Truong X. Tran

Emergence of geometrical optical nonlinearities in photonic crystal fiber nanowires.

We demonstrate analytically and numerically that a subwavelength-core dielectric photonic nanowire embedded in a properly designed photonic crystal fiber cladding shows evidence of a previously unknown kind of nonlinearity (the magnitude of which is strongly dependent on the waveguide parameters) which acts on solitons so as to considerably reduce their Raman self-frequency shift. An explanation of the phenomenon in terms of indirect pulse negative chirping and broadening is given by using the moment method. Our conclusions are supported by detailed numerical simulations.

Interaction between optical fields and their conjugates in nonlinear media.

Motivated by recent experimental results, we demonstrate that the ubiquitous pulse propagation equation based on a single generalized nonlinear Schrödinger equation is incomplete and inadequate to explain the formation of the so called negative-frequency resonant radiation emitted by optical solitons. The origin of this deficiency is due to the absence of a peculiar nonlinear coupling between the positive and negative frequency components of the pulse spectrum during propagation, a feature that the slowly-varying envelope approximation is unable to capture. We therefore introduce a conceptually new model, based on the envelope of the analytic signal, that takes into account the full spectral dynamics of all frequency components, is prone to analytical treatment and retains the simulation efficiency of the nonlinear Schrödinger equation. We use our new equation to derive from first principles the phase-matching condition of the negative-frequency resonant radiation observed in previously reported experiments.

Theory of Raman multipeak states in solid-core photonic crystal fibers

We provide a full theoretical understanding of the recent observations of excitation of Raman two-peak states in solid-core photonic crystal fibers. Based on a “gravity-like” potential approach we derive simple equations for the “magic” peak power ratio and the temporal separation between pulses forming these two-peak states. We develop a model to calculate the magic input power of the input pulse around which the phenomenon can be observed. We also predict the existence of exotic multipeak states that strongly violate the perturbative pulse splitting law, and we study their stability and excitation conditions.

An accurate envelope equation for light propagation in photonic nanowires: new nonlinear effects

We derive a set of new unidirectional evolution equations for photonic nanowires, i.e. waveguides with sub-wavelength core diameter. Contrary to previous approaches, our formulation simultaneously takes into account both the vector nature of the electromagnetic field and the full variations of the effective modal profiles with wavelength. This leads to the discovery of new, previously unexplored nonlinear effects which have the potential to affect soliton propagation considerably. We specialize our theoretical considerations to the case of perfectly circular silica strands in air, and we support our analysis with detailed numerical simulations.

Understanding Raman-shifting multipeak states in photonic crystal fibers: two convergent approaches.

In this Letter we give theoretical explanations for the recent observations of the excitation of Raman-shifting pulse pairs in solid-core photonic crystal fibers. The formation of these pairs is surprisingly common in the deep anomalous dispersion regime of a large variety of highly nonlinear optical fibers, away from zero group-velocity dispersion points. We have developed two different theoretical models, which agree very well in their conclusions. A qualitative and a quantitative explanation of pair formation is provided, and the existence of multipeak states is predicted.

Raman-induced temporal condensed matter physics in gas-filled photonic crystal fibers.

Raman effect in gases can generate an extremely long-living wave of coherence that can lead to the establishment of an almost perfect temporal periodic variation of the medium refractive index. We show theoretically and numerically that the equations, regulate the pulse propagation in hollow-core photonic crystal fibers filled by Raman-active gas, are exactly identical to a classical problem in quantum condensed matter physics - but with the role of space and time reversed - namely an electron in a periodic potential subject to a constant electric field. We are therefore able to infer the existence of Wannier-Stark ladders, Bloch oscillations, and Zener tunneling, phenomena that are normally associated with condensed matter physics, using purely optical means.

Dirac soliton stability and interaction in binary waveguide arrays

We analyze the stability of a recently found exact analytical spatial soliton in binary waveguide arrays—an analog of the relativistic Dirac soliton. We demonstrate that this soliton class is very robust. The soliton dynamics and different scenarios of soliton interactions are systematically investigated.

Mimicking the nonlinear dynamics of optical fibers with waveguide arrays: towards a spatiotemporal supercontinuum generation

We numerically demonstrate the formation of the spatiotemporal version of the so-called diffractive resonant radiation generated in waveguide arrays with Kerr nonlinearity when a long pulse is launched into the system. The phase matching condition for the diffractive resonant radiation that we have found earlier for CW beams also works well in the spatiotemporal case. By introducing a linear potential, one can introduce a continuous shift of the central wavenumber of a linear pulse, whereas in the nonlinear case one can demonstrate that the soliton self-wavenumber shift can be compensated by the emission of diffractive resonant radiation, in a very similar fashion as it is done in optical fibers. This work paves the way for designing unique optical devices that generate spectrally broad supercontinua with a controllable directionality by taking advantage of the combined physics of optical fibers and waveguide arrays.

Linear and nonlinear photonic Jackiw-Rebbi states in interfaced binary waveguide arrays

We study analytically and numerically the optical analogue of the Jackiw-Rebbi states in quantum field theory. These solutions exist at the interface of two binary waveguide arrays which are described by two Dirac equations with opposite sign masses. We show that these special states are topologically robust not only in the linear regime, but also in nonlinear regimes (with both focusing and de-focusing nonlinearity). We also reveal that one can generate the Jackiw-Rebbi states starting from Dirac solitons.

Optical analog of spontaneous symmetry breaking induced by tachyon condensation in amplifying plasmonic arrays

We study analytically and numerically an optical analogue of tachyon condensation in amplifying plasmonic arrays. Optical propagation is modeled through coupled-mode equations, which in the continuous limit can be converted into a nonlinear one-dimensional Dirac-like equation for fermionic particles with imaginary mass, i.e. fermionic tachyons. We demonstrate that the vacuum state is unstable and acquires an expectation value with broken chiral symmetry, corresponding to the homogeneous nonlinear stationary solution of the system. The quantum field theory analogue of this process is the condensation of unstable fermionic tachyons into massive particles. This paves the way for using amplifying plasmonic arrays as a classical laboratory for spontaneous symmetry breaking effects in quantum field theory.