(Nanowerk Spotlight) Lasers power modern life, from fiber optic networks that drive internet communications to precision tools that enable complex surgery. Most commercial lasers emit light at fixed wavelengths – specific colors that cannot be adjusted once manufactured. This limitation constrains their use in emerging applications like advanced medical imaging and next-generation optical computing, which require lasers that can dynamically shift their color output.
Quantum dots offer a potential solution. These engineered semiconductor particles, each just nanometers across, possess a remarkable property: their size determines the exact color of light they emit. By adjusting particle dimensions during manufacturing, scientists can create quantum dots that produce any desired wavelength of light with exceptional efficiency.
Converting this efficient light emission into a practical laser presents substantial challenges. Laser operation requires intense light buildup between mirrors, generating heat that rapidly damages quantum dot materials. Traditional solid-state designs, which embed quantum dots in rigid films, cannot effectively dissipate this heat. Even with sophisticated cooling systems, these lasers typically operate only in brief pulses rather than producing the continuous output needed for real-world applications.
Researchers at Zhejiang University have developed a solution: liquid-state vertical-cavity surface-emitting lasers (VCSELs) that use flowing quantum dot solutions in a microfluidic system to maintain stable operation. Their system continuously flows a solution containing quantum dots through microscopic channels between specialized mirrors. As particles heat up and degrade, they’re automatically replaced with fresh ones from a reservoir. This approach significantly improves heat dissipation, maintaining stable laser operation at power levels that would otherwise degrade conventional designs.
The research team engineered quantum dots that produce red, green, or blue laser light by precisely controlling the particles’ composition and structure. When excited by an external light source, these particles emit colored light that reflects between two mirrors to build up the intense, coherent beam characteristic of laser operation.
The breakthrough centers on the cooling system’s design, which is particularly crucial for vertical-cavity surface-emitting lasers (VCSELs). These devices rely on highly reflective mirrors to confine light within a microcavity, making heat dissipation a key challenge. The quantum dot solution flows through channels thinner than a human hair, efficiently carrying away heat and maintaining stable VCSEL operation. Computer simulations demonstrated this approach maintains stable temperatures even under intense excitation that would normally degrade the quantum dots. The study found that stable lasing was achieved at high repetition rates (up to 1 MHz), though true continuous-wave operation remains an ongoing goal.
To further enhance heat dissipation, the researchers integrated the VCSEL with a microfluidic channel, allowing continuous liquid circulation to regulate temperature and maintain stable lasing. The figure below illustrates how this system works.
Integration of a VCSEL with a microfluidic cooling system. The quantum dot solution circulates through narrow channels, dissipating heat and stabilizing laser operation at high excitation levels. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Replacing traditional glass mirrors with metallic ones further improved heat dissipation. This modification limits temperature increases to just 25 degrees Celsius above ambient conditions, even at high power levels. The liquid format also enables real-time adjustments to laser properties by changing the quantum dot solution composition.
The system works with different types of quantum dots, including CdSe/CdZnSe/CdZnS and InP/ZnSe/ZnS varieties, with the latter offering cadmium-free options better suited for biomedical applications. The researchers developed a theoretical framework explaining how quantum dot concentration, cavity design, and thermal effects influence laser performance. They found that lasing requires a minimum optical density (O.D.) of ~0.2, corresponding to a quantum dot volume fraction of ~0.7%. This understanding helped optimize the current system and provides guidelines for future improvements.
The new cooling approach maintained stable operation at up to 1 million pulses per second in liquid-state VCSELs—a key milestone for high-speed pulsed operation. This suggests that, with further optimization, continuous-wave lasing could become feasible. This performance demonstrates progress toward practical applications in displays, biological imaging, and chemical sensing.
The technology addresses a major barrier to electrically powered quantum dot lasers. Previous attempts to create such devices failed primarily due to heat buildup. While the microfluidic cooling system does not yet enable direct electrical pumping, it provides a potential solution to the persistent thermal challenge limiting such devices.
The researchers now aim to improve power handling and efficiency while exploring ways to miniaturize the system for integrated photonic devices. The ability to dynamically control laser color while maintaining stable operation could enable new applications in telecommunications, computing, and sensing that current laser technologies cannot support.
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