We investigate the ultrafast terahertz response of electrostatically gated graphene upon optical excitation. We observe that the photoinduced terahertz absorption increases in charge neutral graphene but decreases in highly doped graphene. We show that this transition from semiconductor-like to metal-like response is unique for zero bandgap materials such as graphene. In charge neutral graphene photoexcited hot carriers effectively increase electron and hole densities and increase the conductivity. In highly doped graphene, however, photoexcitation does not change net conducting carrier concentration. Instead, it mainly increases electron scattering rate and reduce the conductivity.

Plasmonic color filters employing a single optically-thick nanostructured metal layer have recently generated considerable interest as an alternative to colorant-based color filtering technologies, due to their reliability, ease of fabrication, and high color tunability. However, their relatively low transmission efficiency (~30%) needs to be significantly improved for practical applications. The present work reports, for the first time, a novel plasmonic subtractive color filtering scheme that exploits the counter-intuitive phenomenon of extraordinary low transmission (ELT) through an ultrathin nanostructured metal film. This approach relies on a fundamentally different color filtering mechanism than that of existing plasmonic additive color filters, and achieves unusually high transmission efficiencies of 60 ~ 70% for simple architectures. Furthermore, owing to short-range interactions of surface plasmon polaritons at ELT resonances, our design offers high spatial resolution color filtering with compact pixel size close to the optical diffraction limit (~λ/2), creating solid applications ranging from imaging sensors to color displays.

Layer-stacking domain walls in bilayer graphene are emerging as a fascinating one-dimensional system that features stacking solitons structurally and quantum valley Hall boundary states electronically. The interactions between electrons in the 2D graphene domains and the one-dimensional domain-wall solitons can lead to further new quantum phenomena. Domain-wall solitons of varied local structures exist along different crystallographic orientations, which can exhibit distinct electrical, mechanical and optical properties. Here we report soliton-dependent 2D graphene plasmon reflection at different 1D domain-wall solitons in bilayer graphene using near-field infrared nanoscopy. We observe various domain-wall structures in mechanically exfoliated graphene bilayers, including network-forming triangular lattices, individual straight or bent lines, and even closed circles. The near-field infrared contrast of domain-wall solitons arises from plasmon reflection at domain walls, and exhibits markedly different behaviours at the tensile- and shear-type domain-wall solitons. In addition, the plasmon reflection at domain walls exhibits a peculiar dependence on electrostatic gating. Our study demonstrates the unusual and tunable coupling between 2D graphene plasmons and domain-wall solitons.

Phonon polaritons are quasiparticles resulting from strong coupling of photons with optical phonons. Excitation and control of these quasiparticles in 2D materials offer the opportunity to confine and transport light at the nanoscale. Here, we image the phonon polariton (PhP) spectral response in thin hexagonal boron nitride (hBN) crystals as a representative 2D material using amplitude-and phase-resolved scattering scanning near-field optical microscopy (s-SNOM) using broadband mid-IR synchrotron radiation. The large spectral bandwidth enables the simultaneous measurement of both out-of-plane (780 cm −1 ) and in-plane (1370 cm −1 ) hBN phonon modes. In contrast to the strong in-plane mode, the out-of-plane PhP mode response is weak. Measurements of the PhP wavelength reveal a proportional dependence on sample thickness for thin hBN flakes, which can be understood by a general model describing two-dimensional polariton excitation in ultrathin materials. KEYWORDS: phonon polariton, boron nitride, near-field spectroscopy, synchrotron infrared nanospectroscopy (SINS) P honon polaritons 1 (PhPs) result from coupling of photons with optical phonons in polar crystals. Unlike plasmon polaritons, 2−4 which usually span a very broad energy range, PhPs provide a spectrally selective response related to the optical phonon modes in the infrared to terahertz spectral range. The PhPs can have strong spatial confinement and may enable potential applications for enhanced IR light−matter interaction, 5,6 high-density IR data storage, 7 coherent thermal emission, 8 development of metamaterials, 9,10 and frequencytunable terahertz wave generation. 11 Hexagonal boron nitride (hBN) is a convenient model system for studying PhPs in ultrathin materials because hBN flakes with different thicknesses are easy to prepare with high quality and no dangling bonds as a two-dimensional (2D) van der Waals material. It has two Reststrahlen bands in the midinfrared region spanning the transverse (ω TO ) and longitudinal (ω LO ) phonon frequencies of the out-of-plane mode (ω TO = 760, ω LO = 825 cm −1 ) and the in-plane mode (ω TO = 1370, ω LO = 1614 cm −1 ). Both the lower frequency out-of-plane mode and the higher frequency in-plane mode have negative real parts of the dielectric function and thus have the potential to support PhPs. 12 Furthermore, hBN is a promising material for nanotechnology applications due to its 2D layered structure, excellent electrical insulation, and chemical and thermal stability. It has attracted great interest as a substrate for highmobility graphene, 13−17 as an ideal dielectric layer and spacer for 2D heterostructures, 18,19 and for its intrinsic UV lasing response. 20 Recently, the hBN PhP related behavior started to attract much interest. 21−24 Dai et al. studied PhPs in thin hBN flakes with laser-based scattering scanning near-field optical microscopy (s-SNOM) and observed a thickness-dependent PhP dispersion 21 in the upper spectral range around the in-plane (∼1370 cm −1 ) vibration phonon mode. However, they ...

Surface plasmons 1 , collective oscillations of conduction electrons, hold great promise for the nanoscale integration of photonics and electronics 1-4 . However, nanophotonic circuits based on plasmons have been significantly hampered by the difficulty in achieving broadband plasmonic waveguides that simultaneously exhibit strong spatial confinement, a high quality factor and low dispersion. Quantum plasmons, where the quantum mechanical effects of electrons play a dominant role, such as plasmons in very small metal nanoparticles 5,6 and plasmons affected by tunnelling effects 7 , can lead to novel plasmonic phenomena in nanostructures. Here, we show that a Luttinger liquid 8,9 of one-dimensional Dirac electrons in carbon nanotubes 10-13 exhibits quantum plasmons that behave qualitatively differently from classical plasmon excitations. The Luttinger-liquid plasmons propagate at 'quantized' velocities that are independent of carrier concentration or excitation wavelength, and simultaneously exhibit extraordinary spatial confinement and high quality factor. Such Luttinger-liquid plasmons could enable novel low-loss plasmonic circuits for the subwavelength manipulation of light.Quantum-confined electrons in one dimension behave as a Luttinger liquid, a strongly correlated electronic matter distinctly different from the quasi-free electrons described by the Fermi liquid 8,9 . A defining characteristic of the Luttinger liquid is the spin-charge separation, where the spin and charge excitations propagate at different speeds. The elementary charge excitations of the Luttinger liquid are one-dimensional quantum plasmons, which differ substantially from their classical counterparts. Classically, plasmons are determined by the free electron density and effective mass, as in Drude conductivity, but this description completely breaks down for Luttinger-liquid plasmons, which are instead determined by the electron Fermi velocity and the number of quantum conducting channels 10,11 . Metallic singlewalled carbon nanotubes (SWNTs), with their extraordinary one-dimensional quantum confinement, provide the ideal platform to explore such Luttinger-liquid plasmons. Due to this strong quantum confinement, Luttinger-liquid plasmons in SWNTs with a diameter of 1 nm should persist to visible frequencies before the first intersubband transition appears 14 . In addition, the forbidden backscattering of Dirac electrons 15,16 , evidenced by ballistic transport up to micrometre lengths 17-19 in metallic SWNTs, can lead to strongly confined but low-loss Luttingerliquid plasmons. The experimental observation of such Luttinger-liquid plasmons in SWNTs has remained an outstanding challenge for over a decade, although previous electrical transport and photoemission measurements have shown the presence of Luttinger liquid in SWNTs 12,13,20 .Here, we report the first observation of Luttinger-liquid plasmons in SWNTs using infrared scattering-type scanning nearfield optical microscopy (s-SNOM) 21-23 . We show that the Luttinger-liquid plasmons can ...

During the past decades, major advances have been made in both the generation and detection of infrared light; however, its efficient wavefront manipulation and information processing still encounter great challenges. Efficient and fast optoelectronic modulators and spatial light modulators are required for mid-infrared imaging, sensing, security screening, communication and navigation, to name a few. However, their development remains elusive, and prevailing methods reported so far have suffered from drawbacks that significantly limit their practical applications. In this study, by leveraging graphene and metasurfaces, we demonstrate a high-performance free-space mid-infrared modulator operating at gigahertz speeds, low gate voltage and room temperature. We further pixelate the hybrid graphene metasurface to form a prototype spatial light modulator for high frame rate single-pixel imaging, suggesting orders of magnitude improvement over conventional liquid crystal or micromirror-based spatial light modulators. This work opens up the possibility of exploring wavefront engineering for infrared technologies for which fast temporal and spatial modulations are indispensable.

A plasmonic interferometric biosensor that consists of arrays of circular aperture-groove nanostructures patterned on a gold film for phase-sensitive biomolecular detection is demonstrated. The phase and amplitude of interfering surface plasmon polaritons (SPPs) in the proposed device can be effectively engineered by structural tuning, providing flexible and efficient control over the plasmon line shape observed through SPP interference. Spectral fringes with high contrast, narrow linewidth, and large amplitude have been experimentally measured and permit the sensitive detection of protein surface coverage as low as 0.4 pg mm(-2). This sensor resolution compares favorably with commercial prism-based surface plasmon resonance systems (0.1 pg mm(-2)) but is achieved here using a significantly simpler collinear transmission geometry, a miniaturized sensor footprint, and a low-cost compact spectrometer. Furthermore, we also demonstrate superior sensor performance using the intensity interrogation method, which can be combined with CCD imaging to upscale our platform to high-throughput array sensing. A novel low-background interferometric sensing scheme yields a high sensing figure of merit (FOM*) of 146 in the visible region, surpassing that of previous plasmonic biosensors and facilitating ultrasensitive high-throughput detection.

We propose an ultrasensitive terahertz (THz) sensor consisting of a subwavelength graphene disk and an annular gold ring within a unit cell. The interference between the resonances arising from the graphene disk and the gold ring gives rise to Fano type resonances and enables ultrasensitive sensing. Our full wave electromagnetic simulations show frequency sensitivity as high as 1.9082 THz per refractive index unit (RIU) and a figure of merit (FOM) of 6.5662. Furthermore, the sensing range can be actively tuned by adjusting the Fermi level of graphene.