Chene Tradonsky

Manipulating the spatial coherence of a laser source.

An efficient method for controlling the spatial coherence has previously been demonstrated in a modified degenerate cavity laser. There, the degree of spatial coherence was controlled by changing the size of a circular aperture mask placed inside the cavity. In this paper, we extend the method and perform general manipulation of the spatial coherence properties of the laser, by resorting to more sophisticated intra-cavity masks. As predicted from the Van Cittert Zernike theorem, the spatial coherence is shown to depend on the geometry of the masks. This is demonstrated with different mask geometries: a variable slit which enables independent control of spatial coherence properties in one coordinate axis without affecting those in the other; a double aperture, an annular ring and a circular aperture array which generate spatial coherence functional forms of cosine, Bessel and comb, respectively.

Conversion of out-of-phase to in-phase order in coupled laser arrays with second harmonics

A novel method for converting an array of out-of-phase lasers into one of in-phase lasers that can be tightly focused is presented. The method exploits second harmonic generation and can be adapted for different laser arrays geometries. Experimental and calculated results, presented for negatively coupled lasers formed in a square, honeycomb, and triangular geometries are in good agreement.

Observing Dissipative Topological Defects with Coupled Lasers

Topological defects have been observed and studied in a wide range of systems, such as cosmology, spin systems, cold atoms, and optics, as they are quenched across a phase transition into an ordered state. These defects limit the coherence of the system and its ability to approach a fully ordered state, so revealing their origin and control is becoming an increasingly important field of research. We observe dissipative topological defects in a one-dimensional ring of phased-locked lasers, and show how their formation is related to the Kibble-Zurek mechanism and is governed in a universal manner by two competing time scales. The ratio between these two time scales depends on the system parameters, and thus offers the possibility of enabling the system to dissipate to a fully ordered, defect-free state that can be exploited for solving hard computational problems in various fields.Topologically protected defects have been observed and studied in a wide range of fields, such as cosmology, spin systems, cold atoms and optics as they are quenched across a phase transition into an ordered state. Revealing their origin and control is becoming increasingly important field of research, as they limit the coherence of the system and its ability to approach a fully ordered state. Here, we present dissipative topological defects in a 1-D ring network of phaselocked lasers, and show how their formation is related to the Kibble-Zurek mechanism and is governed in a universal manner by two competing time scales of the lasers, namely the phase locking time and synchronization time of their amplitude fluctuations. The ratio between these two time scales depends on the system parameters such as gain and coupling strength, and thus offers the possibility to control the probability of topological defects in the system. Enabling the system to dissipate to the fully ordered, defect-free state can be exploited for solving hard combinatorial optimization problems in various fields. As opposed to unitary systems where quenching is obtained via external cooling mostly through the edges, our dissipative system is kept strictly uniform even for fast quenches. INTRODUCTION Topological defects have been extensively studied in cosmology, spin systems and optics [1]. Their origin and scaling behavior was first explained by Kibble-Zurek (KZ) mechanism [2, 3], which was experimentally studied in various systems, such as atomic gases [4-9], nonlinear optics [10], and condensed matter systems [11-13]. Due to complexity and experimental limitations, these systems do not fulfil the exact KZ mechanism, and only deal with limited aspects [5, 11, 14, 15]. The formation of defects in the Zurek’s approach relies on the notion of competing time scales in the system, and the density of defects follows a power-law behavior with respect to the rate at which the phase transition is crossed [13]. The topologically protected defects may prevent the system from reaching a globally stable state with spontaneous symmetry breaking and long range ordering [8]. In most of these works the phase transition into an ordered state was crossed by cooling the system from the outside, whereby the external cooling rate determined the density of defects. For the most interesting regime of fast cooling, it is hard to maintain uniformity over the entire system [4]. Here we present and characterize a new mechanism to form topological defects without external cooling in a dissipative system of coupled laser network [16-18]. In coupled lasers and polaritons networks, losses depend on the relative phase between all oscillators [18-21]. This provides dissipative coupling [20] that can drive the system to a stable steady state phased-locked solution with minimal loss that can be directly mapped to the ground state of the classical XY spin Hamiltonian [18, 19, 21]. However, when the dissipative dynamics is highly over-damped [22] and occurs on a complex landscape, the system fails to reach the globally stable solution and gets stuck in local minima. For a 1-D system on a closed ring, such local minima are topological defects [8], characterized by a non-zero phase circulated over the ring that must be an integer multiple of 2π (winding number or topological charge) [23]. The degeneracy of the co-existing degenerate

Spin-controlled twisted laser beams: intra-cavity multi-tasking geometric phase metasurfaces

Novel multi-tasking geometric phase metasurfaces were incorporated into a modified degenerate cavity laser as an output coupler to efficiently generate spin-dependent twisted light beams of different topologies. Multiple harmonic scalar vortex laser beams were formed by replacing the laser output coupler with a shared-aperture metasurface. A variety of distinct wave functions were obtained with an interleaving approach - random interspersing of geometric phase profiles within shared-aperture metasurfaces. Utilizing the interleaved metasurfaces, we generated vectorial vortices by coherently superposing of scalar vortices with opposite topological charges and spin states. We also generated multiple partially coherent vortices by incorporating harmonic response metasurfaces. The incorporation of the metasurface platforms into a laser cavity opens a pathway to novel types of nanophotonic functionalities and enhanced light-matter interactions, offering exciting new opportunities for light manipulation.

Teaching an old laser new tricks: Solving the inverse scattering problem rapidly

Inverse scattering problems, namely reconstructing the structures of objects from their scattered intensity distributions occur in many fields of science and technology[1], such as tomographic imaging, seismology, single shot X-ray scattering[2] and imaging. Solving the inverse scattering problem where all the phase information is lost, is generally very difficult. The difficulty is alleviated by resorting to some a priori knowledge such as the boundaries within which the object lies (compact support), sparsity or other spatial features. Then it is possible to reconstruct the object using iterative algorithms, such as the well-known Gerchberg-Saxton algorithm. Unfortunately, the algorithms are time consuming and do not always converge to the right solution even with advanced computational resources.

Efficient in-phase locking of coupled lasers

In-phase locked array of lasers, where all have common frequencies and phases, can serve as single powerful laser with the high beam quality of an individual laser. Talbot and Fourier diffractions are commonly used for strong coupling between the lasers in the array. When used separately, each has some disadvantages. Talbot diffraction can lead to efficient out-of-phase locking, but requires additional diffractive elements to achieve stable in-phase locking. Fourier diffraction can directly lead to in-phase locking, but with low efficiency, high alignment sensitivity and possible deleterious damage to elements.

Digital degenerate cavity laser

In the past, we investigated degenerate cavity lasers (DCL) which allows manipulation of both near-field and far-field properties of the output beam. The DCL was comprised of a gain medium, two lenses in a 4f telescope configuration, an output coupler at one end and a back mirror at the other end. With the DCL we investigated topological defects in arrays of coupled lasers[1], simulation of classical spins arrays in a frustrated geometry[2], beam focusing after scattering media[3], and lasers with controllable coherence functions for speckles reduction[4]. In these investigations, the DCL usually included metallic masks of holes and filters that had to be specifically designed and fabricated for each application.

Intra-cavity spin controlled geometric phase metasurface

Geometric phase metasurface (GPM) elements are two dimensional space variant gradient structures, which enable exotic light manipulation. Such structures consist of a dense assembly of resonant optical nanoantennas, the size parameters and orientation of which dictate local light-matter interactions. The GPM elements have been extensively studied, showing that they can control of the phase, amplitude, polarization and orbital angular momentum of light beams [1-4]. The GPM elements have been used as flat optical elements with unique features, as polarization control elements, and as spectro-polarimetric devices.

Lasing with propagation invariant shaped beams

Control of the propagation properties of complex beams is desired for many applications. Here we present a novel method to generate propagation invariant shaped beams. Our method is based on a modified degenerate cavity (MDC) [1], [2], which has a huge number of degrees of freedom (300, 000 modes in our system), that can be coupled and controlled. Specifically, the MDC allows direct access to both the x-space and k-space components of the laser beam. Accordingly, placing two amplitude masks, one in x-space and one in k-space, enables control of the output beam. Varying the geometric properties of the mask in x-space changes the shape of the output beam, and varying the geometric properties of the mask in k-space breaks the degeneracy between modes and forms spatial correlations (partial spatial coherence) in the output beam [1].

Rapid manipulation of the spatial coherence

Efficient method for manipulating the spatial coherence of a laser is presented. Different mutual intensity coherence functions, such as cosine or Bessel functions, are obtained, and number of modes is controlled in 1D and 2D.