Shuchang Liu

Liquid metal-based plasmonics

We demonstrate that liquid metals support surface plasmon-polaritons (SPPs) at terahertz (THz) frequencies, and can thus serve as an attractive material system for a wide variety of plasmonic and metamaterial applications. We use eutectic gallium indium (EGaIn) as the liquid metal injected into a polydimethylsiloxane (PDMS) mold fabricated by soft lithography techniques. Using this approach, we observe enhanced THz transmission through a periodic array of subwavelength apertures. Despite of the fact that the DC conductivity of EGaIn is an order of magnitude smaller than many conventional metals, we clearly observe well-defined transmission resonances. This represents a first step in developing reconfigurable and tunable plasmonic devices that build upon well-developed microfluidic capabilities.

Graphene-based tunable metamaterial terahertz filters

We propose and describe a micro-machined tunable metamaterial terahertz filter based on graphene. The device structure consists of periodic metallic rings with several gaps where tunable graphene stripes are located. We demonstrate that the filter resonance frequency can be adjusted easily by varying the conductivity of graphene and implement this by changing the number of stacked graphene layers. Moreover, the proposed design is scalable, in the sense that the resonance frequency tuning can be controlled by scaling the inner and outer radius of the metal rings. Using numerical simulations and terahertz time-domain spectroscopy measurements of the fabricated samples, we show that the resonance frequency of the structure can be altered by 40% (i.e., from ∼0.2 THz to ∼0.12 THz) by simply tuning the conductivity of graphene. Importantly, the active area of the device is ≪0.1% of the total unit cell area, which can boost the device speed upon electrostatic actuation.

Reconfigurable plasmonic devices using liquid metals.

We experimentally demonstrate an approach to create reconfigurable plasmonic devices in which the geometry of the device can be changed dramatically. The specific embodiment we present utilizes eutectic gallium indium (EGaIn), a metal that is liquid at room temperature, which is injected into or withdrawn from channels encapsulated by a polydimethylsiloxane (PDMS) bullseye mold fabricated on a gold coated substrate. Using terahertz (THz) time-domain spectroscopy, we measure the enhanced transmission properties of a single subwavelength aperture surrounded by differing numbers of concentric annular EGaIn rings. The results obtained from different device geometries, with either a single or multiple rings, are performed using a single device, demonstrating true reconfigurability. We explain the properties of the observed temporal waveforms using a simple time-domain model. This represents, we believe, a first step in developing more complex reconfigurable plasmonic devices.

Coherent Detection of Multiband Terahertz Radiation Using a Surface Plasmon-Polariton Based Photoconductive Antenna

We characterize a dipole antenna structure that allows for coherent detection of narrowband terahertz radiation with enhanced sensitivity at the resonant frequency. The antenna incorporates a corrugated metal structure that surrounds the dipole. Each periodically spaced groove in the corrugation couples an approximate replica of the incident THz pulse to a surface plasmon-polariton pulse, which then propagates towards and is detected by the dipole. We use numerical simulations to validate the experimental data. Based on these results, we describe a multiband dipole antenna detector that allows for enhanced sensitivity at multiple frequencies. This device can also be used as an emitter.

Reconfigurable liquid metal based terahertz metamaterials via selective erasure and refilling to the unit cell level

We demonstrate a technique for selectively erasing and refilling unit cells of terahertz (THz) metamaterials. The structures are formed by injecting eutectic gallium indium (EGaIn), a liquid metal at room temperature, into microchannels within a polydimethylsiloxane (PDMS) mold fabricated using conventional soft lithography techniques. The thin oxide layer that forms on the surface of EGaIn can be locally dissolved via exposure to hydrochloric acid (HCl) introduced at the surface of the gas permeable PDMS mold. In the absence of the oxide skin, the liquid metal retracts to a position where a stable new oxide layer can be formed, effectively erasing the liquid metal structure in the presence of HCl. After erasing selected structures, EGaIn can be re-injected into microchannels to yield the initial structure. In the case of small unit cells, we show that mechanical pressure can be used to effectively erase individual elements. We use THz time-domain spectroscopy to characterize the distinct transmission properties for each of these different structures.

Terahertz Corrugated and Bull's-Eye Antennas

The radiation and temporal properties of one-dimensional and two-dimensional (Bulls-eye) corrugated antennas are investigated numerically and experimentally at terahertz. The thickness of the antenna is miniaturized, within the fabrication limits, by using the transverse slot resonance rather than the longitudinal resonance. Square and triangular corrugations are discussed. The comparison between these two profiles shows that, in terms of return loss and gain, the antenna is robust to the corrugation shape, which alleviates the fabrication complexity. The temporal analysis of the Bulls eye antenna is also shown to demonstrate the contributions coming from the leakage of the surface wave at each groove. This insight allows us to engineer the temporal shape of the output pulse by varying independently the depth of each groove. The antennas presented here hold promise for manipulating with very low profiles pulse- and beam-shapes of THz radiation.

Self-referenced measurements of the dielectric properties of metals using terahertz time-domain spectroscopy via the excitation of surface plasmon-polaritons

We present experimental measurements that show direct determination of the dielectric properties of various metals relevant to plasmonics. In contrast to traditional measurements that typically rely on transmission and reflectance measurements, we launch surface plasmon-polaritons on a variety of different substrates and measure the propagation properties using terahertz time-domain spectroscopy. Surprisingly, we find that the extracted values for the dielectric constant for these metals differ by orders of magnitude from published data. In order to validate the obtained results, we separately measure the 1/e decay length, both along the propagation direction and normal to the metal surface, and show that the results are fully self-consistent with experimental data. The generality of the measurement technique makes it a useful tool to estimate the properties not only of planar conducting substrates but also a wide variety of more complex plasmonic structures.

Reconfigurable terahertz metamaterial device with pressure memory

We demonstrate a liquid metal-based reconfigurable terahertz (THz) metamaterial device that is not only pressure driven, but also exhibits pressure memory. The discrete THz response is obtained by injecting eutectic gallium indium (EGaIn) into a microfluidic structure that is fabricated in polydimethylsiloxane (PDMS) using conventional soft lithography techniques. The shape of the injected EGaIn is mechanically stabilized by the formation of a thin oxide surface layer that allows the fluid to maintain its configuration within the microchannels despite its high intrinsic surface energy. Although the viscosity of EGaIn is twice that of water, the formation of the surface oxide layer prevents flow into a microchannel unless a critical pressure is exceeded. Using a structure in which the lateral channel dimensions vary, we progressively increase the applied pressure beyond the relevant critical pressure for each section of the device, enabling switching from one geometry to another (split ring resonator to closed ring resonator to an irregular closed ring resonator). As the geometry changes, the transmission spectrum of the device changes dramatically. When the external applied pressure is removed between device geometry changes, the liquid metal morphology remains unchanged, which can be regarded as a form of pressure memory. Once the device is fully filled with liquid metal, it can be erased through the use of mechanical pressure and exposure to acid vapors.

Engineering the properties of terahertz filters using multilayer aperture arrays

We experimentally demonstrate the ability to create additional transmission resonances in a double-layer aperture array by varying the interlayer gap spacing. In the case of periodic aperture arrays, these additional resonances are sharply peaked, while for random aperture arrays the resonances are broad. Surprisingly, these additional resonances only occur when the interlayer gap spacing is greater than half the aperture spacing on a single array. Since there is no corresponding periodicity in the random arrays, these resonances occur regardless of how small the gap spacing is made. This phenomenon can be accurately modeled only if the correct frequency-dependent complex dielectric function of a metal film perforated with subwavelength apertures is used. Using THz time-domain spectroscopy, we are able to directly obtain the complex dielectric response function from the THz experimental transmission measurements. We conclude by demonstrating several passive free-space THz filters using multilayer aperture arrays. Importantly, we show that the magnitude of the lowest order resonance can be approximately maintained, while the background transmission can be significantly suppressed leading to a significant improvement in the optical filter fidelity.

Near-field terahertz imaging using sub-wavelength apertures without cutoff.

We demonstrate near-field imaging capabilities of a conical waveguide without cutoff using broadband terahertz (THz) radiation. In contrast to conventional conically tapered waveguides, which are characterized by strong suppression of transmission below the cutoff frequency, the proposed structure consists of two pieces, such that there is an adjustable gap along the length of the waveguide. We also ensure that the sidewalls are thin in the vicinity of the gap. The combination of these geometrical features allow for significantly enhanced transmission at frequencies below the cutoff frequency, without compromising the mode confinement and, consequently, the spatial resolution when used for imaging applications. We demonstrate near-field imaging with this probe simultaneously at several frequencies, corresponding to three regimes: above, near and below the cutoff frequency. We observe only mild degradation in the image quality as the frequency is reduced below the cutoff frequency. These results suggest that further refinements in the probe structure will allow for improved imaging capabilities at frequencies well below the cutoff frequency.