Simeon Trendafilov

Polarization properties of chiral fiber gratings

Recent experiments (Kopp et al 2007 J. Opt. Soc. Am. B 24 A48) have demonstrated that the polarization sensitivity of chiral fiber gratings depends strongly on the grating symmetry: double-helix fibers are polarization sensitive while single-helix fibers are not. A coupled-mode perturbation theory is developed and used to explain the polarization properties of chiral fiber gratings. Features of the transmission spectrum such as multiple dips in the spectrum and circular dichroism are also derived and attributed to chiral Bragg scattering of the core modes into the cladding modes of the fiber.

Femtosecond laser damage threshold of pulse compression gratings for petawatt scale laser systems

Laser-induced femtosecond damage thresholds of Au and Ag coated pulse compression gratings were measured using 800 nm laser pulses ranging in duration from 30 to 200 fs. These gratings differ from conventional metal-on-photoresist pulse compression gratings in that the gratings patterns are generated by etching the fused silica substrate directly. After etching, the metal overcoating was optimized based on diffraction efficiency and damage threshold considerations. The experiment on these gratings was performed under vacuum for single-shot damage. Single-shot damage threshold, where there is a 0% probability of damage, was determined to be within a 400-800 mJ/cm(2) range. The damage threshold exhibited no clear dependence on pulse width, but showed clear dependence on gold overcoat surface morphology. This was confirmed by electromagnetic field modeling using the finite element method, which showed that non-conformal coating morphology gives rise to significant local field enhancement near groove edges, lowering the diffraction efficiency and increasing Joule heating. Large-scale gratings with conformal coating have been installed successfully in the 500 TW Scarlet laser system.

Experimental Demonstration of Phase Modulation and Motion Sensing Using Graphene-Integrated Metasurfaces

Strong interaction of graphene with light accounts for one of its most remarkable properties: the ability to absorb 2.3% of the incident lights energy within a single atomic layer. Free carrier injection via field-effect gating can dramatically vary the optical properties of graphene, thereby enabling fast graphene-based modulators of the light intensity. However, the very thinness of graphene makes it difficult to modulate the other fundamental property of the light wave: its optical phase. Here we demonstrate that considerable phase control can be achieved by integrating a single-layer graphene (SLG) with a resonant plasmonic metasurface that contains nanoscale gaps. By concentrating the light intensity inside of the nanogaps, the metasurface dramatically increases the coupling of light to the SLG and enables control of the phase of the reflected mid-infrared light by as much as 55° via field-effect gating. We experimentally demonstrate graphene-based phase modulators that maintain the amplitude of the reflected light essentially constant over most of the phase tuning range. Rapid nonmechanical phase modulation enables a new experimental technique, graphene-based laser interferometry, which we use to demonstrate motion detection with nanoscale precision. We also demonstrate that by the judicious choice of a strongly anisotropic metasurface the graphene-controlled phase shift of light can be rendered polarization-dependent. Using the experimentally measured phases for the two orthogonal polarizations, we demonstrate that the polarization state of the reflected light can be by modulated by carrier injection into the SLG. These results pave the way for novel high-speed graphene-based optical devices and sensors such as polarimeters, ellipsometers, and frequency modulators.

Laser induced periodic surface structure formation in germanium by strong field mid IR laser solid interaction at oblique incidence.

Laser induced periodic surface structures (LIPSS or ripples) were generated on single crystal germanium after irradiation with multiple 3 µm femtosecond laser pulses at a 45° angle of incidence. High and low spatial frequency LIPSS (HSFL and LSFL, respectively) were observed for both s- and p-polarized light. The measured LSFL period for p-polarized light was consistent with the currently established LIPSS origination model of coupling between surface plasmon polaritons (SPP) and the incident laser pulses. A vector model of SPP coupling is introduced to explain the formation of s-polarized LSFL away from the center of the damage spot. Additionally, a new method is proposed to determine the SPP propagation length from the decay in ripple depth. This is used along with the measured LSFL period to estimate the average electron density and Drude collision time of the laser-excited surface. Finally, full-wave electromagnetic simulations are used to corroborate these results while simultaneously offering insight into the nature of LSFL formation.

Transmission-Line Model and Propagation in a Negative-Index, Parallel-Plate Metamaterial to Boost Electron-Beam Interaction

A negative-index metawaveguide (NIMW) comprised of stacks of planar metal sheets loaded with periodic split-ring resonator apertures is modeled using a transmission-line (TL) analysis, and analyzed in detail. The proposed model provides a useful tool to extract the wave impedance and effective homogenized parameters of the metamaterial, ideal for design and optimization purposes. The dominant mode of the NIMW is transverse-magnetic, with a large longitudinal component of the electric field that enables its strong interaction with an electron beam and opens new opportunities for microwave generation and radiation detection. The inherent bi-anisotropy of its loaded inclusions is also examined within our TL theoretical framework, and a solution design to suppress its effect is presented. Finally, since the proposed geometry is shown to support an additional transverse-electromagnetic mode due to strong spatial dispersion, proper excitation of the NIMW is discussed and simulated.

Switching of Mid-Infrared Light Using Plasmonic Fano-Resonant Meta-Surfaces Integrated with Graphene

We experimentally demonstrate that graphene can strongly modulate the scattered light from Fano-resonant plasmonic metasurfaces. The Modulation depth of 1000% is achieved at around 7μm as the graphene carrier concentration changes.

Amplitude and Phase Modulation of Light Using Fano-Resonant Meta-Surfaces Integrated with Graphene

We experimentally demonstrate amplitude and phase modulation in graphene-integrated Fano-resonant plasmonic metasurfaces. Order of magnitude modulation depth is achieved in mid-IR. Strong coupling between trapped graphene and metallic plasmons is predicted in high-mobility graphene.

Laser induced periodic surface structure formation in germanium above laser damage fluence by mid IR femtosecond laser irradiation

Laser induced periodic surface structures (LIPSS) were generated via interaction of multiple 90 femtosecond 1900 - 3600 nm mid IR laser pulses (3 -10,000) on single crystal Ge targets. For specific laser parameters, both low and high frequency LIPSS are found together, which are oriented perpendicular to each other. Study of polarization dependence of LIPSS revealed that orientation and symmetry of interaction could be controlled by rotating polarization of laser pulses. Low frequency LIPSS formation was consistent with surface plasmon coupling of laser pulses with excited Ge.

Optical isolator /polarizer based on a rectangular waveguide with helical grooves

Rectangular waveguide with slanted grooves in its sidewalls can be used as an optical isolator/polarizer due to the chirality effect. Even the crudest implementations of chirality are shown to exhibit high circular dichroism.