The aim of this research program is to demonstrate new techniques for sensing and nonlinear spectroscopy using terahertz pulses with very high peak electric fields. Our approach is to combine our recent work on plasmon-induced field enhancements with newly developed techniques for high-energy terahertz pulse generation. Plasmonic interactions near subwavelength metal structures give rise to tiny regions of enhanced electromagnetic energy density. When these structures are excited by high-energy ultrashort pulses, the resulting field strengths will be huge, well into the regime of nonlinear optics. The combination of these two ideas, optimized for terahertz radiation, will open up a new realm of terahertz nonlinear optics. Intellectual Merit The intellectual merit of this research program lies in the development of sensitive new techniques for time-resolved spectroscopic studies based on nonlinear interactions induced by strong terahertz fields. In addition, the ability to detect very small quantities of an analyte using terahertz nonlinear optical effects, with high temporal and spatial resolution, will reveal new dynamical processes previously obscured by limitations of sample size and inhomogeneity. Finally, this work will enable the study of nonlinear surface plasmon propagation, and will contribute to our understanding of high-field plasmon dynamics from terahertz to optical frequencies. Broader Impacts By generating the highest terahertz fields yet reported and studying their interaction with materials, this work will establish the possibility of exploiting higher-order nonlinear interactions. It will also demonstrate new techniques for terahertz-based sensing, which are of great technological importance. More broadly, this research will change our view of the limits of science in the THz regime by establishing a new discipline of high-field terahertz light-matter interactions.
The terahertz portion of the electromagnetic spectrum lies between the realms of electronics and photonics. At lower frequencies, we are all familiar with radios, cell phones, and microwave devices. At higher frequencies, we know about lasers and infrared remote controls for consumer electronics. In between, the terahertz regime has been known as "the gap", because sources, detectors, and components for manipulating this radiation are all much less mature. Our research project is directed towards the development of some of these missing components and capabilities, using the ideas of waveguides, as well as engineered metal-dielectric surfaces known as metamaterials. One good example of our work can be found in considering a simple waveguide geometry, formed by placing two metal plates parallel to each other, with a gap in between. This parallel-plate waveguide (PPWG) can guide radiation, with low loss and low distortion, if the width of the gap is chosen correctly. Indeed, several different guided modes are possible, depending on the pattern and orientation of the electric field of the guided wave. We have recently pioneered the use of one particular PPWG mode, the so-called TE1 mode, in which the electric field is always parallel to the plate surfaces. The TE1 mode has a number of interesting advantages, including the possibility for extremely low losses. One aspect of our research was to build a waveguide which would allow us to characterize these losses experimentally, and confirm the theoretical predictions of extremely low-loss propagation. Another interesting aspect of the TE1 mode is that radiation propagates at different speeds, depending on its frequency and the plate spacing. We can take advantage of this fact by varying the plate spacing, so that (for a given frequency) we can channel and guide radiation in unusual ways. Using this idea, we have demonstrated a universal filter for terahertz radiation, unusual optical components such as a fish-eye lens, and the first example of a terahertz mirage in which radiation propagates around an obstruction, rendering it effectively invisible. We have also continued our collaboration with researchers at Los Alamos and Sandia National Laboratories, in studies of terahertz metamaterial films. Metamaterials are artificial structures composed of a collection of sub-wavelength metal elements, which can exhibit properties that are not found in natural materials. In our work, we leverage the discovery of switchable metamaterials, where the metamaterial behavior can be switched on and off by applying a few volts to the surface of the film. We have shown that such films can be divided up into a collection of pixels, with each element independently switchable. This can be used to control a terahertz wavefront, in the manner of a spatial light modulator. We have also shown that, by shaping our pixels into a series of long thin rectangles, we can effectively create a grating across the terahertz wave front. This grating can be switched on and off, since each pixel (column) is independently switchable. The terahertz radiation diffracted from this grating therefore has a very large on-off ratio (since, when no voltage is applied to any of the columns, there is no grating to give rise to a diffracted beam). As a result, this device switches a terahertz beam on and off with a very high dynamic range: the ratio of the beam in the "on" state to that in the "off" state is greater than 100, which is the largest ratio ever demonstrated for an electrically controlled terahertz modulator. Our results are very promising for the development of future wireless communications and imaging systems which will rely on terahertz radiation.
