Matthias Wulf

Free-carrier-induced soliton fission unveiled by in situ measurements in nanophotonic waveguides

Solitons are localized waves formed by a balance of focusing and defocusing effects. These nonlinear waves exist in diverse forms of matter yet exhibit similar properties including stability, periodic recurrence and particle-like trajectories. One important property is soliton fission, a process by which an energetic higher-order soliton breaks apart due to dispersive or nonlinear perturbations. Here we demonstrate through both experiment and theory that nonlinear photocarrier generation can induce soliton fission. Using near-field measurements, we directly observe the nonlinear spatial and temporal evolution of optical pulses in situ in a nanophotonic semiconductor waveguide. We develop an analytic formalism describing the free-carrier dispersion (FCD) perturbation and show the experiment exceeds the minimum threshold by an order of magnitude. We confirm these observations with a numerical nonlinear Schrödinger equation model. These results provide a fundamental explanation and physical scaling of optical pulse evolution in free-carrier media and could enable improved supercontinuum sources in gas based and integrated semiconductor waveguides.

Unravelling Nonlinear Spectral Evolution Using Nanoscale Photonic Near-Field Point-to-Point Measurements

We demonstrate nanoscale photonic point-to-point measurements characterizing a single component inside an all-optical signal-processing chip. We perform spectrally resolved near-field scanning optical microscopy on ultrashort pulses propagating inside a slow light photonic crystal waveguide, which is part of a composite sample. A power study reveals a reshaping of the pulses spectral density, which we model using the nonlinear Schrödinger equation. With the model, we are able to identify the various physical processes governing the nonlinear pulse propagation. Finally, we contrast the near-field measurements with transmission measurements of the complete composite sample to elucidate the importance of gaining local information about the evolution of the spectral density.

Imaging of electric and magnetic fields near plasmonic nanowires.

Near-field imaging is a powerful tool to investigate the complex structure of light at the nanoscale. Recent advances in near-field imaging have indicated the possibility for the complete reconstruction of both electric and magnetic components of the evanescent field. Here we study the electro-magnetic field structure of surface plasmon polariton waves propagating along subwavelength gold nanowires by performing phase- and polarization-resolved near-field microscopy in collection mode. By applying the optical reciprocity theorem, we describe the signal collected by the probe as an overlap integral of the nanowire’s evanescent field and the probe’s response function. As a result, we find that the probe’s sensitivity to the magnetic field is approximately equal to its sensitivity to the electric field. Through rigorous modeling of the nanowire mode as well as the aperture probe response function, we obtain a good agreement between experimentally measured signals and a numerical model. Our findings provide a better understanding of aperture-based near-field imaging of the nanoscopic plasmonic and photonic structures and are helpful for the interpretation of future near-field experiments.

Soliton dynamics in semiconductor photonic crystals

Semiconductor optical waveguides have been the subject of intense study as both fundamental objects of study, as well as a path to photonic integration. In this talk I will focus on the nonlinear evolution of optical solitons in photonic crystal waveguides made of semiconductor materials. The ability to independently tune the dispersion and the nonlinearity in photonic crystal waveguides enables the examination of completely different nonlinear regimes in the same platform. I will describe experimental efforts utilizing time-resolved measurements to reveal a number of physical phenomena unique to solitons in a free carrier medium. The experiments are supported by analytic and numerical models providing a deeper insight into the physical scaling of these processes.

Harmonic generation in plasmonic nanowires

Since its discovery, nonlinear optics, whereby light can be used to control and interact with light, has generated enormous interest in many scientific areas [1]. Plasmonic modes, where light is bound to a metal-dielectric interface, and surface plasmon polaritons (SPPs) in particular, are known for their high field confinement and enhancements. Hence these modes are attractive candidates to increase nonlinear optical responses and play an important role in a variety of applications ranging from biosensing, ultrafast switching, quantum optics and fully functional nanophotonic circuitry [2]. Here, we present first observations of the efficient harmonic generation in a guided plasmonic nanosystem, paving the way towards the guidance and control of nonlinear effects at the nanoscale.

Analysis of soliton fission induced by free-carriers

Previously we presented measurements of soliton fission induced by free-carriers. Here we report the derivation of a normalized free-carrier perturbation parameter to describe both those experiments and to extract more general properties of this mechanism.

Soliton fission in nanophotonic waveguides unveiled by near-field measurements

We demonstrate time-resolved measurements of soliton fission in nanophotonic waveguides for the first time. Using near-field measurements, we directly observe the nonlinear pulse evolution along the 1.5 millimetre device in-situ.

Tracking the spectral evolution of slow light en route

This paper reports on experimental investigations of the evolution of the spectral density of an ultrashort laser pulse as it propagates through a composite sample consisting of a slow-light photonic crystal (PhC) waveguide and polymer access waveguides. A novel method of near-field scanning optical microscopy was used to measure the spectrum at different locations, and for different powers, only inside the PhC waveguide. This is in contrast to traditional transmission measurements where the evolution of the spectrum due to the entire system is observed.

Tracking the temporal and spectral evolution of femtosecond pulses on plasmonic nanowires

Summary form only given. Plasmonic structures offer a promising platform for all-optical signal processing and ultrafast switching devices due to the achievable field enhancement [1]. In this regard, plasmonic nanowires (NWs), which are thin and narrow gold stripes of varying width, are especially interesting since they can guide surface plasmon polaritons (SPPs) and confine the electro-magnetic field in two-dimensions [2]. To date pulse propagation on these structures has been only characterized by input-output measurements [3]. Here, we present a local investigation of ultrashort pulse propagation on these plasmonic NWs using a near-field scanning optical microscope that allows us to measure the temporal as well as the spectral evolution of the pulses. We determine the group velocity of the guided mode and find strong indications of spectral reshaping due to nonlinear processes. We measure the temporal dynamics of SPP pulses on plasmonic NWs with widths ranging from 40 to 200 nm to investigate the dependency of the group index ng and the propagation length of the guided mode on the geometrical cross-section. As Fig. 1a shows the SPP is slowed down for decreasing NW widths. The experimentally determined group index ng drops from a value of 2.2 at a width of 40 nm to 1.8 at a width of 180 nm which is in good agreement with numerical calculations. At the same time the measured propagation length decreases from 25 to 3 μm also following the trend of the numerical calculations. By spectrally analyzing the field picked up by the aperture near-field probe at different positions along the plasmonic NW, we visualize the evolution of the local power spectral density (PSD) of the SPP pulse as it propagates. One exemplary measurement is shown in Fig. 1b, presenting 4 spectra taken at different locations along a 40 nm wide plasmonic NW for an input power of 5 mW. We observe that the PSD broadens as the SPP pulse propagates further along the nanowire. We will also present the power dependence of the observed broadening to elucidate the nonlinear processes occurring en route. In conclusion, we locally investigated the dynamics of ultrashort pulse propagation along plasmonic NWs. Local measurements in the time domain allowed us to unambiguously extract the group index and propagation length for different nanowire widths. In addition, we are able to locally determine the PSD, which indicates nonlinear processes in pulse propagation on plasmonic nanowires.