Higher-order light modes track water molecules leaving a surface in real time

In a study published in Frontiers of Optoelectronics, researchers describe how a rolled-up silicon nitride microtube cavity, engineered to support a family of higher-order whispering-gallery resonances, can watch water molecules peel away from an oxide surface under a focused laser beam — and do so at the sub-monolayer level, in real time, and with spatial resolution along the tube’s axis.

The work, led by Mingquan Deng, Xing Ma and Jiawei Wang together with colleagues at several institutions, addresses a long-standing gap in optical sensing. Conventional label-free optical sensors excel at measuring bulk quantities such as refractive index or humidity, but when only a fraction of a single molecular layer interacts with the sensor, macroscopic refractivity is no longer a meaningful quantity. Characterising water on solid surfaces at that scale has typically required scanning tunnelling microscopy or low-energy electron diffraction; an optical demonstration had remained elusive.

The team’s device is built with a nanomembrane origami technique, in which a strained multilayer film — silicon nitride sandwiched with alumina layers — is patterned, etched and then self-rolls into a microtube roughly 10 to 20 micrometres in diameter. The cavity wall is only about 250 nanometres thick, thin enough that the evanescent optical field extends strongly outside the tube, right where molecules adsorb and desorb. A U-shaped design lifts the tube centre away from the substrate, reducing leakage losses and yielding a quality factor of about 2000.

The crucial innovation lies in the tube’s geometry. The centre of the flat nanomembrane is tailored into a parabolic-like lobe before rolling, which acts as an optical quasi-potential well along the tube axis. Instead of a single resonance confined to a plane, each whispering-gallery mode splits into a set of higher-order axial modes, numbered l = 0 up to l = 4, each with a distinct spatial distribution of electric-field intensity along the tube. Because defects in the amorphous silicon nitride emit broadband photoluminescence between 600 and 800 nanometres when pumped, simply shining a 532-nanometre laser on the tube lights up the entire mode family at once, and spatially resolved measurements confirmed that the measured mode profiles agree with numerical solutions of the quasi-Schrödinger equation for the lobe-shaped potential well.

The sensing experiment itself is elegantly simple. The microtube sits in a sealed chamber of roughly 0.7 cubic centimetres, where a saturated potassium sulphate solution holds the humidity at about 60 percent at room temperature. The group of axial modes around 677.5 to 683.5 nanometres, all sharing azimuthal mode number M = 101, is tracked continuously. At low pump intensity the fundamental mode’s resonant wavelength stays stable in thermal equilibrium. Then, at a chosen moment, the laser power is increased tenfold, and the resonances begin to blueshift steadily — the optical signature of water molecules leaving the surface as the focused beam, concentrated into a spot of about one square micrometre, locally heats the tube.

How much water actually leaves? Applying perturbation theory, which links the fractional resonance shift linearly to the fraction of desorbed molecules, the team calculated that the observed saturation blueshift of about −0.31 nanometres corresponds to the removal of a water layer roughly 0.1 nanometres thick — about 33 percent of a monolayer. Given a spectral resolution of around 20 picometres, the estimated noise-equivalent detection limit reaches down to a change of roughly 2 percent of a single water monolayer. The modest total desorption, the authors note, reflects the hydrophobic character of the alumina surface.

The kinetics of the desorption tell a two-stage story. In the first 30 seconds after the power increase — Stage I — the blueshift is rapid and intense, and it is well fitted by a pseudo-second-order kinetic model with a time-scale factor k2 of −1.30 per second and a maximum desorption of −0.29 relative to the initial adsorbed amount. Beyond 30 seconds the shift deviates from that single-model prediction, so the researchers introduced a combined model mixing pseudo-first-order and pseudo-second-order dynamics. Fitting Stage II, from 30 to 190 seconds, showed the pseudo-second-order mechanism still dominates, with a weighting factor of 2.46 against 0.73 for the pseudo-first-order contribution, until the blueshift saturates at around 190 seconds.

The real payoff comes from the higher-order modes. Because each axial mode samples a different stretch of the tube, the mode set acts like an array of spatially distributed probes. With the laser focused at the lobe centre, the lower-order modes, whose fields concentrate there, blueshift the most and desorb fastest, with the fundamental mode showing the largest shift and the shortest time-scale factor. When the laser spot was displaced to 4.4 micrometres from the centre, the picture inverted: the fundamental mode’s shift shrank to −0.19 nanometres, while the fourth-order axial mode, whose field maximum sits near that position, took over with the largest shift of −0.25 nanometres. By modelling the laser-induced desorption as a Gaussian perturbation along the axis and calculating how the whole optical quasi-potential well deforms, the team reproduced the measured mode-dependent shifts and time constants, demonstrating that the mode family genuinely resolves where the perturbation sits.

What distinguishes this from the group’s earlier work is the trigger. Previous studies of water adsorption and release on oxide surfaces with similar WGM sensors relied on external thermal control, such as a liquid helium cryostat, sweeping the whole environment’s temperature. Here the desorption is caused by the same tightly focused beam that excites the photoluminescence, making the measurement genuinely local and in situ. The temporal resolution of the spectral acquisition system is about one millisecond, although the desorption experiments used 5-second integration steps to maximise signal-to-noise.

The authors envisage the technique extending beyond water. Because the platform can be built from diverse oxide and semiconductor nanomembranes and probed with nothing more than a focused laser, they suggest applications in photodegradation, biological transformation and protein–protein interaction studies — any process in which light drives local molecular change at a surface.

Subject of Research: Laser-induced local water desorption from oxide surfaces tracked in real time using higher-order axial whispering-gallery modes in a nanomembrane microtube cavity

Article Title: Unveiling local molecular desorption dynamics using higher-order optical resonances

Article References: Deng, M., Dou, X., Wang, X., Yin, Y., Guan, X., Ma, L., Ma, X., & Wang, J. (2025). Unveiling local molecular desorption dynamics using higher-order optical resonances. Frontiers of Optoelectronics, 18(1), Article 15. https://doi.org/10.1007/s12200-025-00159-1

Image Credits: AI Generated

DOI: 10.1007/s12200-025-00159-1

Keywords: whispering-gallery modes, microtube cavity, molecular desorption, water molecules, nanomembrane origami, silicon nitride, perturbation theory, sub-monolayer sensing, photoluminescence, oxide surface, desorption kinetics, spatially resolved sensing