Mykhailo Tymchenko

Gradient Nonlinear Pancharatnam-Berry Metasurfaces

We apply the Pancharatnam-Berry phase approach to plasmonic metasurfaces loaded by highly nonlinear multiquantum-well substrates, establishing a platform to control the nonlinear wave front at will based on giant localized nonlinear effects. We apply this approach to design flat nonlinear metasurfaces for efficient second-harmonic radiation, including beam steering, focusing, and polarization manipulation. Our findings open a new direction for nonlinear optics, in which phase matching issues are relaxed, and an unprecedented level of local wave front control is achieved over thin devices with giant nonlinear responses.

Hyperbolic metasurfaces: surface plasmons, light-matter interactions, and physical implementation using graphene strips [Invited]

We investigate ultrathin metasurfaces defined by anisotropic conductivity tensors using a Green’s function approach, focusing on their exciting plasmonic interactions and dramatic enhancement of light-matter interactions for hyperbolic dispersion. We apply our analytical formulation to explore several practical implementations at THz and near infrared frequencies, including electrically and magnetically-biased graphene sheets – a natural isotropic elliptic metasurface – and densely-packed arrays of graphene ribbons modelled through an effective medium approach. This latter configuration allows the electrical control of their band diagram topology – from elliptic to hyperbolic, going through the extremely anisotropic σ-near-zero case – providing unprecedented control over the confinement and direction of plasmon propagation while simultaneously boosting the local density of states. Finally, we study the influence of the strip granularity to delimit the accuracy of effective medium theory to model the electromagnetic interactions with hyperbolic metasurfaces. Our findings may lead to the development of ultrathin reconfigurable plasmonic devices able to provide extreme confinement and dynamic guidance of light while strongly interacting with their surroundings, with direct application in sensing, imaging, hyperlensing, on-chip networks, and communications.

Large-Area Nanoimprinted Colloidal Au Nanocrystal-Based Nanoantennas for Ultrathin Polarizing Plasmonic Metasurfaces

We report a low-cost, large-area fabrication process using solution-based nanoimprinting and compact ligand exchange of colloidal Au nanocrystals to define anisotropic, subwavelength, plasmonic nanoinclusions for optical metasurfaces. Rod-shaped, Au nanocrystal-based nanoantennas possess strong, localized, plasmonic resonances able to control polarization. We fabricate metasurfaces from rod-shaped nanoantennas tailored in size and spacing to demonstrate Au nanocrystal-based quarter-wave plates that operate with extreme bandwidths and provide high polarization conversion efficiencies in the near-to-mid infrared.

Infrared beam-steering using acoustically modulated surface plasmons over a graphene monolayer

We model and design a graphene-based infrared beamformer based on the concept of leaky-wave (fast traveling wave) antennas. The excitation of infrared surface plasmon polaritons (SPPs) over a ‘one-atom-thick’ graphene monolayer is typically associated with intrinsically ‘slow light’. By modulating the graphene with elastic vibrations based on flexural waves, a dynamic diffraction grating can be formed on the graphene surface, converting propagating SPPs into fast surface waves, able to radiate directive infrared beams into the background medium. This scheme allows fast on–off switching of infrared emission and dynamic tuning of its radiation pattern, beam angle and frequency of operation, by simply varying the acoustic frequency that controls the effective grating period. We envision that this graphene beamformer may be integrated into reconfigurable transmitter/receiver modules, switches and detectors for THz and infrared wireless communication, sensing, imaging and actuation systems.

Nonlocal response of hyperbolic metasurfaces

We analyze and model the nonlocal response of ultrathin hyperbolic metasurfaces (HMTSs) by applying an effective medium approach. We show that the intrinsic spatial dispersion in the materials employed to realize the metasurfaces imposes a wavenumber cutoff on the hyperbolic isofrequency contour, inversely proportional to the Fermi velocity, and we compare it with the cutoff arising from the structure granularity. In the particular case of HTMSs implemented by an array of graphene nanostrips, we find that graphene nonlocality can become the dominant mechanism that closes the hyperbolic contour - imposing a wavenumber cutoff at around 300k(0) - in realistic configurations with periodicity L<π/(300k(0)), thus providing a practical design rule to implement HMTSs at THz and infrared frequencies. In contrast, more common plasmonic materials, such as noble metals, operate at much higher frequencies, and therefore their intrinsic nonlocal response is mainly relevant in hyperbolic metasurfaces and metamaterials with periodicity below a few nm, being very weak in practical scenarios. In addition, we investigate how spatial dispersion affects the spontaneous emission rate of emitters located close to HMTSs. Our results establish an upper bound set by nonlocality to the maximum field confinement and light-matter interactions achievable in practical HMTSs, and may find application in the practical development of hyperlenses, sensors and on-chip networks.

Manipulation and Steering of Hyperbolic Surface Polaritons in Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) is a natural hyperbolic material that supports both volume-confined hyperbolic polaritons and sidewall-confined hyperbolic surface polaritons (HSPs). In this work, efficient excitation, control, and steering of HSPs are demonstrated in hBN through engineering the geometry and orientation of hBN sidewalls. By combining infrared nanoimaging and numerical simulations, the reflection, transmission, and scattering of HSPs are investigated at the hBN corners with various apex angles. It is also shown that the sidewall-confined nature of HSPs enables a high degree of control over their propagation by designing the geometry of hBN nanostructures.

Giant nonlinear processes in plasmonic metasurfaces

Nonlinear metasurfaces based on coupling locally-enhanced plasmonic response to intersubband transitions of multi-quantum-wells have recently provided second-order susceptibilities several orders of magnitude larger than any other non-linear flat structure measured so far. Here, we demonstrate that a proper design of the plasmonic metasurface inclusions can dramatically enhance the overall non-linear response of the structure. The optimized metasurfaces are then applied to achieve efficient second-harmonic generation (SHG) and differential frequency generation (DFG) in the infrared and terahertz frequency bands, respectively. In case of low-power impinging beams (≈1W), our simulations predict large conversion efficiencies of around 0.8% and 0.01% for SHG and DFG, outperforming previously reported efficiencies in 1-2 orders magnitude and confirming the suitability of this type of plasmonic metasurfaces as a highly-efficient flat platform for non-linear photonics.

Nonlinear optics with quantum-engineered intersubband metamaterials

Intersubband transitions in n-doped semiconductor heterostructures provide the possibility to quantum engineer one of the largest known nonlinear optical responses in condensed matter systems, limited however to electric field polarized normal to the semiconductor layers. Here we show that by coupling of electromagnetic modes in plasmonic metasurfaces with quantum-engineered intersubband transitions in semiconductor heterostructures one can create ultra-thin highlynonlinear metasurfaces for normal light incidence. Structures discussed here represent a novel kind of hybrid metalsemiconductor metamaterials in which exotic optical properties are produced by coupling electromagneticallyengineered modes in dielectric and plasmonic nanostructures with quantum-engineered intersubband transitions in semiconductor heterostructures. Record values of effective optical nonlinearities of over 400 nm/V are experimentally measured for metasurfaces optimized for efficient second harmonic generation at 9.7 μm pump wavelength under normal incidence.

Internal nanostructure diagnosis with hyperbolic phonon polaritons in hexagonal boron nitride

Imaging materials and inner structures with resolution below the diffraction limit has become of fundamental importance in recent years for a wide variety of applications. We report subdiffractive internal structure diagnosis of hexagonal boron nitride by exciting and imaging hyperbolic phonon polaritons. On the basis of their unique propagation properties, we are able to accurately locate defects in the crystal interior with nanometer resolution. The precise location, size, and geometry of the concealed defects are reconstructed by analyzing the polariton wavelength, reflection coefficient, and their dispersion. We have also studied the evolution of polariton reflection, transmission, and scattering as a function of defect size and photon frequency. The nondestructive high-precision polaritonic structure diagnosis technique introduced here can be also applied to other hyperbolic or waveguide systems and may be deployed in the next-generation biomedical imaging, sensing, and fine structure analysis.

Manipulation and steering of hyperbolic surface polaritons in hexagonal boron nitride (Conference Presentation)

Hexagonal boron nitride (hBN) is a natural hyperbolic material that supports both volume-confined hyperbolic polaritons (HPs) and sidewall-confined hyperbolic surface polaritons (HSPs). In this work, we demonstrate efficient excitation, control and steering of HSPs in hBN through engineering the geometry and orientation of hBN sidewalls. By combining infrared (IR) nano-imaging and numerical simulations, we investigate the reflection, transmission and scattering of HSPs at the hBN corners with various apex angles. We show that the sidewall-confined nature of HSPs enables a high degree of control over their propagation by designing the geometry of hBN nanostructures.