Abstract-Broadband and narrowband laser-based terahertz source and its application for resonant and non-resonant excitation of antiferromagnetic modes in NiO

O. V. Chefonov, A. V. Ovchinnikov, C. P. Hauri, and M. B. Agranat

 Schematic diagram of a versatile THz source with tunable spectrum. (a) – the main elements of the laser system for generating optical pulses with different temporal shape. AODDL is an acousto-optic dispersion delay line, MZI is a Mach-Zehnder interferometer. (b), (c), (d) – examples of temporal shapes of laser pulses at the output of the temporal compressor. NC is a non-linear organic crystal for generating THz radiation (THz).

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-19-27273

A versatile table-top high-intense source of terahertz radiation, enabling to generate pulses of both broadband and narrowband spectra with a tunable frequency up to 3 THz is presented. The terahertz radiation pulses are generated by optical rectification of femtosecond pulses of Cr:forsterite laser setup in nonlinear organic crystal OH1. Electric field strengths of broadband and narrowband terahertz pulses were achieved close to 20 MV/cm and more than 2 MV/cm, correspondingly. Experiments on excitation of spin subsystem oscillations of an antiferromagnetic NiO were carried out. Selective excitation of 0.42 THz mode was observed for the first time at room temperature by a narrowband terahertz pulses tuned close to mode frequency.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Broadband phase shift engineering for terahertz waves based on dielectric metasurface

Qianyi Mu, Hengzhi Lin, Fei Fan, Jierong Cheng, Xianghui Wang, Sheng Jiang,




https://www.sciencedirect.com/science/article/abs/pii/S0030401818309143
Broadband terahertz (THz) phase shift engineering and zero-dispersion waveplates based on dielectric metasurface have been investigated, of which structure is a periodical rectangular scattering units on silicon substrates. By designing proper geometric parameters of metasurface structure, the value, dispersion and bandwidth of the phase shift curves can be effectively manipulated. Based on this, the broadband half waveplate (HWP) and quarter waveplate (QWP) have been designed and fabricated. The experimental results show that the HWP can work in the broad range of 0.7–1.35 THz with the polarization conversion ratio (PCR) of close to 100% and the transmission of over 70%. And the QWP can operate in the range of 0.7$\sim$0.85THz with the PCR of over 90% and the transmission of over 70%. The method of phase shift engineering based on dielectric metasurfaces and these broadband zero-dispersion waveplates have great potential in promoting the performance of THz application systems.

Abstract-Broadband terahertz ZnO photonic crystals fabricated by 3D printing

Carmen R.Tubío, José Antonio Nóvoa, Jorge Martín, Francisco Guitián, José Ramón Salgueiro, Alvaro Gil, 

https://www.sciencedirect.com/science/article/pii/S0272884218334837

ZnO has been demonstrated to be a promising material for optoelectronic applications in the terahertz (THz) spectral range. However, there have been no reports regarding the fabrication of ZnO structures to control THz radiation, as photonic crystal (PCs) materials. Here, we focus on the development of a functional ZnO PCs for the THz band. The functionality is provided by the design that is based on a woodpile lattice with the appropriate periodicity for the THz region. Using a three-dimensional (3D) printing technique, we fabricate 3D ZnO based PCs with an effective lattice constant to generate a bandgap at the THz region. The functionality of the resulting PC was evaluated using THz time-domain spectroscopy combined with theoretical simulations. The results demonstrate the 3D woodpile lattice structure has good optical properties in the THz frequency region. These results serve as a step for the development of new THz functional devices for optoelectronics applications.

Abstract-Broadband terahertz rotator with an all-dielectric metasurface

Quanlong Yang, Xieyu Chen, Quan Xu, Chunxiu Tian, Yuehong Xu, Longqing Cong, Xueqian Zhang, Yanfeng Li, Caihong Zhang, Xixiang Zhang, Jiaguang Han, and Weili Zhang

Fig. 1. (a) Conceptual description of the metasurface based on two identical dielectric antennas to manipulate the polarization of the terahertz wave. 𝛼$\alpha$ and 𝛽$\beta$ represent the orientations of two dielectric antennas (marked by the orange arrows), and 𝛾$\gamma$ is the effective optical axis orientation from the superposition of two antennas (marked by the navy arrow). (b) Schematic illustration of the two silicon antennas with geometrical parameters 𝑊=45  μm$W=45\mathrm{\mu m}$, 𝐿=180  μm$L=180\mathrm{\mu m}$, 𝐻=200  μm$H=200\mathrm{\mu m}$, and period 𝑃=375  μm$P=375\mathrm{\mu m}$. (c),(d) Schematic diagrams for high-quality polarization generation. Without introducing the phase gradient, both the 𝑥$x$-polarized and 𝑦$y$-polarized light propagates in the normal direction forming dispersive polarization states within the frequency range of interest. The phase gradient enables spatial separation of the two orthogonal polarization components, giving rise to pure linearly polarized components within a broad frequency range.

https://www.osapublishing.org/prj/fulltext.cfm?uri=prj-6-11-1056&id=399205

Polarization manipulation is essential in developing cutting-edge photonic devices ranging from optical communication displays to solar energy harvesting. Most previous works for efficient polarization control cannot avoid utilizing metallic components that inevitably suffer from large ohmic loss and thus low operational efficiency. Replacing metallic components with Mie resonance-based dielectric resonators will largely suppress the ohmic loss toward high-efficiency metamaterial devices. Here, we propose an efficient approach for broadband, high-quality polarization rotation operating in transmission mode with all-dielectric metamaterials in the terahertz regime. By separating the orthogonal polarization components in space, we obtain rotated output waves with a conversion efficiency of 67.5%. The proposed polarization manipulation strategy shows impressive robustness and flexibility in designing metadevices of both linear- and circular-polarization incidences.

© 2018 Chinese Laser Press

Abstract-Broadband THz radiation via the inverse spin Hall and Rashba-Edelstein effects (Conference Presentation)

Jingbo Qi

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10732/107323I/Broadband-THz-radiation-via-the-inverse-spin-Hall-and-Rashba/10.1117/12.2322834.short

One major challenge in the next generation THz (1012) technology is to develop highly efficient, ultra-broadband and low-cost terahertz emitters with a gapless spectrum. Up-to-date, most broadband and table-top THz emitters are based on the femtosecond laser excitations, taking advantage exclusively of the charge property of the electron. Here, we introduce two novel types of broadband spin-based THz emitters composed of the ferromagnetic metallic heterostructures [1-4], e.g. (Co, Fe)/Pt and Fe/Ag/Bi. We have carried out detailed thickness-dependent experiments in these samples. Such investigations not only enable us to clarify the intrinsic mechanisms behind the THz radiation - the inverse spin Hall effect (ISHE) and the inverse Rashba-Edelstein effect (IREE), but also help to determine the key parameters to optimize the THz emission. The emitted THz wave, with its phase and polarization easily manipulated by changing the film stacking order and the magnetization direction, has an ultra-broadband width (~0.1-20 THz) and strong amplitude (comparable to the conventional nonlinear crystals). We also demonstrate that the THz radiation arising from both the ISHE and IREE can be selectively superimposed with each other. [1] Seifert, T. et al. Efficient metallic spintronic emitters of ultrabroadband terahertz radiation. Nat. Photon. 10, 483 (2016). [2] Yang, D. et al. Powerful and Tunable THz Emitters Based on the Fe/Pt Magnetic Heterostructure. Adv. Opt. Mater. 4, 1944 (2016). [3] Wu, Y. et al. High-performance THz emitters based on ferromagnetic/nonmagnetic heterostructures. Adv. Mater. 29, 1603031 (2017). [4] Zhou. C et al. Broadband terahertz generation via the interface inverse Rashba-Edelstein effect (submitted).

Abstract-A broadband tunable terahertz negative refractive index metamaterial

Fang Ling, Zheqiang Zhong, Renshuai Huang,  Bin Zhang

https://www.nature.com/articles/s41598-018-28221-3

A strategy to greatly broaden negative refractive index (NRI) band, reduce loss and ease bi-anisotropy of NRI metamaterials (MMs) has been proposed at terahertz frequencies. Due to the symmetric structure of the MM, the transmission and refractive index are independent to polarizations of incident radiations, and a broadband NRI is obtainable for the range of the incident angle from 0° to 26°. In addition, THz MMs’ properties such as transmission, phase and negative refraction exhibit a real-time response by controlling the temperature. The results indicate that the maximum bands of the negative and double-negative refraction are 1.66 THz and 1.37 THz for the temperature of 40 °C and 63 °C, respectively. The figure of merit of the MMs exceeds 10 (that is, low loss) as the frequency increases from 2.44 THz to 2.56 THz in the working temperature range, and the maximum figure of merit is 83.77 at 2.01 THz where the refractive index is −2.81 for a given temperature of 40 °C. Furthermore, the negative refraction of the MMs at the low loss band is verified by the classical method of the wedge, and the symmetric slab waveguide based on the proposed MM has many unique properties.

Abstract-Lateral heterogeneous integration of quantum cascade lasers

Yang Yang, Andrew Paulsen, David Burghoff, John L. Reno, and Qing Hu

https://pubsdc3.acs.org/doi/10.1021/acsphotonics.8b00507

Broadband terahertz radiation potentially has extensive applications, ranging from personal health care to industrial quality control and security screening. While traditional methods for broadband terahertz generation rely on bulky and expensive mode-locked lasers, frequency combs based on quantum cascade lasers (QCLs) can provide an alternative compact, high power, wideband terahertz source. QCL frequency combs incorporating a heterogeneous gain medium design can obtain even greater spectral range by having multiple lasing transitions at different frequencies. However, despite their greater spectral coverage, the comparatively low gain from such gain media lowers the maximum operating temperature and power. Lateral heterogeneous integration offers the ability to cover an extensive spectral range while maintaining the competitive performance offered from each homogeneous gain media. Here, we present the first lateral heterogeneous design for broadband terahertz generation: by combining two different homogeneous gain media, we have achieved a two-color frequency comb spaced by 1.5 THz.

Abstract-Investigation of broadband terahertz generation from metasurface

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-26-11-14241

The nonlinear metamaterials have been shown to provide nonlinear properties with high nonlinear conversion efficiency and in a myriad of light manipulation. Here we study terahertz generation from nonlinear metasurface consisting of single layer nanoscale split-ring resonator array. The terahertz generation due to optical rectification by the second-order nonlinearity of the split-ring resonator is investigated by a time-domain implementation of the hydrodynamic model for electron dynamics in metal. The results show that the nonlinear metasurface enables us to generate broadband terahertz radiation and free from quasi-phase-matching conditions. The proposed scheme provides a new concept of broadband THz source and designing nonlinear plasmonic metamaterials.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Broadband controllable terahertz quarter-wave plate based on graphene gratings with liquid crystals

Yun-Yun Ji, Fei Fan, Xiang-Hui Wang, and Sheng-Jiang Chang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-10-12852

Developing the broadband controllable or tunable terahertz (THz) polarization and phase devices are in an urgent need. In this paper, we demonstrate a broadband controllable THz quarter-wave plate (QWP) with double layers of graphene grating and a layer of liquid crystals. The double layer graphene gratings can achieve a switchable QWP to switch between linear-to-linear and linear-to-circular polarization states with over 0.35THz bandwidth in the ON or OFF state by applying biased electric field on the graphene grating or not. Moreover, this QWP based on the structure of periodic gradient grating can significantly enhance the phase difference between two orthogonally polarized components compared to that based on equal-periodic grating structure because of the additional phase distribution of the gradient structures. Furthermore, we incorporate liquid crystals into the graphene grating to form a tunable QWP, of which operating frequency can be continuously tuned in a wide frequency range by electrically controlling the molecular director of the liquid crystals. The results show that the graphene periodic gradient grating with LCs not only broadens the operating bandwidth, but also reduces the external electric field. This device offers a further step in the development of THz polarization and phase devices for potential applications in THz polarized imaging, spectroscopy, and communication.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Dual-band superposition induced broadband terahertz linear-to-circular polarization converter

                                                                   

Xiao-Fei Zang, Su-Ji Liu, Han-Hong Gong, Yajun Wang, and Yi-Ming Zhu

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-35-4-950

A reflective broadband terahertz (THz) linear-to-circular (LTC) polarization converter based on a single-layer ultrathin metasurface is designed and experimentally demonstrated. Two different-size rectangular ultrathin metasurface micro-structures are proposed to realize such a broadband THz LTC polarization converter with bandwidth ranging from 0.832 to 1.036 THz. The phase delay between two orthogonal resonance modes is −90°±5°$-90°\pm 5°$. These qualities are realized mainly by combining two separated LTC polarization conversion frequencies and the benefit of the coupling between two different-size rectangles. The calculated results indicate that the bandwidth of the LTC polarization converter is controlled via the dimensions and period of the structure. This kind of ultrathin broadband THz polarization converter can be widely applied into wireless communication, imaging, and detection, and can widen the path to designing novel functional THz devices.

© 2018 Optical Society of America