Controlling light with light is a long-sought goal for computing and communication technologies. Achieving this capability would allow optical signals to be processed without converting them into electrical signals, potentially enabling faster and more energy-efficient devices. In recent years, researchers have begun exploring an unexpected platform for this purpose: soft matter.
Soft-matter photonics investigates how materials such as liquids, liquid crystals, gels, and polymers can self-organize into structures that manipulate light. Unlike conventional solid-state photonic components, which require precise nanofabrication, soft materials can spontaneously form functional optical geometries. Some soft materials also exhibit nonlinear optical behavior. For example, through the Kerr effect, their refractive index can change in response to intense light, enabling one beam to influence another and allowing ultrafast optical switching on picosecond timescales.
As reported in Advanced Photonics , an international team of researchers introduced a different approach: a nanosecond optical switch based on resonant stimulated-emission depletion (STED) in a liquid crystal cavity. Rather than relying on refractive index changes, this method manipulates the stored optical energy inside a resonant structure.
The researchers created a micrometer-sized droplet of liquid crystal doped with a fluorescent dye. The droplet acts as a resonant cavity that supports whispering gallery modes, where light circulates along its perimeter and becomes amplified. The droplet is placed in water, and its surface is brought into contact with several tapered polymer waveguides that channel light in and out of the cavity.
When an initial laser pulse is sent through a waveguide to excite the dye, the droplet begins to lase, emitting its own light. However, if a second, red-shifted light pulse is sent through the same waveguide before the lasing begins, it triggers stimulated emission and depletes the excited dye molecules. Instead of producing whispering gallery mode laser emission, the stored energy is transferred to amplify this second pulse. In effect, the system switches which wavelength dominates the output, achieving light-by-light control without electrical input.
A key innovation lies in how the solid waveguide connects to the liquid droplet. In solid materials, the contact area between a spherical cavity and a cylindrical waveguide would be too small for efficient light transfer. However, because the droplet is liquid, it slightly changes shape when it touches the waveguides due to surface tension and interfacial forces, which creates a stable and efficient optical connection. The researchers note that this self-formed contact would be difficult to achieve with solid materials, clearly highlighting an advantage of soft matter for photonic interconnections.
The approach is also remarkably energy efficient. In conventional stimulated-emission depletion applications, such as super-resolution microscopy, the depletion pulse must typically be orders of magnitude stronger than the excitation pulse because it interacts with the sample only once. In contrast, in this resonant cavity, the depletion light circulates multiple times, repeatedly interacting with the excited molecules. This multipass effect greatly enhances efficiency, reducing the required depletion energy by more than a hundredfold compared with nonresonant conditions.
Beyond demonstrating nanosecond all-optical switching, the platform offers practical advantages over traditional photonic technologies. The spherical cavities can form through faster self-assembly processes, avoiding the numerous production steps needed for hard materials. This opens the door to biocompatible and even flexible photonic devices, where complex circuits can potentially be replicated using soft imprint lithography, produced at low temperatures, and made from less toxic materials.
The researchers position this work as a foundational step toward a new class of soft, bio-inspired optical technologies. “We presented a self-assembled soft-matter microphotonic element, a soft-matter photonic switch that uses the concept of light-by-light manipulation at very low light intensity. As such, this is a rare example of a photonic device based on self-organizing properties of soft matter that could be a building block of a futuristic, bio-inspired soft photonic platform,” states corresponding author Professor Igor Muševič (University of Ljubljana and Jožef Stefan Institute, Slovenia).
For details, see the original Gold Open Access article by V. Sharma et al., “ Light control of lasing from liquid-crystal micro-droplet light switch ," Adv. Photon . 8(2), 026009 (2026), doi: 10.1117/1.AP.8.2.026009
Advanced Photonics
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Light control of lasing from liquid-crystal micro-droplet light switch
4-Mar-2026