Lasers are often described as sources of pure light, but real devices can behave less like a perfectly tuned instrument and more like a crowded orchestra. At low power, a laser may emit predominantly at one wavelength and in one spatial pattern. As the pump energy rises, however, additional modes can begin to oscillate. These competing modes can broaden the spectrum, destabilize the beam, reduce coherence, and limit the usefulness of the laser in communications, sensing, imaging, manufacturing, and quantum technologies. The central challenge is that the physical mechanisms that allow a laser to produce more power can also make it easier for unwanted modes to survive.

Photonic-crystal slab cavities offer a way to confront that problem at the level of the device’s architecture. A photonic crystal is a material patterned with a carefully arranged, repeating variation in refractive index. Because light responds to this periodic structure in much the same way that electrons respond to the periodic atomic lattice of a solid, certain optical frequencies and propagation directions can be allowed or suppressed. In a slab cavity, the patterned layer confines light in the plane of the structure while the contrast between the slab and its surrounding materials helps guide the field vertically. By shaping the pattern, engineers can control resonance wavelengths, radiation losses, mode volumes, and the direction in which light escapes.

The appeal of these cavities is their ability to compress and manipulate light within microscopic dimensions. A resonant optical mode can circulate in the cavity and interact repeatedly with the gain medium, strengthening stimulated emission. In principle, that can produce efficient lasing from a compact footprint. Yet miniaturization introduces its own complications. A cavity that strongly confines light may have limited capacity to handle heat or distribute the optical field over a large active region. If many cavities are placed together to increase power, their resonances can interact, and small fabrication variations may cause different elements to lase at slightly different wavelengths. The result can be a powerful source that no longer behaves as a clean single-mode laser.

The study’s significance lies in its emphasis on a general cavity design rather than a solution tailored to only one experimental geometry. The researchers describe an approach intended to make the essential optical properties of the cavity scalable. In this context, scalability means more than simply enlarging a device. A useful design must preserve the conditions that select the desired mode while increasing the active area or combining multiple emitting regions. It must also manage the relationship between in-plane confinement, vertical radiation, optical feedback, and the far-field beam. These variables are tightly coupled: changing one can improve power extraction while damaging spectral purity or beam quality.

A key technical issue in high-power single-mode operation is modal discrimination. For a laser to remain single mode, the intended resonance must experience enough net gain to dominate all alternatives. Net gain is determined by the balance between amplification in the active material and losses caused by absorption, scattering, radiation, and imperfect confinement. A successful cavity design therefore needs to give the target mode a clear advantage. Photonic-crystal engineering can create that advantage by adjusting the geometry of holes, lattice features, defect regions, and coupling pathways. The design can favor one resonant field distribution while increasing loss or reducing feedback for competing modes.

Scaling also raises the question of how individual optical elements communicate with one another. If separate cavities are coupled too weakly, an array may behave as a collection of independent emitters, producing multiple frequencies and a poor-quality combined beam. If they are coupled too strongly or in the wrong configuration, the system may develop several nearly degenerate supermodes, making mode control even harder. A general cavity strategy must navigate this balance, coordinating the phase and spectrum of the emitted light without sacrificing the ability to pump a larger region. This is one reason photonic-crystal slabs are attracting attention: their geometry can be engineered to control both local resonances and collective optical behavior.

The potential impact extends far beyond laboratory demonstrations. High-power, narrow-linewidth lasers are essential wherever light must be directed, measured, encoded, or amplified with precision. In optical communications, a stable single-frequency source can improve the reliability and information capacity of transmission systems. In lidar and three-dimensional sensing, spectral purity and beam quality can sharpen distance measurements and reduce interference. In spectroscopy, a narrow and stable wavelength allows researchers to resolve subtle signatures of molecules and materials. Industrial systems could benefit from compact sources that deliver more power without requiring large optical assemblies, while emerging quantum technologies depend on lasers with tightly controlled frequency, phase, and spatial mode.

The work also highlights a broader shift in photonics: the move from designing isolated optical components to creating general rules for complex, scalable light sources. As devices become smaller and more integrated, performance increasingly depends on architecture rather than on a single material or isolated cavity. The researchers’ framework points toward photonic-crystal lasers that can be designed systematically for a desired combination of power, mode purity, efficiency, and manufacturability. The achievement is not simply the promise of a brighter laser. It is the possibility of making high-power single-mode emission a repeatable design outcome rather than a fragile compromise. If validated across materials, wavelengths, and fabrication platforms, such an approach could help transform nanophotonic lasers from elegant laboratory structures into practical engines for the next generation of optical technology.

Subject of Research: Scalable high-power single-mode lasing using photonic-crystal slab cavity design

Article Title: General photonic-crystal slab cavity design for scalable high-power single-mode lasing

Article References: Zhou, M., Pan, M., Gautam, C. et al. “General photonic-crystal slab cavity design for scalable high-power single-mode lasing.” Light: Science & Applications 15, 353 (2026). https://doi.org/10.1038/s41377-026-02448-6

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02448-6

Keywords: photonic crystal, slab cavity, high-power laser, single-mode lasing, nanophotonics, optical cavities, laser scalability, coherent light sources

Carbon, one of the most fundamental elements to life and technology, has long mystified scientists when it comes to its behavior in liquid form. Unlike most materials, carbon does not simply melt under pressure; instead, under normal conditions, it sublimates directly from solid to gas. To transform carbon into a liquid state, extraordinarily high pressures and temperatures are necessary — approximately 4500 degrees Celsius, a temperature that exceeds the melting point of all known materials. This extreme environment has rendered laboratory studies of liquid carbon all but impossible until now, as no container material can withstand such conditions.

The research team overcame this monumental challenge by employing laser-driven compression techniques that induce phase changes on ultrafast timescales. Using the powerful DIPOLE100-X laser to generate shock waves, solid carbon samples were compressed and heated, briefly entering the liquid phase for mere billionths of a second. During this fleeting interval, European XFEL’s ultrashort X-ray pulses probed the atomic arrangement of the liquid carbon, an observation feat previously thought unachievable. The combination of laser-induced compression and X-ray diffraction thus provided a direct, time-resolved window into the liquid state.

This experimental setup represented a unique synergy between two state-of-the-art technologies. The DIPOLE100-X laser, developed by the UK’s Science and Technology Facilities Council, delivers high-energy pulses that precisely drive compression waves in the sample. Simultaneously, the European XFEL’s X-ray laser produces pulses lasting just quadrillionths of a second, allowing investigators to capture diffraction patterns before the sample relaxes or vaporizes. Such time-resolved diffraction data reveal how carbon atoms rearrange themselves as the material transitions from solid diamond-like order into a complex liquid structure.

Importantly, this experiment was conducted at the HED-HIBEF (High Energy Density Helmholtz International Beamline for Extreme Fields) station of the European XFEL, which was specifically designed for research involving extreme states of matter. The collaboration brought together numerous international institutions, combining expertise in laser physics, high-pressure science, and advanced X-ray diagnostics to tackle one of the longstanding frontiers of materials science.

Analysis of the diffraction patterns yielded surprising insights into the fundamental nature of liquid carbon. Contrary to earlier assumptions, the atomic structure of the liquid phase closely resembles that of solid diamond, exhibiting a coordination number of four — each carbon atom maintaining four nearest neighbors. This structural motif is reminiscent of water’s hydrogen bonding network, imparting liquid carbon with unique properties and complexity. The study thus confirms theoretical models and simulations that predicted such a water-like local order but lacked experimental validation until now.

Another critical achievement of the study was the precise determination of carbon’s melting point under extreme pressure. Prior theoretical approaches provided widely varying predictions, but the experimental data now substantially narrow this uncertainty. Accurately knowing the melting curve of carbon is essential not only for fundamental condensed matter physics but also for modeling planetary interiors and processes such as nuclear fusion, where carbon’s behavior under extreme conditions is pivotal.

The fleeting timescales of the experiments also highlight a new paradigm in high-pressure and high-temperature research. The entire laser-X-ray probing sequence lasts only nanoseconds, capturing snapshots of phase transitions as they happen. By systematically varying the delay between the compression pulses and X-ray shots, researchers generated a sequence of diffraction images that effectively stitch together the atomic rearrangements in real time. Through this approach, they constructed a dynamic “movie” of carbon’s transition from solid to liquid, a feat impossible through traditional static experiments.

Professor Dominik Kraus, who leads the Carbon Working Group within the collaboration, emphasized the exceptional nature of the findings: “For the first time, we can see direct experimental evidence of liquid carbon’s structure. It is a complex liquid with properties comparable to water, challenging our understanding of phase transitions at extreme states.” This breakthrough not only validates longstanding theoretical frameworks but also sets the stage for future explorations into exotic forms of matter.

Dr. Ulf Zastrau, head of the High Energy Density group at the European XFEL, underscored the significance of the research tools employed: “The combination of ultrafast, high-energy lasers with X-ray diffraction capabilities gives us a versatile toolkit to dissect matter under previously inaccessible conditions in extraordinary detail.” The capability to rapidly characterize material states under extreme pressure and temperature is poised to revolutionize multiple fields, from planetary science and astrophysics to advanced materials engineering.

Looking forward, the researchers anticipate that improvements in automation and data processing will dramatically accelerate such experiments. Currently, the acquisition and interpretation of data can take several hours, but enhanced computational frameworks may reduce this to a matter of seconds, enabling real-time experimentation and decision-making. This development will expand opportunities for probing a wider range of materials and phenomena, democratizing access to extreme matter research.

The success of this initial DIPOLE-XFEL experiment represents a landmark moment for science and technology. It underscores how state-of-the-art instrumentation and international collaboration can conquer experimental frontiers once deemed impossible. With liquid carbon finally accessible to direct study, new insights are expected to cascade into planetary geology, energy research, and condensed matter physics, fundamentally enriching our understanding of matter’s behavior at nature’s most extreme edges.

Subject of Research:
Not applicable

Article Title:
The structure of liquid carbon elucidated by in situ X-ray diffraction

News Publication Date:
21-May-2025

Web References:
http://dx.doi.org/10.1038/s41586-025-09035-6

References:
D. Kraus, et al.: The structure of liquid carbon elucidated by in situ X-ray diffraction, Nature, 2025

Image Credits:
HZDR / M. Künsting

Keywords

Liquid carbon, high-pressure physics, X-ray diffraction, European XFEL, laser compression, DIPOLE100-X, phase transition, diamond structure, ultrafast measurement, extreme matter, melting point, planetary interiors