Abstract-Enhanced Terahertz Detection Efficiency via Grating-Assisted Noncollinear Electro-Optic Sampling

Alexei Halpin, Wei Cui, Aidan W. Schiff-Kearn, Kashif Masud Awan, Ksenia Dolgaleva, and Jean-Michel Ménard

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.12.031003

Sensitive detection of phase-locked terahertz (THz) pulses is routinely achieved using electro-optic sampling (EOS), a technique that harnesses second-order optical nonlinearities in a crystalline semiconductor. The thickness and electro-optic coefficient of such a crystal set the detection sensitivity in EOS, while its linear optical properties often determine the accessible detection bandwidth through the phase-matching conditions. Here, we demonstrate how a periodically patterned structure on the incident surface of an EOS crystal can be used to overcome bandwidth limitations by enabling noncollinear propagation between the THz waves and a near-infrared (NIR) gating pulse. The concept is demonstrated with phase gratings etched on a 1-mm-thick gallium phosphide ($\mathrm{Ga}P$) semiconductor. The use of a diffracted NIR gating pulse to retrieve the THz waveform results in a sensitivity enhancement of a factor of approximately 2 with regard to the spectral maximum at 2 THz and exceeding an order of magnitude at 5 THz. This extends the spectral detection bandwidth up to 5 THz, which is twice as large as that achievable in a similar configuration with collinear THz and NIR pulses, facilitating the broadband detection of weak signals in THz spectroscopy.

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Abstract-Terahertz diffraction enhanced transparency probed in the near field

Alexei Halpin, Niels van Hoof, Arkabrata Bhattacharya, Christiaan Mennes, and Jaime Gomez Rivas

https://journals.aps.org/prb/accepted/ba076O10Q0913a31a24c61171bda23fbf5c1b5280

Electromagnetically induced transparency in metamaterials allows to engineer structures which transmit narrow spectral ranges of radiation while exhibiting a large group index. Implementation of this phenomenon frequently calls for strong near-field coupling of bright (dipolar) resonances to dark (multipolar) resonances in the meta-molecules comprising the metamaterials. The sharpness and contrast of the resulting transparency windows thus depends strongly on how closely these meta-moleucles can be placed to one another, placing constraints on fabrication capabilities. In this manuscript we demonstrate that the reliance on near-field interaction strength can be relaxed, and the magnitude of the electromagnetic induced transparency enhanced, by exploiting the long range coupling between meta-molecules in periodic lattices. By placing dolmen structures resonant at THz frequencies in a periodic lattice, we show a significant increase of the transparency window when the in-plane diffraction is tuned to the resonant frequency of the meta-molecules, as confirmed by direct mapping of the THz near-field amplitude across a lattice of dolmens. Through the direct interrogation of the dark resonance in the near-field we show the interplay of near- and far-field couplings in optimizing the response of planar dolmen arrays via diffraction induced transparency.

Abstract-Visualizing near-field coupling in terahertz dolmens

Alexei Halpin1,a), Christiaan Mennes2, Arkabrata Bhattacharya1, and Jaime Gómez Rivas

http://aip.scitation.org/doi/abs/10.1063/1.4978031

Strong interactions between resonant structures in the near-field occur at length scales shorter than the wavelength, and can be exploited for modifying the propagation of electromagnetic radiation. Dolmen-like structures, formed by a rod supporting a dipolar (bright) resonance and two orthogonal rods with a quadrupolar (dark) resonance at the same frequency, represent a geometry of significant interest for near-field electromagnetic coupling. These structures demonstrate electromagnetically induced transparency (EIT) through coupling between these resonances, concurrently providing a sharp spectral selectivity in transmission and large group velocity reduction. We use near-field terahertz scanning microscopy to map the electric fields in the vicinity of a metallic dolmen in both amplitude and phase. In this way, we directly measure the interaction between bright and dark modes in the time-domain, revealing the physics resulting in EIT. We experimentally demonstrate the hybridization of bright and dark modes accompanying the near-field coupling, as well as the excitation of the dark mode at the frequency of the far-field transparency.