The unique optical properties of metals are at the core of many areas of research and applications, including plasmonics, metamaterials, superlensing and subdiffraction focusing, optical antennas and ...surface enhanced Raman scattering. One important length scale inherent in metamaterials and plasmonics research activities in the microwave, terahertz, infrared, visible and ultraviolet ranges is the skin depth of metal, which remains at the submicrometre level throughout the broad spectral range. One prominent question is whether terahertz electromagnetic waves can be controlled on the nanoscale to achieve new functionalities in the sub-skin-depth regime. Here, we show that a λ/30,000 slit on metal film acts as a nanogap-capacitor charged by light-induced currents, enhancing the electric field by orders of magnitudes.
The nano-size metallic aperture array has been fabricated using conventional semiconductor fabrication techniques and focused ion beam. The micron size patterns were transferred on SOI wafer using ...photographic patterning process, and the plateau pyramidal structures were fabricated using wet Si etching with alkaline solution followed by thermal oxidation. In order to reveal the plateau type pyramid, the backside Si bulk etching was performed. The metallic Al thin film was deposited and the nano-apertures with the diameters ranging from 190
nm to 380
nm were fabricated using focused ion beam milling method. The skin-depth of the Al thin film at 532
nm wavelength is known to be ∼10
nm. The deposited Al thin film is thick enough to become opaque. Circular types of periodic grooves were fabricated around the sub-wavelength size nano-aperture in order to examine the transmittance of the light coupled to circular periodic grooves. The far-field measurements for the optical transmittance through a sub-wavelength size nano-probe have been investigated and found to be up to 10
3 fold increase, compared to the output intensity of aperture without groove patterns.
We show that terahertz waves transmit through a lambda/30,000 nanogap separating two conducting planes. The field enhancement responsible for the light funneling has a 1/f dependence, which ...originates from the charging time of the gap.
We observe that rectangular holes fabricated in a gold film with a sub-skindepth thickness show blue-shifted resonance frequencies as the width increases. For rectangular holes on a finite substrate ...its thickness and the hole width are crucial factors determining the resonance frequency.
We demonstrate that platinum nanorods across the gap of a nanoresonator shift or overdamp the resonance depending on the rod-size. Moreover, what is striking is that the thinner nanorods almost ...completely turn off both the fundamental and higher mode resonances, analogous to the pinch harmonics.
We measure field enhancement through nanogap by using both terahertz time domain spectroscopy (TDS) and terahertz imaging technique with continuous wave (CW) radiation source. Far-field amplitude is ...connected to the near-field enhancement through the Kirchhoff formalism. With TDS, we found that a field enhancement of 200 at 0.1 THz for the 500 nm slit. To confirm our TDS results, we also perform the transmission experiment with 70 nm width nanogap structure perforated in thin gold films on Si substrates by using 0.2 THz CW source.
We have measured transmission properties of a composite structure consisting of nano-rods on a long (a y = 300 micron) nano gap (70 nm) on Au film in broad frequency range of 0.1 THz to 1.0 THz using ...THz time-domain spectroscopy. The normalized transmittance with no nano-bridge or nano-rod structure in the middle shows a half-wavelength resonance: the resonance frequency is ~c/(2na y ) where n is the index of refraction of the substrate. The nano-size bridge at the center of the nano gap gives changes the resonance characteristics profoundly, because in essence, the length a y now halves. Mostly the same resonance-changing behavior is expected with a nano-rod structure fabricated by Pt-deposition method using a focused ion beam (FIB). This small rod also acts as a bridge dividing the length of the rectangle. The structure dependent resonance allows to detect nano-size particles and to tailor resonance characteristics with feature sizes of lambda/10,000.
