A Johns Hopkins Applied Physics Laboratory (APL) team developed infrared (IR) metasurface imaging lenses designed to selectively focus specific states of polarized light (linear and circular) to different locations on a detector array. The lenses' operational characteristics make them well suited to miniaturize future optical sensor systems planned for deployment on small platforms or personnel that cannot support the volume or mass of large optical sensor systems.

The Stokes reflectometer concept presented here takes a novel approach to microwave remote sensing of soils. It relies on a constellation of low-power, low-mass, continuous wave, ultrahighfrequency transmitters and receivers. The systems are arranged in a forward scattering configuration over a range of angles of incidence and scattering set to cross the Brewster angle to retrieve soil properties. Proof of concept is provided by a series of COMSOL Multiphysics finite element runs to model Stokes parameters (SP) of forward-scattered RF signals through layers of varying compositions, dielectric constants, and surface roughness, assuming no sources of contamination during transmission. Significant variations for all four SP were demonstrated. At frequencies around 430 MHz, contamination during transmission is due to Faraday rotation (FR) in the ionosphere. A hybrid-polarimetric configuration minimized ionospheric contamination and provided for simplified expressions to retrieve FR angles and scattering coefficients from the received polarimetric power. With these, SP can be correctly calculated to soil moisture retrievals.

Here we consider a tunable superconducting cavity that can be used either as a tunable coupler to a qubit inside the cavity or as a tunable low noise, low temperature, RF filter. Our design consists of an array of radio-frequency superconducting quantum interference devices (rf SQUIDs) inside a superconducting cavity. This forms a tunable metamaterial structure which couples to the cavity through its magnetic plasma frequency. By tuning the resonant frequency of the metamaterial through an applied magnetic flux, one can tune the cavity mode profile. This allows us to detune the cavity initially centered at 5.593 GHz by over 200 MHz. The maximum quality factor approaches that of the empty cavity, which is 4.5 × 106. The metamaterial electromagnetic response is controlled via a low-frequency or dc magnetic flux bias, and we present a control line architecture that is capable of applying sufficient magnetic flux bias with minimal parasitic coupling. Together this design allows for an in-situ tunable cavity which enables low-temperature quantum control applications.

Overcoming the diffraction limit, which enables focusing much less than the wavelength, requires tailoring the evanescent spectrum of an aperture's field distribution. We model and simulate a corrugated near field plate, which can generate a sub-wavelength focus in inhomogeneous background media. All reactive coupling, between the metasurface near field plate and the focusing domain and among the corrugations in the metasurface, is taken into consideration with the finite element method, which we solve in combination with a constraint to generate a desired focus. Various geometries for the near field plate are considered and we demonstrate that the proposed method can effectively create a deeply sub-wavelength focus within a layered medium having properties resembling brain tissue. Such a device could find use as a detector of biological signals or for hyperthermic treatment near the skin surface.

The emergence of metasurface technology and its accompanying design principles are enabling the development of optical components with multiple functionalities, e.g., polarization discrimination and focusing. In this report, we highlight our experimental results associated with the characterization of a reflective mid-wave infrared (mid-IR) metasurface designed for both imaging and polarization-specific beam splitting in the 4.2 through 4.8 m region of the electromagnetic (EM) spectrum. A large area metasurface (1 cm diameter), fabricated using nano-lithography, was observed to exhibit a high degree of discrimination between transverse electric (TE) and magnetic (TM) polarized light with near diffraction limited focusing.

We have designed, fabricated, and experimentally verified a highly sensitive plasmonic sensing device in the terahertz frequency range. For a proof of concept of the sensing phenomenon, we have chosen the so-called fishnet structure based on circular hole array insensitive to the polarization of the incident wave. We employ the localized resonance associated with the cutoff frequency (electric plasma frequency) of the hole array to investigate its sensing capability. A thin-film overlayer deposited on the surface of the metallic apertures causes an amplitude modulation and a shift in the resonant frequency of the terahertz transmission. The frequency shift and the amplitude modulation were investigated as a function of the refractive index and the thickness of the overlayer for determining the sensing potential of the proposed structure. Measurements carried out using terahertz time-domain spectroscopy show good agreement with the numerical predictions. The results we obtained indicate that the proposed device could be very promising for enhancing the sensing capabilities of the refractive index changes involved in bio-applications, for instance.

We report on the observation of ultrafast impact ionization and carrier generation in high resistivity silicon induced by intense subpicosecond terahertz transients. Local terahertz peak electric fields of several MV cm−1 are obtained by field enhancement in the near field of a resonant metallic antenna array. The carrier multiplication is probed by the frequency shift of the resonance of the antenna array due to the change of the local refractive index of the substrate. Experimental results and simulations show that the carrier density in silicon increases by over seven orders of magnitude in the presence of an intense terahertz field. The enhancement of the resonance shift for illumination from the substrate side in comparison to illumination from the antenna side is consistent with our prediction that the back illumination is highly beneficial for a wide range of nonlinear processes.

Electron microscopy and electron diffraction techniques rely on electron sources.Those sources require strong electric fields to extract electrons from metals, either by the photoelectric effect, driven by multiphoton absorption of strong laser fields, or in the static field emission regime.Terahertz (THz) radiation, commonly understood to be nonionizing due to its low photon energy, is here shown to produce electron field emission.We demonstrate that a carrier-envelope phase-stable single-cycle optical field at THz frequencies interacting with a metallic microantenna can generate and accelerate ultrashort and ultrabright electron bunches into free space, and we use these electrons to excite and ionize ambient nitrogen molecules near the antenna.The associated UV emission from the gas forms a novel THz wave detector, which, in contrast with conventional photon-counting or heat-sensitive devices, is ungated and sensitive to the peak electric field in a strongly nonlinear fashion.

We present a new technique for permanent metamaterial reconfiguration via optically induced mass transfer of gold. This mass transfer, which can be explained by field-emission induced electromigration, causes a geometric change in the metamaterial sample. Since a metamaterial's electromagnetic response is dictated by its geometry, this structural change massively alters the metamaterial's behavior. We show this by optically forming a conducting pathway between two closely spaced dipole antennas, thereby changing the resonance frequency by a factor of two. After discussing the physics of the process, we conclude by presenting an optical fuse that can be used as a sacrificial element to protect sensitive components, demonstrating the applicability of optically induced mass transfer for device design.

Summary form only given. Ultrafast electron field emission from gold resonant antennas induced by strong terahertz (THz) transient is investigated using two THz pulse experiments. It is shown that UV emission from nitrogen plasma generated by liberated electrons is a good indication of the local electric field at the antenna tip. Using this method resonant properties of antennas fabricated on high resistivity silicon are investigated in the strong field regime. Decrease of antenna Q-factor due to ultrafast carrier multiplication in the substrate is observed.

Using a metamaterial composite, we demonstrate a bandpass filter that has only a single transmission mode from 0 to >25 THz. This usable bandwidth matches, or exceeds, that of currently used THz sources.

We present a metamaterial, consisting of a cross structure and a metal mesh filter, that forms a composite with greater functional bandwidth than any terahertz (THz) metamaterial to date. Metamaterials traditionally have a narrow usable bandwidth that is much smaller than common THz sources, such as photoconductive antennas and difference frequency generation. The composite structure shown here expands the usable bandwidth to exceed that of current THz sources. To highlight the applicability of this combination, we demonstrate a series of bandpass filters with only a single pass band, with a central frequency (f0) that is scalable from 0.86–8.51 THz, that highly extinguishes other frequencies up to >240 THz. The performance of these filters is demonstrated in experiment, using both air biased coherent detection and a Fourier transform infrared spectrometer (FTIR), as well as in simulation. We present equations—and discuss their scaling laws—which detail the f0 and full width at half max (Δf) of the pass band, as well as the required geometric dimensions for their fabrication using standard UV photolithography and easily achievable fabrication linewidths. With these equations, the geometric parameters and Δf for a desired frequency can be quickly calculated. Using these bandpass filters as a proof of principle, we believe that this metamaterial composite provides the key for ultra-broadband metamaterial design.

Using a THz camera and THz bandpass filters, we measure the frequency-resolved beam profile emitted from a two-color air plasma. We observe a frequency-independent emission angle from the plasma.

The design of artificial nonlinear materials requires control over internal resonant charge densities and local electric field distributions. We present a MM design with a structurally controllable oscillator strength and local electric field enhancement at terahertz frequencies. The MM consists of a split ring resonator (SRR) array stacked above an array of closed conducting rings. An in-plane, lateral shift of a half unit cell between the SRR and closed ring arrays results in an increase of the MM oscillator strength by a factor of 4 and a 40% change in the amplitude of the resonant electric field enhancement in the SRR capacitive gap. We use terahertz time-domain spectroscopy and numerical simulations to confirm our results. We show that the observed electromagnetic response in this MM is the result of image charges and currents induced in the closed rings by the SRR.

Terahertz (THz) waves have the potential to improve a wide range of devices in the space, defense and semiconductor industries as well as offering the possibility of investigating various molecules of interest in biology, medicine, art etc. For this reason, THz sources, detectors and passive linear components have greatly been developed in recent years. However there is still no any non-linear device available. This is mainly due to the lack of the rigorous investigation of non-linear characteristics of materials in the THz range. We chose to investigate chalcogenide glasses due to their high nonlinear coefficients in the optical range, which might suggest pronounced nonlinear behavior also at THz frequencies. Here we present measurements, both linear and nonlinear, of As2S3 and As2Se3. For the linear measurements we employed a combination of THz time-domain spectroscopy setups. In the low frequency range we used a standard THz-TDS setup based on photoconductive switches while in the higher frequency domain we used an air biased coherent detection (ABCD) setup. This allowed for a wide frequency range (from 0.2 to 18 THz) investigation of the refractive index of the glasses. The nonlinear coefficient in the THz range was investigated using a lithium niobate based setup that allows us to reach electrical field strength exceeding 400 kV/cm. Results from both investigations will be presented during the talk.

We experimentally show that metallic dipole antennas emit ultraviolet radiation when illuminated with ultrashort high-power terahertz pulses due to ultrafast electron field emission from the metal and consecutive ionization and excitation of atmospheric gas molecules.

We report the first observation of THz-field-induced electromigration in sub-wavelength metallic gap structures after exposure to intense single-cycle, sub-picosecond electric field transients of amplitude up to 400 kV/cm.

Development of tunable, dynamic, and broad bandwidth metamaterial designs is a keystone objective for metamaterials research, necessary for the future viability of metamaterial optics and devices across the electromagnetic spectrum. Yet, overcoming the inherently localized, narrow bandwidth, and static response of resonant metamaterials continues to be a challenging endeavor. Resonant perfect absorbers have flourished as one of the most promising metamaterial devices with applications ranging from power harvesting to terahertz imaging. Here, an optically modulated resonant perfect absorber is presented. Utilizing photo‐excited free carriers in silicon pads placed in the capacitive gaps of split ring resonators, a dynamically modulated perfect absorber is designed and fabricated to operate in reflection. Large modulation depth (38% and 91%) in two absorption bands (with 97% and 92% peak absorption) is demonstrated, which correspond to the LC (0.7 THz) and dipole (1.1 THz) modes of the split ring resonators.

We experimentally measure the profile of a THz beam generated by a two-color air plasma as it passes through a beam waist, and show that it can be approximated as a Bessel-Gauss beam.