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Manipulating ultrafast terahertz generation using nonlinear metasurfaces
Xueqian Zhang1,*, Quan Xu1, Weili Zhang2 and Jiaguang Han1,3,**
1Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering, and the Key Laboratory of Optoelectronics Information and Technology, Tianjin University. Tianjin 300072, P. R. China
2School of Electrical and Computer Engineering, Oklahoma State University. Stillwater, Oklahoma 74078, USA
3Guangxi Key Laboratory of Optoelectronic Information Processing, School of Optoelectronic Engineering, Guilin University of Electronic Technology. Guilin 541004, P. R. China
*alearn1988@tju.edu.cn; **jiaghan@tju.edu.cn
Ultrafast terahertz (THz) generation stands at the forefront of THz science and applications. A prime illustration of this is THz time-domain spectroscopy technology, widely utilized in material exploration, non-destructive inspection, and spectral imaging, among other fields. Currently, various methods for ultrafast THz generation exist, including photoconductive antennas, nonlinear crystals, air plasma, and ferromagnetic films. Despite their efficacy in producing broadband THz waves, they often lack sufficient control over the generated waves. Additional external manipulation devices are necessary to regulate THz propagation behavior, inevitably resulting in insertion loss and bandwidth limitations.
Recent advancements in nonlinear metasurfaces offer a promising solution to this challenge. Analogous to conventional linear metasurfaces, they comprise artificial subwavelength structures. It has been demonstrated that leveraging resonating structures in the infrared range, the internal difference frequency generation (DFG) effect under femtosecond laser pump can yield broadband THz waves with high efficiency [1]. It is foreseeable that by leveraging the customizable nature of these structures, the DFG process can be tailored, enabling simultaneous control over the propagation properties of the generated THz waves, including phase, amplitude, and wavefront. Since modulation occurs during the generation process, it naturally circumvents THz insertion loss and is applicable to all generated THz frequencies, rendering the overall generation and manipulation system more compact.
In this talk, we will showcase several of our recent studies on the manipulation of THz generation using nanofilm and nonlinear metasurfaces. In 2021, our experimental investigation unveiled the capacity of an ultrathin ITO film to generate broadband THz waves via Difference Frequency Generation (DFG) when subjected to an oblique pump from a femtosecond laser containing a p-polarized component [2]. Notably, the peak generation of THz waves occurred when the central wavelength of the pump beam aligned with the epsilon-near-zero (ENZ) wavelength of the ITO film. This phenomenon can be attributed to the ENZ property, which induces a substantial field enhancement effect within the ITO film. Leveraging the nonlinear polarization characteristics of the ITO film in THz generation, we proceeded to develop several devices capable of manipulating the wavefront of the generated THz waves.
Among these innovations, we introduced a THz cylindrical Bessel beam generator by coating the ITO film at an axial angle under circularly polarized pump [3]. Additionally, we crafted three focusing THz vortex beam generators with varying topological charges through the patterning of ITO films utilizing the Fresnel zone plate method [4]. Furthermore, we devised generators capable of simultaneously producing multiple vortex beams in specific diffraction orders by employing binary optics methods in the patterning of ITO films [5]. Moreover, our exploration extended to investigating the THz generation effect through four-wave mixing processes in ITO films under two-color pump conditions, revealing precise control over the amplitude and polarization angle of the generated THz waves at the femtosecond level [6].
In the pursuit of enhancing THz generation efficiency and expanding control capabilities, we integrated plasmonic metasurface structures onto the ITO film. Initial experiments focused on designing nano split-ring resonators (SRRs) resonating around the ENZ wavelength of the ITO film. Results demonstrated a remarkable enhancement in THz generation efficiency, outperforming both bare plasmonic metasurfaces and ITO films [7]. This enhancement can be attributed to the dual field enhancement effects stemming from plasmonic resonance and the ENZ effect, offering a promising platform for nonlinear metasurface design.
Further investigations revealed intriguing phenomena regarding the phase control of THz generation. For instance, under circularly polarized pump, the rotation angle (θ) of an SRR led to the generation of left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) THz waves, with phases equal to θ and −θ, respectively. This phenomenon represents a nonlinear version of the Pancharatnam-Berry (PB) phase for DFG processes. Exploiting this phase control scheme, we developed innovative devices, including an angle-dependent circularly polarized THz generator and a vortex beam generator, capitalizing on the manipulation of SRRs to achieve specific phase distributions and polarization states [7].
In addition, by introducing the coupling effect of the classic electromagnetically induced transparency into the structure design, we obtained additional controlling degrees of freedom [8]. The structures were composed by bar resonator (BR) and SRR which functioned as bright and dark modes, respectively. According to the nonlinear selection rule by the structure symmetry, only the SRR could generate THz waves. Two nonlinear metasurface designs were demonstrated. One was composed of one BR and two SRRs in a symmetric geometry, we found that by controlling the coupling between the BR and SRRs through changing their relative position, the resonating strength of the SRRs could be maniplated, so as to the amplitude of the generated THz waves. The other was composed of one BR and one SRR in a chiral symmetry, we found that it could show nonlinear circular dichroism effect through designing the interference between the coupling excitation and direction excitation routes to the SRR, where the relative THz generation amplitude under LCP and RCP pumps can be controlled. Furthermore, using the special designs with near unit and opposite nonlinear circular dichroisms and leveraging the nonlinear PB phase and multiplexing methods, we demonstrated a device that could generate vortex beams of different topological charges under LCP and RCP pumps.
All aforementioned devices operate in a broadband manner, with the generated THz waves exhibiting properties aligned with our intended functionalities. Our proposed approach harnessing nonlinear effects in indium tin oxide (ITO) film and metasurfaces presents a novel avenue for controlling both THz generation and manipulation. Such capabilities hold promise for diverse applications in future THz communications, imaging, and spectroscopy.
References
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[6] Y. Lu, X. Zhang, Q. Xu, W. Jia, X. Feng, X. Chen, Y. Gu, Y. Yang, W. Zhang and J. Han. Two-Color-Driven Controllable Terahertz Generation in ITO Thin Film. ACS Photon. 11, 293 (2024)
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