Split-ring resonators represent the ideal route to achieve optical control of the incident light at THz frequencies. These subwavelength metamaterial elements exhibit broad resonances that can be easily tuned lithographically. We have realized a design based on the interplay between the resonances of metallic split rings and the electronic properties of monolayer graphene integrated in a single device. By varying the major carrier concentration of graphene, an active modulation of the optical intensity was achieved in the frequency range between 2.2 and 3.1 THz, achieving a maximum modulation depth of 18%, with a bias as low as 0.5 V.

Time-varying metasurfaces are emerging as a powerful instrument for the dynamical control of the electromagnetic properties of a propagating wave. Here we demonstrate an efficient time-varying metasurface based on plasmonic nano-antennas strongly coupled to an epsilon-near-zero (ENZ) deeply sub-wavelength film. The plasmonic resonance of the metal resonators strongly interacts with the optical ENZ modes, providing a Rabi level spitting of ∼ 30%. Optical pumping at frequency ω induces a nonlinear polarisation oscillating at 2ω responsible for an efficient generation of a phase conjugate and a negative refracted beam with a conversion efficiency that is more than four orders of magnitude greater compared to the bare ENZ film. The introduction of a strongly coupled plasmonic system therefore provides a simple and effective route towards the implementation of ENZ physics at the nanoscale.Introduction. Time-varying systems and metasurfaces are of interest in view of the fundamental physics questions that have arisen [1][2][3][4][5][6][7] and also in view of the potential applications ranging from perfect lenses to spectral and temporal shaping of light fields [8][9][10][11][12][13][14][15][16][17]. Recent results have shown that thin films of epsilon-near-zero (ENZ) materials with a dielectric permittivity close to zero [18,19] at optical wavelengths in the visible or near-infrared spectral regions are promising candidates to achieve rapid (on the optical wave oscillation timescale) temporal changes of the optical properties [7]. The very large order-of-unity refractive index changes that can be induced optically [20][21][22][23] makes it possible to achieve efficient temporal modulation uniformly across the medium [10, 24] even in deeply subwavelength thin films [25][26][27], resulting in optically-induced negative refraction with unity efficiency [7]. However, the results demonstrated so far rely on high-intensity optical pumping of the ENZ film in order to achieve such large changes in the refractive index. Recently, the combination of ENZ films with plasmonic structures has led to a significant reduction of the required optical powers for the Kerr nonlinear contribution to the refractive index [28]. Coupling between light and matter can be enhanced when two resonant systems with the same optical resonant frequency are brought into close contact [29]. Strong coupling occurs when the strength of the coupling mech- * daniele.faccio@glasgow.ac.uk, r.sapienza@imperial.ac.uk, sha-laev@purdue.edu: † These authors contributed equally. anism (measured by the splitting of the two resonant frequencies [30]) dominates the intrinsic losses in the system thus resulting in a double peaked structure in the absorption spectrum or equivalently, in two well-separated polariton branches in the spectral domain. In the temporal domain, this will give rise to Rabi oscillations between the populations on these two branches and the combination of light-matter states where the matter component can contain a large fraction of the total ener...

Synthetic fractals inherently carry spatially encoded frequency information that renders them as an ideal candidate for broadband optical structures. Nowhere is this more true than in the terahertz (THz) band where there is a lack of naturally occurring materials with valuable optical properties. One example are perfect absorbers that are a direct step toward the development of highly sought after detectors and sensing devices. Metasurface absorbers that can be used to substitute for natural materials suffer from poor broadband performance, while those with high absorption and broadband capability typically involve complex fabrication and design and are multilayered. Here, we demonstrate a polarization-insensitive ultrathin (∼λ/6) planar metasurface THz absorber composed of supercells of fractal crosses capable of spanning one optical octave in bandwidth, while still being highly efficient. A sufficiently thick polyimide interlayer produces a unique absorption mechanism based on Salisbury screen and antireflection responses, which lends to the broadband operation. Experimental peak absorption exceeds 93%, while the average absorption is 83% from 2.82 THz to 5.15 THz. This new ultrathin device architecture, achieving an absorption-bandwidth of one optical octave, demonstrates a major advance toward a synthetic metasurface blackbody absorber in the THz band.

Plasmonic metasurfaces provide unprecedented control of the properties of light. By designing symmetry-breaking nanoholes in a metal sheet and engineering the optical properties of the metal using geometry, highly selective transmission and polarisation control of light is obtained. To date such plasmonic filters have exhibited broad (> 200 nm) transmission linewidths in the NIR and as such are unsuitable for applications requiring narrow passbands, e.g. multi-spectral imaging. Here we present a novel subwavelength elliptical and circular nanohole array in a metallic film that simultaneously exhibits high transmission efficiency, polarisation insensitivity and narrow linewidth. The experimentally obtained linewidth is 79 nm with a transmission efficiency of 44%. By examining the electric and magnetic field distributions for various incident polarisations at the transmission peak we show that the narrowband characteristics are due

Hong-Ou-Mandel (HOM) interference, the bunching of indistinguishable photons at a beam splitter, is a staple of quantum optics and lies at the heart of many quantum sensing approaches and recent optical quantum computers. Here, we report a full-field, scan-free, quantum imaging technique that exploits HOM interference to reconstruct the surface depth profile of transparent samples. We demonstrate the ability to retrieve images with micrometre-scale depth features with a photon flux as small as 7 photon pairs per frame. Using a single photon avalanche diode camera we measure both the bunched and anti-bunched photon-pair distributions at the HOM interferometer output which are combined to provide a lower-noise image of the sample. This approach demonstrates the possibility of HOM microscopy as a tool for label-free imaging of transparent samples in the very low photon regime.