Plasmon enhanced high power photoconductive antenna for THz endoscope

Abstract

Terahertz endoscopy has great potential for detecting cancers in early stage. The terahertz frequency regime is highly sensitive to physiological characteristics related to a water molecule, since the vibration and rotational energy states of the water molecule corresponds to the terahertz frequency regime. However, also the absorption due to water is high, development of a high power terahertz emitter is demanded to implement the terahertz endoscopy for clinical applications. A photoconductive(PC) emitter optically pumped by a Ti:Sapphire oscillator is one of the most widely used terahertz emitter systems in these days. Currently reported high power terahertz PC emitter employs an interdigitated electrode arrays with narrow gaps. An advantage of this structure is that the terahertz wave emitted from the each electrode gap constructively interferes at far field that results in a high intensity of the terahertz wave. If the optical pumping area becomes large, the more number of gaps participate in interference so that a further higher intensity of the terahertz wave can be obtained. To achieve this, however, the photocarrier transition rate should be enhanced to compensate the lowering of intensity of pump beam while it illuminates a wide area. This is the case with any other typical PC emitters. The emission power of terahertz PC emitter can be further enhanced with increasing optical pumping power to obtain higher carrier transition rate. A Ti:Sapphire amplifier laser system can be used for achieving higher power optical pumping than that of the Ti:Sapphire oscillator system, yet the cost efficiency is low. In this work, a THz photoconductive antenna(PCA) with high carrier transition rate under optical pumping of Ti:Sapphire oscillator system is demonstrated via surface plasmon excitation using hierarchically patterned silver nanoislands, namely a plasmon enhanced photoconductive antenna(P-PCA). Silver nanoislands with a diameter of 20nm is hierarchically ...