Mark Lawrence

Towards nanoscale multiplexing with parity-time-symmetric plasmonic coaxial waveguides

We theoretically investigate a nanoscale mode-division multiplexing scheme based on parity-time (PT) symmetric coaxial plasmonic waveguides. Coaxial waveguides support paired degenerate modes corresponding to distinct orbital angular momentum states. PT symmetric inclusions of gain and loss break the degeneracy of the paired modes and create new hybrid modes without orbital angular momentum. This process can be made thresholdless by matching the mode order with the number of gain and loss sections within the coaxial ring. Using both a Hamiltonian formulation and degenerate perturbation theory, we show how the wavevectors and fields evolve with increased loss/gain and derive sufficient conditions for thresholdless transitions. As a multiplexing filter, this PT symmetric coaxial waveguide could help double density rates in on-chip nanophotonic networks.

Nonreciprocal Flat Optics with Silicon Metasurfaces

Metasurfaces enable almost complete control of light through ultrathin, subwavelength surfaces by locally and abruptly altering the scattered phase. To date, however, all metasurfaces obey time-reversal symmetry, meaning that forward and backward traveling waves will trace identical paths when being reflected, refracted, or diffracted. Here, we use full-field calculations to design a passive metasurface for nonreciprocal transmission of both direct and anomalously refracted near-infrared light over nanoscale optical path lengths. The metasurface consists of a 100 nm-thick, periodically patterned Si slab. Owing to the high-quality-factor resonances of the metasurface and the inherent Kerr nonlinearities of Si, this structure acts as an optical diode for free-space optical signals. This structure also exhibits nonreciprocal anomalous refraction with appropriate patterning to form a phase gradient metasurface. Compared to existing schemes for breaking time-reversal symmetry, our platform enables subwavelength nonreciprocity for arbitrary free-space optical inputs and provides a straightforward path to experimental realization. The concept is also generalizable to other metasurface functions, providing a foundation for one-way lensing and holography.

Chemically Responsive Elastomers Exhibiting Unity‐Order Refractive Index Modulation

Chameleons are masters of light, expertly changing their color, pattern, and reflectivity in response to their environment. Engineered materials that share this tunability can be transformative, enabling active camouflage, tunable holograms, and novel colorimetric medical sensors. While progress has been made in creating artificial chameleon skin, existing schemes often require external power, are not continuously tunable, and may prove too stiff or bulky for applications. Here, a chemically tunable, large-area metamaterial is demonstrated that accesses a wide range of colors and refractive indices. An ordered monolayer of nanoresonators is fabricated, then its optical response is dynamically tuned by infiltrating its polymer substrate with solvents. The material shows a strong magnetic response with a dependence on resonator spacing that leads to a highly tunable effective permittivity, permeability, and refractive index spanning negative and positive values. The unity-order index tuning exceeds that of traditional electro-optic and photochromic materials and is robust to cycling, providing a path toward programmable optical elements and responsive light routing.

Giant vacuum friction: PT symmetric spectral singularity and negative frequency resonance (Conference Presentation)

Vacuum consists of a bath of balanced and symmetric positive and negative frequency fluctuations. Media in relative motion or accelerated observers can break this symmetry and preferentially amplify negative frequency modes as in Quantum Cherenkov radiation and Unruh radiation. Here, we show the existence of a universal negative frequency-momentum mirror symmetry in the relativistic Lorentzian transformation for electromagnetic waves. We show the connection of our discovered symmetry to parity-time (PT) symmetry in moving media and the resulting spectral singularity in vacuum fluctuation related effects. We prove that this spectral singularity can occur in the case of two metallic plates in relative motion interacting through positive and negative frequency plasmonic fluctuations (negative frequency resonance). Our work paves the way for understanding the role of PT-symmetric spectral singularities in amplifying fluctuations and motivates the search for PT-symmetry in novel photonic systems. [1] arXiv:1612.02050 [physics.optics]

Quantum phenomena with graphene plasmons (Conference Presentation)

Plasmons in atomic-scale structures exhibit intrinsic quantum phenomena related to both the finite confinement that they undergo and the small number of electrons on which they are supported. Their interaction with two-level emitters is also evidencing strong quantum effects. In this talk I will discuss several salient features of graphene plasmons in this context, and in particular their ability to mediate ultrafast heat transfer, the generation of high harmonics, their interaction with molecules and quantum emitters, and their extreme nonlinearity down to the single-photon level.

Rb atomic vapor interaction with nanophotonic and plasmonic devices (Conference Presentation)

Over the past decades, alkali atoms have been the subject of intensive and diverse research, ranging from fundamental studies on ultra-cold atoms and Bose-Einstein condensates to technological applications. Since they possess only a single valance s-electron, the alkali atoms manifest a simple low-lying electronic structure compared to other atoms. Moreover, unlike conventional solid state systems their dispersion-free features make them an ideal candidate for sensing applications and referencing tasks. These two features have introduced alkali atoms as promising quantum emitters for the new paradigm of hybrid quantum optics and quantum electrodynamics. This new concept benefits from the existing integrated photonics technology for squeezing and confining the light in sub-wavelength scales to substantially enhance the light-atom interaction. The hybrid chip is envisioned to have light sources, waveguides, devices, and detectors to realize a complex quantum network down to a single photon level. In this talk I will discuss about our recent theoretical and experimental works on atomic vapor spectroscopy in the vicinity of the plasmonic and nanophotonic devices. I start from a density matrix-based formalism describing the evolution of Rb vapor atomic levels, excited with an incoherent pump and coupled to a plasmonic lattice. When designed properly, the lattice plasmon mode efficiently captures the spontaneously emitted photons from the excited Rb atoms and a coherent coupling between the lattice mode and the atomic levels would occur. I will elaborate on the effect of pumping rate and decoherence on the steady state of the hybrid system and the feasibility of achieving a lasing state. In the second part of the talk I will present the results of our experiments on Rb vapor coupled to such a plasmonic lattice. Starting from the pumping mechanism, I describe the collisional scheme we employed to transfer the excited Rb atoms from (_^5)P_(3/2) to(_^5)P_(1/2) , hence achieving a population inversion between P and S levels and an optical gain at 795 nm, eventually. I present the experimental results of this atomic vapor interaction with a plasmonic lattice resonating at 795 nm. The spectroscopy of Rb cloud modified with tightly squeezed and enhanced field of the lattice plasmons shows the clear signature of Fano resonances in the passive gas, followed by amplified spontaneous emission in the active gas and the lasing at higher pumping powers. The results of this study would pave the way toward hybrid atom-quantum photonic chips.

Controlling light at the atomic scale (Conference Presentation)

Atomically thin materials such as graphene and molecular aromatic hydrocarbon exhibit unique optical properties that allow us to control the flow of light down to the atomic scale. These materials can sustain collective electron resonances -plasmons- involving a relatively small number of electrons, therefore enabling unprecedented electrical, magnetic, optical, and thermal control of those properties. In this talk, I will review recent progress in this field and present illustrative examples of nonlinear, quantum, and ultrafast phenomena in these materials, along with applications to optical sensing, optoelectronics, and quantum optics.

Bianisotropy: a new route towards non-reciprocal optical metasurfaces (Conference Presentation)

Directional light flow is fundamental to the development of photonic information processors. One all optical way of generating such nonreciprocal transport involves exploiting the nonlinear Kerr effect within an asymmetric arrangement of high Q resonators. However, current demonstrations involve optical paths that are tens to hundreds of microns in length. Here, we show that Kerr based nonreciprocal devices can be further miniaturized to the nanoscale by working with Silicon nanoantenna-based metasurfaces. In the subwavelength regime structural asymmetry alone isn’t enough to generate directionally-dependent field amplification. We overcome this limitation by overlapping a sub-radiant electric dipolar mode with a perpendicular super-radiant magnetic dipole. In this case, breaking out-of-plane inversion symmetry leads to nearfield coupling between the two excitations. Because of interference between nearfield and far field magneto-electric coupling, the electric dipole is suppressed (enhanced) for a normally incident plane wave propagating in the backward (forward) direction. When the metasurface is illuminated with powers of a few 100kW/cm2, the electric field strength within the Si becomes sufficient to change its refractive index, red-shifting the narrow transmission dip. For forward excitation the resonance is shifted by a significant portion of the FWHM, making the metasurface transparent. For backward excitation the much smaller shift renders the transmission very low. We show, for the first time, that bianisotropy provides a means to achieve optical nonreciprocity at the nanoscale. Relying simply on collocated dipolar excitations, our scheme has, in principle, no lower size limitation and could be miniaturised further by making use of gain assisted plasmonics.

A reconfigurable parity-time symmetric meta-atom for polarization and phase control(Conference Presentation)

Metasurfaces offer exotic optical properties, which often originate from carefully designed material geometries. With locked geometries, these metasurfaces are difficult or impossible to change post-fabrication. Here, we theoretically explore a nano-scale coaxial structure capable of adjustably manipulating the polarization, phase, and spatial distribution of light through the introduction of parity-time (PT) symmetric perturbations. Coaxial waveguides possess degenerate modes, corresponding to different orbital angular momentum (OAM) states. The degeneracy of OAM modes can be lifted through the introduction of any non-zero amount of gain and loss into the structure in a way that matches the azimuthal periodicity of the degenerate mode pair. New hybrid complex conjugate modes are created which lose their pure OAM nature and are either amplifying or lossy. We confirm this behavior using both a Hamiltonian formulation and degenerate perturbation theory, and propose this selective excitation and absorption scheme as a new method of filtering for mode division multiplexing in on-chip nanophotonic systems. In addition to the creation of new hybrid modes, we show that these PT-symmetric perturbations in coaxial apertures are capable of converting incident circularly polarized light into linearly polarized light with unity efficiency. Further, due to the localization of field intensity within the gain sections, it is possible to rotate linear polarization and induce up to a pi-phase shift. We describe how our PT-symmetric coaxial aperture could function as a reconfigurable meta-atom for phase, amplitude, and polarization controlled meta-surfaces, and discuss routes toward unity-efficiency, reconfigurable holography.