Defying the Limits of Light Detection

Shorubalko, Kara, and their colleagues at Empa and ETH Zurich recently published a study in ACS Photonics detailing a detector that is smaller than the wavelength of the light it measures.

Empa researchers are developing infrared (IR) detectors for applications such as smart fabrics and self-driving cars, aiming to make them more sustainable, adaptable, and cost-effective than existing technology. The size and composition of the material are critical to its functionality.

Motion sensors, self-driving vehicles, chemical analyzers, and satellites all rely on IR detectors, which traditionally use crystalline semiconductor materials. These materials are challenging to produce, often requiring extreme conditions such as high temperatures and substantial energy input.

A team led by Ivan Shorubalko from the Transport at Nanoscale Interfaces laboratory is exploring a different approach, focusing on miniature IR detectors made from colloidal quantum dots.

The properties of a material depend not only on its chemical composition, but also on its dimensions. If you produce tiny particles of a certain material, they may have different properties than larger pieces of the very same material. This is due to quantum effects, hence the name "quantum dots."

Ivan Shorubalko, Empa

Moungi Bawendi, Louis E. Brus, and Alexey Ekimov will receive the 2023 Nobel Prize in Chemistry for their work in discovering and synthesizing quantum dots. While the underlying science is complex, processing quantum dots is relatively straightforward. Colloidal quantum dots can be applied to various materials through methods such as spin coating or printing, offering a less expensive, more energy-efficient, and versatile alternative to traditional semiconductors.

From Material to Process to Application

Empa has extensive experience with quantum dots. Maksym Kovalenko's research group at the Thin Films and Photovoltaics laboratory has worked on synthesizing quantum dots from various materials for over a decade. Shorubalko and his team integrate these quantum dots to develop functional electrical components, such as IR detectors. They also collaborate with other Empa scientists to refine processing methods and explore new applications for quantum dots and the devices they enable.

For example, in 2023, Empa researchers successfully printed a quantum-dot-based IR detector onto an optical polymer fiber—a task not feasible with traditional IR detectors. To achieve this, device specialist Shorubalko and his doctoral student, Gökhan Kara, collaborated with materials expert Kovalenko, printing expert Yaroslav Romanyuk from the Thin Films and Photovoltaics laboratory, and fiber specialist René Rossi from the Biomimetic Membranes and Textiles lab. The results were published in Advanced Materials Technologies in 2023.

One prospective use for this technology is smart textiles.

“The global textile market is bigger and faster-growing than the consumer electronics market,” added Shorubalko.

Flexible IR detectors could be particularly useful in specialized textiles, such as functional clothes for firefighters or medical textiles for patient monitoring. However, Shorubalko sees immense promise in fashion.

“If detectors and other electronic components are small, inexpensive, and easy to manufacture, we can use them to functionalize our everyday clothing. Current technologies are simply not compatible with textiles,” Shorubalko stated.

Since each detector is made up of multiple quantum dots that are barely five nanometers in size, incredibly compact IR detectors can be produced. This allows the researchers to record additional IR light properties, such as phase or interference, increasing the detector’s versatility.

Unmatched Speed

Shorubalko’s current goal is to improve the speed of the detector. Fast IR detectors are essential for applications such as lidar, a light-based distance detection technology used in self-driving vehicles.

“Today, lidars use silicon-based infrared detectors, which measure IR light with a wavelength of around 905 nanometers,” added the researcher.

Although this wavelength is invisible to the human eye, it can be harmful at high intensities. This limitation means lidar lasers must emit weak light, reducing the system's range. While detectors for longer, safer wavelengths exist, they are prohibitively expensive for widespread use. A quantum dot-based rapid detector could address this challenge, enabling lidar systems that are powerful, safe, and cost-effective.

As for when quantum dot-based IR detectors might become widely available, Shorubalko noted, “Quantum dot IR detectors are already available on the market. I've never seen a technology make the leap from the lab to the market so quickly.”

Despite this progress, the researchers continue to refine the technology, aiming to make it faster, more affordable, adaptable, and sustainable.