A clever glass trick fixes the decade-old photonic crystal laser problem

Yahoo

4 days ago

Photonic-crystal surface-emitting laser (PCSEL) is a promising technology that can completely revolutionize what lasers can achieve. Such a system can produce highly directional, bright beams with pinpoint precision, making it ideal for applications like LiDAR, optical communication, and sensing systems used in autonomous vehicles and defense gadgets.
However, despite their potential, PCSELs have been notoriously difficult to develop. The problem lies in their core design, which relies on special air holes that disappear soon after they are formed.
Now, engineers at University of Illinois Urbana-Champaign (UIUC) have overcome this long-standing hurdle. Instead of air holes, they have proposed embedding silicon dioxide into the photonic crystal layer. According to the scientists, this change allowed them to successfully demonstrate the first room-temperature, eye-safe, photopumped PCSEL using buried dielectric features.
Traditionally, PCSELs are built with air holes in their photonic crystal layer, which helps control how light moves inside the device. However, when scientists try to grow semiconductor material around those holes, which is a necessary step to complete the laser, the atoms shift and fill in the holes. This reshaping ruins the photonic crystal's structure, preventing the laser from working correctly.
To resolve this issue, the study authors filled the photonic crystal layer with silicon dioxide, a solid and stable dielectric material. This change gave the structure strength and prevented it from collapsing during regrowth.
However, the solution introduced a new problem. Silicon dioxide is amorphous, meaning it doesn't have a crystal structure, and growing semiconductors on top of such materials is usually extremely difficult.
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"The first time we tried to regrow the dielectric, we didn't know if it was even possible. Ideally, for semiconductor growth, you want to maintain that very pure crystal structure all the way up from the base layer, which is difficult to achieve with an amorphous material like silicon dioxide," said Erin Raftery, lead researcher and an engineering student at UIUC.
For a laser to function, the semiconductor layers above need to maintain a smooth, continuous crystal structure. The team didn't know if it would be possible, but by carefully controlling the conditions, the engineers managed to grow the semiconductor laterally around the dielectric and then merge the material over the top, in a process known as coalescence.
The result was a buried dielectric PCSEL that could emit a laser beam when excited by light, even at room temperature, and at a wavelength safe for the human eye, something no one had achieved before.
This proof-of-concept lays the foundation for a new generation of surface-emitting lasers that are more precise, stable, and scalable than current technologies. PCSELs will produce high-brightness, narrow, circular beams, making them ideal for everything ranging from communication applications to high-tech weaponry.
The use of solid dielectric material also simplifies the fabrication process and improves device durability. However, this first version still requires external light (photopumping) to power the laser, which limits its practicality in real-world applications.
To make it truly usable, the team plans to design an electrically injected version by adding electrical contacts, allowing the laser to run on standard power sources.
The study is published in the journal IEEE Photonics Journal.