Abstract-Pump wavelength-independent broadband terahertz generation from a nonlinear optical crystal

Kei Takeya, Koutarou Okimura, Kengo Oota, Kodo Kawase, and Hirohisa Uchida

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-43-17-4100

In nonlinear optical (NLO) crystals, the selection of pump light wavelengths for the generation of terahertz (THz) waves is limited due to problems associated with coherence length, refractive index, and absorption by the crystal. Relaxation of this limitation would open up potential light sources for THz generation. One such solution, Cherenkov phase matching, removes the coherence length constraint. In this study, we attempted to generate THz waves from an NLO crystal using femtosecond pulses of various wavelengths. Specifically, 805-nm and 1560-nm femtosecond pulses were used to pump a prism-coupled LiNbO3${\mathrm{LiNbO}}_3$ crystal. Broadband THz-wave generation and a THz-wave output proportional to the square of the pump light intensity were observed at both wavelengths. The generation of THz waves by prism-coupled Cherenkov phase matching was not limited by the wavelength of the pump light. Moreover, THz-wave generation at even greater intensities may be possible by optimizing the pump source and coupling to an NLO crystal.

© 2018 Optical Society of America

Abstract-Broadband terahertz generation via the interface inverse Rashba-Edelstein effect

C. Zhou, Y. P. Liu, Z. Wang, S. J. Ma, M. W. Jia, R. Q. Wu, L. Zhou, W. Zhang, M. K. Liu, Y. Z. Wu, and J. Qi, 

https://journals.aps.org/prl/accepted/7f07dY95Df118160072f78c38ad2d3d9134dfaf91

Novel mechanisms for electromagnetic wave emission in the terahertz (THz) frequency regime emerging at the nanometer scale have recently attracted intense attention for the purpose of searching next-generation broadband THz emitters. Here, we report broadband THz emission, utilizing the interface inverse Rashba-Edelstein effect. By engineering the symmetry of the Ag/Bi Rashba interface, we demonstrate a controllable THz radiation (\textasciitilde 0.1-5 THz) waveform emitted from metallic Fe/Ag/Bi heterostructures following photo-excitation. We further reveal that this type of THz radiation can be selectively superimposed on the emission discovered recently due to the inverse Spin Hall effect, yielding a unique film thickness dependent emission pattern. Our results thus offer new opportunities for versatile broadband THz radiation using the interface quantum effects. Terahertz (THz) radiation from 0.1--30 THz accesses a diverse group of low-energy elementary excitations in solid-state systems [1], holding great promises for imaging, sensing and security applications [2]. One major challenge in the next generation THz technology is to search novel mechanism(s) providing efficient and broadband THz radiation with a gapless spectrum [3-5]. To date, most broadband THz emission devices [2-5] are based on the femtosecond laser excitations, taking advantage exclusively of the nonlinear or dynamic properties of the electrons. Recently, the emerging ultrafast spintronics [6-13], however, offers an alternative route to the THz emission with the spin-degree of freedom, by converting spin current bursts into THz pulses. In this way, one can effectively generate, control, and detect the spin currents, as well as utilize such spin-to-charge conversion [6-13] within the sub-picosecond timescale to yield efficient ultra-broadband THz emission. Such ultrafast spin-to-charge conversion process in all previous works is mostly based on the inverse Spin Hall effect (ISHE) (14-15), which happens inside the bulk of a metallic system with a strong spin-orbit coupling (SOC). In contrast, the inverse Rashba-Edelstein effect (IREE) occurring at the interfaces with broken inversion symmetry can also provide efficient spin-to-charge conversion [16-19]. In the IREE, the generated charge current in two-dimensional electron gas can be described by [19]$$j_c\propto {\lambda }_{IREE}{j}_s\times \hat{z}$$where ${\lambda }_{IREE}$ is the IREE coefficient which is proportional to the Rashba parameter ${\alpha }_R$, $\hat{z}$ is the direction of the potential gradient (interfacial electric field) perpendicular to the interface, and $j_s$ is the spin current. Although the IREE has been intensively studied under equilibrium or quasi-equilibrium conditions in magnetoresistance measurements [17], non-local spin valves [18], and ferromagnetic resonance experiments [19], it is still elusive whether the IREE can work in femtosecond timescale, and play a vital role in the THz emission. In this work, we report the observation of THz radiation via the interface IREE in the metallic Fe/Ag/Bi heterostructures, which strongly suggests the effect of the interface IREE on the spin-to-charge conversion in femtosecond timescale. This observation brings us to a novel mechanism of emitting ...

Abstract-Broadband terahertz generation via the interface inverse Rashba-Edelstein effect

C. Zhou, Y. P. Liu, Z. Wang, S. J. Ma, M. W. Jia, R.Q. Wu, L. Zhou, W. Zhang, M. K. Liu, Y. Z. Wu, J. Qi

https://128.84.21.199/abs/1804.04765v1

Novel mechanisms for electromagnetic wave emission in the terahertz (THz) frequency regime emerging at the nanometer scale have recently attracted intense attention for the purpose of searching next-generation broadband THz emitters. Here, we report a new mechanism for broadband THz emission, utilizing the interface inverse Rashba-Edelstein effect. By engineering the symmetry of the Ag/Bi Rashba interface, we demonstrate a controllable THz radiation (~0.1-5 THz) waveform emitted from metallic Fe/Ag/Bi heterostructures following photo-excitation. We further reveal that this type of THz radiation can be selectively superimposed on the emission discovered recently due to the inverse Spin Hall effect, yielding a unique film thickness dependent emission pattern. Our results thus offer new opportunities for versatile broadband THz radiation using the interface quantum effects.

Abstract- Broadband Oscillator-Free THz Pulse Generation and Radiation Based on Direct Digital-to-Impulse Architecture

M. Mahdi Assefzadeh,  Aydin Babakhani,

http://ieeexplore.ieee.org/document/8022938/

Broadband 0.03-1.1 THz signal generation and radiation are demonstrated based on an oscillator-free direct digital-to-impulse architecture with a 1.9-ps full width at half maximum and 130-GHz 3-dB bandwidth (BW) (200-GHz 10-dB BW) centered at 160 GHz. The radiated pulse achieves a peak pulse effective isotropic-radiated power of 19.2 dBm and peak pulse-radiated power of 2.6 mW. An ON/OFF impulse-shaping technique is introduced and implemented to suppress undesired ringing and to increase dc-to-radiated efficiency. The frequency-comb spectrum of the radiated pulse train with 5.2-GHz repetition rate is measured up to 1.1 THz. At a distance of 4 cm, the measured received SNR at 1 and 1.1 THz is 28 and 22 dB, respectively. A 1.1-THz tone is measured with a 10-dB spectral width of 2 Hz, demonstrating an extremely narrow spectral line width (two parts per trillion). Time-domain picosecond pulses are characterized using a custom femtosecond-laser-based terahertz time-domain spectroscopy system. Coherent spatial combining from two widely spaced chips is demonstrated. It is shown that the starting time of the radiated pulses is locked to the edge of the input digital trigger with a timing jitter of 270 fs. The chip is fabricated in a 130-nm SiGe BiCMOS process technology.

Abstract-Towards broadband terahertz generation due to coherent phonon–polariton excitations in centrosymmetric media

Eugenijus Gaižauskas, Virgilijus Vaičaitis, Olga Fedotova, and Oleg Khasanov  »View Author Affiliations

http://www.opticsinfobase.org/ome/abstract.cfm?uri=ome-5-3-623

Optical Materials Express, Vol. 5, Issue 3, pp. 623-628 (2015)
http://dx.doi.org/10.1364/OME.5.000623

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In this work, four-wave difference-frequency generation in centrosymmetric media doped with three-level quantum systems is investigated theoretically. Terahertz radiation generation in the coherent regime of interaction, where the durations of optical pulses are shorter than the relaxation time of coherences induced in two- and one-photon resonant transitions, is analyzed. It is shown that the coherent regime of four-wave frequency mixing enhances the efficiency of radiation generation in the terahertz frequency range. Quantum confined systems (e.g., atoms and molecules in nanoparticles) should be considered as possible media for practical implementation of the method.

© 2015 Optical Society of America