In this work, we investigate intrinsically chiral optical effects – specifically absorption circular dichroism (CD) at normal incidence – in elliptical nanohole arrays (ENHAs) with square lattice realized in thin films of silver, gold, and aluminum on glass. Our purpose is twofold: first, we aim at clarifying the origin of CD and its relation to surface plasmon polariton (SPP) properties upon symmetry reduction in the plasmonic metasurface. Second, we optimize the parameters for CD enhancement, with specific attention on aluminum in the visible-near UV spectral range. The choice of the square lattice (as compared to our previous study on ENHA in Au with a triangular lattice [Petronjievic et al. Opt. Quantum Electronics (2020) 52:176]) yields a more complete picture of chiral properties in ENHAs, and it allows comparing the results of two different simulation methods under ideal conditions, as discussed below. The results shed light on the subtle interplay between two concurring mechanisms of symmetry reduction, namely elliptical nanohole shape and tilting of nanohole axis with respect to the symmetry axes of the array. Moreover, they give guidelines for optimizing the CD from the near-IR (using Au or Ag) to the UV spectral region (using Al metal). Thus, the present work sets the bases for applications of the ENHA to chiroptical spectroscopies, notably in the near-UV region which is especially interesting for various kinds of biomolecules.
The relation between polarization singularities and optical properties in metasurfaces is a hot topic in nanophotonics. In this work, we focus on dielectric metasurfaces supporting nondegenerate photonic bands, leading to symmetry-protected Bound States in a Continuum (BICs) that become Circularly Polarized States (CPS) upon symmetry reduction. First, we discuss how BICs and CPS lead to polarization singularities in the far field, whose winding numbers – or topological charges – follows from the symmetry of the lattice. Then, we determine the polarization properties via the Stokes parameters, focusing on the conditions for the occurrence of a nonvanishing circular polarization. Finally, we calculate the optical response in reflection and the degree of circular dichroism. The results shed light on the role of polarization singularities and symmetry in determining the optical chirality.
Light-matter interaction is crucial in many application domains of nanophotonics, including biosensing, trapping at the nanoscale, nonlinear optics, and lasing. Many approaches, mainly based on photonic and plasmonic resonant structures, have been investigated to enhance and tailor the interaction, but those based on all-dielectric metasurfaces have several unique advantages: low loss, easy excitation and readout, possibility of engineering the optical field distribution with many degrees of freedom, and electric tuning. Here we show that properly designed all-dielectric metasurfaces can support silicon-slot quasi-bound states in the continuum modes resonating in the near-infrared, strongly confining light in air and, consequently, enhancing light-matter interaction. Some samples of the designed metasurface have been fabricated in a silicon-on-sapphire wafer by e-beam lithography and reactive ion etching. The optical characterization of the chip has confirmed the excitation of the quasi-bound state in the continuum resonant modes, with measured Q-factor values exceeding 700.
Mach-Zehnder (MZ) modulators in Silicon-on-Insulator (SOI) are key components for integrated silicon photonic devices. Reducing their energy dissipation is a crucial step for applications of silicon photonics, especially in large data centers. In this work, we combine band-edge slow light structures consisting of silicon waveguide gratings with a periodic (interleaved) p-n junction. The slow-light structures consist of a waveguide grating with wide/narrow sections realized in a 300-nm thick silicon layer, on top of an unetched silicon layer of 50 to 150 nm thickness, fully embedded in SiO2. The grating gives rise to a photonic stop band and to a slow-light region close to the lowest band edge. The profile of the p-n junction varies periodically along the waveguide with interleaved n and p regions. This structure maximizes the spatial overlap between the optical mode and the depletion regions, yielding a further improvement of modulator efficiency beyond the slow-light effect.
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Under an applied reverse bias, the silicon refractive index is modified by the plasma dispersion effect, thus the waveguide grating acts as a phase shifter. The modulator efficiency VpiLpi is strongly improved in comparison with modulators without slow light or with a lateral p-n junction. Thanks to the optimized overlap between electric field and depletion regions, this improvement takes place over a spectral interval that is much larger than the slow-light bandwidth. Insertion losses due to free carriers are also lower than in conventional modulators. The advantage of combining slow-light grating waveguides with an interleaved p-n junction is especially pronounced at low driving voltage (of the order of 1V), where the dissipated energy can be as low as 0.4 pJ/bit over an optical bandwidth of several 10 nm. Thus, the present modulator structure is promising in view of realizing integrated MZ modulators with low power dissipation.
In this work, we study chiral effects in well-known 2D plasmonic nanohole arrays with a triangular unit cell. The chirality can be induced by moving from circular to elliptical nanohole shape and tilting the ellipse away from the array symmetry. This symmetry breaking induces a different absorption of the circularly polarized light of opposite handedness, i.e. circular dichroism. We numerically study circular dichroism at normal incidence in elliptic nanoholes in Au in the spectral range 400-1000 nm. CD arises in transmission and absorption spectra in the same wavelength region of extraordinary optical transmission, indicating highly resonant light-metasurface coupling mechanisms. We focus on its dependence on the elliptic nanohole tilt and further proceed with the ellipse radii optimization. The optimized CD is on the order of 80%, and it is robust with respect to the radii and rotation angle variations. Moreover, such samples could be produced by means of low-cost nanosphere lithography, which makes them interesting for applications in enhanced sensing of chiral biomolecules.
We present the characterization of Silicon-on-insulator (SOI) photonic-crystal based 2D grating-couplers (2D-GCs) fabricated by CEA-Leti in the frame of the FP7 Fabulous project, which is dedicated to the realization of devices and systems for low-cost and high-performance passives-optical-networks. On the analyzed samples different test structures are present, including 2D-GC connected to another 2D-GC by different waveguides (in a Mach-Zehnder like configuration), and 2D-GC connected to two separate 2D-GCs, so as to allow a complete assessment of different parameters. Measurements were carried out using a tunable laser source operating in the extended telecom bandwidth and a fiber-based polarization controlling system at the input of device-under-test. The measured data yielded an overall fiber-to-fiber loss of 7.5 dB for the structure composed by an input 2D-GC connected to two identical 2D-GCs. This value was obtained at the peak wavelength of the grating, and the 3-dB bandwidth of the 2D-GC was assessed to be 43 nm. Assuming that the waveguide losses are negligible, so as to make a worst-case analysis, the coupling efficiency of the single 2D-GC results to be equal to -3.75 dB, constituting, to the best of our knowledge, the lowest value ever reported for a fully CMOS compatible 2D-GC. It is worth noting that both the obtained values are in good agreement with those expected by the numerical simulations performed using full 3D analysis by Lumerical FDTD-solutions.
In this contribution, we use a rigorous electro-optical model to study randomly rough crystalline silicon solar cells with the absorber thickness ranging from 1 to 100 μm. We demonstrate a significant efficiency enhancement, particularly strong for thin cells. We estimate the “region of interest” for thin-film photovoltaics, namely the thickness range for which the energy conversion efficiency reaches maximum. This optimal thickness results from the opposite trends of current and voltage as a function of the absorber thickness. Finally, we focus on surface recombination. In our design, the cell efficiency is limited by recombination at the rear (silicon absorber/back reflector) interface, and therefore engineering the front surface to a large extent does not reduce the efficiency. The presented model of roughness adds a significant functionality to previous approaches, for it allows performing rigorous calculations at a much reduced computational cost.
Efficient photovoltaic conversion of solar energy requires optimization of both light absorption and carrier collection. This manuscript reviews theoretical studies of thin-film silicon solar cells with various kinds of ordered and disordered photonic structures. Light trapping capabilities of these systems are analyzed by means of rigorous coupled-wave analysis and compared with the so-called Lambertian limit as given by a fully randomizing light scatterer. The best photonic structures are found to require proper combinations of order and disorder, and can be fabricated starting from pre-patterned rough substrates. Carrier collection is studied by means of analytic models and by full electro-optical simulations. The results indicate that thin-film silicon solar cells can outperform bulk ones with comparable material quality, provided surface recombination is kept below a critical level, which is compatible with present-day surface passivation technologies.
In this work we theoretically investigate the light trapping properties of one- and two-dimensional periodic patterns
etched in crystalline silicon solar cells with anti-reflection coating and back-reflector, in a wide range of active material
thicknesses. The resulting short-circuit current (taken as the figure of merit for efficiency) and the optical spectra are
compared with those of an unpatterned cell, and with the ultimate limits to light trapping in the case of a Lambertian
(isotropic) scatterer. Photonic patterns are found to give a substantial absorption enhancement, especially for twodimensional
patterns and for thinner cells, thanks to physical mechanisms like reduction of reflection losses, diffraction
of light into the cell, and coupling into the resonant optical modes of the structure.
Here we discuss the experimental characterization of the spatial far-field profiles for the confined modes in a
photonic crystal cavity of the L3 type, finding a good agreement with FDTD simulations. We then link the
far-field profiles to relevant features of the cavity mode near-fields, using a simple Fabry-Perot resonator model.
Finally, we describe a technique for independent all-electrical control of the wavelength of quantum dots in
separated L3 cavities, coupled by a waveguide, by electrical isolation via proton implantation
We present a theoretical study of amorphous and crystalline thin-film solar cells with a periodic pattern on a sub-micron
scale realized in the silicon layer and filled with silicon dioxide right below a properly designed antireflection coating.
The study and optimization of the PV structure as a function of all the photonic crystals parameters allows to identify the
different roles of the periodic pattern and of the etching depth in determining an increase of the absorption. From one
side, the photonic crystal acts as an impendence matching layer, thus minimizing reflection of incident light over a
particularly wide range of frequencies. Moreover a strong absorption enhancement is observed when the incident light
is coupled into the quasi guided modes of the photonic slab. We compare the efficiency of this structure to that of PV
cells characterized by the sole antireflection coating. We found a substantial increase of the short-circuit current when
the parameters are properly optimized, demonstrating the advantage of a wavelength-scale, photonic-crystal based
approach.
During the last years, much attention has been paid to photonic crystals (PC) for different applications, but
only recently they have been proposed and showed useful for applications in solar cells. Little work has been
done in the actual manufacture and characterization of a complete solar cell with a two-dimensional photonic
crystal (2D-PC) on its front surface, conceived as a periodic distribution of the dielectric constant in the plane
(the surface of the solar cell) and involving sub-wavelength motifs. In this case, the photonic crystal effect is
different from the one happening in slabs or suspended membranes. Despite the partial vertical confinement,
there may be some reasons that can justify the use of photonic crystal front surface with sub-wavelength
motifs. Experimental results on actual devices with a photonic crystal nanopatterned layer will be shown,
along with reflectivity studies on PC lattices with different symmetries and shapes.
Photonic modes in 1-D and 2-D silicon-on-insulator photonic
crystal waveguides periodic or containing line-defects, are fully
explored by means of angle- and polarization-resolved
micro-reflectance measurements. Both quasi-guided and truly guided
photonic modes are probed with a frequency-wave vector range that
is greatly expanded under attenuated total reflectance
configuration. It is shown that the presence of a supercell
repetition in the direction perpendicular to a line defect leads
to the simultaneous excitation of defect and bulk modes folded in
a reduced Brillouin zone. Consequently, the group-velocity
dispersion of the defect modes corresponding to different
polarizations of light can be fully determined. We show also that
the measured dispersion is in good agreement with full 3D
calculations based on expansion in the waveguide modes.
Bulk polystyrene opals have been grown. Variable incidence angle reflectance spectroscopy is used to probe their photonic band structures. Several different structures are observed and accounted for by theoretical calculations of photonic bands and density of states. The results yield a clear distinction between diffraction in the direction of propagation by the (111) family planes (leading to the formation of the stop band) and diffraction in other directions by higher-order planes (corresponding to excitation of photonic modes in the crystal).
We report on third-harmonic (TH) generation emitted from 1D photonic slabs etched into Silicon-on-Insulator (SOI) planar waveguides, as compared to the bare waveguide and (100) Silicon bulk responses. 130-fs laser pulses at ~ 810 nm and ~1550 nm have been chosen as a pump to excite TH signals in reflection and diffraction directions. The measured angles of in-plane diffracted third-harmonic beams agree with those predicted by nonlinear diffraction equations. The nonlinear reflectance as a function of the angle of incidence and azimuthal orientation of the structure has been measured. The near-infrared measurements have revealed that, whenever the pump frequency is resonant with a photonic mode, a substantial enhancement of the harmonic signal occurs. This nonlinear mechanism is in principle a very sensitive spectroscopic tool in determining and mapping the photonic band diagram of the system above the light line. The agreement between experimental data and ad hoc simulations of the nonlinear behavior of the system sheds new light on the nonlinear optical response of these nanostructured materials.
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