A New Energy Milestone for Chip Lasers
After two decades of effort, a chip-based Mamyshev oscillator delivers femtosecond pulses for supercontinuum and terahertz spectroscopy
A photonic-chip laser has pushed integrated ultrafast sources into a higher-energy regime, delivering femtosecond pulses strong enough to drive supercontinuum generation and terahertz spectroscopy without extra amplification.
The device, developed by researchers at EPFL and HZDR, uses an integrated Mamyshev oscillator built from erbium-ion-implanted silicon nitride waveguides. Laser pulses circulate through a 42-centimeter cavity folded onto a compact photonic chip, passing between spectrally offset Bragg gratings and a nonlinear waveguide. Stronger pulses broaden enough to pass through the filters and sustain mode-locking, while weaker light is rejected, avoiding the need for a physical saturable absorber.
The laser produced a 176 megahertz pulse train with energies of about one nanojoule, more than two orders of magnitude above previous photonic-chip ultrafast sources. After compression, the pulses reached 147 femtoseconds, while the device maintained stable mode-locking for more than 10 hours.
That output translated into spectroscopy-relevant demonstrations. Without extra amplification, the chip laser drove 1.5-octave supercontinuum generation in a silicon nitride waveguide and powered a compact terahertz time-domain spectroscopy system. The latter reached a bandwidth of five terahertz and 90 decibels of dynamic range, then distinguished flour from lactose and measured the thickness of a silicon wafer.
“For more than twenty years, a high-pulse-energy femtosecond laser on chip was widely regarded as a holy grail of integrated photonics,” said corresponding author Tobias Kippenberg in EPFL’s press release. “Our result shows that it is not only possible, but that it can be achieved with a surprisingly elegant architecture.”
The authors say the next step is to bring pulse compression and supercontinuum generation onto the same photonic chip, moving the platform closer to integrated frequency combs for spectroscopy and precision measurement.
Raman Hotspots on the Move
Magnetically reconfigurable nanoprobe swarms create dynamic SERS hotspots that improve signal strength and reproducibility in biological environments
A new in vivo Raman sensing platform uses magnetically reconfigurable nanoprobe swarms to create dynamic surface-enhanced Raman spectroscopy (SERS) hotspots in biological environments, making signal enhancement more reproducible inside living systems.
The platform addresses a trade-off in in vivo SERS. Solid plasmonic substrates can provide reproducible enhancement but are poorly suited to living, moving biological environments; colloidal nanoprobes can enter biological fluids and tissues but often suffer from unstable aggregation and variable signals.
The team designed multilayer nanoprobes with an iron oxide magnetic core, gold and silver plasmonic layers, and a silica coating for stability in biological environments. Under rotating or oscillating magnetic fields, the probes assembled into vortex-like or ribbon-like swarms, continually forming and breaking nanoscale gaps that act as transient SERS hotspots.
Simulations and model-analyte tests suggested that this motion improves detection in two ways: by regenerating hotspots across the swarm and by creating microflows that help draw analytes into those enhancement zones. The dynamic swarms produced stronger and more reproducible Raman signals than dispersed probes or static aggregates, with enhancement factors above 10⁷.
The same principle carried into biological tests. Swarming nanoprobes amplified blood-associated Raman bands with low variability in undiluted rabbit blood, and ultrasound-guided magnetic control produced a stable intravascular swarm with more than tenfold Raman enhancement in a rabbit ear vein model.
While the study remains a proof of concept, with further work needed on specificity, biocompatibility, clearance, and clinical integration, the platform shifts in vivo SERS toward a more active sensing model, where hotspots can be assembled, moved, and refreshed inside biological environments.
Computational Spectroscopy on a Chip
A silicon photonics platform combines photonic crystal waveguides, microcavity resonances, and thermal tuning to reconstruct mid-infrared spectra
A mid-infrared computational spectrometer has combined photonic crystal waveguides, microcavity resonances, and thermal tuning to reconstruct spectra on a compact silicon photonics platform.
The device is built around microcavity-coupled photonic crystal waveguides (MPCWs) designed to generate distinct spectral responses across the 2 µm wavelength region. Each unit combines a photonic crystal waveguide with a side-coupled microcavity. Sharp photonic band edges provide high-contrast spectral sampling, while non-uniform microcavity resonances add extra wavelength-dependent structure across the passband.
The chip used eight MPCW units, each paired with a titanium microheater so its spectral response could be tuned by changing the heating power. Because light slows near the photonic crystal band edge, small temperature changes were enough to shift the sampling response, keeping the device compact and relatively power-efficient.
To reconstruct unknown spectra, the researchers used an alternating optimization algorithm that separates sparse and smooth spectral components while automatically selecting regularization parameters. This avoided the more computationally intensive parameter searches used in conventional convex optimization approaches.
In tests with narrow, broadband, and mixed input spectra, the chip reconstructed signals across a 100 nanometer mid-infrared window and distinguished peaks only 0.5 nanometers apart. A proof-of-concept carbon dioxide absorption measurement also showed how the platform could be used for gas sensing, although fluctuations in the gas cell limited the fidelity of that demonstration.
Because the photonic crystal design can be scaled by changing its dimensions, the authors suggest that the same architecture could be adapted for compact spectrometers across other wavelength ranges, including applications in chemical sensing, environmental monitoring, and biomedical diagnostics.
A Dual View of the Water Interface
Transmission XAS and electron-yield detection provide parallel bulk and interface-sensitive views of water near gold
Water at a solid surface can behave differently from the liquid around it, but separating those two views spectroscopically is not straightforward. A new soft X-ray absorption spectroscopy (XAS) setup now measures both at once, using the same liquid cell to compare bulk water with water at a gold interface.
The method pairs transmission XAS with electron-yield detection. In transmission mode, soft X-rays pass through a controlled liquid layer, giving a spectrum dominated by bulk water. At the same time, drain current from a gold-coated membrane provides an electron-yield spectrum that is weighted toward the water–gold interface, because Auger electrons escape only from shallow depths in liquid water.
Using oxygen K-edge XAS, the team compared the bulk and interfacial views of water across liquid layers ranging from 20 nanometers to 40 micrometers. The bulk spectrum showed the expected pre-edge, main-edge, and post-edge features. At the water–gold interface, the pre-edge feature shifted to higher energy and merged with the shoulder of the main-edge peak, reflecting changes in the electronic structure of water molecules interacting with the gold surface.
The setup also helped define the conditions needed for interface-sensitive measurements. Irradiating from the gold-coated membrane side gave a clearer electron-yield spectrum from the water–gold interface, whereas thicker liquid layers increased the contribution from bulk water and could distort the interfacial spectrum.
By measuring both sides of the interface problem at once, the method could help researchers follow how liquids behave near catalysts, electrodes, and biological membranes.
Measuring Morphogens in the Middle Distance
Using living zebrafish embryos, a new study bridges the gap between local diffusion measurements and tissue-scale morphogen transport
A study of early zebrafish embryos has tracked Squint transport across micrometer-scale distances, showing how local tissue geometry and receptor binding slow a key developmental signal.
Squint is a Nodal morphogen involved in early embryonic patterning, where concentration gradients help cells interpret their position. Existing measurements tend to capture either very local diffusion, using fluorescence correlation spectroscopy, or larger tissue-scale movement, using fluorescence recovery after photobleaching. To date, the intermediate distances between those scales have been harder to follow directly.
To bridge that gap, the team developed single-plane illumination microscopy-based spatial fluorescence cross-correlation spectroscopy, or SPIM-sFCCS. The method uses light-sheet imaging to cross-correlate fluorescence fluctuations between pixels, estimating how long labeled molecules take to move across micrometer-scale distances in living tissue. A revised fitting model and binning strategy helped account for camera-pixel crosstalk and embryo movement.
Applied to early zebrafish embryos, SPIM-sFCCS showed that Squint diffusion depended on the width of the spaces between cells. In narrow intercellular gaps, Squint slowed over distances comparable to the gap itself; in wider spaces, diffusion stayed closer to the faster local values measured by conventional FCS.
The receptor experiments supported a binding-based mechanism. Reducing Acvr2b, a receptor that binds Squint, increased Squint diffusion in narrow spaces, as did overexpression of the Nodal inhibitor Lefty2. A second morphogen involved in embryonic patterning showed a similar spacing-dependent slowdown, suggesting that local geometry and receptor interactions may shape transport beyond Squint alone.
More broadly, SPIM-sFCCS gives researchers access to the intermediate length scales between local diffusion and tissue-scale gradient formation, with future development potentially allowing more detailed maps of morphogen transport in live embryos.
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