Abstract-Terahertz generation measurements of multilayered GeTe–Sb2Te3 phase change materials

Kotaro Makino, Kosaku Kato, Yuta Saito, Paul Fons, Alexander V. Kolobov, Junji Tominaga, Takashi Nakano, and Makoto Nakajima

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-44-6-1355

Multilayered structures of GeTe and Sb2Te3${\mathrm{Sb}}_2{\mathrm{Te}}_3$ phase change material, also referred to as interfacial phase change memory (iPCM), provide superior performance for nonvolatile electrical memory technology in which the atomically controlled structure plays an important role in memory operation. Here, we report on terahertz (THz) wave generation measurements. Three- and 20-layer iPCM samples were irradiated with a femtosecond laser, and the generated THz radiation was observed. The emitted THz pulse was found to be always 𝑝$p$ polarized independent of the polarization of the excitation pulse. Based on the polarization dependence as well as the flip of the THz field from photoexcited Sb2Te3${\mathrm{Sb}}_2{\mathrm{Te}}_3$ and Bi2Te3${\mathrm{Bi}}_2{\mathrm{Te}}_3$, the THz emission process can be attributed to the surge current flow due to the built-in surface depletion layer formed in 𝑝$p$-type semiconducting iPCM materials.

© 2019 Optical Society of America

Abstract-Terahertz emission from gold nanorods irradiated by ultrashort laser pulses of different wavelengths

Keisuke Takano, Motoki Asai, Kosaku Kato, Hideaki Komiyama, Akihisa Yamaguchi, Tomokazu Iyoda, Yuzuru Tadokoro, Makoto Nakajima, Michael I. Bakunov

https://www.nature.com/articles/s41598-019-39604-5

Electron photoemission and ponderomotive acceleration by surface enhanced optical fields is considered as a plausible mechanism of terahertz radiation from metallic nanostructures under ultrafast laser excitation. To verify this mechanism, we studied experimentally terahertz emission from an array of gold nanorods illuminated by intense (~10–100 GW/cm2) femtosecond pulses of different central wavelengths (600, 720, 800, and 1500 nm). We found for the first time that the order of the dependence of the terahertz fluence on the laser intensity is, unexpectedly, almost the same (~4.5–4.8) for 720, 800, and 1500 nm and somewhat higher (~6.6) for 600 nm. The results are explained by tunneling currents driven by plasmonically enhanced laser field. In particular, the pump-intensity dependence of the terahertz fluence is more consistent with terahertz emission from the sub-cycle bursts of the tunneling current rather than with the ponderomotive mechanism.

Abstract-Visible Measurement of Terahertz Power Based on Capsulized Cholesteric Liquid Crystal Film

Lei Wang,  Hongsong Qiu, Thanh Nhat Khoa Phan, Kosaku Kato, Boyoung Kang, Keisuke Takano, Yanqing Lu, Lujian Chen, Peng Lv, Kehan Yu, Wei Wei,  Biaobing Jin, Makoto Nakajima

file:///C:/Users/Randy/Downloads/applsci-08-02580.pdf

We demonstrate a new method to detect terahertz (THz) power using a temperature supersensitive capsulized cholesteric liquid crystal film based on the thermochromic and thermodiffusion effect, which is clearly observed. A quantitative visualization of the THz intensity up to 4.0 × 103 mW/cm2 is presented. The diameter of the color change area is linearly dependent on the THz radiation power above 0.07 mW in the steady state. Moreover, the THz power can be detected for 1 sec of radiation with a parabolic relation to the color change area. The THz power meter is robust, cost-effective, portable, and even flexible, and can be used in applications such as THz imaging, biological sensing, and inspection.

Abstract-Enhanced detection sensitivity of terahertz magnetic nearfield with cryogenically-cooled magnetooptical sampling in terbium-gallium-garnet

Applied Physics Letters

Takayuki Kurihara,  Hongsong Qiu,   Kosaku Kato, Hiroshi Watanabe, Makoto Nakajima

https://aip.scitation.org/doi/abs/10.1063/1.5037521

Due to its efficient coupling with electron spins, the application of terahertz magnetic nearfield in metallic microstructures has been attracting attention. While paramagnetic materials that exhibit magneto-optical effect have been known to enable visualization of the terahertz magnetic fields (magneto-optical sampling), the low field-detection sensitivity has been setting a practical limit to the broader application of such a method. Here we propose and experimentally demonstrate that the terahertz magnetic nearfield-detection sensitivity of magneto-optical sampling with terbium-gallium-garnet crystal can be drastically enhanced by cooling the crystal down to cryogenic temperatures in accordance with Curie's law. Our result paves the way for the efficient characterization of the terahertz magnetic nearfield in planer metamaterials.

Abstract-Enhancing terahertz magnetic near field induced by a micro-split-ring resonator with a tapered waveguide

Hongsong Qiu, Takayuki Kurihara, Hirofumi Harada, Kosaku Kato, Keisuke Takano, Tohru Suemoto, Masahiko Tani, Nobuhiko Sarukura, Masashi Yoshimura, and Makoto Nakajima

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-43-8-1658

Substantial enhancement of terahertz magnetic near field achieved by the combination of a tapered metallic waveguide and a micro-split-ring resonator is demonstrated. The magnetic near field is probed directly via the magneto-optic sampling with a Tb3Ga5O12${\mathrm{Tb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ crystal. The incident terahertz wave with a half-cycle waveform is generated by using the pulse-front tilting method. The magnetic near field at the resonant frequency is enhanced by more than 30 times through the combination of the waveguide and the resonator. The peak amplitude of the magnetic field with a damped oscillation waveform in the time domain is up to 0.4 T. The resonant frequency can be tuned by adopting different resonator designs. The mechanism of the enhancement is analyzed by performing calculations based on the finite element method. The strong terahertz magnetic near field enables the excitation of large-amplitude spin dynamics and can be utilized for an ultrafast spin control.

© 2018 Optical Society of America

Abstract-Significant Volume Expansion as a Precursor to Ablation and Micropattern Formation in Phase Change Material Induced by Intense Terahertz Pulses

Kotaro Makino, Kosaku Kato, Keisuke Takano, Yuta Saito, Junji Tominaga, Takashi Nakano, Goro Isoyama, Makoto Nakajima,

https://www.nature.com/articles/s41598-018-21275-3

With rapid advances occurring in terahertz (THz) radiation generation techniques, the interaction between matter and intense THz fields has become an important research topic. Among different types of THz radiation sources, the free electron laser (FEL) is a promising experimental tool that is expected to pave the way for new forms of material processing, control of phase transitions, and serve as a test bench for extreme operating conditions in high-speed small-size electrical and magnetic devices through the exploitation of strong THz electrical and magnetic fields without the presence of interband electronic excitation. In the current work, we irradiated Ge2Sb2Te5phase change memory material with intense THz pulse trains from an FEL and observed THz-induced surface changes due to damage as a precursor to ablation and the formation of fine surface undulations whose spatial period is comparable to or slightly smaller than the wavelength of the excitation THz pulses in the material. The formation of undulations as well as the fact that no significant thermal effect was observed below the volume expansion threshold suggests that THz-induced effects mainly contributed to the observed changes. To the best of our knowledge, this is the first experimental observation of THz-induced undulations (so-called “LIPSS”), which are of potential importance for laser material processing.