Controlling Light With Light: A Breakthrough in Soft-Matter Photonics

The quest for faster, more energy-efficient computing has long been centered on a fundamental challenge: the limitations of electricity. Although traditional electronics rely on the movement of electrons, photonic devices utilize light, offering the potential for drastically higher speeds and lower power consumption. However, creating the “brains” of such a system—the logic gates—has required materials that can manipulate light with precision. Now, a breakthrough in squishy photonic switches is opening a recent door toward flexible, low-power logic.

An international team of scientists, led by researchers at the University of Ljubljana, has developed a method to control light using only light, without the need for electricity or the alteration of a material’s physical properties. By utilizing soft materials like liquid crystals and polymers, the team has created a device that can act as a photonic switch, a critical component for any future computer that processes information using photons instead of electrons.

This innovation moves away from traditional “hard” photonics, which typically rely on silicon. Instead, it embraces “soft matter”—materials such as gels and polymers that are easier to manufacture and more environmentally friendly. The primary hurdle for soft-matter photonics has been the ability to manipulate light without using intense energy pulses that could damage the material or change its refractive index. The new approach overcomes this by employing very low light intensities to achieve a “light-controlling-light” effect.

The inspiration for this device came from an unexpected place. Igor Muševič, a professor of physics at the University of Ljubljana, conceived the idea while attending a conference in San Francisco. He was listening to a presentation by Stefan W. Hell, who was awarded the Nobel Prize in Chemistry in 2014 for the development of stimulated emission depletion (STED) microscopy. STED microscopy uses two lasers to create an incredibly compact beam for scanning objects, a process Muševič recognized as a form of manipulating light with light.

The Mechanics of a Liquid Crystal Photonic Switch

At the heart of this technology is a liquid crystal photonic switch. The device consists of a spherically shaped bead of liquid crystal, which maintains its form through its elastic material properties and molecular forces. This bead is infused with a fluorescent dye and suspended between four upright, cone-shaped polymer structures that serve as waveguides, guiding light into and out of the system.

The process begins when a laser pulse is sent through one of the polymer waveguides. This light is transferred into the liquid crystal, which excites the fluorescent dye. The photons then enter a state known as whispering gallery mode resonance. In this state, the photons are reflected back inside the spherical surface of the liquid crystal repeatedly, circulating within the cavity until they are eventually reflected back into one of the waveguides and emitted as a laser beam.

The “switch” occurs when a second laser pulse, of a different color, is fired into the waveguides. If this second pulse—referred to as the STED beam—is fired less than a nanosecond after the first pulse, it interacts with the already-excited dye molecules. This triggers a process called stimulated emission, where the dye emits photons identical to those of the second pulse while simultaneously depleting the energy from the first pulse.

Because the STED beam is amplified by this interaction and the first pulse is diminished to the point that it is not emitted, the researchers have successfully demonstrated a way to control the outcome of the first light pulse using the second. This capability is the essence of a photonic switch: the second pulse effectively “tells” the first pulse whether or not to exit the device.

Efficiency and Energy Savings

One of the most significant advantages of this liquid crystal approach is its energy efficiency. Previous soft-matter techniques often required intense light fields to change the material’s physical properties, such as the index of refraction, to redirect light. This was energy-intensive and potentially stressful for the materials.

The University of Ljubljana team reports that their new method reduces the energy required by more than a factor of 100. This efficiency is possible because the STED laser pulse circulates repeatedly within the crystal; a single photon can deplete the energy of many dye molecules from the initial pulse, maximizing the impact of a very low-intensity beam.

Why Soft Matter Outperforms Hard Materials

While silicon photonics is a well-established field, the use of soft matter provides several distinct technical and manufacturing advantages. Igor Muševič notes that the manufacturing process for these squishy switches is greatly simplified. For instance, the liquid crystal bead can be inserted into the device in less than a second, whereas creating a similar structure using hard materials is a complex and difficult process.

soft-matter devices can be produced at much lower temperatures than silicon or other hard materials, reducing the energy footprint of the manufacturing process itself. The flexibility of liquid crystals also allows for greater engineering freedom. Because researchers can create various types of cavities and geometries with soft matter, there is significantly more “engineering space” to optimize the devices for different functions.

Miha Ravnik, a theoretical physicist at the University of Ljubljana, emphasizes that this control of light by light is the fundamental requirement for creating soft-matter photonic logic gates. By controlling when light is generated and in which direction it travels, researchers can perform logical operations—the basic “AND,” “OR,” and “NOT” functions—that power all computing.

The Path Toward Photonic Computing

The long-term implications of this research point toward a future of photonic computing and photonic neural networks. Unlike current electronic neural networks, which generate significant heat and consume massive amounts of electricity, photonic systems could theoretically operate with extremely low energy losses and calculation speeds that far exceed current limits.

However, the researchers are realistic about the timeline. Miha Ravnik admits that this technology is not yet in a position to compete with current neural network implementations. The transition from a single photonic switch to a full-scale computing architecture is a massive leap that will require further development in scaling and integration.

Despite the distance to commercial application, the proof-of-concept provides a roadmap for a new class of “squishy” hardware that is flexible, efficient, and potentially more sustainable than the rigid silicon chips that currently dominate the tech industry.

Key Takeaways: Soft-Matter Photonic Switches

• Mechanism: Uses a liquid crystal bead and fluorescent dye to create a “light-controlling-light” switch via stimulated emission.
• Energy Efficiency: Reduces the energy needed for light manipulation by more than a factor of 100 compared to previous soft-matter methods.
• Manufacturing: Soft materials allow for lower production temperatures and faster assembly than silicon-based photonics.
• Inspiration: Based on the principles of STED microscopy, a technique recognized by the 2014 Nobel Prize in Chemistry.
• Future Goal: Serves as a foundational step toward energy-efficient photonic logic gates and neural networks.

As research continues at the University of Ljubljana and other international institutions, the next phase of development will likely focus on scaling these individual switches into functional logic circuits. While a photonic replacement for the modern CPU remains a distant goal, the ability to manipulate light with such low energy in flexible materials marks a pivotal shift in optical engineering.

We invite our readers to share their thoughts on the future of photonic computing in the comments below. Do you believe “squishy” hardware will eventually replace silicon? Let us know and share this article with your network.