Near Zero Index THz Perfect Metasurface Absorber using Inverted Conformal Mapping

摘要

We describe here the use of a metasurface geometry previously reported by the author, based upon geometric inversion of a set of conformal mapping contours, for application in the THz bandwidth as the basis for an uncooled microbolometer in downhole chemical spectroscopy. The resulting geometry forms a nearly continuous series of perfect absorption resonances in a broad bandwidth of the THz gap by an ultrathin (A/1600) metasurface. The metasurface is derived from a geometric inversion of the Rhodonea, or more commonly called four-leaf roses, conformal mapping contours and was found to exhibit a near zero index metamaterial behavior. The near zero index properties of the metasurface lead to an absorption phenomenom characterized by surface plasmon resonances that confine the absorption mechanism within the ultrathin metasurface plane and make the absorption properties of the microbolometer design practically independent of the material properties of the remaining laminae. This unusual feature allows the metasurface to be integrated on a single dielectric support layer with a single VO_2 material thermometric layer which is now able to be operated at downhole elevated temperatures within its metal-insulator-transition region. Within this transition region the VO_2 layer is effectively a metallic electrical conductor and exhibits more than an order of magnitude enhancement in its thermometric properties. This leads to a metasurface microbolometer design with predicted maximum detectivity D* = 2.2 × 10~(10)cm(Hz)~(1/2)/W and noise equivalent difference temperature NEDT of 1 mK at a modulation frequency of 50 Hz. These levels of THz detector performance conventionally would be achievable only with cryogenically cooled technologies and could represent a significant step in the effort towards deploying miniaturized uncooled THz sensor devices into oilfield exploration and production applications.