We report the observation of nonlinear interactions in quadratic nonlinear crystals having a geometrically twisted susceptibility pattern. The quasi-angular-momentum of these crystals is imprinted on the interacting photons during the nonlinear process so that the total angular momentum is conserved. These crystals affect three basic physical quantities of the output photons: energy, translational momentum, and angular momentum. Here we study the case of second-order harmonic vortex beams, generated from a Gaussian pump beam. These crystals can be used to produce multidimensional entanglement of photons by angular momentum states or for shaping the vortex’s structure and polarization.

The control of heat flow is a formidable challenge due to lack of good thermal insulators. Promising new opportunities for heat flow control were recently theoretically discovered for radiative heat flow in near field, where large heat flow contrasts may be achieved by tuning electronic excitations on surfaces. Here we show experimentally that the phase transition of ${\mathrm{VO}}_2$ entails a change of surface polariton states that significantly affects radiative heat transfer in near field. In all cases the Derjaguin approximation correctly predicted radiative heat transfer in near field, but it underestimated the far field limit. Our results indicate that heat flow contrasts can be realized in near field that can be larger than those obtained in far field.

Rare events of extremely high optical intensity are experimentally recorded at the output of a mode-locked fiber laser that operates in a strongly dissipative regime of chaotic multiple-pulse generation. The probability distribution of these intensity fluctuations, which highly depend on the cavity parameters, features a long-tailed distribution. Recorded intensity fluctuations result from the ceaseless relative motion and nonlinear interaction of pulses within a temporally localized multisoliton phase.

We present a solid-state tunable terahertz source that exploits the theory of coupled oscillators to simultaneously achieve high output power and frequency tuning. Our proposed structure effectively generates and combines high power harmonics from multiple synchronized solid-state oscillators in a loop configuration. We study the dynamics of the system, find the stable modes, and show how the structure can dynamically select a desired coupling mode. Using this method, we fabricated 0.29 and 0.32 THz tunable sources with peak output powers of 0.76 and 0.5 mW both in a standard 65 nm bulk complementary metal-oxide semiconductor technology. This power level is around 10 000 times larger than the state of the art which demonstrates that the proposed concept achieves significantly higher output power for a given solid-state process.

Numerical simulations of the geodynamo in the presence of heterogeneous heating are presented. We study the dynamics and the structure of the magnetic field when the equatorial symmetry of the flow is broken. If the symmetry breaking is sufficiently strong, the $m=0$ axial dipolar field is replaced by a hemispherical magnetic field, dominated by an oscillating $m=1$ magnetic field. Moreover, for moderate symmetry breaking, a bistability between the axial and the equatorial dipole is observed. In this bistable regime, the axial magnetic field exhibits chaotic switches of its polarity, involving the equatorial dipole during the transition period. This new scenario for magnetic field reversals is discussed within the framework of Earth’s dynamo.

We experimentally demonstrate selective control of the $Q$ and transmission of an individual resonance of an optical microcavity by optically controlling its intracavity loss via inverse Raman scattering. A strongly overcoupled resonance is brought into critical coupling with continuous tuning of the on-resonance transmission by $>9\mathrm{dB}$ and reduction of the intrinsic $Q$ factor by more than a factor of five. Adjacent resonances experience minimal disturbance and can be selectively controlled by tuning the control beam to the appropriate control resonance. These dynamics are analogous to Zeno effects observed in decoherence-driven atomic ensembles and two-level systems.

We develop a set of laser rate equations that accurately describes mechanical amplification in optomechanical oscillators driven by photothermal or radiation pressure forces. In the process we introduce a set of parameters describing gain, stored energy, slope efficiency, and saturation power of the mechanical laser. We identify the three-phonon parametric interactions as a microscopic mechanism enabling self-oscillation. Our theory shows remarkable agreement with our experimental data, demonstrating that optomechanical self-oscillation is essentially a “phonon lasing” process in which an optical pump generates coherent acoustic phonons.

We demonstrate that textured closed surfaces, i.e., particles made of perfect electric conductors (PECs), are able to support localized electromagnetic resonances with properties resembling those of localized surface plasmons (LSPs) in the optical regime. Because of their similar behavior, we name these types of resonances as spoof LSPs. As a way of example, we show the existence of spoof LSPs in periodically textured PEC cylinders and the almost perfect analogy to optical plasmonics. We also present a metamaterial approach that captures the basic ingredients of their electromagnetic response.

We demonstrate that ultrafast nonlinear dynamics gives rise to reciprocity breaking in a random photonic medium. Reciprocity breaking is observed via the suppression of coherent backscattering, a manifestation of weak localization of light. The effect is observed in a pump-probe configuration where the pump induces an ultrafast step change of the refractive index during the dwell time of the probe light in the material. The dynamical suppression of coherent backscattering is reproduced well by a multiple scattering Monte Carlo simulation. Ultrafast reciprocity breaking provides a distinct mechanism in nonlinear optical media, which opens up avenues for the active manipulation of mesoscopic transport, random lasers, and photon localization.

Recently it was discovered that periodic lattices of metamaterial scatterers show optical activity, even if the scatterers or lattice show no 2D or 3D chirality, if the illumination breaks symmetry. We demonstrate that such “pseudochirality” is intrinsic to any single planar metamaterial scatterer and in fact has a well-defined value at a universal bound. We argue that in any circuit model, a nonzero electric and magnetic polarizability derived from a single resonance automatically imply strong bi-anisotropy, i.e., magnetoelectric cross polarizability at the universal bound set by energy conservation. We confirm our claim by extracting polarizability tensors and cross sections for handed excitation from transmission measurements on near-infrared split ring arrays, and electrodynamic simulations for diverse metamaterial scatterers.

The effect of turbulent clustering of water droplets on radiative transfer is investigated by means of both a three-dimensional direct numerical simulation of particle-laden homogeneous isotropic turbulence and a radiative transfer simulation based on a Monte Carlo photon tracing method. The results show that turbulent clustering causes the formation of void regions of droplets and hence increases the direct transmittance. This effect decreases as the turbulent Reynolds number increases and is estimated to be negligibly small under the conditions in real clouds.

Well-resolved direct numerical simulations of 2D Rayleigh-Bénard convection in a porous medium are presented for Rayleigh numbers $\mathrm{Ra}\le 4\times {10}^4$ which reveal that, contrary to previous indications, the linear classical scaling for the Nusselt number, $\mathrm{Nu}\sim \mathrm{Ra}$, is attained asymptotically. The flow dynamics are analyzed, and the interior of the vigorously convecting system is shown to be increasingly well described as $\mathrm{Ra}\to \infty$ by a simple columnar “heat-exchanger” model with a single horizontal wave number $k$ and a linear background temperature field. The numerical results are approximately fitted by $k\sim {\mathrm{Ra}}^{0.4}$.

By using ultrathin nonlinear media, the phase velocity of nonlinear polarization waves can be continuously adjusted. We realized nonlinear Cherenkov radiation (NCR) in an anomalous dispersion medium experimentally, which breaks the minimum speed limit of NCR. This modified nonlinear Cherenkov radiation has two new special states with respect to its prior, i.e., the degenerate state and extreme state. Also, periodically arranged ultrathin nonlinear media can enhance NCR and realize the nonlinear Smith-Purcell effect.

In preparation for real space studies of magnetic domains in a pump-probe setup at free-electron laser sources, it is necessary to develop an imaging method compatible with the linearly polarized radiation available at these sources. We present results from a prototype experiment performed at the synchrotron source BESSY II, using a modification of existing phase retrieval techniques. Our results show that it is possible to image magnetic domains in real space using linear polarized light, and we introduce the concept of a reliability map of our reconstructions using Gabor transforms.

During bubble or droplet coalescence, there is a puzzling tendency for the coalesced bubble or droplet to be preferentially placed closer to the larger of its two parents. We confirm that this preference is a function of parent size ratio by directly visualizing coalescing air bubbles on an oil-water interface and coalescing water droplets immersed in oil. We find that the final position of the coalesced sphere is controlled by surface energy release and is related to the parent size ratio by a power-law relationship. 

We present a method for exciting surface plasmon polaritons (SPPs) caused by a magnetic field component perpendicular to the direction of slit. The excitation mechanism is based on the spatially oscillating induced current along the edges of the slit under obliquely incident electromagnetic waves. Our finding distinguishes itself from previous mechanisms based on transverse electric fields and unveils the missing point of the SPP-excitation problem in a nanoslit. The use of a magnetic field for SPP excitation can be highly efficient and even comparable to that with an electric field, so that their composition can lead to selective unidirectional excitation.

We reveal the existence of a new codimension-1 curve that involves a topological change in the structure of the chaotic invariant sets (attractors and saddles) in generic three-dimensional dissipative systems with Shilnikov saddle foci. This curve is related to the spiral-like structures of periodicity hubs that appear in the biparameter phase plane. We show how this curve configures the spiral structure (via the doubly superstable points) originated by the existence of Shilnikov homoclinics and how it separates two regions with different kinds of chaotic attractors or chaotic saddles. Inside each region, the topological structure is the same for both chaotic attractors and saddles.

We present a study on swelling-induced circumferential buckling of tubular shaped gels. Inhomogeneous stress develops as the gel swells under mechanical constraints, which gives rise to spontaneous buckling instability without an external force. Full control over the postbuckling pattern is experimentally demonstrated. A simple analytical model is developed using elastic energy to predict stability and postbuckling patterns upon swelling. Analysis reveals that the height to diameter ratio is the most critical design parameter to determine the buckling pattern, which agrees well with experimental and numerical results.

The scaling and statistical properties of three-dimensional compressible turbulence are studied using high-resolution numerical simulations and a heuristic model. The two-point statistics of the solenoidal component of the velocity field are found to be not significantly different from those of incompressible turbulence, while the scaling exponents of the velocity structure function for the compressive component become saturated at high orders. Both the simulated flow and the heuristic model reveal the presence of a power-law tail in the probability density function of negative velocity divergence (high compression regime). The power-law exponent is different from that in Burgers turbulence, and this difference is shown to have a major contribution from the pressure effect, which is absent in the Burgers turbulence.

An asymmetric, reciprocal, diffraction-free transmission of linearly polarized waves in a new diodelike, three-layer, ultrathin, chiral structure is studied theoretically and experimentally. The exploited physical mechanism is based on the maximization of the cross-polarized transmission in one direction due to the polarization selectivity dictated by the peculiar eigenstate combination, which is efficiently controlled by the electromagnetic tunneling through the metallic subwavelength mesh sandwiched between these layers. Simulation and microwave experiment results demonstrate a nearly total intensity transmission at normal incidence in one direction and a small intensity transmission in the opposite direction.