Subdiffractive Propagation Research Articles

Observation of directional leaky polaritons at anisotropic crystal interfaces

Extreme anisotropy in some polaritonic materials enables light propagation with a hyperbolic dispersion, leading to enhanced light-matter interactions and directional transport. However, these features are typically associated with large momenta that make them sensitive to loss and poorly accessible from far-field, being bound to the material interface or volume-confined in thin films. Here, we demonstrate a new form of directional polaritons, leaky in nature and featuring lenticular dispersion contours that are neither elliptical nor hyperbolic. We show that these interface modes are strongly hybridized with propagating bulk states, sustaining directional, long-range, sub-diffractive propagation at the interface. We observe these features using polariton spectroscopy, far-field probing and near-field imaging, revealing their peculiar dispersion, and – despite their leaky nature – long modal lifetime. Our leaky polaritons (LPs) nontrivially merge sub-diffractive polaritonics with diffractive photonics onto a unified platform, unveiling opportunities that stem from the interplay of extreme anisotropic responses and radiation leakage.

Nature of spatiotemporal light bullets in bulk Kerr media.

We present a detailed experimental investigation which uncovers the nature of light bullets generated from self-focusing in a bulk dielectric medium with Kerr nonlinearity in the anomalous group velocity dispersion regime. By high dynamic range measurements of three-dimensional intensity profiles, we demonstrate that the light bullets consist of a sharply localized high-intensity core, which carries the self-compressed pulse and contains approximately 25% of the total energy, and a ring-shaped spatiotemporal periphery. Subdiffractive propagation along with dispersive broadening of the light bullets in free space after they exit the nonlinear medium indicate a strong space-time coupling within the bullet. This finding is confirmed by measurements of a spatiotemporal energy density flux that exhibits the same features as a stationary, polychromatic Bessel beam, thus highlighting the nature of the light bullets.

Transfer and conversion of images based on EIT in atom vapor.

Transfer and conversion of images between different wavelengths or polarization has significant applications in optical communication and quantum information processing. We demonstrated the transfer of images based on electromagnetically induced transparency (EIT) in a rubidium vapor cell. In experiments, a 2D image generated by a spatial light modulator is used as a coupling field, and a plane wave served as a signal field. We found that the image carried by coupling field could be transferred to that carried by signal field, and the spatial patterns of transferred image are much better than that of the initial image. It also could be much smaller than that determined by the diffraction limit of the optical system. We also studied the subdiffraction propagation for the transferred image. Our results may have applications in quantum interference lithography and coherent Raman spectroscopy.

Modulation instability in filamentary self-compression

We numerically analyze filamentary propagation for various media and input pulse parameters and show that temporal self-compression can greatly benefit from refocusing events. Analyzing the dynamical behavior in the second focal spot, it turns out that a dispersive temporal break-up may appear due to the emission of a hyperbolic shock wave from the self-steepened trailing edge of the pulse. This break-up event in the refocus enhances the self-compression capabilities of laser filaments, enabling up to 12-fold temporal compression. Only slightly perturbing the input pulse parameters, we further identify a regime in which refocusing events give rise to extended subdiffractive propagation in a weakly ionized channel.

Broad angle phase matching in subdiffractive photonic crystals

We show that photonic crystals made of materials with normal dispersion allow broad angular range phase matching in nonlinear wave mixing processes if tuned to the subdiffractive (or equivalently self-collimated) beam propagation regimes for the frequencies of both interacting waves. This allows efficient parametric mixing of narrow beams. We demonstrate this idea by numerical simulation of the second harmonic generation in two-dimensional photonic crystal in particular nonlinear material (AlGaAs) in planar waveguide geometry.

Comparison of Waveguiding Properties of Plasmonic Voids and Plasmonic Waveguides

Here we compare different nanoscale waveguiding geometries involving plasmonic materials for subdiffractive propagation at optical frequencies. Deriving closed-form analytical formulas to analyze the different structures, we show how the presence of a plasmonic background may produce robust, highly confined guided wave propagation as compared with the dual setups involving plasmonic particles in a transparent background. Advantages and disadvantages of different scenarios for realizing right-handed and left-handed propagation in one-dimensional (1D) and two-dimensional (2D) waveguides are highlighted and discussed.

Subdiffraction propagation of images using saturated absorption of optical transition

We show how well-developed saturated absorption spectroscopy could be adopted for subdiffraction propagation of optical images. We employ pump beams with appropriate spatial profiles that produce controllable inhomogeneous absorption and dispersion of the medium. We discuss advantages of using super-Gaussian and Laguerre-Gaussian pump beams.

Self collimation of ultrasound in a three-dimensional sonic crystal

We present the experimental demonstration of self-collimation (subdiffractive propagation) of an ultrasonic beam inside a three-dimensional (3D) sonic crystal. The crystal is formed by two crossed steel cylinders structures in a woodpilelike geometry disposed in water. Measurements of the 3D field distribution show that a narrow beam, which diffractively spreads in the absence of the sonic crystal, is strongly collimated in propagation inside the crystal, demonstrating the 3D self-collimation effect.

Kerr-induced spontaneous Bessel beam formation in the regime of strong two-photon absorption

We study the effect of Two-Photon Absorption (TPA) nonlinear losses on Gaussian pulses, with power that exceeds the critical power for self-focusing, propagating in bulk kerr media. Experiments performed in fused silica and silicon highlight a spontaneous reshaping of the input pulse into a pulsed Bessel beam. A filament is formed in which sub-diffractive propagation is sustained by the Bessel-nature of the pulse.

Nondiffractive propagation of sound in sonic crystals

In this work we present the study of the nondiffractive propagation regime of ultrasonic waves in a two-dimensional periodic acoustic media (sonic crystal) both from the theoretical and the experimental point of view. An analytical treatment of this regime sonic wave propagation is presented, which allows evaluating the parameters of the nondiffractive regime, just as the minimum size of the nondiffractively propagating acoustic beam. Experimentally, we report on the first experimental demonstration of the subdiffractive propagation (self-collimation) of an ultrasound beam in a sonic crystal formed by steel cylinders in water. The experimental measurements show a good agreement with the theoretical predictions, where the effect of the finite size of the crystal was also considered. The preliminary results on self-collimation of sound in three-dimensional sonic crystal, in progress at the present moment, will be also presented.

Midband solitons in an array of quadratically nonlinear cavities

We predict the existence of discrete midband solitons in an array of coupled quadratically nonlinear cavities driven by an external optical field at the second harmonic. The tilted holding beam provides both subdiffractive propagation and maximum group velocity for the fundamental harmonic. This configuration allows for the existence of identical but counterpropagating midband solitons for the same system parameters. We study numerically the interaction dynamics of these solitons.

Ultrashort laser pulse filamentation from spontaneous X Wave formation in air

The description of ultrashort laser pulse filamentation in condensed media as a spontaneous formation of X waves is shown to apply also to filaments generated in air. Within this framework, a simple explanation is brought for several features of the filament such as the subdiffractive propagation and the energy flux from the weakly localized tails of the X-waves to the intense core.

Subdiffractive propagation of ultrasound in sonic crystals

We present an experimental demonstration of the subdiffractive propagation (self-collimation) of an ultrasound beam in a two-dimensional sonic crystal formed by a square array of steel cylinders inmersed in water. Measurements show that the diffractive spreading of a narrow beam is strongly reduced along the spatially modulated direction of the crystal. The effect of the finite crystal length is theoretically analyzed, resulting in a frequency shift of the subdiffractive point in good correspondence with the experimental results.

Towards observation of sub-diffractive pulse propagation in photonic crystals

It is shown that sub-diffractive (self-collimating) propagation of ultrashort pulses of narrow light beams is possible in two dimensional planar photonic crystal slabs (in particular made with air holes in Si3N4 slab) where the sub-diffractive propagation of monochromatic beam of visible light has been demonstrated previously both theoretically and numerically. We found that the sub-diffractive propagations of the pulses of duration of more than 40fs are indistinguishable from that of monochromatic beams. However, for the pulses with duration less than 40fs their propagation peculiarities associated with asymmetric spatiotemporal misshaping, spatial and temporal broadening come into play. The effects are pronounced for the pulses of duration less than 20fs. The phenomena are shown for both TM and TE polarized light by means of numerical integration of 2D Maxwell’s equations with finite-difference time-domain technique.

Quantitative analysis of subdiffractive light propagation in photonic crystals

We present a qualitative study of the subdiffractive light beam propagation in photonic crystals, based on the theoretical analysis of full Maxwell equation system beyond the slowly varying envelope and paraxial approximations. We calculate the zero diffraction curve in the parameter space, we calculate the weak asymptotical broadening (spreading in transverse direction), and we also analyze the dependence of subdiffractive propagation on the polarization orientation (both for TM and TE polarization).

Subdiffractive light in bi-periodic arrays of modulated fibers

We study theoretically the linear and nonlinear propagation of light in one-dimensional bi-periodic arrays of fibers, with the propagation constant periodically modulated along the propagation direction. We predict analytically and observe numerically subdiffractive propagation along such fiber arrays, and characterize the light propagation properties. We also predict novel subdiffractive discrete solitons in the presence of Kerr nonlinearity, both of focusing and defocusing cases, which are essentially different from the usual discrete solitons in waveguide arrays.