Near-infrared modulation by means of GeTe/SOI-based metamaterial

Today, nanophotonics still lacks components for modulation that can be easily implementable in existing silicon-on-insulator (SOI) technology. Chalcogenide phase change materials (PCMs) are promising candidates for tuning in the near infrared: at the nanoscale, thin layers can provide enough contrast to control the optical response of a nanostructure. Moreover, all-dielectric metamaterials allow for resonant behavior without having ohmic losses in the telecom range. Here, a novel hybridization of a SOI-based metamaterial with PCM GeTe is experimentally investigated. A metamaterial based on Si nanorods, covered by a thin layer of GeTe, is designed and fabricated. Switching GeTe from amorphous to crystalline leads to a rather high resonance-governed reflection contrast at 1.55 μm. Additional confocal Raman imaging is done to differentiate the crystallized zones of the metamaterials' unit cell. The findings are in good agreement with numerical analysis and show good perspectives of all-dielectric tunable near-infrared nanophotonics.

Diatom frustules decorated with zinc oxide nanoparticles for enhanced optical properties

Zinc oxide (ZnO) nanoparticles were synthesized on diatomite (DE) surface by a low temperature sol gel technique, starting from zinc acetate dihydrate (Zn(CH₃COO)₂ · 2H₂O) solution in water/ethyl alcohol, in presence of triethanolamine (TEA) with functions of Zn²⁺ chelating agent, catalyst and mediator of nanoparticle growth on DE surface. Microstructural features were investigated by field emission scanning electron microscopy and x-ray diffraction. ZnO crystalline nanoparticles, well distributed both on the surface and into the porous architecture of diatomite, were obtained just after the synthesis carried out at 80 °C without the need of calcination treatments. The optical properties of ZnO/DE hybrid powders were measured for the first time by means of photoacoustic spectroscopy (PAS). A new method to retrieve both the optical absorption and scattering coefficients from PAS is here discussed for powder aggregates. The fingerprint of the zinc oxide nanoparticles has been highlighted in the Mie scattering resonance in the UV-Vis range, and in the enhancement of the optical absorption with respect to diatomite.

All-optical tuning of EIT-like dielectric metasurfaces by means of chalcogenide phase change materials

Electromagnetically induced transparency (EIT) is a pump-induced narrowband transparency window within an absorption line of the probe beam spectrum in an atomic system. In this paper we propose a way to bring together the all-dielectric metamaterials to have EIT-like effects and to optically tune the response by hybridizing them with a layer of a phase change material. We propose a design of the metamaterial based on Si nanoresonators that can support an EIT-like resonant response. On the top of the resonators we consider a thin layer of a chalcogenide phase change material, which we will use to tune the optical response. Our choice is Ge₂Sb₂Te₅ (GST), since it has two stable phases at room temperature, namely amorphous and crystalline, between which it can be switched quickly, nonvolatively and reversibly, sustaining a large number of switching cycles. They differ in optical properties, while still having moderately low losses in telecom range. Since such dielectric resonators do not have non-radiative losses of metals around 1550nm, they can lead to a high-Q factor of the EIT-like response in this range. Firstly, we optimize the starting structure so that it gives an EIT-like response at 1550 nm when the GST layer is in the amorphous state. Our starting design uses glass as a substrate, but we also consider implementation in SOI technology. If we then switch the thin layer of GST to its crystalline phase, which has higher losses, the EIT-like response is red shifted, providing around 10:1 contrast at 1550nm. This reversible tuning can be done with an ns visible pulsed laser. We discuss the results of the simulation of the dielectric metasurface for different configurations and the tuning possibility.

Polarization insensitive infrared absorbing behaviour of one-dimensional multilayer stack: a fractal approach

The control and tailoring of infrared absorbance/emittance is a crucial task for all those applications involving thermal radiation management and detection. We theoretically investigated the peculiar absorbing/emitting behaviour of pre-fractal Cantor multilayers, in order to design a polarization-insensitive multilayer stack absorbing over a wide angular lobe in the mid wavelength infrared range (8-10 μm). Using transfer matrix method, we explored the spectral properties arising from both the material and the geometrical dispersion. We considered several combinations of the constituent materials: SiO2 was combined with TiO2 and Si, respectively.

Trace gas analysis from glazes by means of a compact photothermal deflection spectroscopy apparatus

A portable photothermal deflection spectroscopy apparatus for accurate analysis of gas mixtures containing volatile organic substances is presented. The main technical aspects of the photothermal deflection cell, and of the gas sampling equipment are discussed in details. Finally, we present a straightforward industrial application of the apparatus concerning the qualitative and quantitative analysis of the gas emissions from thermal treatment of ceramic glazes.

Pre-fractal multilayer structure for polarization- insensitive temporally and spatially coherent thermal emitter

We investigate electromagnetic wave propagation through one-dimensional stacks arranged as truncated pre-fractal Cantor multilayer. Taking into account materials' dispersion as well as real absorptive losses, we studied the spectral and spatial emissivity in both on-axis and off-axis direction. The typical cavity mode resonances associated to the pre-fractal structure are exploited to design a polarization-insensitive infrared emitter pertaining both temporal and spatial coherence.

Evidence of multipolar response of Bacteriorhodopsin by noncollinear second harmonic generation

Noncollinear second harmonic generation from a Bacteriorhodopsin (BR) oriented multilayer film was systematically investigated by varying the polarization state of both fundamental beams. Both experimental results and theoretical simulations, show that the resulting polarization mapping is an useful tool to put in evidence the optical chirality of the investigated film as well as the corresponding multipolar contributions to the nonlinear.

Circular dichroism in the optical second-harmonic emission of curved gold metal nanowires

Here we report the experimental observation of circular dichroism in the second-harmonic field (800-400 nm conversion) generated by self-organized gold nanowire arrays with subwavelength periodicity (160 nm). Such circular dichroism, raised by a nonlinear optical extrinsic chirality, is the evident signature of the sample morphology. It arises from the curvature of the self-assembled wires, producing a lack of symmetry at oblique incidence. The results were compared, both in the optical linear and nonlinear regime, with a reference sample composed of straight wires. Despite the weak extrinsic optical chirality of our samples (not observable by our optical linear measurements), high visibility (more than 50%) was obtained in the second-harmonic generated field.

Optical polarization based logic functions (XOR or XNOR) with nonlinear Gallium nitride nanoslab

We present a scheme of XOR/XNOR logic gate, based on non phase-matched noncollinear second harmonic generation from a medium of suitable crystalline symmetry, Gallium nitride. The polarization of the noncollinear generated beam is a function of the polarization of both pump beams, thus we experimentally investigated all possible polarization combinations, evidencing that only some of them are allowed and that the nonlinear interaction of optical signals behaves as a polarization based XOR. The experimental results show the peculiarity of the nonlinear optical response associated with noncollinear excitation, and are explained using the expression for the effective second order optical nonlinearity in noncollinear scheme.

Transmission function properties for multi-layered structures: application to super-resolution

We discuss the properties of the transmission function in the k-space for a generic multi-layered structure. In particular we analytically demonstrate that a transmission greater than one in the evanescent spectrum (amplification of the evanescent modes) can be directly linked to the guided modes supported by the structure. Moreover we show that the slope of the phase of the transmission function in the propagating spectrum is inversely proportional to the ability of the structure to compensate the diffraction of the propagating modes. We apply these findings to discuss several examples where super-resolution is achieved thanks to the simultaneous availability of the amplification of the evanescent modes and the diffraction compensation of the propagating modes.

Bound and free waves in non-collinear second harmonic generation

We analyze the relationship between the bound and the free waves in the noncollinear SHG scheme, along with the vectorial conservation law for the different components arising when there are two pump beams impinging on the sample with two different incidence angles. The generated power is systematically investigated, by varying the polarization state of both fundamental beams, while absorption is included via the Herman and Hayden correction terms. The theoretical simulations, obtained for samples which are some coherence length thick show that the resulting polarization mapping is an useful tool to put in evidence the interference between bound and free waves, as well as the effect of absorption on the interference pattern.

Mapping the nonlinear optical susceptibility by noncollinear second-harmonic generation

We present a method, based on noncollinear second-harmonic generation, to evaluate the nonzero elements of the nonlinear optical susceptibility. At a fixed incidence angle, the generated signal is investigated by varying the polarization state of both fundamental beams. The resulting polarization charts allows us to verify if Kleinman's symmetry rules can be applied to a given material or to retrieve the absolute value of the nonlinear optical tensor terms, from a reference measurement. Experimental measurements obtained from gallium nitride layers are reported. The proposed method does not require an angular scan and thus is useful when the generated signal is strongly affected by sample rotation.

Wide band negative magnetic permeability materials (NMPM) with composite metalsemiconductor structures based on the Drude model, and applications to negative-refractive index (NIM)

Composite structures based on metal open rings and thin wires are well established, for obtaining efficient negative index materials (NIM), acting as metamaterials in the long wavelength regime. The main losses are due both to metal absorption and to the inner electric resistance of metals; to overcome this latter loss we propose a new metal-semiconductor structure dimensioned by direct synthesis method, which offers an almost perfect Drude-like effective magnetic permeability. The choice of particular semiconductor components allows to get a negative resistance for the current induced by the electromagnetic field, which cancels that of the metal but puts a limit to the spectral response of the metamaterial. We consider some parasite effects, such as bianisotropy and incorrect values of structural parameters, to see limitations and features of this new NIM technology.

Coherent control of stimulated emission inside one-dimensional photonic crystals

In this paper, the quasinormal mode (QNM) theory is applied to discuss the quantum problem of an atom embedded inside a one-dimensional (1D) photonic band gap (PBG) cavity pumped by two counterpropagating laser beams. The e.m. field is quantized in terms of the QNMs in the 1D PBG and the atom modeled as a two-level system is assumed to be weakly coupled to just one of the QNMs. The main result of the paper is that the decay time depends on the position of the dipole inside the cavity, and can be controlled by the phase difference of the two laser beams.

Second quantization and atomic spontaneous emission inside one-dimensional photonic crystals via a quasinormal-modes approach

An extension of the second quantization scheme based on the quasinormal-modes theory to one-dimensional photonic band gap (PBG) structures is discussed. Such structures, treated as double open optical cavities, are studied as part of a compound closed system including the electromagnetic radiative external bath. The electromagnetic field inside the photonic crystal is successfully represented by a new class of modes called quasinormal modes. Starting from this representation we introduce the Feynman's propagator to calculate the decay rate of a dipole inside a PBG structure, related to the density of modes, in the presence of the vacuum fluctuations outside the one-dimensional cavity.

Slowing light in chi2 photonic crystals

A study of parametric nonlinear frequency down-conversion in photonic crystals reveals that under suitable conditions the probe field can be slowed down to approximately 11 m/s. The effect arises as a result of the simultaneous availability of global phase-matching conditions, field localization, and gain experienced by the probe beam. Together, these effects conspire to yield tunneling velocities previously reported only for coherently resonant interactions, i.e., electromagnetic induced transparency, in Bose-Einstein condensates, hot atomic gases, and doped crystals.

Quasinormal-mode description of waves in one-dimensional photonic crystals

Quasinormal-mode treatment is extended to the description of scalar field behavior in one-dimensional photonic crystals. A one-dimensional photonic crystal is a particular configuration of an open cavity, where discontinuities of the refractive index give rise to field confinement. This paper presents, for a one-dimensional photonic crystal, a discussion about the completeness of the quasinormal-mode representation and, moreover, a discussion on the complex eigenfrequencies, as well as the corresponding field distribution. The concept of density of modes is also discussed in terms of quasinormal modes.

Dynamics of counterpropagating pulses in photonic crystals: enhancement and suppression of stimulated emission processes

Using numerical methods, we study the propagation of counterpropagating pulses in finite photonic crystals. We show that linear interference and localization effects combine to either enhance or suppress stimulated emission processes, depending on the initial phase difference between the input pulses. We consider the example of second harmonic generation, where we find a maximum contrast of three orders of magnitude in nonlinear conversion efficiency as a function of the input phase difference between incident pulses. We interpret these results by viewing the photonic crystal as an open cavity, with a field-dependent, electromagnetic density of modes sensitive to initial and boundary conditions.

Transit time of chirped pulses through one-dimensional, nonabsorbing barriers

Experiments show that the transit times of chirped, narrow-band pulses that move across nonabsorbing, one-dimensional barriers are modified dramatically by the interplay between the chirp and the transmission function of the sample. In an experiment we monitored 0.9-ns chirped, nearly Gaussian pulses as they traversed a 450-mum GaAs etalon. At certain wavelengths pulse transit times can be superluminal or even negative. To explain these phenomena we have proposed a generalization of the transit time for chirped pulses that is still meaningful even when the transit times are superluminal or negative. Our predictions agree well with the experimental results.

Quantum properties of optical field in photonic band gap structures

A theoretical analysis of the quantum behaviour of radiation field's propagation in photonic band gaps structures is performed. In these initial calculations we consider linear inhomogeneous and nondispersive media.

Simultaneously phase-matched enhanced second and third harmonic generation

Second and third harmonic generation via a chi((2)) three-wave mixing process can occur with high conversion efficiency in a one-dimensional photonic band gap structure. We find that it is possible to simultaneously achieve enhancement and exact phase-matching conditions of second harmonic and sum frequency generation, omega+2 omega-->3 omega. It is also remarkable that high conversion efficiencies persist under tuning conditions that correspond to a phase mismatch. While these conditions are quite unusual and cannot be achieved in any known bulk material, we show that they can be easily obtained in finite layered structures by using and balancing an interplay between material dispersion and the geometrical dispersion introduced by the structure.

Photonic band edge effects in finite structures and applications to chi 2 interactions

Using the concept of an effective medium, we derive coupled mode equations for nonlinear quadratic interactions in photonic band gap structures of finite length. The resulting equations reveal the essential roles played by the density of modes and effective phase matching conditions necessary for the strong enhancement of the nonlinear response. Our predictions find confirmation in an experimental demonstration of significant enhancement of second harmonic generation near the photonic band edge. The measured conversion efficiency is in good agreement with the conversion efficiency predicted by the effective-medium model.

Group velocity, energy velocity, and superluminal propagation in finite photonic band-gap structures

We have analyzed the notions of group velocity V(g) and energy velocity V(E) for light pulses propagating inside one-dimensional photonic band gap structures of finite length. We find that the two velocities are related through the transmission coefficient t as V(E)=/t/(2)V(g). It follows that V(E)=V(g) only when the transmittance is unity (/t/(2)=1). This is due to the effective dispersive properties of finite layered structures, and it allows us to better understand a wide range of phenomena, such as superluminal pulse propagation. In fact, placing the requirement that the energy velocity should remain subluminal leads directly to the condition V(g)<or=c//t/(2). This condition places a large upper limit on the allowed group velocity of the tunneling pulse at frequencies of vanishingly small transmission.

Enhancement of chi((2)) cascading processes in one-dimensional photonic bandgap structures

We theoretically analyze the nonlinear phase shifts induced by cascaded chi((2)):chi((2)) processes in one-dimensional photonic bandgap structures. We find that the enhancement of the density of modes near the band edge, coupled with a suitable choice of relative phase mismatch, leads to a remarkable new effect: The relative phase shift of the fundamental field on transmission can be of the order of pi over a distance of 7mum , with input intensities of the order of only 10 MW/cm(2).

Dispersive properties of finite, one-dimensional photonic band gap structures: applications to nonlinear quadratic interactions

We discuss the linear dispersive properties of finite one-dimensional photonic band-gap structures. We introduce the concept of a complex effective index for structures of finite length, derived from a generalized dispersion equation that identically satisfies the Kramers-Kronig relations. We then address the conditions necessary for optimal, phase-matched, resonant second harmonic generation. The combination of enhanced density of modes, field localization, and exact phase matching near the band edge conspire to yield conversion efficiencies orders of magnitude higher than quasi-phase-matched structures of similar lengths. We also discuss an unusual and interesting effect: counterpropagating waves can simultaneously travel with different phase velocities, pointing to the existence of two dispersion relations for structures of finite length.

Spectral transmission properties of a self-similar optical Fabry-Perot resonator

The spectral transmission properties of a self-similar optical Fabry-Perot resonator are theoretically studied. The structure considered can be realized by alternation of two dielectric layers of different refractive indices such that the highest refractive-index layers belong to a triadic Cantor set. The transmission spectrum exhibits localized peaks inside forbidden frequency band gaps. This feature could be useful in the design of novel filters. Moreover the self-similarity of the structure is reflected in a weak self-similarity in the transmission spectrum. A comparison with a regular periodic structure is also presented.

Diffuse nonlinearity with a thermal gradient in waveguide prism coupling

Prism coupling of radiation inside a linear film over a nonlinear substrate is studied with a numerical method, showing that the change of coupling as a function of the input power in the case of a quasi-continuous operation (thermal nonlinearity) is influenced by a thermal gradient generated in the linear film. A comparison with experimental data confirms these calculations.

Nonlinear coupling in a planar waveguide

The thermal nonlinear response of a waveguide produced by sputtering a Corning 7059 glass film over a nonlinear substrate(Corning CS2-61) is studied with a pump-probe technique in a quasi-cw regime. The coupling efficiency is found to beafunction of pump power and chopping frequency. The coupling zone is scanned with a probe beam to examine the spatial dependence of the coupling efficiency.

Photothermal deflection technique for measuring thermal nonlinearities in semiconductor glasses

The photothermal deflection technique is used to determine the thermal nonlinearity in samples of glasses doped with CdS(x)Se(1-x). Values of the nonlinear refractive index are derived at low frequencies; they compare with recently obtained dc values in similar, but not equal, glasses.

Test measurements of the photothermal deflection method to determine the thermal diffusivity of solids

The photothermal deflection spectroscopy method is employed in thermal diffusivity measurements of solids. Thermal diffusivity at room temperature in standard materials such as Poco-graphite and S.S. 1.4970 stabilized austenltlc steel Is measured and compared with the tabulated values.

Optical nonlinearities in a lyotropic liquid crystal

The existence of an optical nonlinearity in a lyotropic liquid crystal has been demonstrated in an optical waveguide. A large hysteresis, of origin to be clarified, is found in the optical response in all mesophases of potassium capryllate.