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-Quantized conductance observed during sintering of silver nanoparticles by intense terahertz pulses

Keisuke Takano, Hirofumi Harada, Masashi Yoshimura, Makoto Nakajima,

https://www.researchgate.net/publication/324555773_Quantized_conductance_observed_during_sintering_of_silver_nanoparticles_by_intense_terahertz_pulses

We show that silver nanoparticles, which are deposited on a terahertz-receiving antenna, can be sintered by intense terahertz pulse irradiation. The conductance of the silver nanoparticles between the antenna electrodes is measured under the terahertz pulse irradiation. The dispersant materials surrounding the nanoparticles are peeled off, and conduction paths are created. We reveal that, during sintering, quantum point contacts are formed, leading to quantized conductance between the electrodes with the conductance quantum, which reflects the formation of atomically thin wires. The terahertz electric pulses are sufficiently intense to activate electromigration, i.e., transfer of kinetic energy from the electrons to the silver atoms. The silver atoms move and atomically thin wires form under the intense terahertz pulse irradiation. These findings may inspire nanoscale structural processing by terahertz pulse irradiation.

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.

Abstract-Energy loss of terahertz electromagnetic waves by nano-sized connections in near-self-complementary metallic checkerboard patterns

Keisuke Takano, Yoku Tanaka, Gabriel Moreno, Abdallah Chahadih, Abbas Ghaddar, Xiang-Lei Han, François Vaurette, Yosuke Nakata, Fumiaki Miyamaru, Makoto Nakajima, Masanori Hangyo,Tahsin Akalin

https://arxiv.org/abs/1708.03107

The design of a self-complementary metallic checkerboard pattern achieves broadband, dispersion-less, and maximized absorption, concentrating in the deep subwavelength resistive connections between squares, without any theoretical limitation on the energy absorbing area. Here, we experimentally and numerically investigate the electromagnetic response in the limit of extremely small connections. We show that finite conductivity and randomness in a near-self-complementary checkerboard pattern plays a crucial role in producing a frequency-independent energy loss in the terahertz frequency region. Here metals behave like an almost perfect conductor. When the checkerboard pattern approaches the perfect self-complementary pattern, the perfect conductor approximation spontaneously breaks down, owing to the finite conductivity at the nano- scale connection, leading to broadband absorption. It is also shown that the random connections between metallic squares also lead to broadband and maximized energy loss through scattering loss, similar to finite conductivity.

Abstract-Terahertz wave emission from plasmonic chiral metasurfaces

Takahiro Matsui, Satoshi Tomita , Motoki Asai, Yuzuru Tadokoro, Keisuke Takano, Makoto Nakajima, Masanori
http://link.springer.com/article/10.1007%2Fs00339-016-9657-y

Plasmonic chiral metasurfaces with pinwheel-like structures are fabricated on silver films using a focused ion-beam milling technique. In time-domain spectroscopy, we observe terahertz (THz) wave emission from metasurfaces irradiated by a near-infrared Ti:sapphire ultrashort pulsed laser. The origin of the THz wave generation is likely to be tunnelling ionization accompanied with photoelectron acceleration by ponderomotive force. Numerical simulation is carried out toward improvement of the chiral metasurfaces for better emission of circularly polarized THz waves.

Abstract-Study of terahertz field induced coherer

Makoto Nakajima, Yuzuru Tadokoro, Keisuke Takano, and Masanori Hangyo
https://www.osapublishing.org/abstract.cfm?URI=nlo-2015-NTh2B.6

The electrical resistance falls down several orders for aluminum particles by single-shot terahertz intense pulse irradiation. We confirmed that the electron conduction paths are induced in a direction parallel to polarization of incident terahertz pulse.

© 2015 OSA

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Abstract-Trapping waves with terahertz metamaterial absorber based on isotropic Mie resonators

Riad Yahiaoui, Kenichiro Hanai, Keisuke Takano, Tsubasa Nishida, Fumiaki Miyamaru, Makoto Nakajima, and Masanori Hangyo
https://www.osapublishing.org/ol/abstract.cfm?uri=ol-40-13-3197

Quasi-monodisperse dielectric particles organized in a periodic hexagonal network on an aluminum surface are exploited numerically and experimentally as a single-layered near-perfect absorber in the terahertz regime. Of particular interest are titanium dioxide (TiO2) microspheres because of their large dielectric permittivity and isotropic shape leading to Mie resonances with insensitive polarization. Absorption higher than 80% at normal incidence covering two distinct ranges of frequencies is demonstrated experimentally. Furthermore, the performance of the metamaterial absorber is kept over a wide range of incident angles.

© 2015 Optical Society of America

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Abstract-Extraordinary optical transmission through incommensurate metal hole arrays in the terahertz region

Yoji Jimba, Keisuke Takano, Masanori Hangyo, and Hiroshi Miyazaki  »View Author Affiliations
http://www.opticsinfobase.org/josab/abstract.cfm?uri=josab-30-9-2476

 By using coupled-mode analysis, we investigate numerically the extraordinary optical transmission (EOT) of incommensurate metal hole arrays (IMHAs) in the terahertz region to study how the degree of long-range and short-range orders affect EOT. In IMHAs, the holes of one set of metal hole arrays (MHAs) are regarded to work as impurities to the holes of another set of MHAs. Therefore, the transmittance spectra are expected to be significantly modified from those of the constituent MHAs. It is found that the resonance transmission frequencies of IMHAs almost coincide with those of the composite MHAs despite the fact that the surface waves of each MHA are mutually perturbed strongly due to the presence of short-range disorder. This indicates that the presence of long-range order is essential to establish EOT. These observations allow more flexible design of narrow bandpass filters in the terahertz region.