Abstract-Tunable cavity-enhanced terahertz frequency-domain optical Hall effect

Review of Scientific Instruments

Sean Knight, Stefan Schöche,  Philipp Kühne, Tino Hofmann,  Vanya Darakchieva, Mathias Schubert, 

Three-dimensional surface rendering of model-calculated cavity-enhanced field-reversal THz-OHE data for an AlInN/AlN/GaN HEMT structure grown on a sapphire substrate as functions of frequency ν, external cavity distance dgap, and angle of incidence Φa. Data for ΔM13,31 = M13,31(+B) − M13,31(−B) are shown as an example. Values within the range of −0.02 to 0.02 are omitted for clarity. The green horizontal plane at Φa = 45° indicates the instrumental settings for the angle of incidence in this work. Model parameters are given in the text. Note that the model-calculated plot for ΔM23,32 is similar in appearance to that of ΔM13,31 and is excluded here for brevity.

https://aip.scitation.org/doi/abs/10.1063/5.0010267

Presented here is the development and demonstration of a tunable cavity-enhanced terahertz (THz) frequency-domain optical Hall effect (OHE) technique. The cavity consists of at least one fixed and one tunable Fabry–Pérot resonator. The approach is suitable for the enhancement of the optical signatures produced by the OHE in semi-transparent conductive layer structures with plane parallel interfaces. Tuning one of the cavity parameters, such as the external cavity thickness, permits shifting of the frequencies of the constructive interference and provides substantial enhancement of the optical signatures produced by the OHE. A cavity-tuning optical stage and gas flow cell are used as examples of instruments that exploit tuning an external cavity to enhance polarization changes in a reflected THz beam. Permanent magnets are used to provide the necessary external magnetic field. Conveniently, the highly reflective surface of a permanent magnet can be used to create the tunable external cavity. The signal enhancement allows the extraction of the free charge carrier properties of thin films and can eliminate the need for expensive superconducting magnets. Furthermore, the thickness of the external cavity establishes an additional independent measurement condition, similar to, for example, the magnetic field strength, THz frequency, and angle of incidence. A high electron mobility transistor (HEMT) structure and epitaxial graphene are studied as examples. The tunable cavity-enhancement effect provides a maximum increase of more than one order of magnitude in the change of certain polarization components for both the HEMT structure and epitaxial graphene at particular frequencies and external cavity sizes.

Abstract-A Stereolithographically Fabricated Polymethacrylate Broadband THz Absorber

Serang Park, Zackery Z. Clark, Yanzeng Li, Michael McLamb, Tino Hofmann

https://arxiv.org/abs/1909.13662

Additive manufactured THz optics have been introduced as an efficient alternative to their commercial counterparts. Among various additive manufacturing methods, stereolithography provides superior spatial resolution and surface finish. However, examples of stereolithographically fabricated components for THz applications are still scarce. In this paper, we report on the fabrication process and performance of a stereolithographically fabricated broadband absorber for the THz spectral range. Simple THz transmission experiments were carried out for the absorber and bulk reference samples. The experimental results indicated that the fabricated absorber effectively absorbs incident signal in the investigated THz spectral range.

Abstract-Terahertz to mid-infrared dielectric properties of polymethacrylates for stereolithographic single layer assembly

Serang Park, Yanzeng Li, Daniel B. Fullager, Stefan Schöche, Craig M. Herzinger, Glenn D. Boreman, Tino Hofmann

https://arxiv.org/abs/1907.13299

The fabrication of terahertz (THz) optics with arbitrary shapes via poly-methacrylate-based stereolithography is very attractive as it may offer a rapid, low-cost avenue towards optimized THz imaging applications. In order to design such THz optical components appropriately, accurate knowledge of the complex dielectric function of the materials used for stereolithographic fabrication is crucial. In this paper we report on the complex dielectric functions of several polymethacrylates frequently used for stereolithographic fabrication. Spectroscopic ellipsometry data sets from the THz to mid-infrared spectral range were obtained from isotropically cross-linked polymethacrylate samples. The data sets were analyzed using stratified layer optical model calculations with parameterized model dielectric functions. While the infrared spectral range is dominated by a number of strong absorption features with Gaussian profiles, these materials are found to exhibit only weak absorption in the THz frequency range. In conclusion, we find that thin transmissive THz optics can be efficiently fabricated using polymethacrylate-based stereolithographic fabrication.

Abstract-Metalized Poly-methacrylate Off-Axis Parabolic Mirrors for Terahertz Imaging Fabricated by Additive Manufacturing

Daniel B. Fullager, Serang Park, Clark Hovis, Yanzeng Li, Jesse Reese, Erin Sharma,  Susanne Lee,  Christopher Evans, Glenn D. Boreman, Tino Hofmann

https://link.springer.com/article/10.1007/s10762-019-0568-9

Terahertz radiation sources are currently one of the most widely used non-ionizing illumination mechanisms for security applications and also find increasing utilization in quality control of commercial products. Presently, a majority of these applications rely on scanning rather than direct imaging and implicitly suffer from temporal latency due to post processing. The monetary and temporal cost associated with procuring commercially manufactured optics that are suitable for imaging leads to fundamental limitations in the ability to rapidly develop application-specific imaging modalities using terahertz sources. Herein, we show a novel method for the rapid prototyping of metallic coated poly-methacrylate parabolic reflectors fabricated by stereolithographic 3D printing. Images comparing the performance of a commercially available off-axis parabolic reflector to our metalized poly-methacrylate prototype, which was designed to be identical to the commercially available mirror, are subsequently presented. The images show that at 530 GHz it is possible to produce a metalized poly-methacrylate off-axis paraboloid whose spatial beam profile is nearly identical to that of a commercially available equivalent.