Direct electron beam patterning of electro-optically active PEDOT:PSS.

The optical and electronic tunability of the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has enabled emerging applications as diverse as bioelectronics, flexible electronics, and micro- and nano-photonics. High-resolution spatial patterning of PEDOT:PSS opens up opportunities for novel active devices in a range of fields. However, typical lithographic processes require tedious indirect patterning and dry etch processes, while solution-processing methods such as ink-jet printing have limited spatial resolution. Here, we report a method for direct write nano-patterning of commercially available PEDOT:PSS through electron-beam induced solubility modulation. The written structures are water stable and maintain the conductivity as well as electrochemical and optical properties of PEDOT:PSS, highlighting the broad utility of our method. We demonstrate the potential of our strategy by preparing prototypical nano-wire structures with feature sizes down to 250 nm, an order of magnitude finer than previously reported direct write methods, opening the possibility of writing chip-scale microelectronic and optical devices. We finally use the high-resolution writing capabilities to fabricate electrically-switchable optical diffraction gratings. We show active switching in this archetypal system with >95 % contrast at CMOS-compatible voltages of +2 V and -3 V, offering a route towards highly-miniaturized dynamic optoelectronic devices.

Ultracompact wavefront characterization of femtosecond 3D printed microlenses using double-frequency Ronchi interferometry.

3D printed microoptics have become important tools for miniature endoscopy, novel CMOS-based on-chip sensors, OCT-fibers, among others. Until now, only image quality and spot diagrams were available for optical characterization. Here, we introduce Ronchi interferometry as ultracompact and quick quantitative analysis method for measuring the wavefront aberrations after propagating coherent light through the 3D printed miniature optics. We compare surface shapes by 3D confocal microscopy with optical characterizations by Ronchi interferograms. Phase retrieval gives us the transversal wave front aberration map, which indicates that the aberrations of our microlenses that have been printed with a Nanoscribe GT or Quantum X printer exhibit RMS wavefront aberrations as small as λ/20, Strehl ratios larger than 0.91, and near-diffraction limited modulation transfer functions. Our method will be crucial for future developments of 3D printed microoptics, as the method is ultracompact, ultra-stable, and very fast regarding measurement and evaluation. It could fit directly into a 3D printer and allows for in-situ measurements right after printing as well as fast iterations for improving the shape of the optical surface.

Limits of the detection of microplastics in fish tissue using stimulated Raman scattering microscopy.

We demonstrate the detection sensitivity of microplastic beads within fish tissue using stimulated Raman scattering (SRS) microscopy. The intrinsically provided chemical contrast distinguishes different types of plastic compounds within fish tissue. We study the size-dependent signal-to-noise ratio of the microplastic beads and determine a lower boundary for the detectable size. Our findings demonstrate how SRS microscopy can serve as a complementary modality to conventional Raman scattering imaging in order to detect and identify microplastic particles in fish tissue.

Ultrafast Time Dynamics of Plasmonic Fractional Orbital Angular Momentum.

The creation and manipulation of optical vortices, both in free space and in two-dimensional systems such as surface plasmon polaritons (SPPs), has attracted widespread attention in nano-optics due to their robust topological structure. Coupled with strong spatial confinement in the case of SPPs, these plasmonic vortices and their underlying orbital angular momentum (OAM) have promise in novel light-matter interactions on the nanoscale with applications ranging from on-chip particle manipulation to tailored control of plasmonic quasiparticles. Until now, predominantly integer OAM values have been investigated. Here, we measure and analyze the time evolution of fractional OAM SPPs using time-resolved two-photon photoemission electron microscopy and near-field optical microscopy. We experimentally show the field's complex rotational dynamics and observe the beating of integer OAM eigenmodes at fractional OAM excitations. With our ability to access the ultrafast time dynamics of the electric field, we can follow the buildup of the plasmonic fractional OAM during the interference of the converging surface plasmons. By adiabatically increasing the phase discontinuity at the excitation boundary, we track the total OAM, leading to plateaus around integer OAM values that arise from the interplay between intrinsic and extrinsic OAM.

Hybrid fiber-solid-state laser with 3D-printed intracavity lenses.

Microscale 3D-printing has revolutionized micro-optical applications ranging from endoscopy, imaging, to quantum technologies. In all these applications, miniaturization is key, and in combination with the nearly unlimited design space, it is opening novel, to the best of our knowledge, avenues. Here, we push the limits of miniaturization and durability by realizing the first fiber laser system with intra-cavity on-fiber 3D-printed optics. We demonstrate stable laser operation at over 20 mW output power at 1063.4 nm with a full width half maximum (FWHM) bandwidth of 0.11 nm and a maximum output power of 37 mW. Furthermore, we investigate the power stability and degradation of 3D-printed optics at Watt power levels. The intriguing possibilities afforded by free-form microscale 3D-printed optics allow us to combine the gain in a solid-state crystal with fiber guidance in a hybrid laser concept. Therefore, our novel ansatz enables the compact integration of a bulk active media in fiber platforms at substantial power levels.

Spectrally resolved ultrafast transient dynamics of a femtosecond fiber-feedback optical parametric oscillator.

We report on spectrotemporal transient dynamics in a femtosecond fiber-feedback optical parametric oscillator (FFOPO) system. Burst modulation of the pump beam in combination with dispersive Fourier transformation sampling allows to record single-pulse signal spectra at 41 MHz sampling rate. Therefore, each individual pulse of the signal transients can be spectrally resolved. We characterize the signal output behavior for anomalous as well as for normal intra-cavity dispersion. Amongst steady state output we observed period-doubling cycles and other attractors, which occured at higher intra-cavity nonlinearity levels. The experimental findings are supported by numerical simulations, in order to identify the linear and nonlinear effects, which govern the wavelength tuning behavior of this FFOPO system. We find that steady state operation is preferred and that the wavelength tuning stability of the FFOPO dramatically increases when using a normal dispersion feedback fiber.