Daniel V. Nickel

High-Contrast Terahertz Wave Modulation by Gated Graphene Enhanced by Extraordinary Transmission through Ring Apertures

Gate-controllable transmission of terahertz (THz) radiation makes graphene a promising material for making high-speed THz wave modulators. However, to date, graphene-based THz modulators have exhibited only small on/off ratios due to small THz absorption in single-layer graphene. Here we demonstrate a ∼50% amplitude modulation of THz waves with gated single-layer graphene by the use of extraordinary transmission through metallic ring apertures placed right above the graphene layer. The extraordinary transmission induced ∼7 times near-filed enhancement of THz absorption in graphene. These results promise complementary metal-oxide-semiconductor compatible THz modulators with tailored operation frequencies, large on/off ratios, and high speeds, ideal for applications in THz communications, imaging, and sensing.

High-contrast terahertz modulator based on extraordinary transmission through a ring aperture.

We demonstrated extraordinary THz transmission through ring apertures on a metal film. Transmission of 60% was obtained with an aperture-to-area ratio of only 1.4%. We show that the high transmission can be suppressed by over 18 dB with a thin layer of free carriers in the silicon substrate underneath the metal film. This result suggests that CMOS-compatible terahertz modulators can be built by controlling the carrier density near the aperture.

Terahertz vibrational modes of the rigid crystal phase of succinonitrile.

Succinonitrile (N ≡ C-CH2-CH2-C ≡ N), an orientationally disordered molecular plastic crystal at room temperature, exhibits rich phase behavior including a solid-solid phase transition at 238 K. In cooling through this phase transition, the high-temperature rotational disorder of the plastic crystal phase is frozen out, forming a rigid crystal that is both spatially and orientationally ordered. Using temperature-dependent terahertz time-domain spectroscopy, we characterize the vibrational modes of this low-temperature crystalline phase for frequencies from 0.3 to 2.7 THz and temperatures ranging from 20 to 220 K. Vibrational modes are observed at 1.122 and 2.33 THz at 90 K. These modes are assigned by solid-state density functional theory simulations, corresponding respectively to the translation and rotation of the molecules along and about their crystallographic c-axis. In addition, we observe a suppression of the phonon modes as the concentration of dopants, in this case a lithium salt (LiTFSI), increases, indicating the importance of doping-induced disorder in these ionic conductors.

High-Q terahertz Fano resonance with extraordinary transmission in concentric ring apertures

We experimentally demonstrate a polarization-independent terahertz Fano resonance with extraordinary transmission when light passes through two concentric subwavelength ring apertures in the metal film. The Fano resonance is enabled by the coupling between a high-Q dark mode and a low-Q bright mode. We find the Q factor of the dark mode ranges from 23 to 40, which is 3~6 times higher than Q of bright mode. We show the Fano resonance can be tuned by varying the geometry and dimension of the structures. We also demonstrate a polarization dependent Fano resonance in a modified structure of concentric ring apertures.

High-pressure cell for terahertz time-domain spectroscopy

We introduce a sample cell that can be used for pressure-dependent terahertz time-domain spectroscopy. Compared with traditional far-IR spectroscopy with a diamond anvil cell, the larger aperture permits measurements down to much lower frequencies as low as 3.3 cm-1 (0.1 THz), giving access to new spectroscopic results. The pressure tuning range reaches up to 34.4 MPa, while the temperature range is from 100 to 473 K. With this large range of tuning parameters, we are able to map out phase diagrams of materials based on their THz spectrum, as well as to track the changing of the THz spectrum within a single phase as a function of temperature and pressure. Pressure-dependent THz-TDS results for nitrogen and R-camphor are shown as an example.

Resonant Transmission of Ring Aperture for Switching Terahertz Waves

We demonstrated extraordinary THz transmission through ring apertures on metal film. Transmission of 60% was obtained with an aperture-to-area ratio of only 1.4%. Silicon-based electro-optic switch for THz waves can be built with this structure.

One-Dimensional Terahertz Imaging of Surfactant-Stabilized Dodecane-Brine Emulsions

Terahertz line-images of surfactant-stabilized dodecane(C12H26)-brine emulsions are obtained by translating the emulsified region through the focus of a terahertz time-domain spectrometer. From these images, dodecane content, emulsion size, and stability can be extracted to evaluate the efficacy of the surfactant in solubilizing the dodecane. In addition, the images provide insight into the dynamics of concentrated emulsions after mixing.

Hindered molecular reorientation of lithium ion doped succinonitrile in the terahertz range

The THz range permittivity of succinonitrile exhibits Debye-like behavior in its plastic-crystal phase, with a characteristic Cole-Cole relaxation time of 65ps ±17ps which increases considerably in the presence of ionic dopants.

Terahertz conductivity of lithium salt-succinonitrile plastic crystals

The terahertz conductivity of succinonitrile decreases when doped with lithium salts, contrary to their DC behavior. In addition, a lattice phonon mode at 1.1 THz is observed in only the crystalline phase of the undoped succinonitrile.

Terahertz time domain spectroscopy of branched alkanes

N(ν) and α(ν) for branched alkanes were measured using THz-TDS and compared to their linear structure counterparts. There is an overall decrease in n(ν) and an increase in α(ν), contrary to the predictions of the additive model for polarizibility.