THz diffractive lenses have recently gathered a lot of attention as an effective alternative to conventional THz lenses which are bulky, thick, expensive and suffer from strong wavefront (geometric and chromatic) aberrations. It has already been shown that employing a direct binary search technique on the actual height profile of diffractive elements (DEs) can yield designs with better or comparable performance metrics with respect to kinoforms. Such search techniques are however, limited by their exponential time complexity in an unstructured design solution space. If the solution can be proven to exist, we can further perform a gradient descent optimization along with the binary search to overcome the time complexity required to arrive at the desired solution.
The semi-analytic and measured efficiency for all diffractive lenses under both narrowband and broadband focusing is > 80% based on our modified DBS employed design prediction. The modified DBS is observed to converge much faster for both 1D and 2D diffractive lens cases (>10X) with respect to conventional direct binary search based design prediction. For 2D diffractive lenses, the figure of merit is initially high and takes a longer time to converge to the desired solution, which can be understood from the fact that the number of “dielectric” pixels in a 2D lens is much greater than its 1D analogue. Furthermore, since the direct binary search is an iterative algorithm, it convergence depends a lot on the initial random pixel height profile, which is not the case in the modified DBS method.
Conventional plasmonic materials are typically fabricated using a single homogenous metal and structured to obtain useful functionality. Alternatively, structures are occasionally made in which several homogenous materials are deposited using a layer-by-layer process, such as metal-dielectric-metal structures . However additional control over the propagation properties of surface plasmon-polaritons should be possible if the metal conductivity could also be varied spatially. This is not straightforward using conventional microfabrication techniques.
We demonstrate the ability to vary the conductivity spatially using a conventional inkjet printer, yielding either step-wise changes or continuous changes in the conductivity. We accomplish this using a commercially available inkjet printer, where one inkjet cartridge is filled with conductive silver ink and a second cartridge is filled with resistive carbon ink. By varying the fractional amounts of the two inks in each printed dot, we can spatially vary the conductivity. The silver ink has a DC conductivity that is only a factor of six lower than the bulk silver, while the carbon ink acts as a lossy dielectric at terahertz frequencies. Both inks sinter immediately after being printed on a treated PET transparency.
We demonstrate the utility of this approach with both plasmonics and metamaterial applications, demonstrating the ability to control beam profiles, create new filter capabilities and hide images in THz metasurfaces.
We analyze the terahertz properties of complex oxide hetero-structures with record-high carrier concentration approaching 1015 cm-2. Our results evidence a large room temperature terahertz conductivity, which corresponds to 3X to 6X larger mobility than what is extracted from electrical measurements. That is, in spite of a relatively lower mobility, when taking into account its ultra-large carrier concentration, the 2DEG in complex oxide hetero-structures can still attain a large terahertz conductivity, which is comparable with that in traditional high-mobility semiconductors or large-area CVVD graphene films. Moreover, we also discuss the perspectives off these hetero-structures for terahertz and high frequency electronic applications.