Efficient Raman shifting of microjoule pulses in N2-filled anti-resonant fiber
Yishai Eisenberg,
Yi-Hao Chen,
Wenchao Wang,
Francesco Tani,
Michael Frosz,
Jeffrey Moses,
Chris Xu,
Frank W. Wise
Journal of the Optical Society of America B-Optical Physics
42
2291-2295
(2025)
| Journal
We demonstrate efficient frequency down-conversion of femtosecond pulses based on the interplay of Raman-enhanced self-phase modulation and impulsive redshifting in gas-filled anti-resonant hollow-core fiber. With 20 µJ and 140 fs pulses at 1030 nm launched into a short length of fiber, pulses with durations below 100 fs are generated between 1100 and 1300 nm, with over 26% efficiency and above 5 µJ energy, for peak powers between 50 and 100 MW. The modest experimental requirements and highly efficient conversion make this a practical source for wavelength-specific applications.
Optical frequency shifter based on continuous-wave pump fields
Anica Hamer,
Frank Vewinger,
Michael Frosz,
Simon Stellmer
Practical implementations of quantum information networks require frequency conversion of individual photons. Approaches based on a molecular gas as the nonlinear medium cover a wide range of the optical spectrum and promise high efficiency at negligible background. We present polarization-preserving frequency conversion in a hydrogen-loaded hollow-core fiber using continuous-wave pump fields. We demonstrate conversion efficiency at the level of a few per mille, discuss various limitations and loss mechanisms, and present a route to increase conversion efficiency to near unity.
Optofluidic Waveguides for the Label-Free Study of Silk Protein Aggregates
Jan R. Heck,
Zenon Toprakcioglu,
Tobias E. Naegele,
Michael Frosz,
Tuomas P. J. Knowles,
Tijmen G. Euser
Methods for studying protein aggregation are crucial to understanding the associated disease pathologies and for functional biomaterial synthesis in nature and in the laboratory. The ideal measurement platform is low-volume, label-free, and noncontact, as well as easily integrated into continuous-flow microfluidic experiments to provide scalability. Current approaches realize only a subset of these requirements. Here, we demonstrate a new technique for studying protein aggregates and in situ aggregation within hollow-core photonic crystal fibers. These optofluidic waveguides allow us to perform continuous-flow microfluidic label-free analysis of silk fibroin protein in the form of preformed nanofibrillar aggregates and on the native protein as it undergoes aggregation in situ in the optofluidic waveguide. We demonstrate label-free ultraviolet absorbance measurements on both calibration-standard nanospheres and silk fibroin aggregates as well as monitoring the aggregation of native silk fibroin protein solution via simultaneous ultraviolet absorbance and intrinsic fluorescence measurements in situ. This technique forms a platform for the study of protein aggregation that is low volume, label-free, and optical, thereby providing a valuable optofluidic tool for a range of protein biophysics.
Nonlinear Metafiber: On-fiber 3D Nanoprinted Metalenses to Enhance Ultrafast Supercontinuum Generation in Suspended Core Fibers
Shahrzad Hosseinabadi,
Johannes Hofmann,
Torsten Wieduwilt,
Xue Qi,
Michael Frosz,
Markus A. Schmidt
Supercontinuum generation (SCG) using ultrashort pulses is a highly efficient technique for achieving broad nonlinear frequency conversion, with suspended core fibers (SCFs) being particularly effective due to their high modal field concentration and precise dispersion control. However, their small core sizes, typically a few micrometers, pose significant challenges for light incoupling, resulting in a low and unstable coupling that often requires complex high numerical aperture bulk optics that are both costly and difficult to integrate. This work addresses this key challenge by introducing the concept of nonlinear metafibers. By implementing tailored metalenses directly on the end faces of SCFs using advanced 3D nanoprinting, we demonstrate alignment-free and highly robust coupling of broadband ultrashort pulses into small-core SCFs. This first demonstration of a nonlinear metafiber achieves full all-fiber integration, eliminating the need for bulky external optical components and facilitating broadband soliton-based SCG. The flexibility of this novel approach, which effectively overcomes a fundamental problem in nonlinear photonics, has broad applicability in various fields including quantum technology and life sciences. In addition, the concept extends beyond SCFs to other fiber types and on-chip waveguides, paving the way for new opportunities in nonlinear photonics and integrated optics. This study establishes nonlinear metafibers as a transformative platform with the potential to advance applications in which efficient, compact, and robust nonlinear photonic systems are critical.
Frequency conversion in a hydrogen-filled hollow-core fiber: power scaling, background, and bandwidth
Anica Hamer,
Frank Vewinger,
Michael Frosz,
Simon Stellmer
Large-area quantum networks based on optical fibers allow photons at near-infrared wavelengths to travel with minimal loss. Quantum frequency conversion is a method to alter the wavelength of a single photon while maintaining its quantum state. Most commonly, nonlinear crystals are employed for this conversion process, where near-unity conversion efficiency at high fidelity has been demonstrated. Still, the crystal-based conversion process is plagued by strong background noise, very limited spectral bandwidth, and inhomogeneous temperature profiles at strong pump fields. In the previous work, we have demonstrated frequency conversion in hydrogen-filled hollow-core fibers and claimed that this conversion process does not compromise performance at strong pump fields, is essentially free of background noise, and is intrinsically broadband. Here, we demonstrate that these three claims are justified: we demonstrate the quadratic scaling with pump field intensity, quantify the background level, and present coarse tuning over a range of 10 nm.
Non-destructive real-time characterization of anti-resonant hollow-core fibers using Fabry-Pérot interferometry
Michael Frosz,
Michael Bergler,
Patrick Uebel
Optics Express
33
22961-22973
(2025)
| Journal
| PDF
Reliable industrial manufacturing of anti-resonant hollow-core fibers (AR-HCFs) requires non-destructive, in-line real-time measurements of the fiber structure during drawing. Such a method was recently developed, but it suffered from measurement deviations as the fiber rotated, as well as other disadvantages. Here we demonstrate a greatly improved measurement principle based on Fabry-Pérot interference, which allows for direct measurement of the wall thickness of the AR cladding elements, the gap between capillaries, jacket thickness, and jacket inner diameter. The core diameter can also be inferred from these measurements. The method is therefore more robust, provides more useful information, and enables a significant improvement in the uniformity and length of AR-HCFs.
Ultra-broadband UV/VIS spectroscopy enabled by resonant dispersive wave emission of a frequency comb
Adrian Kirchner,
Alexander Eber,
Lukas Fürst,
Emily Hruska,
Michael Frosz,
Francesco Tani,
Birgitta Bernhardt
Optics Express
33
7005-7015
(2025)
| Journal
| PDF
We introduce an agile light source bridging from the near ultraviolet to the visible spectral region by covering more than 240 THz through resonant dispersive wave (RDW) emission in a gas-filled hollow-core fiber waveguide. The light source allows tuning of a 20 nm (FWHM) spectrum from ∼340 nm to 465 nm (645 to ∼885 THz) with conversion efficiencies of (1.5 ± 0.4) %, providing spectral powers up to (2.6 ± 1) mW/nm. This technique is showcased for spectroscopy with broadband absorption measurements of nitrogen dioxide, a molecular species of major atmospheric relevance. To our knowledge, this is the first demonstration of absorption spectroscopy with an RDW-based light source. The presented measurements indicate conservation of the coherence of the frequency comb seeding the frequency up-conversion process, paving the way towards ultra-broadband (dual) comb molecular spectroscopy across the highly relevant ultraviolet and visible range.
Giant Helical Dichroism in Twisted Hollow-Core Photonic Crystal Fibers
We show that twisted single-ring hollow-core fibers can exhibit strong helical dichroism, i.e., a different transmission depending on the orbital angular momentum of the launched light. Experimentally, we observe loss differences of at least 40 dB/m over a broad spectral range (>60 THz). We investigate the effect via analytical and numerical studies and show that considerably higher differential loss can be achieved over a broader spectral range (>180 THz). Our observation provides new routes for controlling the polarization state, extends previous studies of circularly dichroic waveguides, and has many potential applications, such as the realization of new polarizing elements in previously inaccessible spectral regions, chiral sensing, broadband generation of vortex beams, and optical communication.
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