be complicated by the broad bandwidth of the THz source and by the phase-sensitive detection of the scattered radiation. We have analyzed the use of tip-sample distance modulation for the removal of background signals. We find that significant background signals, originating from scattering off the probe tip, can be observed even after modulation. These background signals result from path-length difference modulation, and thus depend on phase-sensitive detection. We use a dipole antenna model to explain the spatial variation of this signal. Since it originates from the tip only, it can be used to characterize free-space terahertz wave fronts with sub-wavelength resolution.
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    A mode-matching analysis of dielectric-filled resonant cavities coupled to terahertz parallelplate waveguides
    (Optical Society of America, 2012-09-07) Astley, Victoria; Reichel, Kimberly S.; Jones, Jonathan; Mendis, Rajind; Mittleman, Daniel M.
    We use the mode-matching technique to study parallel-plate waveguide resonant cavities that are filled with a dielectric. We apply the generalized scattering matrix theory to calculate the power transmission through the waveguide-cavities. We compare the analytical results to experimental data to confirm the validity of this approach.
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    Modifying terahertz waveguide geometries: Bends, tapers, and grooves
    (2012) Astley, Victoria; Mittleman, Daniel M.
    Terahertz waveguides are the focus of considerable research interest due to their potential for sensing, imaging and communications applications. Two of the most promising designs are the metal wire waveguide and the parallel-plate waveguide. The metal wire waveguide exhibits excellent low loss and low dispersion characteristics. However, the radiation is only weakly coupled to the wire and the beam extends a great distance from the waveguide, which can lead to high bending loss. In my research I show that this large beam extent also gives a high degree of flexibility in the geometry required to couple radiation into the waveguide or between waveguide sections. I also show that the traditional formalism of bending loss is incomplete, and that there is an optimum radius of curvature to reduce loss. The relationship between the beam extent and the radius of the wire presents the possibility of a tapered waveguide to confine the radiation as it propagates. I here present experimental data and simulations results to verify this subwavelength confinement at the tip of a tapered metal wire waveguide, which is of great interest for near-field imaging applications. The parallel-plate waveguide is another design frequently employed due to its low loss and low dispersion characteristics. Resonant structures may also be easily incorporated into the waveguide for sensing and filtering applications. One such structure is a single rectangular groove, which serves as a notch filter with a very narrow linewidth when the transverse-electric (TE) mode of the waveguide is excited, though its physical origin is poorly understood. In this work I present a detailed experimental and theoretical study of the rectangular resonant cavity in a TE-mode parallel-plate waveguide, particularly with respect to its potential as a microfluidic refractive index sensor. This study is extended to include the possibility of two grooves, in both coupled and non-coupled geometries, and their efficacy as multichannel or high-resolution single-channel microfluidic sensors.
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    Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor
    (JoVE, 2012) Astley, Victoria; Reichel, Kimberly; Mendis, Rajind; Mittleman, Daniel M.
    Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic samples with a wide range of possible sensor designs such as interferometers and resonators [1,2]. Most of the existing RI sensing applications focus on biological materials in aqueous solutions in visible and IR frequencies, such as DNA hybridization and genome sequencing. At terahertz frequencies, applications include quality control, monitoring of industrial processes and sensing and detection applications involving nonpolar materials. Several potential designs for refractive index sensors in the terahertz regime exist, including photonic crystal waveguides [3], asymmetric splitring resonators [4], and photonic band gap structures integrated into parallel-plate waveguides [5]. Many of these designs are based on optical resonators such as rings or cavities. The resonant frequencies of these structures are dependent on the refractive index of the material in or around the resonator. By monitoring the shifts in resonant frequency the refractive index of a sample can be accurately measured and this in turn can be used to identify a material, monitor contamination or dilution, etc. The sensor design we use here is based on a simple parallel-plate waveguide [6,7]. A rectangular groove machined into one face acts as a resonant cavity (Figures 1 and 2). When terahertz radiation is coupled into the waveguide and propagates in the lowest-order transverse-electric (TE1) mode, the result is a single strong resonant feature with a tunable resonant frequency that is dependent on the geometry of the groove [6,8]. This groove can be filled with nonpolar liquid microfluidic samples which cause a shift in the observed resonant frequency that depends on the amount of liquid in the groove and its refractive index [9]. Our technique has an advantage over other terahertz techniques in its simplicity, both in fabrication and implementation, since the procedure can be accomplished with standard laboratory equipment without the need for a clean room or any special fabrication or experimental techniques. It can also be easily expanded to multichannel operation by the incorporation of multiple grooves [10]. In this video we will describe our complete experimental procedure, from the design of the sensor to the data analysis and determination of the sample refractive index.