arXiv subjects

Koorosh Sadri

Publications and source records attributed to Koorosh Sadri.

6 recordsLinked to original sources

arXiv2026

Extreme sensitivity of nonlinear trajectories enhances optical spectral broadening

The generation of a wide spectrum of light in highly nonlinear optical fibers has broad application in spectroscopy, microscopy and medical imaging. To generate such a 'supercontinuum', an ultrashort pulse of light is injected into a highly nonlinear fiber. Then, in a process called self-phase modulation, the nonlinearity of the fiber causes the spectrum to start broadening as the pulse becomes chirped during propagation, seeding further cascaded nonlinear processes. The dynamics associated with supercontinuum generation are captured mathematically as a pulse profile evolving in time and occupying a single spatial mode. Here, we theoretically and experimentally demonstrate that a waveguide composed of multiple coupled cores - a photonic molecule' - rather than just a single core, gives rise to greater self-phase modulation for a given input power. This is perhaps counterintuitive because it may be naively expected that the strongest nonlinear effects would be achieved by concentrating all optical power in one waveguide. The increased broadening arises due to the extreme sensitivity of trajectories near a separatrix of the nonlinear dynamics. This sensitivity leads to a distortion of the temporal shape of the pulse, resulting in a broader spectrum. The effect is reminiscent of the sensitivity associated with exceptional points in coupled-resonator systems, but does not suffer in the same way from the parasitic effects of noise. This suggests that by including multiple cores, a straightforward modification of conventional nonlinear fiber design, supercontinuum sources seeded by self-phase modulation can generate a significantly wider spectrum. More broadly, this demonstrates that the sensitivity to initial conditions associated with nonlinear dynamics may be utilized to generate stronger nonlinear effects in optics.

physics.optics↗

arXiv2026

Quantum Geometry in the Continuum: Solitons in Shallow Lattices

The quantum geometry of electronic, photonic, and atomic lattice systems quantifies the distance in Hilbert space between Bloch states at neighboring lattice momenta. This quantity has profound implications for flat-band systems especially, characterizing surprising behavior such as superfluidity and superconductivity when the group velocity is zero and no transport would be expected for non-interacting particles. However, when the band is not flat, the effects of quantum geometry are often intertwined with and partly masked by the band dispersion. Here, we show that in weakly interacting bosonic systems in the critical dimension (i.e., two dimensions for Kerr nonlinearity), the deviation from critical behavior due to the presence of the lattice is governed by the quantum geometry, which is directly proportional to the fourth-order dispersion. Furthermore, we identify the family of continuous lattice potentials that saturates the bound on the quantum metric for a given effective mass tensor.

cond-mat.quant-gas↗

arXiv2026

Hyperuniform Disorder in Photonic Crystal Slabs with Intrinsic non-Hermiticity

Hyperuniform disorder is a type of correlated disorder characterized by vanishing spectral density at small wavevectors, making the configuration effectively homogeneous on long length scales. In photonics, hyperuniform disorder is promising for generating isotropic photonic pseudogaps and engineering photonic crystal waveguides. However, these studies are largely restricted to idealized lossless settings, although all photonic systems necessarily have loss. In this work, light propagation in photonic crystal slabs with imposed hyperuniform disorder is investigated theoretically and numerically. The system is intrinsically non-Hermitian due to radiative loss, with non-Hermiticity appearing as a complex effective mass of a quadratic photonic band. A theoretical framework for disorder scattering is analytically derived in Hermitian and non-Hermitian quadratic bands with real and complex effective mass, respectively. In contrast to the power law behavior $|\mathbf{k}|^α$ observed in the Hermitian case (where $α$ is the hyperuniformity exponent), the scattering loss in the non-Hermitian band is given by $C_0+C_{β_2}\cdot|\mathbf{k}|^{β_2}$, where $C_0$ is a finite constant and the exponent $β_2\leq 2$. Our theoretical predictions are verified with tight-binding and Finite-Difference Time-Domain simulations with realistic photonic crystal parameters, based on recent experiments.

physics.optics↗

arXiv2025

Stealthy-Hyperuniform Wave Dynamics in Two-Dimensional Photonic Crystals

Hyperuniform structures are spatial patterns whose fluctuations disappear on long length scales, making them effectively homogeneous when observed from afar. Mathematically, this means that their spectral density, $\tildeρ({\bf k})$, approaches zero for low wavenumber, $|\textbf{k}|$. Crystalline lattices are hyperuniform, as are certain quasicrystals, maximally random jammed packing of spheres, and electrons in the fractional quantum Hall state. Stealthy-hyperuniformity is an even stronger constraint on the spectral density: it requires that $\tildeρ({\bf k})$ is strictly zero in a finite range of wavevectors around $\mathbf{k}=\mathbf{0}$, called the stealthy regime, or exclusion region. Since the degree of scattering by disorder is, to leading order, proportional to $\tildeρ({\bf k})$, waves propagating through such structures may do so without scattering for sufficiently long wavelengths and short distances. Here, we measure scattering by disorder in photonic crystal slabs with stealthy-hyperuniform disorder by measuring the linewidths of the photonic bands. We observe the transition between the stealthy and non-stealthy regimes, marked by a sharp increase in linewidth. We also observe the effects of multiple scattering in the stealthy regime, which implies diminishing transparency. Moreover, we show that residual single scattering in the stealthy regime arises from an intrinsically non-Hermitian effect: propagating light has a complex effective mass due to radiative loss out of the slab.

physics.optics↗

arXiv2022

Accessible maps in a group of classical or quantum channels

We study the problem of accessibility in a set of classical and quantum channels admitting a group structure. Group properties of the set of channels, and the structure of the closure of the analyzed group $G$ plays a pivotal role in this regard. The set of all convex combinations of the group elements contains a subset of channels that are accessible by a dynamical semigroup. We demonstrate that accessible channels are determined by probability vectors of weights of a convex combination of the group elements, which depend neither on the dimension of the space on which the channels act, nor on the specific representation of the group. Investigating geometric properties of the set $\mathcal{A}$ of accessible maps we show that this set is non-convex, but it enjoys the star-shape property with respect to the uniform mixture of all elements of the group. We demonstrate that the set $\mathcal{A}$ covers a positive volume in the polytope of all convex combinations of the elements of the group.

quant-ph↗

arXiv2021

Quasi-inversion of quantum and classical channels in finite dimensions

We introduce the concept of quasi-inverse of quantum and classical channels, prove general properties of these inverses and determine them for a large class of channels acting in an arbitrary finite dimension. Therefore we extend the previous results of [1] to arbitrary dimensional channels and to the classical domain. We demonstrate how application of the proposed scheme can increase on the average the fidelity between a given random pure state and its image transformed by the quantum channel followed by its quasi-inversion.

quant-ph↗