Inverted pin beams for robust long-range propagation through atmospheric turbulence.

We introduce a new, to the best of our knowledge, class of optical beams, which feature a spatial profile akin to an "inverted pin." In particular, we asymptotically find that close to the axis, the transverse amplitude profile of such beams takes the form of a Bessel function with a width that gradually increases during propagation. We examine numerically the behavior of such inverted pin beams in turbulent environments as measured via the scintillation index and show that they outperform Gaussian beams (collimated and focused) as well as Bessel beams and regular pin beams, which are all optimized, especially in the moderate and strong fluctuation regimes.

Reconfigurable chirality with achiral excitonic materials in the strong-coupling regime.

We introduce and theoretically analyze the concept of manipulating optical chirality via strong coupling of the optical modes of chiral nanostructures with excitonic transitions in molecular layers or semiconductors. With chirality being omnipresent in chemistry and biomedicine, and highly desirable for technological applications related to efficient light manipulation, the design of nanophotonic architectures that sense the handedness of molecules or generate the desired light polarization in an externally controllable manner is of major interdisciplinary importance. Here we propose that such capabilities can be provided by the mode splitting resulting from polaritonic hybridization. Starting with an object with well-known chiroptical response-here, for a proof of concept, a chiral sphere-we show that strong coupling with a nearby excitonic material generates two spectral branches that retain the object's high chirality density, which manifest most clearly through anticrossings in circular-dichroism or differential-scattering dispersion diagrams. These windows can be controlled by the intrinsic properties of the excitonic layer and the strength of the interaction, enabling thus the post-fabrication manipulation of optical chirality. Our findings are further verified via simulations of circular dichroism of a realistic chiral architecture, namely a helical assembly of plasmonic nanospheres embedded in a resonant matrix.

Absolute Chiral Sensing in Dielectric Metasurfaces Using Signal Reversals.

Sensing molecular chirality at the nanoscale has been a long-standing challenge due to the inherently weak nature of chiroptical signals, and nanophotonic approaches have proven fruitful in accessing these signals. However, in most cases, complete sensing of the chiral part of the molecule's refractive index (magnitude and sign of both its real and imaginary part) has not been possible, while the strong inherent signals from the nanostructures themselves obscure the weak chiroptical signals. Here, we propose a dielectric metamaterial system that overcomes these limitations and allows for complete measurements of the total chirality and discrimination of the effects of its real and imaginary part, possible also in an absolute manner via the application of a crucial signal reversal (excitation with reversed polarization) that enables chirality measurements without the need for sample removal. As proof of principle, we demonstrate signal enhancements by a factor of 200 for ultrathin, subwavelength, chiral samples over a uniform and accessible area.

Chiral Metamaterials with PT Symmetry and Beyond.

Optical systems with gain and loss that respect parity-time (PT) symmetry can have real eigenvalues despite their non-Hermitian character. Chiral systems impose circularly polarized waves which do not preserve their handedness under the combined space- and time-reversal operations and, as a result, seem to be incompatible with systems possessing PT symmetry. Nevertheless, in this work we show that in certain configurations, PT symmetric permittivity, permeability, and chirality is possible; in addition, real eigenvalues are maintained even if the chirality goes well beyond PT symmetry. By obtaining all three constitutive parameters in realistic chiral metamaterials through simulations and retrieval, we show that the chirality can be tailored independently of permittivity and permeability; thus, in such systems, a wide control of new optical properties including advanced polarization control is achieved.

Quantum dot based 3D printed woodpile photonic crystals tuned for the visible.

The development of dynamically responsive 3D photonic elements, which is crucial for the design of active integrated photonic circuits, requires the incorporation of material systems with fast and tunable response. To this end, semiconductor quantum dots have been widely used to perform as the active material system to be integrated; nonetheless, multiple-step processing is usually required for the active functions to be preserved, thereby restricting functionality of integrated 3D quantum photonic elements mostly to the infrared. Here, we report a simple scheme for the realization of visible light active 3D photonic devices by combining direct laser writing with two-photon absorption and in situ synthesis of cadmium sulfide (CdS) nanoparticles. The novel active 3D printable hybrid material is synthesized by crosslinking precursors of CdS quantum dots into a photo-structurable organic-inorganic zirconium-silicon hybrid composite integrating functional properties of both high spatial resolution and high third-order nonlinearity into the photonic matrix. As a proof-of-demonstration for 3D printed active photonic devices, woodpile photonic crystals with an inlayer periodicity down to 500 nm are successfully fabricated showing clear photonic stop bands in the visible spectral region, while for the first time, evidence of an ultrafast dynamic response in the visible is also demonstrated.

Finite-Size Effects in Metasurface Lasers Based on Resonant Dark States.

The quest for subwavelength coherent light sources has recently led to the exploration of dark-mode based surface lasers, which allow for independent adjustment of the lasing state and its coherent radiation output. To understand how this unique design performs in real experiments, we need to consider systems of finite size and quantify finite-size effects not present in the infinite dark-mode surface laser model. Here we find that, depending on the size of the system, distinct and even counterintuitive behavior of the lasing state is possible, determined by a balanced competition between multiple loss channels, including dissipation, intentional out-coupling of coherent radiation, and leakage from the edges of the finite system. The conclusions are crucial for the design of future experiments that will enable the realization of ultrathin coherent light sources.

Novel Lasers Based on Resonant Dark States.

The route to miniaturization of laser systems has so far led to the utilization of diverse materials and techniques for reaching the desired laser oscillation at small scales. Unfortunately, at some point all approaches encounter a trade-off between the system dimensions and the Q factor, especially when going subwavelength, mostly because the radiation damping is inherent to the oscillating mode and can thus not be controlled separately. Here, we propose a metamaterial laser system that overcomes this trade-off and offers radiation damping tunability, along with many other features, such as directionality, subwavelength integration, and simple layer-by-layer fabrication.

Mechanism of the metallic metamaterials coupled to the gain material.

We present evidence of strong coupling between the gain material and the metallic metamaterials. It is of vital importance to understand the mechanism of the coupling of metamaterials with the gain medium. Using a four-level gain system, the numerical pump-probe experiments are performed in several configurations (split-ring resonators (SRRs), inverse SRRs and fishnets) of metamaterials, demonstrating reduction of the resonator damping in all cases and hence the possibility for loss compensation. We find that the differential transmittance ΔT/T can be negative in different SRR configurations, such as SRRs on the top of the gain substrate, gain in the SRR gap and gain covering the SRR structure, while in the fishnet metamaterial with gain ΔT/T is positive.

Lasing threshold control in two-dimensional photonic crystals with gain.

We demonstrate how the lasing threshold of a two dimensional photonic crystal containing a four-level gain medium is modified, as a result of the interplay between the group velocity and the modal reflectivity at the interface between the cavity and the exterior. Depending on their relative strength and the optical density of states, we show how the lasing threshold may be dramatically altered inside a band or, most importantly, close to the band edge. The idea is realized via self-consistent calculations based on a finite-difference time-domain method. The simulations are in good agreement with theoretical predictions.

Dissipative soliton acceleration in nonlinear optical lattices.

An effective mechanism for dissipative soliton acceleration in nonlinear optical lattices under the presence of linear gain and nonlinear loss is presented. The key idea for soliton acceleration consists of the dynamical reduction of the amplitude of the effective potential experienced by the soliton so that its kinetic energy eventually increases. This is possible through the dependence of the effective potential amplitude on the soliton mass, which can be varied due to the presence of gain and loss mechanisms. In contrast to the case where either the linear or the nonlinear refractive index is spatially modulated, we show that when both indices are modulated with the same period we can have soliton acceleration and mass increasing as well as stable soliton propagation with constant non-oscillating velocity. The acceleration mechanism is shown to be very robust for a wide range of configurations.

Discrete X-wave formation in nonlinear waveguide arrays.

We present experimental evidence for the spontaneous formation of discrete X waves in AlGaAs waveguide arrays. This new family of optical waves has been excited, for the first time, by using the interplay between discrete diffraction and normal temporal dispersion, in the presence of Kerr nonlinearity. Our experimental results are in good agreement with theoretical predictions.

X - waves in nonlinear normally dispersive waveguide arrays.

We theoretically demonstrate that optical discrete X-waves are possible in normally dispersive nonlinear waveguide arrays. We show that such X-waves can be effectively excited for a wide range of initial conditions and in certain occasions can be generated in cascade. The possibility of observing this family of waves in AlGaAs array systems is investigated in terms of pertinent examples.