Tobias Steinle

Nonreciprocal plasmonics enables giant enhancement of thin-film Faraday rotation

Light propagation is usually reciprocal. However, a static magnetic field along the propagation direction can break the time-reversal symmetry in the presence of magneto-optical materials. The Faraday effect in magneto-optical materials rotates the polarization plane of light, and when light travels backward the polarization is further rotated. This is applied in optical isolators, which are of crucial importance in optical systems. Faraday isolators are typically bulky due to the weak Faraday effect of available magneto-optical materials. The growing research endeavour in integrated optics demands thin-film Faraday rotators and enhancement of the Faraday effect. Here, we report significant enhancement of Faraday rotation by hybridizing plasmonics with magneto-optics. By fabricating plasmonic nanostructures on laser-deposited magneto-optical thin films, Faraday rotation is enhanced by one order of magnitude in our experiment, while high transparency is maintained. We elucidate the enhanced Faraday effect by the interplay between plasmons and different photonic waveguide modes in our system.

Towards integration of a liquid-filled fiber capillary for supercontinuum generation in the 1.2–2.4 μm range

We demonstrate supercontinuum generation in unspliced as well as in integrated CS(2)-filled capillary fibers at different pump wavelengths of 1030 nm, 1510 nm, and 1685 nm. A novel method for splicing a liquid-filled capillary fiber to a standard single-mode optical fiber is presented. This method is based on mechanical splicing using a direct-laser written polymer ferrule using a femtosecond two-photon polymerization process. We maintain mostly single-mode operation despite the multi-mode capability of the liquid-filled capillaries. The generated supercontinua exhibit a spectral width of over 1200 nm and 1000 nm for core diameters of 5 μm and 10 μm, respectively. This is an increase of more than 50 percent compared to previously reported values in the literature due to improved dispersion properties of the capillaries.

Watt-level optical parametric amplifier at 42 MHz tunable from 1.35 to 4.5 μm coherently seeded with solitons

We report on an optical parametric amplifier at high repetition rate of 41.7 MHz seeded by an optical soliton from a tapered fiber. Gap-free signal tuning from 1.35 μm to 1.95 μm with corresponding idler wavelengths from 2.2 μm to 4.5 μm is demonstrated. The system provides up to 1.8 W average power at 1.4 μm, more than 1.1 W up to 1.7 μm, and more than 400 mW up to 4.0 μm with a signal pulse duration of 200 to 300 fs. It is directly pumped by a solid-state oscillator providing up to 7.4 W at 1.04 μm wavelength with 425 fs pulse duration. Soliton-seeding is shown to lead to excellent pulse-to-pulse stability, but it introduces a timing-jitter on the millisecond timescale. Using a two-stage concept the timing-jitter is efficiently suppressed due to the passive synchronization of both conversion stages.

Mid-infrared Fourier-transform spectroscopy with a high-brilliance tunable laser source: investigating sample areas down to 5 μm diameter

We demonstrate highly sensitive infrared spectroscopy of sample volumes close to the diffraction limit by coupling a femtosecond fiber-feedback optical parametric oscillator (OPO) to a conventional Fourier-transform infrared (FTIR) spectrometer. The high brilliance and long-term stable infrared radiation with 1e(2)-bandwidths up to 125 nm is easily tunable between 1.4 μm and 4.2 μm at 43 MHz repetition rate and thus enables rapid and low-noise infrared spectroscopy. We demonstrate this by measuring typical molecular vibrations in the range of 3 μm. Combined with surface-enhanced infrared spectroscopy, where the confined electromagnetic near-fields of resonantly excited metal nanoparticles are employed to enhance molecular vibrations, we realize the spectroscopic detection of a molecular monolayer of octadecanethiol. In comparison to conventional light sources and synchrotron radiation, our compact table-top OPO system features a significantly improved performance making it highly suitable for rapid analysis of minute amounts of molecular species in life science and medicine laboratories.

Multi-Watt femtosecond optical parametric master oscillator power amplifier at 43 MHz.

We present a high repetition rate mid-infrared optical parametric master oscillator power amplifier (MOPA) scheme, which is tunable from 1370 to 4120nm. Up to 4.3W average output power are generated at 1370nm, corresponding to a photon conversion efficiency of 78%. Bandwidths of 6 to 12nm with pulse durations between 250 and 400fs have been measured. Strong conversion saturation over the whole signal range is observed, resulting in excellent power stability. The system consists of a fiber-feedback optical parametric oscillator that seeds an optical parametric power amplifier. Both systems are pumped by the same Yb:KGW femtosecond oscillator.

Beam switching and bifocal zoom lensing using active plasmonic metasurfaces

Compact nanophotonic elements exhibiting adaptable properties are essential components for the miniaturization of powerful optical technologies such as adaptive optics and spatial light modulators. While the larger counterparts typically rely on mechanical actuation, this can be undesirable in some cases on a microscopic scale due to inherent space restrictions. Here, we present a novel design concept for highly integrated active optical components that employs a combination of resonant plasmonic metasurfaces and the phase-change material Ge3Sb2Te6. In particular, we demonstrate beam switching and bifocal lensing, thus, paving the way for a plethora of active optical elements employing plasmonic metasurfaces, which follow the same design principles.

Ultra-stable high average power femtosecond laser system tunable from 1.33 to 20 μm.

A highly stable 350 fs laser system with a gap-free tunability from 1.33 to 2.0 μm and 2.13 to 20 μm is demonstrated. Nanojoule-level pulse energy is achieved in the mid-infrared at a 43 MHz repetition rate. The system utilizes a post-amplified fiber-feedback optical parametric oscillator followed by difference frequency generation between the signal and idler. No locking or synchronization electronics are required to achieve outstanding free-running output power and spectral stability of the whole system. Ultra-low intensity noise, close to the pump lasers noise figure, enables shot-noise limited measurements.

Compact, low-noise, all-solid-state laser system for stimulated Raman scattering microscopy

We present a highly stable and compact laser source for stimulated Raman scattering (SRS) microscopy. cw-seeding of an optical parametric amplifier pumped by a bulk femtosecond Yb-oscillator and self-phase modulation in a tapered fiber allow for broad tunability without any optical or electronic synchronization. The source features noise levels of the Stokes beam close to the shot-noise limit at MHz modulation frequencies. We demonstrate the superior performance of our system by SRS imaging of micrometer-sized polymer beads.

High repetition rate mid-infrared supercontinuum generation from 1.3 to 5.3 μm in robust step-index tellurite fibers

We demonstrate broadband supercontinuum generation over two infrared octaves, spanning from 1.3 to 5.3 μm, with an output power of 150 mW in robust step-index tellurite fibers with core diameters between 3.5 and 4.3 μm. As a pump source, we use femtosecond mid-IR pulses from a home-built post-amplified optical parametric oscillator tunable between 1.5 and 4.0 μm at a 43 MHz repetition rate. We study the influence of core size, pump wavelength, and fiber length to optimize the spectral bandwidth. A key requirement for efficient spectral broadening is a low and rather flat average anomalous dispersion over a wide spectral range that can be tailored accordingly by changing the fiber core diameter. Numerical simulations based on the generalized nonlinear Schrodinger equation are in good agreement with experimental results.

Synchronization-free all-solid-state laser system for stimulated Raman scattering microscopy

We introduce an extremely simple and highly stable system for stimulated Raman scattering (SRS) microscopy. An 8-W, 450-fs Yb:KGW bulk oscillator with 41 MHz repetition rate pumps an optical parametric amplifier, which is seeded by a cw tunable external cavity diode laser. The output radiation is frequency doubled in a long PPLN crystal and generates 1.5-ps long narrowband pump pulses that are tunable between 760 and 820 nm with >50 mW average power. Part of the oscillator output is sent through an etalon and creates Stokes pulses with 100 mW average power and 1.7 ps duration. We demonstrate SRS microscopy at a 30-μs pixel dwell time with high chemical contrast, signal-to-noise ratio in excess of 45 and no need for balanced detection, thanks to the favorable noise properties of the bulk solid-state system. Cw seeding intrinsically ensures low spectral drift. We discuss its application to chemical contrast microscopy of freshly prepared plant tissue sections at different vibrational bands.