Subwavelength imaging using plasmonic lenses at terahertz frequencies

The effects of diffraction at terahertz frequencies limit the spatial resolution of imaging systems. One approach to achieve subwavelength resolution is near-field imaging using a subwavelength aperture, however, the low transmission through subwavelength apertures limits the sensitivity of this approach. Plasmonic lenses in the form of bullseye structures, consisting of a circular subwavelength aperture with concentric periodic corrugations, have demonstrated enhanced transmission and beam confinement. This thesis discusses the design criteria of plasmonic lenses optimized for 325 GHz. Fabrication for optical applications is traditionally achieved by nanolithography. Since the scale of plasmonic structures depends on the wavelength, precision micromilling techniques are well suited for terahertz applications. Theoretical simulations are obtained using a finite-difference time-domain solver and the performance the devices are evaluated using a customized terahertz testbed. The prospect of using plasmonic lenses in a terahertz imaging configuration for the diagnosis of cancer is also discussed

Enhanced transmission and beam confinement using bullseye plasmonic lenses at THz frequencies

A major limitation in terahertz (THz) imaging applications is the relatively poor diffraction limited spatial resolution. A common approach to achieve subwavelength resolution is near-field imaging using a subwavelength aperture, but the low transmission efficiency through the aperture limits the sensitivity of this method. Bullseye structures, consisting of a single subwavelength circular aperture surrounded by concentric periodic corrugations, have been shown to enhance transmission through subwavelength apertures. At optical wavelengths, the fabrication of bullseye structures has been traditionally achieved by lithographic or chemical processes. Since the scale of plasmonic structures depends on the incident wavelength, precision micromilling techniques are well suited for THz applications. In this paper we describe a diamond micromilling process for the fabrication a plasmonic lenses operating at 325 GHz. Theoretical simulations are obtained using an FDTD solver and the performance of the lens is measured using a customized THz test bed.Peer reviewed: YesNRC publication: Ye

Enhanced terahertz transmission through bullseye plasmonics lenses fabricated using micromilling techniques

Imaging applications at terahertz frequencies are, in general, limited to relatively low spatial resolution due to the effects of diffraction. By using a subwavelength aperture in the near-field, however, it is possible to achieve subwavelength resolution, although low transmission through the aperture limits the sensitivity of this approach. Plasmonic lenses in the form of bullseye structures, which consist of a circular subwavelength aperture surrounded by concentric periodic corrugations, have demonstrated enhanced transmission, thereby increasing the utility of near-field imaging configurations. In this paper, the design, fabrication, and experimental performance of plasmonic lenses optimized for 300 GHz are discussed. While nanofabrication techniques are required for optical applications, microfabrication techniques are sufficient for terahertz applications. The process flow for fabricating a double-sided bullseye structure using a precision micromilling technique is described. Transmission and beam profile measurements using a customized terahertz testbed are presented.Peer reviewed: YesNRC publication: Ye

Enhanced terahertz transmission through bullseye plasmonics lenses fabricated using micromilling techniques

Imaging applications at terahertz frequencies are, in general, limited to relatively low spatial resolution due to the effects of diffraction. By using a subwavelength aperture in the near-field, however, it is possible to achieve subwavelength resolution, although low transmission through the aperture limits the sensitivity of this approach. Plasmonic lenses in the form of bullseye structures, which consist of a circular subwavelength aperture surrounded by concentric periodic corrugations, have demonstrated enhanced transmission, thereby increasing the utility of near-field imaging configurations. In this paper, the design, fabrication, and experimental performance of plasmonic lenses optimized for 300 GHz are discussed. While nanofabrication techniques are required for optical applications, microfabrication techniques are sufficient for terahertz applications. The process flow for fabricating a double-sided bullseye structure using a precision micromilling technique is described. Transmission and beam profile measurements using a customized terahertz testbed are presented.Peer reviewed: YesNRC publication: Ye

Search for continuous gravitational waves from neutron stars in globular cluster NGC 6544

We describe a directed search for continuous gravitational waves in data from the sixth initial LIGO science run. The target was the nearby globular cluster NGC 6544 at a distance of ≈ 2.7     kpc . The search covered a broad band of frequencies along with first and second frequency derivatives for a fixed sky position. The search coherently integrated data from the two LIGO interferometers over a time span of 9.2 days using the matched-filtering F -statistic. We found no gravitational-wave signals and set 95% confidence upper limits as stringent as 6.0 × 10 − 25 on intrinsic strain and 8.5 × 10 − 6 on fiducial ellipticity. These values beat the indirect limits from energy conservation for stars with characteristic spin-down ages older than 300 years and are within the range of theoretical predictions for possible neutron-star ellipticities. An important feature of this search was use of a barycentric resampling algorithm which substantially reduced computational cost; this method is used extensively in searches of Advanced LIGO and Virgo detector data.by Anand Sengupta et al.