Jonathan Y. Suen

Characterization and modeling of a terahertz photoconductive switch

We examine the terahertz (THz) performance of an ErAs:GaAs photoconductive switch under varying bias conditions and optical drive power. Despite THz power up to 287 μW, saturation effects were not seen. In addition, the THz power spectra were measured with a Fourier transform infrared spectrometer, and the roll-off was found to be invariant to bias voltage and consistent with a THz pulsewidth of 1.59 ps and a peak power of 3.1 W. These results are confirmed by a large-signal, high-frequency circuit model that suggests that further increase in THz power and efficiency are possible through an increase in the mode-locked laser power and reduction in its pulse width. The model is useful in designing both the laser and photoconductive switches to maximize available power and efficiency.

All-dielectric metasurface absorbers for uncooled terahertz imaging

Imaging in the terahertz (THz) range of the electromagnetic spectrum is difficult owing to the lack of high-power sources and efficient detectors. For decades, there has been tremendous effort to fashion focal plane arrays for THz imaging owing to the great number of potential applications. Here, we propose and demonstrate an alternative approach which utilizes all-dielectric metasurface absorbers that act as universal converters of radiation. Incident THz waves are absorbed by the metasurface, converted to heat, and subsequently detected by an infrared camera. We realize a metasurface consisting of sub-wavelength cylindrical resonators that achieve diffraction-limited imaging at THz frequencies without cooling. The low thermal conductivity and diffusivity significantly limit the thermal conduction between neighboring pixels, thus improving the spatial resolution and imaging time. Similar to conventional metallic-based metamaterials, our all-dielectric metasurface absorber can be scaled to other bands of the electromagnetic spectrum, offering a blueprint to achieve novel uncooled bolometric imaging.

THz Imaging Based on Water-Concentration Contrast

Terahertz medical imaging has emerged as a promising new field because of its non-ionizing photon energy and its acute sensitivity to water concentration. To better understand the primary contrast mechanism in THz imaging of tissues, the reflectivity of varying water concentrations was measured. Using a pulsed THz reflective imaging system, a 0.3 mm thin paper sample with varying water concentrations was probed and from the measured data a noise equivalent delta water concentration (NEΔWC) of 0.054% was derived. The system is based on a photoconductive pulsed source and time-gated waveguide-mounted Schottky diode receiver. It operates at a center frequency of 500 GHz with 125 GHz of noise-equivalent bandwidth and at a standoff of 4 cm, the imaging system achieved a spot size of 2.2 mm. The high water sensitivity of this system was exploited to image burned porcine (pig) skin models in reflection using differences in water content of burned and unburned skin as the contrast mechanism. The obtained images of the porcine skin burns are a step towards the ability to quantify burn injuries using THz radiation.

Directed Energy Active Illumination for Near-Earth Object Detection

On 15 February 2013, a previously unknown ~20 m asteroid struck Earth near Chelyabinsk, Russia, releasing kinetic energy equivalent to ~570 kt TNT. Detecting objects like the Chelyabinsk impactor that are orbiting near Earth is a difficult task, in part because such objects spend much of their own orbits in the direction of the Sun when viewed from Earth. Efforts aimed at protecting Earth from future impacts will rely heavily on continued discovery. Ground-based optical observatory networks and Earth-orbiting spacecraft with infrared sensors have dramatically increased the pace of discovery. Still, less than 5% of near-Earth objects (NEOs) ≥100 m/~100 Mt TNT have been identified, and the proportion of known objects decreases rapidly for smaller sizes. Low emissivity of some objects also makes detection by passive sensors difficult. A proposed orbiting laser phased array directed energy system could be used for active illumination of NEOs, enhancing discovery particularly for smaller and lower emissivity objects. Laser fiber amplifiers emit very narrow-band energy, simplifying detection. Results of simulated illumination scenarios are presented based on an orbiting emitter array with specified characteristics. Simulations indicate that return signals from small and low emissivity objects is strong enough to detect. The possibility for both directed and full sky blind surveys is discussed, and the resulting diameter and mass limits for objects in different observational scenarios. The ability to determine both position and speed of detected objects is also discussed.

Global Distribution of Water Vapor and Cloud Cover—Sites for High-Performance THz Applications

Absorption of terahertz radiation by atmospheric water vapor is a serious impediment for radio astronomy and for long-distance communications. Transmission in the THz regime is dependent almost exclusively on atmospheric precipitable water vapor (PWV). Though much of the Earth has PWV that is too high for good transmission above 200 GHz, there are a number of dry sites with very low attenuation. We performed a global analysis of PWV with high-resolution measurements from the Moderate Resolution Imaging Spectrometer (MODIS) on two NASA Earth Observing System (EOS) satellites over the year of 2011. We determined PWV and cloud cover distributions and then developed a model to find transmission and atmospheric radiance as well as necessary integration times in the various windows. We produced global maps over the common THz windows for astronomical and satellite communications scenarios. Notably, we show that, up through 1 THz, systems could be built in excellent sites of Chile, Greenland, and the Tibetan Plateau, while Antarctic performance is good to 1.6 THz. For a ground-to-space communication link up through 847 GHz, we found several sites in the Continental United States where mean atmospheric attenuation is less than 40 dB, which is not an insurmountable challenge for a link.

Modeling of Terabit Geostationary Terahertz Satellite Links From Globally Dry Locations

While terahertz (THz) communication systems, operating from 100 GHz to 1 THz, have the potential to exploit wide swaths of unused spectrum for ultra-high bitrate communication, there are significant challenges. Particularly, the strong absorption of water vapor can result in very high atmospheric attenuation. We modeled a ground to geostationary satellite link and found that using large aperture THz stations, patterned after the 12.5 m Atacama Large Microwave Array dish and the 3.5 m Herschel Space Observatory optics, worst 10th percentile data rates in excess of one terabit per second in the THz bands are possible. The key is to site ground stations in dry regions. We locate these by coupling our link model, which selects optimum modulation and carrier bandwidth, with global, high-resolution satellite water vapor measurements. We present detailed maps showing modeled link performance over the surface of the Earth. Smaller apertures on aircraft and balloons are also able to exceed 1 terabit/second due to their location above nearly all water vapor. Compared to free-space optical links, evidence suggests THz systems are superior where fog, cloud cover and clear-air turbulence are of concern.

Multifunctional metamaterial pyroelectric infrared detectors

Pyroelectric materials enable the construction of high-performance yet low-cost and uncooled detectors throughout the infrared spectrum. These devices have been used as broadband sensors and, when combined with an interferometric element or filter, can provide spectral selectivity. Here we propose the concept of and demonstrate a new architecture that uses a multifunctional metamaterial absorber to directly absorb the incident longwave IR (8–12 μm) energy in a thin-film lithium niobate layer and also to function as the contacts for the two-terminal detector. Our device achieves a narrowband (560 nm FWHM at 10.73 μm), yet highly efficient (86%) absorption. The metamaterial creates high field concentration, reducing temperature fluctuation noise, and lowering device capacitance and loss tangent noise. The metamaterial design paradigm applied to detectors thus results in a very fast planar device with a thermal time constant of 28.9 ms with a room temperature detectivity, D*, of 107  cm W/Hz.

Graphene metamaterial modulator for free-space thermal radiation.

We proposed and demonstrated a new metamaterial architecture capable of high speed modulation of free-space space thermal infrared radiation using graphene. Our design completely eliminates channel resistance, thereby maximizing the electrostatic modulation speed, while at the same time effectively modulating infrared radiation. Experiment results verify that our device with area of 100 × 120 µm2 can achieve a modulation speed as high as 2.6 GHz. We further highlight the utility of our graphene metamaterial modulator by reconstructing a fast infrared signal using an equivalent time sampling technique. The graphene metamaterial modulator demonstrated here is not only limited to the thermal infrared, but may be scaled to longer infrared and terahertz wavelengths. Our work provides a path forward for realization of frequency selective and all-electronic high speed devices for infrared applications.

Effects of asteroid rotation on directed energy deflection

Asteroids that threaten Earth could be deflected from their orbits using laser directed energy or concentrated solar energy to vaporize the surface; the ejected plume would create a reaction thrust that pushes the object away from its collision course with Earth. One concern regarding directed energy deflection approaches is that asteroids rotate as they orbit the Sun. Asteroid rotation reduces the average thrust and changes the thrust vector imparting a time profile to the thrust. A directed energy system must deliver sufficient flux to evaporate surface material even when the asteroid is rotating. Required flux levels depend on surface material composition and albedo, thermal and bulk mechanical properties of the asteroid, and asteroid rotation rate. In the present work we present results of simulations for directed energy ejecta-plume asteroid threat mitigation. We use the observed distribution of asteroid rotational rates, along with a range of material and mechanical properties, as input to a thermal-physical model of plume generation. We calculate the expected thrust profile for rotating objects. Standoff directed energy schemes that deliver at least 10 MW/m2 generate significant thrust for all but the highest conceivable rotation rates.

Optical modeling for a laser phased-array directed energy system

We present results of optical simulations for a laser phased array directed energy system. The laser array consists of individual optical elements in a square or hexagonal array. In a multi-element array, the far-field beam pattern depends on both mechanical pointing stability and on phase relationships between individual elements. The simulation incorporates realistic pointing and phase errors. Pointing error components include systematic offsets to simulate manufacturing and assembly variations. Pointing also includes time-varying errors that simulate structural vibrations, informed from random vibration analysis of the mechanical design. Phase errors include systematic offsets, and time-varying errors due to both mechanical vibration and temperature variation in the fibers. The optical simulation is used to determine beam pattern and pointing jitter over a range of composite error inputs. Results are also presented for a 1 m aperture array with 10 kW total power, designed as a stand-off system on a dedicated asteroid diversion/capture mission that seeks to evaporate the surface of the target at a distance of beyond 10 km. Phase stability across the array of λ/10 is shown to provide beam control that is sufficient to vaporize the surface of a target at 10 km. The model is also a useful tool for characterizing performance for phase controller design in relation to beam formation and pointing.