Multimode fibers provide a means of scaling the peak power of ultrafast fiber lasers by orders of magnitude. While large mode area (LMA) fibers have been widely utilized in fiber amplifiers, these fibers often sacrifice practical benefits of fiber, such as flexibility, and increase system cost and complexity. In addition, the mode-field area of effectively single-mode LMA fibers is smaller than what can be achieved in multimode fibers. Recent work has shown that nonlinear interactions in multimode graded-index fiber can cause a highly multimode field to self-organize into the fundamental mode in a condensation-like process that is robust even with fiber perturbations or disorder.
Building on this, we developed a series of Yb:fiber, mode-locked lasers utilizing normal-dispersion, multimode graded-index fiber. In experiments, we observe that the transition from continuous wave lasing to mode-locking is characterized by a beam cleaning process, whereby the highly multimode (speckled) beam of the continuous wave field transforms into a low-order mode beam. Remarkably, experiments and numerical simulations show that the pulses can consist not just the fundamental mode, but can even comprise multiple transverse modes. Our theoretical analysis shows this to be a consequence of a surprising kind of mode-locking – spatiotemporal mode-locking - which relies on strong intermode interactions and spatial filtering. Our initial experiments yield MW-power pulses after external compression, rivaling the best results with flexible LMA fibers. Meanwhile, simulations show that nearly GW peak powers should be possible, making spatiotemporal mode-locking extremely attractive for high-power ultrafast laser development.
A new scheme for ultrasensitive laser gyroscopes that utilizes the physics of exceptional points will be presented. By exploiting the properties of such non-Hermitian degeneracies, we show that the rotation-induced frequency splitting becomes proportional to the square root of the gyration speed (√𝛀)- thus enhancing the sensitivity to low angular rotations by orders of magnitudes. In addition, at its maximum sensitivity limit, the measurable spectral splitting is independent of the radius of the rings involved. Our work paves the way towards a new class of ultrasensitive miniature ring laser gyroscopes on chip.
Student contribution: In recent years, non-Hermitian degeneracies, also known as exceptional points (EPs), have emerged as a new paradigm for engineering the response of optical systems. This class of degeneracies represents points in parameter space where the eigenvalues and their corresponding eigenvectors simultaneously coalesce [1,2]. Among the large set of non-conservative photonic systems, parity-time (PT) symmetric arrangements are of particular interest since they provide an excellent platform to study the physics and properties of non-Hermitian degeneracies [3,4]. So far, the abrupt nature of the phase transitions at EPs has led to a number of new functionalities such as loss-induced transparency , unidirectional invisibility [6,7], and single mode lasing [8-11]. In addition, it has been suggested that the bifurcation properties associated with second-order exceptional points can be utilized to achieve enhanced sensitivity in micro-resonator arrangements . Of interest is to use even higher-order exceptional points that in principle could further amplify the effect of perturbations. While such higher-order singularities have been theoretically studied in a number of recent works [13,14], their experimental realization in the optical domain has so far remained out of reach. In this paper, for the first time, we show the emergence of third order exceptional points in ternary parity-time-symmetric coupled resonator lasers by judiciously designing the gain/loss distribution and coupling strengths following a recursive bosonic quantization procedure. Subsequently, the nature of the third order exceptional point is confirmed through the cubic root response of this ternary system to external perturbations. Our work may pave the way towards the utilization of higher order exceptional points in designing ultrasensitive photonic arrangements.
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In recent years, non-Hermitian degeneracies, also known as exceptional points (EPs), have emerged as a new paradigm for engineering the response of optical systems. Among many different non-conservative photonic configurations, parity-time (PT) symmetric arrangements are of particular interest since they provide an excellent platform to explore the physics of exceptional points. In this talk, I will present some of the work in our group, where the intriguing properties of exceptional points that are arising in judiciously designed parity-time-symmetric systems are utilized to address two of the long standing challenges in the field of integrated photonics: enforcing single mode lasing in intrinsically multimode cavities, and enhancing sensitivity of micro-resonators.
In this paper we show how to systematically design anti-reflective metasurfaces for the mid-infrared wavelength range. To do so, we have utilized a multilayer arrangement involving a judiciously nano-perforated surface, having air holes, arranged in a hexagonal fashion. By exploiting an effective medium approach, we optimized the dimensions of the surface features in our design. Here, we report a broadband reflectivity 3.5 − 5.5 μm that is below 10% over a broad range of incident angles 00 ≤ θ𝑖 ≤ 700 , irrespective of the incident polarization (TE, TM). Our experimental results are in excellent agreement with full-wave finite element simulations. This systematic approach can be used to design a wide variety of patterned metasurfaces, capable of controlling the phase of the incident optical field.
In recent years, the concept of parity-time (PT) symmetry has received considerable attention in the field of optics and photonics. In PT-symmetric arrangements, the interaction between gain/loss-contrast and coupling leads to the formation of exceptional points in parameter space. At these junctures, not only the eigenvalues but also the eigenvectors tend to merge, resulting in a sudden reduction of the dimensionality of the eigen-space. Consequently, in the vicinity of such points, the eigenfrequencies are strongly affected by external perturbationsas the system regains its original dimensionality. This unique behavior can be utilized to fundamentally enhance the sensitivity of micro-resonators. Here, we experimentally investigate this effect in integrated semiconductor PT-symmetric microring lasers that are biased at exceptional points. Using this arrangement, we demonstrate >10- fold enhancement in sensitivity. Our results also show that unlike standard microcavities, the parity-time symmetric system responds to the square-root of the perturbation. Our work provides a new avenue for enhancing the sensitivity of optical integrated sensors.
Parity time (PT) symmetric systems are known to exhibit two distinct phases: those associated with an unbroken and broken symmetry. In the domain of optics, PT-symmetry can be established by incorporating a balanced distribution of gain and loss in a system. Under linear conditions, in a coupled dimer, composed of two cavities or waveguides, if the gain-loss contrast increases beyond a critical value with respect to the coupling constant, a transition is expected from the unbroken symmetry to the broken symmetry regime. However, in the presence of nonlinearity, this transition behavior can be drastically modified. We here study a system of two coupled semiconductor-based resonators that are lasing around an exceptional point. The quantum wells in such structures not only provide gain but also lead to strong levels of saturable loss in the absence of any optical pumping. Interestingly, in sharp contrast with linear PT-symmetric configurations, such nonlinear processes are capable of reversing the order in which the symmetry breaking occurs. If the ratio of the net loss to coupling is less than unity in one of the cavities, as the pumping level in the other resonator is increased, the nonlinear eigenmodes move from an unbroken symmetric state to a broken one. Moreover, in this nonlinear domain, the structural form of the resulting solutions are isomorphic to the corresponding linear eigenvectors expected above and below the phase transition point. Experimental results are in good agreement with these predictions.
Exploiting a controllable technique for red and blue shifting of quantum well’s bandgap energy, we have fabricated LED sources accessing a wide frequency spectrum along with all-optical intensity modulator devices. . We demonstrate bandgap tuning of InGaAsP multiple quantum well structures by utilizing an impurity free vacancy diffusion technique. Substantial modification of the bandgap energy toward the red and blue part of the spectrum has been observed using SiO2 , SiOyNx , SiNx capping layers and by controlling the associated oxygen and nitrogen content. The resulting degree of tuning, up to 120nm red-shift and 140nm blue-shift of the band to band wavelength emission, has been studied using room temperature photoluminescence, in agreement with the emission spectra obtained from semiconductor LED devices fabricated on this platform. The intensity modulator devices has been fabricated along with LED sources compatible for the selected frequency, designed to reach minimum material losses and residual amplitude modulation.
Parity-time (PT) symmetric complex structures can exhibit peculiar properties which are otherwise unattainable in traditional Hermitian systems. This is achieved by judiciously involving balanced regions of gain and loss. Here we investigate the scattering properties of PT-symmetric diffraction gratings. The presence of the imaginary potential can modify the light transport properties in their far field. This is an outcome of a local power flow taking place between the gain and loss regions in the near field. We show that for a certain gain/loss contrast, all the negative diffraction orders can be eliminated while the positive diffraction orders can be amplified.
In recent years, the ever-increasing demand for high-capacity transmission systems has driven remarkable advances in technologies that encode information on an optical signal. Mode-division multiplexing makes use of individual modes supported by an optical waveguide as mutually orthogonal channels. The key requirement in this approach is the capability to selectively populate and extract specific modes. Optical supersymmetry (SUSY) has recently been proposed as a particularly elegant way to resolve this design challenge in a manner that is inherently scalable, and at the same time maintains compatibility with existing multiplexing strategies.
Supersymmetric partners of multimode waveguides are characterized by the fact that they share all of their effective indices with the original waveguide. The crucial exception is the fundamental mode, which is absent from the spectrum of the partner waveguide. Here, we demonstrate experimentally how this global phase-matching property can be exploited for efficient mode conversion. Multimode structures and their superpartners are experimentally realized in coupled networks of femtosecond laser-written waveguides, and the corresponding light dynamics are directly observed by means of fluorescence microscopy. We show that SUSY transformations can readily facilitate the removal of the fundamental mode from multimode optical structures. In turn, hierarchical sequences of such SUSY partners naturally implement the conversion between modes of adjacent order. Our experiments illustrate just one of the many possibilities of how SUSY may serve as a building block for integrated mode-division multiplexing arrangements. Supersymmetric notions may enrich and expand integrated photonics by versatile optical components and desirable, yet previously unattainable, functionalities.
PT-symmetric optical structures represent a new generation of artificial optical systems which utilize gain and loss in a balanced fashion in order to perform a desired task. Such non-Hemitian arrangements exhibit interesting properties which are otherwise unattainable in passive Hermitian systems. As a result, since the first experimental demonstration of PT-symmetry in coupled optical configurations, there has been a flurry of activities in understanding and utilizing PT-symmetric processes in optics. Here we review recent developments in the newly emerging field of PT-symmetric optics.
We provide a brief report on our recent work on dielectric and metallic colloidal nanosuspensions with negative polarizability where we observed robust propagation of self-trapped light over a long distance. Our results open up new opportunities in developing soft-matter systems with tunable optical nonlinearities.
We experimentally demonstrate single longitudinal mode operation in microring laser using the concept of PT symmetry.
A PT-symmetric coupled resonator arrangement can considerably enhance the maximum achievable gain of single mode
microring cavity. The method is broadband thus work well for inhomogenously broadened gain mediums. It doesn’t rely
on any additional component to ensure its mode selective performance, and it is robust with respect to fabrication
inaccuracies. This result may pave the way for a novel way of designing integrated laser sources based on PT symmetry.
We show that the concept of supersymmetry (SUSY) can be utilized as a versatile tool to design integrated optical
structures with desirable eigenmode spectra. Our approach relies on the intriguing ability of SUSY transformations
to systematically construct a “superpartner” structure that shares all of its propagation constants with the original
waveguide. This approach can be employed to any given one-dimensional refractive index landscapes and
establishes perfect global phase matching condition between an, in principle, arbitrarily large number of guided
modes, while separating the fundamental mode of the original waveguide. In doing so, SUSY transformations also
relate the field distributions of the paired modes, in turn allowing for mode conversion with unity efficiency. Here,
the concept of supersymmetry is illustrated through several examples of one-dimensional waveguides. These include
the step-index (slab) waveguide as well as parabolic and exponential index profiles. In all cases the superpartner can
be obtained analytically. The unique properties of coupled superpartner arrangements make them an ideal platform
for integrated mode filtering and multiplexing applications. The key idea behind this is that global phase matching
allows each mode from the original waveguide to interact freely with the neighboring guides, while the fundamental
mode remains isolated. Here, this whole set of modes can be simultaneously manipulated, attenuated/amplified, or
passed through to higher-order SUSY arrangements.
We show that nonlinear optical structures involving a balanced gain-loss profile can act as optical diodes. This is made
possible by exploiting the interplay between the fundamental symmetries of parity (P) and time (T), with optical
nonlinear Kerr effects. This unidirectional propagation is demonstrated for the case of a PT -symmetric nonlinear coupler
and a PT-symmetric Bragg grating.
We report our experimental and theoretical progress towards elucidating the nonlinear optical response
of nanosuspensions. To date, we have devised a fiber-optic variant of the Z-scan method to accurately
measure the nonlinearity of liquid nanosuspensions. Furthermore, we shall show that the optical
nonlinearity may be properly accounted theoretically by including both the virial coefficients for the
soft-condensed matter system in addition to the exponential term, which does not account for particleparticle
interactions, yielding an effective or renormalized Kerr effect in many cases.
Group-index birefringence in silicon-on-insulator photonic wire waveguides is determined through a polarization beating technique and a Fabry-Pérot resonance method. A large group-index birefringence, up to 0.67, is obtained as a result of the structural asymmetry and high field confinement of our waveguides. The group index and linear propagation loss are also determined. In particular, the group index is found to be as large as 4.45 due to the significant change in the effective mode index of the waveguide as a function of the wavelength. The effects of structure size on the measured losses and group indices are analyzed. Our experimental results are in good agreement with our simulations, and the method employed is found to be effective in analyzing the linear properties of submicrometer optical waveguide structures.
Discrete optical phenomena occur in one dimensional periodic arrays of parallel coupled waveguides. Coupling between
adjacent waveguides leads to a novel form of "discrete" diffraction, periodic dispersion relations involving multiple
bands, and contain regions of both normal, anomalous and zero diffraction. This in turn impacts many linear diffraction
phenomena, for example the Talbot effect. It also results in a diversity of phenomena in nonlinear optics including novel
types of spatial solitons, beam break-up also known as filamentation, and solitonic interactions. The fundamental
concepts are reviewed here and interesting examples of discrete optical phenomena discussed.
The propagation of solitons along the interface between two dielectric nonlinear media was investigated theoretically extensively in the 1980s but never realized experimentally. Recently we predicted that the required small index differences between the media and hence solitons can be created at the interface between continuous and periodic discrete media consisting of arrays of weakly coupled waveguides. Our theoretical analysis has predicted the existence of stable solitons with power thresholds both in the centre and at the edge of the Brillouin zone. We have observed both of these discrete surface solitons with power thresholds in both Kerr and quadratically nonlinear media. Spatial solitons with fields in neighboring channels either in phase or pi out of phase with one another have been identified.
Discrete nonlinear optical systems exhibit unique properties unknown from wave propagation in bulk materials. Among them are the possibilities to form highly localized discrete solitons and the ability of a wide beam to propagate without diffraction and modulational instability across the array. The interaction between a highly localized discrete soliton and a non-diffracting beam has potential applications for all optical routing and switching. We present our results on the experimental investigation of this kind of beam interactions in a one-dimensional AlGaAs array at a wavelength of 1550 nm. A discrete soliton, almost completely confined to a single waveguide, was excited and the interaction with a wide beam of the same or orthogonal polarization was studied. We confirmed that the wide beam is able to drag the soliton over multiple waveguides towards itself while the soliton is able to maintain its original, highly confined shape. The outcome of the coherent interaction depends on the power of the wide beam and the relative phase between the two beams. This phase-dependence is due to linear interference in the case of co-polarized beams and due to four-wave mixing for orthogonally polarized beams.
Since they have been predicted and observed six years ago, photorefractive spatial solitons have attracted substantial research interest. Photorefractive solitons bring about several new fundamental aspects related to solitons in general. Perhaps the single most important aspect is being the first system in which solitons were demonstrated. This has enabled the study of interactions between 2D solitons in a full 3D medium, which has revealed a fundamentally new property of interacting solitons: conversation of angular momentum when the solitons are bound to each other in a spiraling configuration. Another key property of the photorefractive nonlinearity that has had a major impact on soliton research, is its non-instantaneous nature. This has allowed us to generate a new type of 'self-trapped' light beams: incoherent solitons, which are made of partially spatially incoherent light or of temporally and spatially incoherent white light. In this review, we start from the formation mechanism of photorefractive spatial solitons, and especially focus on the bright screening solitons. We then describe the waveguides induced by these solitons, and use this understanding to explain and demonstrate soliton interactions, which are probably the most fascinating features of all solitons in nature, because they shows how a soliton is related to areal particle. Then, we describe Incoherent Solitons and end by discussing several ideas on how to utilize the photorefractive solitons for useful applications.