Nonlinear Metasurface Fresnel Zone Plates for Terahertz Generation and Manipulation.

We introduce a nanoengineered nonlinear metasurface based optical element that acts as an emitting Fresnel zone plate of terahertz (THz) waves. We show that the nonlinear zone plate generates broadband THz radiation and focuses each generated frequency on a different focal point along the optical axis. Therefore, a narrow beam waist and spectral selectivity of both the bandwidth and central frequency are achieved. Furthermore, we measure and analyze the temporal structure of the focused THz electric field and show that it comprises of few cycles with an axially varying carrier frequency in agreement with the calculated dispersion of the zone plate. This demonstration of controlled emission and focusing of THz waves opens the door for the development of a wide variety of additional holographic metasurface-based THz emitters and can lead to the development of efficient, active, integrated, and ultracompact optical devices for the THz spectral region.

Direct space to time terahertz pulse shaping with nonlinear metasurfaces.

We present a method for the generation of THz pulses with tailored temporal shape from nonlinear metasurfaces. The method is based on single-cycle THz emission by the metasurface inclusions. We show that the spatial amplitude and phase structure of the nonlinear response is mapped to the temporal shape of pulses emitted at certain angles. We specifically show a method for reconstruction of desired pulses, generation of few-cycles pulses with tailored carrier-envelope and all-optical control over the pulse shape by the pump pulse characteristics.

Nonlinear Surface Lattice Resonance in Plasmonic Nanoparticle Arrays.

We study experimentally second-harmonic generation from arrays of split-ring resonators at oblique incidence and find conditions of more than 30-fold enhancement of the emitted second harmonic with respect to normal incidence. We show that these conditions agree well with a nonlinear Rayleigh-Wood anomaly relation and the existence of a surface lattice resonance at the second harmonic. The existence of a nonlinear surface lattice resonance is theoretically confirmed by extending the coupled dipole approximation to the nonlinear case. We further show that the localized surface plasmon modes that collectively contribute to the surface lattice resonance are inherently dark modes that become highly bright due to the collective interaction.

Nonlinear Plasmonic Metasurface Terahertz Emitters for Compact Terahertz Spectroscopy Systems.

Nonlinear plasmonic metasurfaces provide new and promising means to produce broadband terahertz (THz) radiation, due to their compact size and functionalities beyond those achievable with conventional THz emitters. However, they were driven to date only by amplified laser systems, which are expensive and have a large footprint, thus limiting the range of their potential applications. Here we study for the first time the possibility to drive metasurface emitters by low-energy near-infrared femtosecond pulses. We observe broadband THz emission from 40 nm thick metasurfaces and achieve near-infrared to THz conversion efficiencies as high as those of 2500-fold thicker ZnTe crystals. We characterize the THz emission properties and use the metasurface emitter to perform a spectroscopic measurement of α-lactose monohydrate. These results show that nonlinear plasmonic metasurfaces are suitable for integration as emitters in existing compact THz spectroscopy and imaging systems, enhancing their functionalities, and opening the door for a variety of new applications.

Generation of spatiotemporally tailored terahertz wavepackets by nonlinear metasurfaces.

The past two decades have witnessed an ever-growing number of emerging applications that utilize terahertz (THz) waves, ranging from advanced biomedical imaging, through novel security applications, fast wireless communications, and new abilities to study and control matter in all of its phases. The development and deployment of these emerging technologies is however held back, due to a substantial lack of simple methods for efficient generation, detection and manipulation of THz waves. Recently it was shown that uniform nonlinear metasurfaces can efficiently generate broadband single-cycle THz pulses. Here we show that judicious engineering of the single-emitters that comprise the metasurface, enables to obtain unprecedented control of the spatiotemporal properties of the emitted THz wavepackets. We specifically demonstrate generation of propagating spatiotemporal quadrupole and few-cycles THz pulses with engineered angular dispersion. Our results place nonlinear metasurfaces as a new promising tool for generating application-tailored THz fields with controlled spatial and temporal characteristics.