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Lightsails are a promising spacecraft concept that can reach relativistic speeds via propulsion by laser light, allowing travel to nearby stars within a human lifetime. The success of a lightsail mission requires that any motion in the plane transverse to the propagation direction is bounded and damped for the entire acceleration phase. Here, we demonstrate that a previously unappreciated relativistic force, which generalizes the Poynting-Robertson effect, can passively damp this transverse motion. We show that this purely optical effect can be enhanced by two orders of magnitude compared to plane mirror sails by judicious design of the scattering response. We thus demonstrate that exploiting relativistic effects may be a practical means to control the motion of lightsails.

Chip-based terahertz (THz) devices are emerging as versatile tools for manipulating mm-wave frequencies in the context of integrated high-speed communication technologies for potential sixth-generation (6G) wireless applications. The characterization of THz devices is typically performed using far-field techniques that provide limited information about the underlying physical mechanisms producing them. As the library of chip-based functionalities expands, e.g., for tailoring the emission and directional propagation properties of THz antennas and waveguides, novel characterization techniques will likely be beneficial for observing subtle effects that are sensitive to a device's structural parameters. Here we present near-field measurements showing the emission properties of a broadband THz emitter placed in the vicinity of a photonic crystal (PHC) slab. These experiments reveal long-predicted emission properties, but which to our knowledge have yet to be experimentally observed at THz frequencies. We demonstrate three distinct effects between 0.3-0.5 THz: (i) field suppression at frequencies corresponding to quasi-TE bandgaps (ii) a frequency-dependent directed emission along two distinct pathways for two neighboring frequencies, resulting in a local field concentration; (iii) a re-direction of the emission, achieved by rotating the PHC with respect to the dipole orientation. Simulations reveal that the observed behavior can be predicted from the underlying band structure. Our results highlight the opportunities that PHCs can potentially provide for alignment-free, chip-based 6G technologies. Our experimental technique extends the applicability realms of THz spectroscopy and will find use for characterizing the THz modes supported by true samples, whose inherent imperfections cannot realistically be accounted for by simulations, particularly in highly dispersive frequency bands.

Although most soliton research has traditionally considered dominant quadratic dispersion, the recent discovery of pure-quartic solitons has inspired analysis of soliton solutions with large higher orders of dispersion. Here we present analytic expressions for families of bright soliton solutions at arbitrary dispersion orders and practical methods to obtain the associated dispersion relations. These results provide a framework for considering higher order dispersion solitons and show the potential for further investigation of solitons in higher order dispersion systems., Comment: 23 pages, 8 figures

We show theoretically that dark solitons can exist in the presence of pure quartic dispersion, and also in the presence of both quadratic and quartic dispersive effects, displaying a much greater variety of possible solutions and dynamics than for pure quadratic dispersion. The interplay of the two dispersion orders may lead to oscillatory non-vanishing tails, which enables the possibility of bound, potentially stable, multi-soliton states. Dark soliton-like states which connect to low amplitude oscillations are also shown to be possible. Dynamical evolution results corroborate the stability picture obtained, and possible avenues for dark soliton generation are explored., Comment: 4 pages, 5 figures

We consider the motion of a light sail that is accelerated by a powerful laser beam. We derive the equations of motion for two proof-of-concept sail designs with damped internal degrees of freedom. Using linear stability analysis we show that perturbations of the sail movement in all lateral degrees of freedom can be damped passively. This analysis also shows complicated behaviour akin to that associated with exceptional points in PT-symmetric systems in optics and quantum mechanics. The excess heat that is produced by the damping mechanism is likely to be substantially smaller than the expected heating due to the partial absorption of the incident laser beam by the sail., Comment: 7 figures

Temporal solitons are optical pulses that arise from the balance of negative group-velocity dispersion and self-phase modulation. For decades only quadratic dispersion was considered, with higher order dispersion thought of as a nuisance. Following the recent reporting of pure-quartic solitons, we here provide experimental and numerical evidence for an infinite hierarchy of solitons that balance self-phase modulation and arbitrary negative pure, even-order dispersion. Specifically, we experimentally demonstrate the existence of solitons with pure-sextic ($\beta_6$), -octic ($\beta_8$) and -decic ($\beta_{10}$) dispersion, limited only by the performance of our components, and show numerical evidence for the existence of solitons involving pure $16^{\rm th}$ order dispersion. Phase-resolved temporal and spectral characterization reveals that these pulses, exhibit increasing spectral flatness with dispersion order. The measured energy-width scaling laws suggest dramatic advantages for ultrashort pulses. These results broaden the fundamental understanding of solitons and present new avenues to engineer ultrafast pulses in nonlinear optics and its applications., Comment: 7 pages, 5 figures, 1 table

Bulk materials possessing a relative electric permittivity $\varepsilon$ close to zero exhibit giant Kerr nonlinearities. However, harnessing this response in guided-wave geometries is not straightforward, due to the extreme and counter-intuitive properties of epsilon-near-zero materials. Here we investigate, through rigorous calculations of the Kerr nonlinear coefficient, how the remarkable nonlinear properties of such materials can be exploited in several different types of structures, including bulk films, plasmonic nanowires, and metal nanoapertures. We find the largest Kerr nonlinear response when both the modal area and the group velocity are simultaneously minimized, corresponding to omnidirectional field enhancement. The physical insights developed will be key for understanding and engineering nonlinear nanophotonic systems with extreme nonlinearities and point to new design paradigms.

Soliton molecules, bound states composed of interacting fundamental solitons, exhibit remarkable resemblance with chemical compounds and phenomena in quantum mechanics. Whereas optical molecules composed of two or more temporally locked solitons have been observed in a variety of platforms, soliton molecules formed by bound solitons at different frequencies have only recently been theoretically proposed. Here, we report the observation of polychromatic soliton molecules within a mode-locked laser cavity, achieving the desired dispersion by implementing a spectral pulse-shaper. This system supports two or more coincident solitons with different frequencies, but common group velocities. Our results open new directions of exploration in nonlinear dynamics within systems with complex dispersion and offer a convenient platform for optical analogies to mutual trapping and spectral tunneling in quantum mechanics.

The generation of ultrashort pulses hinges on the careful management of dispersion. Traditionally, this has exclusively involved second-order dispersion, while higher-order dispersion was treated as a nuisance to be minimized. Here we show that high-order dispersion can be strategically leveraged to access an uncharted regime of ultrafast laser operation. In particular, we demonstrate a mode-locked laser, with an intra-cavity spectral pulse-shaper, that emits pure-quartic soliton pulses, which arise from the interaction of the fourth-order dispersion and the Kerr nonlinearity. Using phase-resolved measurements we demonstrate that the energy of these pulses is proportional to the third power of the inverse pulse duration. This implies a dramatic increase in the energy of ultrashort pulses compared to those emitted by soliton lasers to date. These results not only demonstrate a novel approach to ultrafast lasers, but more fundamentally, they clarify the use of higher-order dispersion for optical pulse control, opening up a plethora of possibilities in nonlinear optics and its applications., Comment: 7 pages, 4 figures

We introduce a modular approach for efficiently interfacing photonic integrated circuits with deep-sub-wavelength hybrid plasmonic functionality. We demonstrate that an off-the-shelf silicon-on-insulator waveguide can be post-processed into an integrated hybrid plasmonic circuit by evaporating a silica and gold nanolayer. The circuit consists of a plasmonic rotator and a nanofocusser module, which together result in nano-scale, nonlinear wavelength conversion. We experimentally characterize each module, and demonstrate an intensity enhancement of $>200$ in a calculated mode area of $50\,{\rm nm}^2$ at $\lambda = 1320\,{\rm nm}$ using second harmonic generation. This work opens the door to customized plasmonic functionalities on industry-standard waveguides, bridging conventional integrated photonic circuits with hybrid plasmonic devices. This approach promises convenient access to nanometre-scale quantum information processing, nonlinear plasmonics, and single-molecule sensing.

Plasmonic waveguides are an essential element of nanoscale coherent sources, including nanolasers and four-wave mixing (FWM) devices. Here we report how the design of the plasmonic waveguide needs to be guided by the ultimate application. This contrasts with traditional approaches in which the waveguide is considered in isolation. We find that hybrid plasmonic waveguides, with a nonlinear material sandwiched between the metal substrate and a high-index layer, are best suited for FWM applications, whereas metallic wedges are preferred in nanolasers. We also find that in plasmonic nanolasers high-index buffer layers perform better than more traditional low-index buffers., Comment: 11 pages, 6 figures

Photonic cavities are gathering a large interest to enhance the energy transfer between two dipoles, with far-reaching consequences for applications in photovoltaics, lighting sources and molecular biosensing. However, experimental difficulties in controlling the dipoles' positions, orientations and spectra have limited the earlier work in the visible part of the spectrum, and have led to inconsistent results. Here, we directly map the energy transfer of microwaves between two dipoles inside a resonant half-wavelength cavity with ultrahigh control in space and frequency. Our approach extends F\"orster resonance energy transfer (FRET) theory to microwave frequencies, and bridges the gap between the descriptions of FRET using quantum electrodynamics and microwave engineering. Beyond the conceptual interest, we show how this approach can be used to optimize the design of photonic cavities to enhance dipole-dipole interactions and FRET., Comment: Accepted for publication in PRX

We present a rigorous procedure for evaluating the photoelastic coefficients of a layered medium where the periodicity is smaller than the wavelengths of all optical and acoustic fields. Analytical expressions are given for the coefficients of a composite material comprising thin layers of optically isotropic materials. These coefficients include artificial contributions that are unique to structured media and arise from the optical and mechanical contrast between the constituents. Using numerical examples, we demonstrate that the acousto-optic properties of layered structures can be enhanced beyond those of the constituent materials. Furthermore, we show that the acousto-optic response can be tuned as desired., Comment: 9 pages, 4 figures

Silicon is an ideal material for on-chip applications, however its poor acoustic properties limit its performance for important optoacoustic applications, particularly for Stimulated Brillouin Scattering (SBS). We theoretically show that silicon inverse opals exhibit a strongly improved acoustic performance that enhances the bulk SBS gain coefficient by more than two orders of magnitude. We also design a waveguide that incorporates silicon inverse opals and which has SBS gain values that are comparable with chalcogenide glass waveguides. This research opens new directions for opto-acoustic applications in on-chip material systems., Comment: 4 pages, 6 figures

We compute the SBS gain for a metamaterial comprising a cubic lattice of dielectric spheres suspended in a background dielectric material. Theoretical methods are presented to calculate the optical, acoustic, and opto-acoustic parameters that describe the SBS properties of the material at long wavelengths. Using the electromagnetic and strain energy densities we accurately characterise the optical and acoustic properties of the metamaterial. From a combination of energy density methods and perturbation theory, we recover the appropriate terms of the photoelastic tensor for the metamaterial. We demonstrate that electrostriction is not necessarily the dominant mechanism in the enhancement and suppression of the SBS gain coefficient in a metamaterial, and that other parameters, such as the Brillouin linewidth, can dominate instead. Examples are presented that exhibit an order of magnitude enhancement in the SBS gain as well as perfect suppression., Comment: 11 pages, 14 figures

The perfect absorption of light in subwavelength thickness layers generally relies on exotic materials, metamaterials or thick metallic gratings. Here we demonstrate that total light absorption can be achieved in ultra-thin gratings composed of conventional materials, including relatively weakly-absorbing semiconductors, which are compatible with optoelectronic applications such as photodetectors and optical modulators. We fabricate a 41 nm thick antimony sulphide grating structure that has a measured absorptance of A = 99.3% at a visible wavelength of 591 nm, in excellent agreement with theory. We infer that the absorption within the grating is A = 98.7%, with only A = 0.6% within the silver mirror. A planar reference sample absorbs A = 7.7% at this wavelength.

Using full opto-acoustic numerical simulations, we demonstrate enhancement and suppression of the SBS gain in a metamaterial comprising a subwavelength cubic array of dielectric spheres suspended in a dielectric background material. We develop a general theoretical framework and present several numerical examples using technologically important materials. For As$_2$S$_3$ spheres in silicon, we achieve a gain enhancement of more than an order of magnitude compared to pure silicon, and for GaAs spheres in silicon, full suppression is obtained. The gain for As$_2$S$_3$ glass can also be strongly suppressed by embedding silica spheres. The constituent terms of the gain coefficient are shown to depend in a complex way on the filling fraction. We find that electrostriction is the dominant effect behind the control of SBS in bulk media., Comment: 5 pages, 5 figures

A finite element-based modal formulation of diffraction of a plane wave by an absorbing photonic crystal slab of arbitrary geometry is developed for photovoltaic applications. The semi-analytic approach allows efficient and accurate calculation of the absorption of an array with a complex unit cell. This approach gives direct physical insight into the absorption mechanism in such structures, which can be used to enhance the absorption. The verification and validation of this approach is applied to a silicon nanowire array and the efficiency and accuracy of the method is demonstrated. The method is ideally suited to studying the manner in which spectral properties (e.g., absorption) vary with the thickness of the array, and we demonstrate this with efficient calculations which can identify an optimal geometry., Comment: Accepted by Journal of the Optical Society of America A

Temporal optical solitons have been the subject of intense research due to their intriguing physics and applications in ultrafast optics and supercontinuum generation. Conventional bright optical solitons result from the interaction of anomalous group-velocity dispersion and self-phase modulation. Here we report the discovery of an entirely new class of bright solitons arising purely from the interaction of negative fourth-order dispersion and self-phase modulation, which can occur even for normal group-velocity dispersion. We provide experimental and numerical evidence of shape-preserving propagation and flat temporal phase for the fundamental pure-quartic soliton and periodically modulated propagation for the higher-order pure-quartic solitons. Using analytic theory, we derive the approximate shape of the fundamental pure-quartic soliton exhibiting excellent agreement with our experimental observations. Our discovery, enabled by the unique dispersion of photonic crystal waveguides, could find applications in communications and ultrafast lasers.

We demonstrate a controllable enhancement in the electrostrictive properties of a medium using dilute composite artificial materials. Analytical expressions for the composite electrostriction are derived and used to show that enhancement, tunability and suppression can be achieved through a careful choice of constituent materials. Numerical examples with Ag, As$_2$S$_3$, Si and SiO$_2$ demonstrate that even in a non-resonant regime, artificial materials can bring more than a threefold enhancement in the electrostriction., Comment: 8 pages, 5 figures, to appear in Physical Review B

Nanowire arrays exhibit efficient light coupling and strong light trapping, making them well suited to solar cell applications. The processes that contribute to their absorption are interrelated and highly dispersive, so the only current method of optimizing the absorption is by intensive numerical calculations. We present an efficient alternative which depends solely on the wavelength-dependent refractive indices of the constituent materials. We choose each array parameter such that the number of modes propagating away from the absorber is minimized while the number of resonant modes within the absorber is maximized. From this we develop a semi-analytic method that quantitatively identifies the small range of parameters where arrays achieve maximum short circuit currents. This provides a fast route to optimizing NW array cell efficiencies by greatly reducing the geometries to study with full device models. Our approach is general and applies to a variety of materials and to a large range of array thicknesses., Comment: Accepted by ACS Photonics

Solar cell designs based on disordered nanostructures tend to have higher efficiencies than structures with uniform absorbers, though the reason is poorly understood. To resolve this, we use a semi-analytic approach to determine the physical mechanism leading to enhanced efficiency in arrays containing nanowires with a variety of radii. We use our findings to systematically design arrays that outperform randomly composed structures. An ultimate efficiency of 23.75% is achieved with an array containing 30% silicon, an increase of almost 10% over a homogeneous film of equal thickness., Comment: The following article appeared in Appl. Phys. Lett. Copyright 2012 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics

Aperiodic Nanowire (NW) arrays have higher absorption than equivalent periodic arrays, making them of interest for photovoltaic applications. An inevitable property of aperiodic arrays is the clustering of some NWs into closer proximity than in the equivalent periodic array. We focus on the modes of such clusters and show that the reduced symmetry associated with cluster formation allows external coupling into modes which are dark in periodic arrays, thus increasing absorption. To exploit such modes fully, arrays must include tightly clustered NWs that are unlikely to arise from fabrication variations but must be created intentionally., Comment: Accepted by Optics Express

We derive effective photon modes that facilitate an intuitive and convenient picture of photon dynamics in a structured Kramers-Kronig dielectric in the limit of weak absorption. Each mode is associated with an effective line-width which determines the temporal decay rate of the photon. These results are then applied to obtain an expression for the Beer-Lambert-Bouguer law absorption coefficient for unidirectional propagation in structured media consisting of dispersive, weakly absorptive dielectric materials.