Novel aspheric lenses for terahertz imaging

Abstract

We investigate the use of aberration-free aspheric lenses for terahertz imaging and spectroscopy. Replacing the traditionally used off-axis parabolic mirrors with lenses solves the problems of difficult alignment, susceptibility to aberrations, and limitation in terms of the numerical aperture. Carefully designing the lenses enables sub-wavelength imaging resolutions. Since the wavelength of terahertz radiation is much longer compared to optics, we employed geometric optics for designing the aspheric lenses to ensure zero aberration. We use ultra high molecular weight polyethylene as the Iens material, which has a refractive index similar to that of glass for optical wavelengths. The propagation of the beam, hence the focal spot size and the resulting spatial resolution, is determined using KirchhofTs scalar diffraction theory. Simulations show very promising imaging performances, in particular our novel symmetric-pass Iens design. Several experiments were carried out to verify the theoretical predictions. We show that the symmetric-pass Iens is able to achieve sub-wavelength resolution as Iow as 0.61λ. Moreover, in principle, our Iens designs presented in this thesis are applicable to other parts of the electromagnetic spectrum, depending on their application.