We perform a multidimensional characterization of a polarization-entangled photon-pair source using stimulated emission tomography (SET). We measure the frequency-resolved polarization density matrix, which is composed of thousands of individual polarization density matrices, each corresponding to a different frequency pair. The measurement exhibits detailed information about correlations that would be difficult to observe using traditional quantum state tomography. This demonstration exhibits the power of SET to characterize a source of quantum states with multi-dimensional correlations and hyper-entanglement. The SET technique can be applied to a variety of photon-pair-based sources for the optimization and engineering of quantum states.
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.
Four-wave mixing can be stimulated or occur spontaneously: the latter effect, also known as parametric fluorescence,
can be explained only in the framework of a quantum theory of light, and it is at the basis of many
protocols to generate nonclassical states of the electromagnetic field. In this work we report on our experimental
study of spontaneous four wave mixing in microring resonators and photonic crystal molecules integrated on a
silicon on insulator platform. We find that both structures are able to generate signal and idler beams in the
telecom band, at rates of millions of photons per second, under sub-mW pumping. By comparing the experiments
on the two structures we find that the photonic molecule is an order of magnitude more efficient than the
ring resonator, due to the reduced mode volume of the individual resonators.
In this work we focus on photonic crystal (PhC) ridges, which are composed of a dielectric ridge placed on a one-dimensional (1D) PhC, namely a periodic multilayer. In these structures, guided modes are characterized by an asymmetric light confinement, which relies on a photonic band gap (PBG) from the multilayer side and on total-internal-reflection (TIR) in all the other directions. Photonic crystal ridges are known to support guided surface waves, but here we show that at least three different guided modes can be identified, and only one of them seems to possess all the characteristics of a proper guided surface wave.
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.
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
Diffraction-based biosensors are often based on the adsorption of a target material on a grating made of thin layers,
where the adsorption is detected by a modification of the diffracted signal. In this communication we discuss two
strategies for enhancing this detection process. The first is based on the use of grating structures made of porous
elements, where sensing is based on target molecules penetrating into the elements and modifying their effective index of
refraction. The second is a resonant process where the effectiveness of the grating is enhanced by the coupling to surface
electromagnetic states, in particular Bloch surface waves that exist at the interface between a homogeneous medium and
a photonic crystal.
We present a direct-to-device method for stamping porous silicon to produce optical microstructures. The stamping
technique utilizes a reusable silicon stamp fabricated by standard lithographic methods. Large area (9mm2) stamps are
applied to single layer thin films of porous silicon with a force on the order of 1kN. The process affords precise control
over both lateral and vertical dimensions of patterning while maintaining large area uniformity. We demonstrate tunable
imprint depths in the 10nm-120nm range as well as lateral feature sizes down to 0.25μm. Imprinted structures are
characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical diffraction
experiments. By utilizing reusable stamps and a straightforward technique, the overall process can be performed at low-cost
and high throughput. This enables a wide variety of optical microstructures to be readily fabricated. As an
example, we present a porous diffraction grating and demonstrate proof-of-concept sensing capabilities, for exposure to
water vapor as well as small molecules (3-aminopropyltriethoxysilane). Additional device structures enabled by this
fabrication process are also discussed. The stamping process is expected to be applicable to other porous materials such
as porous titania, porous alumina, and porous silica.
Bloch Surface Waves (BSWs) are propagation modes that exist at the interface between a homogeneous medium and a
photonic crystal (PhC). The confinement at the interface of the media relies on total internal reflection in the
homogeneous medium and on the photonic band gap in the PhC. The dispersion relation of BSWs can be easily tailored
through the design of the PhC. This makes BSWs extremely flexible and suitable for applications in the field of optical
sensors, light emitters, and photovoltaic devices, where the capability to confine and amplify the electromagnetic field in
micro- and nano-structures allows for the enhancement of the light-matter interaction. In particular, we present two
different configurations for the detection of Bloch surface waves in silicon nitride multilayers: attenuated total
reflectance and photoluminescence measurements. In the first, we measured a 50-fold enhancement of the diffraction
signal by a protein grating printed on the multilayer when the incident light beam is coupled to the surface waves. In the
second, we observe a significant modification of the spontaneous emission by a monolayer of rhodamine molecules
bonded to the photonic crystal surface. These results may found application in the field of optical sensors, particularly
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.