Resonant third-harmonic generation driven by out-of-equilibrium electron dynamics in sodium-based near-zero index thin films.

We investigate resonant third-harmonic generation in near-zero index thin films driven out-of-equilibrium by intense optical excitation. Adopting the Landau weak coupling formalism to incorporate electron-electron and electron-phonon scattering processes, we derive a novel set of hydrodynamic equations accounting for collision-driven nonlinear dynamics in sodium. By perturbatively solving hydrodynamic equations, we model third-harmonic generation by a thin sodium film, finding that such a nonlinear process is resonant at the near-zero index resonance of the third-harmonic signal.

Phase-matching-free parametric oscillators based on two-dimensional semiconductors.

Optical parametric oscillators are widely used as pulsed and continuous-wave tunable sources for innumerable applications, such as quantum technologies, imaging, and biophysics. A key drawback is material dispersion, which imposes a phase-matching condition that generally entails a complex design and setup, thus hindering tunability and miniaturization. Here we show that the burden of phase-matching is surprisingly absent in parametric micro-resonators utilizing mono-layer transition-metal dichalcogenides as quadratic nonlinear materials.

Out-of-equilibrium electron dynamics of silver driven by ultrafast electromagnetic fields - a novel hydrodynamical approach.

We investigate the ultrafast nonlinear response of silver upon excitation by infrared electromagnetic radiation pulses with a duration of a few femtoseconds. By adopting the Landau weak coupling approach to account for electron-electron and electron-phonon collisions, we solve the Boltzmann equation through the method of moments obtaining a novel set of hydrodynamical equations describing the ultrafast nonlinear dynamics of electrons in silver. While the novel hydrodynamical model that was obtained reduces to the Drude model for small intensities of the driving field, it predicts that absorption saturates for large but experimentally attainable peak intensities of the order of GW cm-2.

Efficient Vortex Generation in Subwavelength Epsilon-Near-Zero Slabs.

We show that a homogeneous and isotropic slab, illuminated by a circularly polarized beam with no topological charge, produces vortices of order 2 in the opposite circularly polarized components of the reflected and transmitted fields, as a consequence of the transverse magnetic and transverse electric asymmetric response of the rotationally invariant system. In addition, in the epsilon-near-zero regime, we find that vortex generation is remarkably efficient in subwavelength thick slabs up to the paraxial regime. This physically stems from the fact that a vacuum paraxial field can excite a nonparaxial field inside an epsilon-near-zero slab since it hosts slowly varying fields over physically large portions of the bulk.

Photo-generated metamaterials induce modulation of CW terahertz quantum cascade lasers.

Periodic patterns of photo-excited carriers on a semiconductor surface profoundly modifies its effective permittivity, creating a stationary all-optical quasi-metallic metamaterial. Intriguingly, one can tailor its artificial birefringence to modulate with unprecedented degrees of freedom both the amplitude and phase of a quantum cascade laser (QCL) subject to optical feedback from such an anisotropic reflector. Here, we conceive and devise a reconfigurable photo-designed Terahertz (THz) modulator and exploit it in a proof-of-concept experiment to control the emission properties of THz QCLs.

One-Dimensional Chirality: Strong Optical Activity in Epsilon-Near-Zero Metamaterials.

We suggest that electromagnetic chirality, generally displayed by 3D or 2D complex chiral structures, can occur in 1D patterned composites whose components are achiral. This feature is highly unexpected in a 1D system which is geometrically achiral since its mirror image can always be superposed onto it by a 180 deg rotation. We analytically evaluate from first principles the bianisotropic response of multilayered metamaterials and we show that the chiral tensor is not vanishing if the system is geometrically one-dimensional chiral; i.

Kapitza homogenization of deep gratings for designing dielectric metamaterials.

We theoretically investigate the homogenization of the dielectric response to transverse electric waves of a transverse grating characterized by the Kapitza condition; i.e., the permittivity is rapidly modulated with a modulation depth scaling as the large wavelength-to-modulation-period ratio.

Terahertz optically tunable dielectric metamaterials without microfabrication.

We theoretically investigate the terahertz (THz) dielectric response of a semiconductor slab hosting a tunable grating photogenerated by the interference of two tilted infrared (IR) plane waves. In the case where the grating period is much smaller than the THz wavelength, we numerically evaluate the ordinary and extraordinary component of the effective permittivity tensor by resorting to electromagnetic full-wave simulation coupled to the dynamics of charge carriers excited by IR radiation. We show that the photo-induced metamaterial optical response can be tailored by varying the grating and it ranges from birefringent to hyperbolic to anisotropic negative dielectric without resorting to microfabrication.

Effective medium theory for Kapitza stratified media: diffractionless propagation.

We show that in the presence of a rapidly modulated dielectric permittivity with a large modulation depth (Kapitza medium) a novel and robust regime of diffractionless electromagnetic propagation occurs. This happens when the mean value to depth ratio of the dielectric profile is comparable to the small ratio between the modulation period and the wavelength. We show that the standard effective medium theory is inadequate to describe the proposed regime and that its occurrence is not substantially hampered by medium losses.

Terahertz active spatial filtering through optically tunable hyperbolic metamaterials.

We theoretically consider infrared-driven hyperbolic metamaterials able to spatially filter terahertz (THz) radiation. The metamaterial is a slab made of alternating semiconductor and dielectric layers whose homogenized uniaxial response, at THz frequencies, shows principal permittivities of different signs. The gap provided by metamaterial hyperbolic dispersion allows the slab to stop spatial frequencies within a bandwidth tunable by changing the infrared radiation intensity.

Miniaturized bending-free solitons by restoring symmetry in periodically biased photorefractives.

We consider optical propagation through a centrosymmetric photorefractive crystal with the externally applied bias voltage modulated along the optical propagation direction. We analytically prove that, if the modulation scale is smaller than the optical diffraction length, the resulting effective nonlinearity has an even parity in the transverse plane for an even-symmetric intensity profile and supports bending-free solitons down to few-micrometer beam widths. Numerical integration of the full photorefractive model for light-matter interaction allows us to confirm the feasibility of these miniaturized solitons and, for longer modulation periods, to investigate the excitation of self-trapped wiggling optical beams.

Transverse and soliton instabilities due to counterpropagation through a reflection grating in Kerr media.

Transverse instabilities are shown to accompany counterpropagation of optical beams through reflection gratings in Kerr media. The instability threshold of continuous waves is analytically derived, and it is shown that the presence of the grating broadens and narrows the stability region of plane waves in focusing and defocusing media, respectively. Furthermore, counterpropagating soliton stability is numerically investigated and compared with the transverse modulation instability analysis, revealing an underlying physical link.

On the limits of validity of nonparaxial propagation equations in Kerr media.

Recent generalizations of the standard nonlinear Schroedinger equation (NLSE), aimed at describing nonparaxial propagation in Kerr media are examined. An analysis of their limitations, based on available exact results for transverse electric (TE) and transverse magnetic (TM) (1+1)-D spatial solitons, is presented. Numerical stability analysis reveals that nonparaxial TM soltions are unstable to perturbations and tend to catastrophically collapse while TE solitons are stable even in the extreme nonparaxial limit.

Photorefractive solitons embedded in gratings in centrosymmetric crystals.

We investigate (1+1D) spatial optical solitons embedded in a fixed-volume grating in centrosymmetric photorefractive crystals. We numerically identify a two-parameter soliton family and deduce both its existence surface and soliton profiles. For shallow gratings, the soliton Fourier spectrum exhibits three lobes located at the reciprocal lattice points -K, 0, and K.

Counterpropagating spatial Kerr soliton in reflection gratings.

We analytically predict the existence of both spatial bright and dark counterpropagating solitons in a reflection grating in the presence of the Kerr nonlinearity. The basic trapping mechanism consists of a twofold balance where diffraction is compensated by self-focusing and reflection is altered by the nonlinear-induced interferometric grating. We find that, whenever the spectral soliton profile lies within the grating stop band, bright and dark solitons exist only if the mutual phase of the counterpropagating solitons is pi or 0, respectively.

Nonparaxial dark solitons in optical Kerr media.

We show that the nonlinear equation that describes nonparaxial Kerr propagation, together with the already reported bright-soliton solutions, admits of (1 + 1)D dark-soliton solutions. Unlike their paraxial counterparts, dark solitons can be excited only if their asymptotic normalized intensity u2infinity is below 3/7; their width becomes constant when u2infinity approaches this value.

Azimuthally polarized spatial dark solitons: exact solutions of Maxwell's equations in a Kerr medium.

Spatial Kerr solitons, typically associated with the standard paraxial nonlinear Schro dinger equation, are shown to exist to all nonparaxial orders as exact solutions of Maxwell's equations in the presence of the vectorial Kerr effect. More precisely, we prove the existence of azimuthally polarized, spatial, dark soliton solutions of Maxwell's equations, while exact linearly polarized (2 + 1)D solitons do not exist. Our ab initio approach predicts the existence of dark solitons up to an upper value of the maximum field amplitude, corresponding to a minimum soliton width of about one-fourth of the wavelength.

Electromagnetic nondiffracting pulses in lossless isotropic plasmalike media.

We introduce a scheme for describing electromagnetic nondiffracting pulses propagating in isotropic and lossless media characterized by a plasma-like refractive index. A family of nondiffracting waves in a dispersive medium is analytically derived in the form of a generalization of X waves propagating in vacuum. It is also shown how the ratio between pulse width and plasma length has a crucial effect on the pulse dynamics.

One-dimensional nondiffracting pulses.

A general expression describing nondiffracting pulses whose transverse profile is a one-dimensional image is presented. The pulse turns out to be expressed as a superposition of two fields, possessing a purely translational dynamics, whose profiles are related to the field distribution on the the waist plane through an Hilbert transformation. The space-time structure of the generally X-shaped pulse is investigated and a simple relation connecting its transverse and the longitudinal widths is established.

Vector electromagnetic X waves.

A vector propagation scheme for describing electromagnetic nondiffracting beams (X waves) is introduced. In particular we show that, from the knowledge of the transverse field components on a given transverse plane and at a fixed instant, it is possible to predict the whole electric field everywhere which in particular allows us to investigate the imaging properties of nondiffracting beam. Furthermore, we show that the longitudinal field component crucially depends on the pulse velocity and that it can be neglected only if the velocity is slightly greater than c.

Universal space-time properties of X waves.

Exact results concerning spatiotemporal universal features of three-dimensional propagation-invariant solutions of the wave equation (X waves) are derived. In particular, relations connecting the pulse transverse extension to the longitudinal coordinate and the propagation velocity to the spatial field distribution are obtained for the whole class of X waves.

Optical propagation in uniaxial crystals orthogonal to the optical axis: paraxial theory and beyond.

We describe monochromatic light propagation in uniaxial crystals by means of an exact solution of Maxwell's equations. We subsequently develop a paraxial scheme for describing a beam traveling orthogonal to the optical axis. We show that the Cartesian field components parallel and orthogonal to the optical axis are extraordinary and ordinary, respectively, and hence uncoupled.

Angular momentum dynamics of a paraxial beam in a uniaxial crystal.

The conservation law governing the dynamics of the radiation angular momentum component along the optical axis (z axis) of a uniaxial crystal is derived from Maxwell's equations; the existence of this law is physically related to the rotational invariance of the crystal around the optical axis. Specializing the obtained general expression for the z component of the angular momentum flux to the case of a paraxial beam propagating along the optical axis, we find that the expression is the same as the corresponding one for a paraxial beam propagating in an isotropic medium of refractive index n(o) (ordinary refractive index of the crystal); besides, we show that the flux is conserved during propagation and that it decomposes into the sum of an intrinsic and an orbital contribution. Investigating their dynamics we demonstrate that they are coupled and, during propagation, an exchange between them exists.