G. O. Andreev

Gate-tuning of graphene plasmons revealed by infrared nano-imaging

Surface plasmons are collective oscillations of electrons in metals or semiconductors that enable confinement and control of electromagnetic energy at subwavelength scales. Rapid progress in plasmonics has largely relied on advances in device nano-fabrication, whereas less attention has been paid to the tunable properties of plasmonic media. One such medium—graphene—is amenable to convenient tuning of its electronic and optical properties by varying the applied voltage. Here, using infrared nano-imaging, we show that common graphene/SiO2/Si back-gated structures support propagating surface plasmons. The wavelength of graphene plasmons is of the order of 200 nanometres at technologically relevant infrared frequencies, and they can propagate several times this distance. We have succeeded in altering both the amplitude and the wavelength of these plasmons by varying the gate voltage. Using plasmon interferometry, we investigated losses in graphene by exploring real-space profiles of plasmon standing waves formed between the tip of our nano-probe and the edges of the samples. Plasmon dissipation quantified through this analysis is linked to the exotic electrodynamics of graphene. Standard plasmonic figures of merit of our tunable graphene devices surpass those of common metal-based structures.

Infrared Nanoscopy of Dirac Plasmons at the Graphene-SiO₂ Interface

We report on infrared (IR) nanoscopy of 2D plasmon excitations of Dirac fermions in graphene. This is achieved by confining mid-IR radiation at the apex of a nanoscale tip: an approach yielding 2 orders of magnitude increase in the value of in-plane component of incident wavevector q compared to free space propagation. At these high wavevectors, the Dirac plasmon is found to dramatically enhance the near-field interaction with mid-IR surface phonons of SiO(2) substrate. Our data augmented by detailed modeling establish graphene as a new medium supporting plasmonic effects that can be controlled by gate voltage.

Infrared spectroscopy and nano-imaging of the insulator-to-metal transition in vanadium dioxide

We present a detailed infrared study of the insulator-to-metal transition IMT in vanadium dioxide VO2 thin films. Conventional infrared spectroscopy was employed to investigate the IMT in the far field. Scanning near-field infrared microscopy directly revealed the percolative IMT with increasing temperature. We confirmed that the phase transition is also percolative with cooling across the IMT. We present extensive near-field infrared images of phase coexistence in the IMT regime in VO2. We find that the coexisting insulating and metallic regions at a fixed temperature are static on the time scale of our measurements. A distinctive approach for analyzing the far-field and near-field infrared data within the Bruggeman effective medium theory was employed to extract the optical constants of the incipient metallic puddles at the onset of the IMT. We found divergent effective carrier mass in the metallic puddles that demonstrates the importance of electronic correlations to the IMT in VO2. We employ the extended dipole model for a quantitative analysis of the observed near-field infrared amplitude contrast and compare the results with those obtained with the basic dipole model.

Near-field spectroscopy of silicon dioxide thin films

We analyze the results of scanning near-field infrared spectroscopy performed on thin films of a-SiO2 on Si substrate. The measured near-field signal exhibits surface-phonon resonances whose strength has a strong thickness dependence in the range from 2 to 300 {nm}. These observations are compared with calculations in which the tip of the near-field infrared spectrometer is modeled either as a point dipole or an elongated spheroid. The latter model accounts for the antenna effect of the tip and gives a better agreement with the experiment. Possible applications of the near-field technique for depth profiling of layered nanostructures are discussed.

Quantitative investigation of a terahertz artificial magnetic resonance using oblique angle spectroscopy

The authors present a spectroscopic analysis of a planar split-ring-resonator (SRR) medium at terahertz frequencies, quantitatively characterizing the associated magnetic resonance. Experimental quantification at terahertz and infrared frequencies of metamaterial optical constants has been primarily absent, largely due to the difficulty of collecting phase information at these frequencies. In this letter, the authors circumvent the need for phase information in the characterization by acquiring the power transmitted through the metamaterial at a series of oblique angles, and relating the multiangle data set to the effective permittivity and permeability through the Fresnel expressions. The resulting measurements reveal the expected resonant permeability of the SRR which exhibits a range of negative values, the minimum value being μ=−0.8 at 1.1THz.