Quantum-inspired multicore optical fiber.

We introduce a new, to the best of our knowledge, type of multicore optical fiber having a quantum-inspired network topology and unique spectral features. Particularly, the connectivity between the cores is generated by unfolding a circular array of coupled quantum oscillators in Fock space. We show that in such a fiber geometry, the eigenvalues of the optical supermodes exhibit partial degeneracy and form a ladder. In turn, this leads to revival dynamics, allowing for a periodic re-imaging of the input intensity. As an example, we present a realistic design with six cores in silica glass platforms.

Hierarchical Construction of Higher-Order Exceptional Points.

The realization of higher-order exceptional points (HOEPs) can lead to orders of magnitude enhancement in light-matter interactions beyond the current fundamental limits. Unfortunately, implementing HOEPs in the existing schemes is a rather difficult task, due to the complexity and sensitivity to fabrication imperfections. Here we introduce a hierarchical approach for engineering photonic structures having HOEPs that are easier to build and more resilient to experimental uncertainties. We demonstrate our technique by an example that involves parity-time symmetric optical microring resonators with chiral coupling among the internal optical modes of each resonator. Interestingly, we find that the uniform coupling profile is not required to achieve HOEPs in this system-a feature that implies the emergence of HOEPs from disorder and provides resilience against some fabrication errors. Our results are confirmed by using full-wave simulations based on Maxwell's equation in realistic optical material systems.

Controlling directional absorption with chiral exceptional surfaces.

Significant efforts have been dedicated to engineering optical systems with predefined, excitation-dependent light absorption. An important concept along this line is that of exceptional points which allow for engineering directional light absorbing schemes. Current systems, however, do not lend themselves to easy design criterion or robust experimental realization. Here we demonstrate that an optical microring resonator coupled to a waveguide terminated with a mirror supports a chiral exceptional surface that can be used as a platform for tailoring directional light absorption in a straightforward fashion. We further demonstrate that this configuration can be used to implement a unidirectional coherent perfect absorber with controllable differential loss by tuning only a single parameter.

Sensing with Exceptional Surfaces in Order to Combine Sensitivity with Robustness.

Exceptional points (EPs) are singularities that arise in non-Hermitian physics. Current research efforts focus only on systems supporting isolated EPs characterized by increased sensitivity to external perturbations, which makes them potential candidates for building next generation optical sensors. On the downside, this feature is also the Achilles heel of these devices: they are very sensitive to fabrication errors and experimental uncertainties. To overcome this problem, we introduce a new design concept for implementing photonic EPs that combine the robustness required for practical use together with their hallmark sensitivity. Particularly, our proposed structure exhibits a hypersurface of Jordan EPs embedded in a larger space, and having the following peculiar features: (1) A large class of undesired perturbations shift the operating point along the exceptional surface (ES), thus, leaving the system at another EP which explains the robustness; (2) Perturbations due to back reflection or backscattering force the operating point out of the ES, leading to enhanced sensitivity. Importantly, our proposed geometry is relatively easy to implement using standard photonics components and the design concept can be extended to other physical platforms such as microwave or acoustics.

Experimental Observation of PT Symmetry Breaking near Divergent Exceptional Points.

Standard exceptional points (EPs) are non-Hermitian degeneracies that occur in open systems. At an EP, the Taylor series expansion becomes singular and fails to converge-a feature that was exploited for several applications. Here, we theoretically introduce and experimentally demonstrate a new class of parity-time symmetric systems [implemented using radio frequency (rf) circuits] that combine EPs with another type of mathematical singularity associated with the poles of complex functions. These nearly divergent exceptional points can exhibit an unprecedentedly large eigenvalue bifurcation beyond those obtained by standard EPs. Our results pave the way for building a new generation of telemetering and sensing devices with superior performance.

Optical revivals in nonuniform supersymmetric photonic arrays.

We investigate the problem of wavepacket revivals in coupled nonuniform linear optical structures. Starting from the photonic Bloch lattices and J(x) arrays, whose propagators are fully periodic, we use cascaded discrete supersymmetric transformations to generate a family of nonuniform isospectral lattices. These new structures exhibit perfect imaging for any initial condition despite the apparent lack of order in their physical parameters. We note, however. that the SUSY-induced disordered coefficients are not random but, rather, inherit some of the features associated with the original array.

Optical parametric amplification via non-Hermitian phase matching.

We introduce the notion of dissipative optical parametric amplifiers (DOPA) and demonstrate that, even in the absence of the Hermitian phase-matching condition in these structures, the signal beam can be amplified when the idler mode suffers optical attenuation. We discuss the optical implementation of this concept in waveguide platforms, and we propose different methods to control the optical loss of these configurations only at the wavelength of the idler component. Surprisingly, this spectrally selective dissipation process allows the signal beam to draw more energy from the pump and, as a result, attains net amplification. Similar results also apply if the losses are introduced only to the signal component. This intriguing feature can open new avenues for building long wavelength light sources and parametric amplifiers by using semiconductor planar structures, where Hermitian phase-matching requirements can be difficult to satisfy without adding stringent geometric constraints or relatively complex fabrication steps.

Accelerating diffraction-free beams in photonic lattices.

We study nondiffracting accelerating paraxial optical beams in periodic potentials, in both the linear and the nonlinear domains. In particular, we show that only a unique class of z-dependent lattices can support a true accelerating diffractionless beam. Accelerating lattice solitons, autofocusing beams and accelerating bullets in optical lattices are systematically examined.

On-chip optical isolation based on nonreciprocal resonant delocalization effects.

We propose on-chip optical isolators that provide large isolation ratios. The isolation mechanism is based on the interplay between magneto-optical effects and resonant delocalization. Our analysis predicts large isolation ratios in the range of -45 to -55 dB between forward and backward propagating optical beams. Both analytical and numerical results are presented, and fabrication feasibility is also discussed.

Linear response theory of open systems with exceptional points.

Understanding the linear response of any system is the first step towards analyzing its linear and nonlinear dynamics, stability properties, as well as its behavior in the presence of noise. In non-Hermitian Hamiltonian systems, calculating the linear response is complicated due to the non-orthogonality of their eigenmodes, and the presence of exceptional points (EPs). Here, we derive a closed form series expansion of the resolvent associated with an arbitrary non-Hermitian system in terms of the ordinary and generalized eigenfunctions of the underlying Hamiltonian. This in turn reveals an interesting and previously overlooked feature of non-Hermitian systems, namely that their lineshape scaling is dictated by how the input (excitation) and output (collection) profiles are chosen. In particular, we demonstrate that a configuration with an EP of order M can exhibit a Lorentzian response or a super-Lorentzian response of order Ms with Ms = 2, 3, …, M, depending on the choice of input and output channels.

Chiral and degenerate perfect absorption on exceptional surfaces.

Engineering light-matter interactions using non-Hermiticity, particularly through spectral degeneracies known as exceptional points (EPs), is an emerging field with potential applications in areas such as cavity quantum electrodynamics, spectral filtering, sensing, and thermal imaging. However, tuning and stabilizing a system to a discrete EP in parameter space is a challenging task. Here, we circumvent this challenge by operating a waveguide-coupled resonator on a surface of EPs, known as an exceptional surface (ES). We achieve this by terminating only one end of the waveguide with a tuneable symmetric reflector to induce a nonreciprocal coupling between the frequency-degenerate clockwise and counterclockwise resonator modes. By operating the system at critical coupling on the ES, we demonstrate chiral and degenerate perfect absorption with squared-Lorentzian lineshape. We expect our approach to be useful for studying quantum processes at EPs and to serve as a bridge between non-Hermitian physics and other fields that rely on radiation engineering.

Resonant delocalization and Bloch oscillations in modulated lattices.

We study the propagation of light in Bloch waveguide arrays exhibiting periodic coupling interactions. Intriguing wave packet revival patterns as well as beating Bloch oscillations are demonstrated. A new resonant delocalization phase transition is also predicted.

Nonlinear optical response of colloidal suspensions.

We experimentally probed the nonlinear optical response of aqueous nano-colloidal suspensions to provide a test of the theoretical approaches that have been proposed for the nonlinearity, namely an exponential model, an artificial Kerr medium, and a non-ideal gas model. The best agreement with experiment is found using the non-ideal gas model for the colloidal suspension which in turn can be used to infer values for the second virial coefficient of the medium and the nonlinear coefficients.

Soliton dynamics and self-induced transparency in nonlinear nanosuspensions.

We study spatial soliton dynamics in nano-particle suspensions. Starting from the Nernst-Planck and Smoluchowski equations, we demonstrate that in these systems the underlying nonlinearities as well as the nonlinear Rayleigh losses depend exponentially on optical intensity. Two different nonlinear regimes are identified depending on the refractive index contrast of the nanoparticles involved and the interesting prospect of self-induced transparency is demonstrated. Soliton stability is systematically analyzed for both 1D and 2D configurations and their propagation dynamics in the presence of Rayleigh losses is examined. The possibility of synthesizing artificial nonlinearities using mixtures of nanosuspensions is also considered.

Robustness and mode selectivity in parity-time (PT) symmetric lasers.

We investigate two important aspects of PT symmetric photonic molecule lasers, namely the robustness of their single longitudinal mode operation against instabilities triggered by spectral hole burning effects, and the possibility of more versatile mode selectivity. Our results, supported by numerically integrating the nonlinear rate equations and performing linear stability analysis, reveals the following: (1) In principle a second threshold exists after which single mode operation becomes unstable, signaling multimode oscillatory dynamics, (2) For a wide range of design parameters, single mode operation of PT lasers having relatively large free spectral range (FSR) can be robust even at higher gain values, (3) PT symmetric photonic molecule lasers are more robust than their counterpart structures made of single microresonators; and (4) Extending the concept of single longitudinal mode operation based on PT symmetry in millimeter long edge emitting lasers having smaller FSR can be challenging due to instabilities induced by nonlinear modal interactions. Finally we also present a possible strategy based on loss engineering to achieve more control over the mode selectivity by suppressing the mode that has the highest gain (i.e. lies under the peak of the gain spectrum curve) and switch the lasing action to another mode.

Non-Hermitian engineering of synthetic saturable absorbers for applications in photonics.

We introduce a new type of synthetic saturable absorber based on quantum-inspired Jx photonic arrays. We demonstrate that the interplay between optical Kerr nonlinearity, interference effects and non-Hermiticity through radiation loss leads to a nonlinear optical filtering response with two distinct regimes of small and large optical transmissions. More interestingly, we show that the boundary between these two regimes can be very sharp. The threshold optical intensity that marks this abrupt "phase transition" and its steepness can be engineered by varying the number of the guiding elements. The practical feasibility of these structures as well as their potential applications in laser systems and optical signal processing are also discussed.

Solitons in dispersion-inverted AlGaAs nanowires.

We demonstrate that optical solitons can exist in dispersion-inverted highly-nonlinear AlGaAs nanowires. This is accomplished by strongly reversing the dispersion of these nano-structures to anomalous over a broad frequency range. These self-localized waves are possible at very low power levels and can form in millimeter long nanowire structures. The intensity and spectral evolution of solitons propagating in such AlGaAs nanowaveguides is investigated in the presence of loss, multiphoton absorption and higher-order dispersion.

Optical solitons in PT periodic potentials.

We investigate the effect of nonlinearity on beam dynamics in parity-time (PT) symmetric potentials. We show that a novel class of one- and two-dimensional nonlinear self-trapped modes can exist in optical PT synthetic lattices. These solitons are shown to be stable over a wide range of potential parameters. The transverse power flow within these complex solitons is also examined.

Beam dynamics in PT symmetric optical lattices.

The possibility of parity-time (PT) symmetric periodic potentials is investigated within the context of optics. Beam dynamics in this new type of optical structures is examined in detail for both one- and two-dimensional lattice geometries. It is shown that PT periodic structures can exhibit unique characteristics stemming from the nonorthogonality of the associated Floquet-Bloch modes. Some of these features include double refraction, power oscillations, and eigenfunction unfolding as well as nonreciprocal diffraction patterns.

Optical beam instabilities in nonlinear nanosuspensions.

We investigate the modulation instability of plane waves and the transverse instabilities of soliton stripe beams propagating in nonlinear nanosuspensions. We show that in these systems the process of modulational instability depends on the input beam conditions. On the other hand, the transverse instability of soliton stripes can exhibit new features as a result of 1D collapse caused by the exponential nonlinearity.