Marko Spasenović

Optical nano-imaging of gate-tunable graphene plasmons

The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, because photons do not possess charge, a way of controlling them efficiently by electrical means has so far proved elusive. A promising way to achieve electric control of light could be through plasmon polaritons—coupled excitations of photons and charge carriers—in graphene. In this two-dimensional sheet of carbon atoms, it is expected that plasmon polaritons and their associated optical fields can readily be tuned electrically by varying the graphene carrier density. Although evidence of optical graphene plasmon resonances has recently been obtained spectroscopically, no experiments so far have directly resolved propagating plasmons in real space. Here we launch and detect propagating optical plasmons in tapered graphene nanostructures using near-field scattering microscopy with infrared excitation light. We provide real-space images of plasmon fields, and find that the extracted plasmon wavelength is very short—more than 40 times smaller than the wavelength of illumination. We exploit this strong optical field confinement to turn a graphene nanostructure into a tunable resonant plasmonic cavity with extremely small mode volume. The cavity resonance is controlled in situ by gating the graphene, and in particular, complete switching on and off of the plasmon modes is demonstrated, thus paving the way towards graphene-based optical transistors. This successful alliance between nanoelectronics and nano-optics enables the development of active subwavelength-scale optics and a plethora of nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing, and strongly enhanced light–matter interactions for quantum devices and biosensing applications.

Plasmon scattering from single subwavelength holes.

We map the complex electric fields associated with the scattering of surface plasmon polaritons by single subwavelength holes of different sizes in thick gold films. We identify and quantify the different modes associated with this event, including a radial surface wave with an angularly isotropic amplitude. This wave is shown to arise from the out-of-plane electric dipole induced in the hole, and we quantify the corresponding polarizability, which is in excellent agreement with electromagnetic theory. Time-resolved measurements reveal a time delay of 38±18  fs between the surface plasmon polariton and the radial wave, which we attribute to the interaction with a broad hole resonance.

Nanohole chains for directional and localized surface plasmon excitation

Arrangements of subwavelength sized holes in metal films are often used to launch surface plasmon polaritons (SPPs) onto metal-dielectric interfaces. They are readily fabricated and can also be used to generate a variety of near- and far-field intensity patterns. We use a short chain of equally spaced subwavelength sized holes to launch SPPs onto a gold-air interface in complex patterns of hotspots. With a phase-sensitive near-field microscope, we visualize the electric field of the excited SPPs. We observe self-images of the chain that we attribute to the Talbot effect. Far from the chain we observe the SPP diffraction orders. We find that when the spacing of the holes is of the order of the wavelength, the revivals do not occur on the well-known Talbot distance as derived in the paraxial limit. We present an alternative expression for the Talbot distance that does hold for these small spacings. We study the behavior of both the revivals and the diffraction orders as a function of the number of holes. We find that the Talbot revivals become more pronounced as the number of holes is increased, which is in accordance with numerical calculations. We anticipate that our findings are interesting for multiplexing sensor applications, where control over the local intensity of SPPs is crucial.

Statistical fluctuations of transmission in slow light photonic-crystal waveguides

We report statistical fluctuations for the transmissions of a series of photonic-crystal waveguides (PhCWs) that are supposedly identical and that only differ because of statistical structural fabrication-induced imperfections. For practical PhCW lengths offering tolerable -3dB attenuation with moderate group indices (n(g) approximately 60), the transmission spectra contains very narrow peaks (Q approximately 20,000) that vary from one waveguide to another. The physical origin of the peaks is explained by calculating the actual electromagnetic-field pattern inside the waveguide. The peaks that are observed in an intermediate regime between the ballistic and localization transports are responsible for a smearing of the local density of states, for a rapid broadening of the probability density function of the transmission, and bring a severe constraint on the effective use of slow light for on-chip optical information processing. The experimental results are quantitatively supported by theoretical results obtained with a coupled-Bloch-mode approach that takes into account multiple scattering and localization effects.

Characterization of bending losses for curved plasmonic nanowire waveguides

We characterize bending losses of curved plasmonic nanowire waveguides for radii of curvature ranging from 1 to 12 microm and widths down to 40 nm. We use near-field measurements to separate bending losses from propagation losses. The attenuation due to bending loss is found to be as low as 0.1 microm(-1) for a curved waveguide with a width of 70 nm and a radius of curvature of 2 microm. Experimental results are supported by Finite Difference Time Domain simulations. An analytical model developed for dielectric waveguides is used to predict the trend of rising bending losses with decreasing radius of curvature in plasmonic nanowires.

Experimental observation of evanescent modes at the interface to slow-light photonic crystal waveguides

We experimentally study the fields close to an interface between two photonic crystal waveguides that have different dispersion properties. After the transition from a waveguide in which the group velocity of light is v(g) ~ c/10 to a waveguide in which it is v(g) ~ c/100, we observe a gradual increase in the field intensity and the lateral spreading of the mode. We attribute this evolution to the existence of a weakly evanescent mode that exponentially decays away from the interface. We compare this to the situation where the transition between the waveguides only leads to a minor change in group velocity and show that, in that case, the evolution is absent. Furthermore, we apply novel numerical mode extraction techniques to confirm experimental results.

Measuring the spatial extent of individual localized photonic states

Waves in disordered media can undergo multiple scattering, resulting in the formation of Anderson-localized states with an associated impeded wave transport. Anderson localization is a universal wave phenomenon, with manifestations in electron transport, 1 sound, 2 matter waves, 3,4 and light. 5,6 The spatial extent of localized states, or localization length, is of primary importance. For example, in systems of finite size, when this length is larger than the length of the sample, disorder has, on average, little effect on wave propagation. 7 Conversely, when this length is smaller than the sample length, strongly confined states with typical lengths shorter than the sample size are likely to occur and wave transport may be severely disrupted. The localization length is an ensemble-averaged quantity, typically obtained by averaging over frequency or many realizations of disorder. Here we show measurements of the spatial extent of individual localized photonic states for a single realization of disorder in a photonic crystal waveguide. To emphasize the difference between the ensemble-averaged localization length and the measured spatial extent of an individual localized state for a single optical frequency for a single realization of disorder, we will use the symbol Lind for the length that we measure. The spatial extent of localized photonic states has been measured before in a two-dimensional waveguide with embedded impurities. 8 In that waveguide the spectral width of the localized resonances is on the order of 10 nm. Close to the band edge of a photonic crystal waveguide, in the technologically important slow light region, localized resonances can have spectral widths on the order of 0.1 nm, associated with small mode volumes. Measuring spectrally narrow resonances with a near-field microscope is a challenge because the tip of the near-field microscope influences the local dielectric constant in its vicinity. This dielectric constant has a paramount influence on the spectral position of a resonance. We show a method to measure the length of a localized state in a photonic crystal waveguide by using the spectral shift to our advantage. We measure Lind with two different methods, a local perturbation method and one based on the inverse participation ratio (IPR), and show that the results are in quantitative agreement. We identify states for which Lind is smaller than the length of the waveguide and show that these states are not observable in traditional transmission measurements, although such states with their small volumes are arguably the most useful for applications in quantum computing and sensing. The idea that disorder in photonic crystals can be used

Cooling and manipulation of a levitated nanoparticle with an optical fiber trap

Accurate delivery of small targets in high vacuum is a pivotal task in many branches of science and technology. Beyond the different strategies developed for atoms, proteins, macroscopic clusters and pellets, the manipulation of neutral particles over macroscopic distances still poses a formidable challenge. Here we report a novel approach based on a mobile optical trap operated under feedback control that enables long range 3D manipulation of a silica nanoparticle in high vacuum. We apply this technique to load a single nanoparticle into a high-finesse optical cavity through a load-lock vacuum system. We foresee our scheme to benefit the field of optomechanics with levitating nano-objects as well as ultrasensitive detection and monitoring.

Measurements of modal symmetry in subwavelength plasmonic slot waveguides

We excite a guided plasmonic mode in slot waveguides of subwavelength width. With a phase- and polarization-sensitive near-field microscope, we measure the electric field of the mode for a range of slot widths from 40 to 120 nm. The field is experimentally found to be antisymmetric across the slot gap. Numerical calculations confirm this symmetry. Calculations also show a confinement of the field to a lateral size ∼10 times smaller than the free-space wavelength.

Nonlinear mode-coupling and synchronization of a vacuum-trapped nanoparticle

We study the dynamics of a laser-trapped nanoparticle in high vacuum. Using parametric coupling to an external excitation source, the linewidth of the nanoparticles oscillation can be reduced by three orders of magnitude. We show that the oscillation of the nanoparticle and the excitation source are synchronized, exhibiting a well-defined phase relationship. Furthermore, the external source can be used to controllably drive the nanoparticle into the nonlinear regime, thereby generating strong coupling between the different translational modes of the nanoparticle. Our work contributes to the understanding of the nonlinear dynamics of levitated nanoparticles in high vacuum and paves the way for studies of pattern formation, chaos, and stochastic resonance.