Friday, December 4, 2020

Abstract-Terahertz Nanoimaging and Nanospectroscopy of Chalcogenide Phase-Change Materials

Chao Chen, Shu Chen, Ricardo P.S.M. Lobo, Carlos Maciel-Escudero, Martin Lewin, Thomas Taubner, Wei Xiong, Ming Xu, Xinliang Zhang, Xiangshui Miao, Peining Li,  Rainer Hillenbrand

 Chalcogenide phase-change materials (PCMs) exhibit optical phonons at terahertz (THz) frequencies, which can be used for studying basic properties of the phase transition and which lead to a strong dielectric contrast that could be exploited for THz photonics applications. Here, we demonstrate that the phonons of PCMs can be studied by frequency-tunable THz scattering-type scanning near-field optical microscopy (s-SNOM). Specifically, we perform spectroscopic THz nanoimaging of a PCM sample comprising amorphous and crystalline phases. We observe phonon signatures, yielding strong s-SNOM signals and, most important, clear spectral differences between the amorphous and crystalline PCM, which allows for distinguishing the PCM phases with high confidence on the nanoscale. We also found that the spectral signature can be enhanced, regarding both signal strength and spectral contrast, by increasing the radius of the probing tip. From a general perspective, our results establish THz s-SNOM for nanoscale structural and chemical mapping based on local phonon spectroscopy.

Thursday, December 3, 2020

Terahertz spectroscopy probes cellular structure of skin


Emma MacPherson: versatile THz technology

University of Warwick and Chinese University of Hong Kong project could assist in skin cancer diagnosis.

Terahertz radiation, falling between the infrared and microwave regions of the spectrum, is attractive for in vivo applications due to its non-invasive and non-ionizing nature.

The limited penetration depths of THz radiation is thought to make it particularly suitable for analysis of the skin, diagnosing burns, scars and cancers.

However the complicated nature of living systems has to date presented a challenge, preventing current THz platforms from obtaining the accurate reflections from target tissues needed to build up images of the skin.

A project at the University of Warwick and the Chinese University of Hong Kong (CUHK) has now developed a THz platform intended to significantly enhance the characterization capabilities of THz spectroscopy, and published its study in Advanced Photonics Research.

The breakthrough involves a novel ellipsometry technique, providing mutliple complementary sets of spectral ratios and significantly boosting the performance of the technique.

A basic ellipsometry approach involves calculating the refractive index of target tissues measured in two directions at right angles to each other. The difference between these refractive indices is termed birefringence, and this is the first time that the THz birefringence of human skin has been measured in vivo according to the project. These properties can provide valuable information on how much water is in the skin and enable the skin thickness to be calculated.

"We wanted to show that we could do in vivo ellipsometry measurements in human skin and calculate the properties of skin accurately," said project leader Emma Pickwell-MacPherson of CUHK's terahertz research group.

"In ordinary terahertz reflection imaging, you have thickness and refractive index combined as one parameter. By taking measurements at multiple angles you can separate the two."

Tailored medicine from THz spectroscopy

The project's experimental platform employed a double-prism architecture mounted on a motorized stage, to provide two alternative optical paths and effectively allow four complementary sets of spectral ratios to be collected from the target.

After initial trials on a model of skin and its outer layer, or stratum corneum (SC), the project applied its platform to the forearms of five human volunteers, and found that the properties of the SC components and the epidermis could be computationally extracted from the spectral data using an algorithm.

The THz dispersion and birefringence sensitivity parameters are effectively probes for the level of hydration and the cellular inhomogeneity in the skin, according to the project, producing results in good agreement with microscope images and the observed biological processes taking place in the SC layer.

A THz platform capable of quantitatively assessing the condition of skin could be useful in clinical scenarios for the monitoring of skin cancer, or to assess the effectiveness of medications and moisturizers. The inherent sensitivity to water molecules could also allow the technique to detect areas of skin where the water circulation is different from the surrounding areas, potentially an early sign of problems.

"If this works well you could go into a clinic, put your arm on a scanner, your occlusion curve would be plotted and a suitable product for your skin could be recommended," commented Emma Pickwell-MacPherson. "We could get more tailored medicine and develop products for different skin responses. It could really fit in with the current focus on tailored medicine.”

Wednesday, December 2, 2020

Abstract-Nonlinear Plasmonic Metasurface Terahertz Emitters for Compact Terahertz Spectroscopy Systems


Mai Tal, Shay,  Keren-Zur, Tal Ellenbogen,

Nonlinear plasmonic metasurfaces provide new and promising means to produce broadband terahertz (THz) radiation, due to their compact size and functionalities beyond those achievable with conventional THz emitters. However, they were driven to date only by amplified laser systems, which are expensive and have a large footprint, thus limiting the range of their potential applications. Here we study for the first time the possibility to drive metasurface emitters by low-energy near-infrared femtosecond pulses. We observe broadband THz emission from 40 nm thick metasurfaces and achieve near-infrared to THz conversion efficiencies as high as those of 2500-fold thicker ZnTe crystals. We characterize the THz emission properties and use the metasurface emitter to perform a spectroscopic measurement of α-lactose monohydrate. These results show that nonlinear plasmonic metasurfaces are suitable for integration as emitters in existing compact THz spectroscopy and imaging systems, enhancing their functionalities, and opening the door for a variety of new applications.

Tuesday, December 1, 2020

Abstract-Printing special surface components for THz 2D and 3D imaging


Bo Yan, Zhigang Wang, Xing Zhao, Lie Lin, Xiaolei Wang, Cheng Gong,  Weiwei Liu

The paper reports an off-axis large focal depth THz imaging system which consists of three 3D printed special surface components (two aspherical mirrors and an axicon). Firstly, the optical design software is used to design and optimize the aspherical parabolic mirror. Secondly, the optimized mirror is prepared by a 3D printing and metal cladding method. Thirdly, a THz axicon is designed for generation of quasi-Bessel Beam and a new geometric theoretical model of oblique incident light for axicon is established. Finally, the imaging system based on the special surface components is constructed. Its maximum diffraction-free distance is about 60 mm, which is 6 times higher than the traditional system. To verify the effectiveness, THz two-dimensional imaging experiments and three-dimensional computed tomography experiment are carried out. The results are consistent with the design and calculations.

Monday, November 30, 2020

Abstract-Achromatic Dielectric Metasurface with Linear Phase Gradient in the Terahertz Domain


Ridong Jia,   Yufei Gao,   Quan Xu,   Xi Feng,   Qingwei Wang,  Jianqiang Gu,   Zhen Tian, Chunmei Ouyang,   Jiaguang Han,   Weili Zhang

The notion of metasurface has inspired the innovation of various functional devices in the terahertz band, but the intrisinc dispersion restricts their application in broadband scenarios. Here, two terahertz achromatic linear‐phase‐gradient metasurface devices are demonstrated, which are a beam deflector and a beam splitter, respectively. The phase and dispersion of the metasurfaces are simultaneously engineered by changing the geometric parameters of the unit cells made of silicon gratings and pillars. The simulated and experimental results demonstrate the achromatic feasibility of the beam deflector from 0.6 to 1.2 THz and of the beam splitter from 0.6 to 1.1 THz. The transmittances and the splitting ratios of the achromatic beam splitter are also analyzed. The metasurface based achromatic beam deflector and splitter presented here not only enrich the terahertz functional devices, but the methods and structures may also promote the research of broadband terahertz metasurfaces.

Friday, November 27, 2020

Abstract-Non-plasmonic improvement in photoconductive THz emitters using nano- and micro-structured electrodes


Abhishek Singh, Malte Welsch, Stephan Winnerl, Manfred Helm, Harald Schneider, 

Simulation of electric field amplitude ((Ex2+Ey2+Ez2)1/2 in V/m) distribution on a slice passing through the center of the emitter in yz-plane in the photoconductor when 10 V bias is applied to the electrodes. Field lines are drawn to show the direction of the electric field. A white dashed line is drawn at a depth (∼ 1 µm) of penetration depth of 800 nm in GaAs.

We investigate here terahertz enhancement effects arising from micrometer and nanometer structured electrode features of photoconductive terahertz emitters. Nanostructured electrode based emitters utilizing the palsmonic effect are currently one of the hottest topics in the research field. We demonstrate here that even in the absence of any plasmonic resonance with the pump pulse, such structures can improve the antenna effect by enhancing the local d.c. electric field near the structure edges. Utilizing this effect in Hilbert-fractal and grating-like designs, enhancement of the THz field of up to a factor of ∼ 2 is observed. We conclude that the cause of this THz emission enhancement in our emitters is different from the earlier reported plasmonic-electrode effect in a similar grating-like structure. In our structure, the proximity of photoexcited carriers to the electrodes and local bias field enhancement close to the metallization cause the enhanced efficiency. Due to the nature of this effect, the THz emission efficiency is almost independent of the pump laser polarization. Compared to the plasmonic effect, these effects work under relaxed device fabrication and operating conditions.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.