Multi-core fibers (MCFs) are promising solutions for high power fiber based devices as they reduce nonlinearity and other unwanted detrimental effects, like transverse mode instability, by transporting, instead of a single high power beam, several low-powered ones to be coherently combined at the fiber output. This method relies on accurate evaluation of the phase differences between signals in different cores, which are significantly impacted by changes in the effective index of the propagating modes. For this to be effective, spatial heat generation must be accounted for. In particular, the heat flux from the doped cores to the external boundary causes a temperature gradient across the fiber, which affects the refractive index distribution, creating the chance for effective index change and thus dephasing of the output beams, which is harmful for beam combining. The results of in-depth numerical analysis on the performance of 9-core and 16-core MCFs under thermal effects are presented by studying the mode phase sensitivity to heat load and by introducing a coupled-mode theory model to study possible optical coupling effects. The effectively single-mode condition is also investigated by calculating the core modal overlap differences between fundamental and higher-order modes.
Multi-core fiber capability to deliver several independent beams in a single structure has been deeply investigated to obtain spatial multiplexing in optical communication. Recently, the coherent beam multiplexing idea has been extended to high power fiber laser field, where multi-core fiber amplifiers, combining low power beams, promise to overcome thermal mode instability, which characterizes single-core fiber amplifiers. Although coherent output beam combination is advantaged in multi-core fiber, the understanding of core phase shifts is necessary to implement efficient beam combination. In presence of thermal load, induced by pump-to-signal conversion quantum defect, a refractive index gradient is formed on the multi-core fiber amplifier cross-section, thus changing core propagation properties and possibly creating unwanted core couplings. In this work a 9-core double-cladding fiber amplifier is numerically investigated by varying the core thermal load, from 2 to 15 W/m, in order to understand the structure propagation mismatch. The 9 cores are organized in a 3×3 regular grid, each core has a diameter of 19 μm and a spacing of 55 μm. Cores numerical aperture is 0.06. The outer cladding has a diameter of 340 μm. A comparison between a rod-type fiber amplifier configuration and a flexible fiber amplifier has been performed. Results show that the cores can be divided in three groups according to their propagation properties: central core, side cores, and corner ones. The phase shift between these groups, or equivalently the effective index difference, becomes higher with the increase of thermal load. These observations are fundamental to implement a model for beam propagation in presence of thermal effect, to investigate the amplification dynamics along z-direction.
In the last few years Yb-doped double cladding fibers have become the key component for the development of reliable and high-performance lasers. Despite an effective cooling of the fiber medium, a significant heat load is generated when high pump power is involved, which alters the mode propagation characteristics, causing unwanted coupling among the modes and destroying the output beam quality. This work presents a new tool for the analysis of the amplification and modal properties of Yb-doped double-cladding fibers, which comprises a full-vector modal solver, based on the finite-element method, an amplifier model and a thermal one. Simulation results, shown for two large pitch fiber designs, both in co-propagating and counter-propagating pumping schemes, have demonstrated the influence of the generated heat load on the overlap integral and on the power evolution of the guided modes.
Thin film solar cells have been widely studied with the focus on how to improve light trapping mechanism and enhance the overall photon-electrons conversion efficiency. The effect of novel plasmonic materials based on wide bandgap semiconductors, such as metals heavily doped zinc oxides and metal nitrides, are here studied in relation to their potential use in thin film solar cells. Here, we show that metal nitrides can play similar roles as gold nanoparticles on a surface of a Si-thin film solar cell, possibly without introducing excessive dissipative absorption, while metals doped zinc oxide nanoparticles could significantly improve the efficiency of thin film solar cells.
Conical holes bored in the active layer of a thin-film silicon solar cell by ion-beam lithography (IBL) show increase of effective optical absorption in the underlying silicon active layer. The optical properties are numerically simulated by the 3D finite-difference time-domain method (3D-FDTD), showing wideband increase of the UV, visible, and IR quantum efficiency. An experimental fabrication procedure is developed using IBL for high wide- area repeatability. A further optimization on the cone shapes is performed in order to make fabrication feasible with plasma etching techniques.
Femtosecond laser fabrication has been used to make hybrid refractive and di ractive micro-optical elements in photo-polymer SZ2080. For applications in micro- uidics, axicon lenses were fabricated (both single and arrays), for generation of light intensity patterns extending through the entire depth of a typically tens-of-micrometers deep channel. Further hybridisation of an axicon with a plasmonic slot is fabricated and demonstrated nu- merically. Spiralling chiral grooves were inscribed into a 100-nm-thick gold coating sputtered over polymerized micro-axicon lenses, using a focused ion beam. This demonstrates possibility of hybridisation between optical and plasmonic 3D micro-optical elements. Numerical modelling of optical performance by 3D-FDTD method is presented.
Chiral patterns are created by focused ion-beam milling nano-grooves with sub-15 nm resolution on thin metal films and arrays of nanoparticles, scattering and absorbing light selectively for left and right circularly polarized light, with high fidelity over fields up to 100 x 100 μm2 without positioning errors. This allows to carry out numerical simulations to estimate light enhancement and circular dichroism both on ideal and realistic particles taken from SEM images, showing doubling of scattering cross-section and enhancement changes up to 5 times controlled by dichroism, with localized field enhancements up to 20000.
3D plasmonic structures extending out of a gold film plane are created by dry etching of the film in the openings of a resist mask defined by electron beam lithography. Conical vertical protrusions (nano-wells) are left, and their optical properties are numerically simulated, showing easily reachable out-of-plane trapping of both dielectric and metal plasmonic nano-spheres, with trapping forces up to 20 pN/W/μm2.
Wideband refractive index spectra in 3D-FDTD are correctly represented by overcoming the polynomial approximations to give accurate field and force/torque results for generalized artificial materials.
A combination of electron- and ion-beam lithographies has been applied to fabricate patterns of plasmonic nanoparticles having tailored optical functions: they create hot-spots at predefined locations on the nanoparticle at specific wavelengths and polarizations of the incident light field. Direct inscribing of complex chiral patterns into uniform nano-disks of sub-wavelength dimensions, over extensive 20-by-20 μm2 areas, is achieved with high fidelity and efficiency; typical groove widths are in 10-30 nm range. Such patterns can perform optical manipulation functions like nano-tweezing and chiral sorting. Fabrication procedures can be optimized to pattern thin 0.1-2.5 μm-thick membranes with chiral nanoparticles having sub-15 nm grooves. Peculiarities of optical force and torque calculations using finite-difference time-domain method are presented.
It is well known that surface-enhanced Raman scattering (SERS) substrates based on metal island films exhibit higher
levels of enhancement when excited through a transparent base material than when excited directly through air.
However, to our knowledge, the origin of this enhancement has never been satisfactorily explained. An initial suggestion
that the additional enhancement was due to a "nearest layer effect" cannot account for the observation of additional
enhancement for monolayer adsorbates. In this paper, finite difference time domain (FDTD) modelling is presented to
show that the electric field intensity in between metal particles at the interface is higher for "far-side" excitation. This is
reasonably consistent with the observed enhancement for silver islands on SiO2. The modelling results are in agreement
with a simple physical model based on Fresnel reflection at the interface. This suggests that the additional enhancement
is due to a near-field enhancement of the electric field due to the phase shift at the dielectric interface, when the light
passes from the higher to the lower region of refractive index.
We studied new three-dimensional tailoring of nano-particles by ion-beam and electron-beam lithographies, aiming for
features and nano-gaps down to 10 nm size. Electron-beam patterning is demonstrated for 2D fabrication in combination
with plasmonic metal deposition and lift-off, with full control of spectral features of plasmonic nano-particles and
patterns on dielectric substrates. We present wide-angle bow-tie rounded nano-antennas whose plasmonic resonances
achieve strong field enhancement at engineered wavelength range, and show how the addition of fractal patterns defined
by standard electron beam lithography achieve light field enhancement from visible to far-IR spectral range and scalable
up towards THz band. Field enhancement is evaluated by FDTD modeling on full-3D simulation domains using complex
material models, showing the modeling method capabilities and the effect of staircase approximations on field
enhancement and resonance conditions, especially at metal corners, where a minimum rounding radius of 2 nm is
resolved and a five-fold reduction of spurious ringing at sharp corners is obtained by the use of conformal meshing.
We report on surface structuring of sapphire, silicon carbide, and silicon by femtosecond laser pulses in multipulse
irradiation mode. The formed ripples on the flat surface or on the vertical walls with hierarchical structures
whose feature sizes are ranging from the irradiation wavelength down to ~ 50 nm are prospective templates for
surface enhanced Raman scattering after coating with plasmonic metals. We study complex patterns of fine
ripples with periods Λr, as small as λ/Rp, where Rp (see manuscript) 3 - 5. The mechanisms suggested for such Rp values
are discussed: temperature and density of breakdown plasma, angle of incidence, mechanism of second harmonic
generation (SHG) and absorption. Predictions of the surface and bulk models of ripple formation are compared
with experimental values of Rp-factor. We propose a model of ripple formation on the surface, which is based on
the known in-bulk sphere-to-plane formation of breakdown plasma in the surface proximity. In semiconductor
4H:SiC normal ripples with periods 190 and 230 nm were recorded with 800 nm and 1030 nm fs-laser pulses
respectively. We show that the period of ripples is defined by the dielectric properties of crystalline (solid) phase
rather than the molten phase in the case of silicon. Generation of SHG on the surface of sample and plasma
nano-bubbles are discussed: surface-SHG is found not important in ripples' formation as revealed by comparative
study of periods on Al2O3 and TiO2 at 800 nm wavelength of irradiation. We propose that ripple periodicity is
pinned to the smallest possible standing wave cavity (λ/n)/2 inside material of refractive index n.
Photonic Crystal Fibers (PCFs) are optical fibers with unique guiding characteristics as well as unusual nonlinear and dispersion properties. Since PCFs offer the possibility to engineer the zero-dispersion wavelength, the dispersion curve and the nonlinear coefficient value, they are very interesting for optical parametric amplification. In the present paper the phase-matching condition has been deeply analyzed in different triangular PCFs configurations. In particular, highly nonlinear PCFs have been designed to achieve flattened dispersion curves around the zero-dispersion wavelength in the C band. Very flat parametric gain, around 16 dB, on a bandwidth up to 35nm can be obtained with short PCF and low pump power level.