Colloidal quantum dots (QDs) are promising gain materials for lasers due to their high material gain, strong absorption of pump light, stability under strong light exposure, and suitability for solution-based processing. However, integrating QDs into laser cavities that fully utilize their potential remains challenging. Previous QD laser designs have included various cavities, but integrating them into flexible, stretchable devices is a significant hurdle. While distributed feedback lasers using QD films on polymers with surface gratings exist, these rely heavily on index contrast with the environment and pump spot size, resulting in asymmetric beam profiles. Liquid crystals (LCs), particularly chiral nematic liquid crystals (CLCs), offer advantages for creating tunable laser cavities due to their helical structure and photonic band gap (PBG). CLCs reflect circularly polarized light matching the helix handedness, acting as self-organizing liquid mirrors. Reactive LCs enable fixing LC configurations through polymerization, changing their mechanical properties. Previous work has demonstrated cholesteric liquid crystal lasers and cholesteric polymer lasers, but the combination of QDs and polymerized CLCs for a flexible, waterproof laser is novel.

The use of colloidal semiconductor nanocrystals, or quantum dots (QDs), as gain material for optically pumped lasers has been explored extensively. While early studies employed femtosecond pulsed lasers, progress has been made towards quasi-continuous-wave operation and electrically pumped systems. The diverse range of cavities employed in QD lasers, from droplets and spheres to sophisticated microresonators, reflects the design flexibility afforded by high material gain and solution-based processing. High-gain QD layers have been fabricated using dropcasting or spincoating, techniques easily adaptable to various platforms. Bendable and stretchable lasers utilizing QD films as optical gain, rather than organic dyes, are attractive for flexible optoelectronics, but limited work exists on QD lasers on bendable films. Existing approaches often involve distributed feedback lasers, which have limitations in terms of feedback reliance on index contrast and pump spot size.

CdSe/CdS core/shell QDs were synthesized, with the CdS shell enhancing optical gain properties. Transient absorption spectroscopy characterized the optical gain, revealing a maximum intrinsic gain of approximately 1500 cm⁻¹ (3600 cm⁻¹ after correcting for local field factor). Polymerized CLC mirrors were fabricated by photopolymerizing a liquid crystalline acrylate mixture with a chiral dopant. A QDCLC laser was constructed by sandwiching a 100 nm QD layer between two ~7 µm CLC mirrors, with additional PVA and glue layers. The laser was optically pumped using a 532 nm nanosecond laser, with emission spectra analyzed at various pump intensities. The free-standing film was created by separating the laser structure from its substrates. The effects of pressure, temperature, and different surrounding media (including water) on lasing characteristics were investigated.

The fabricated QDCLC laser demonstrated stable, narrow-linewidth (80 pm) lasing at 630 nm under nanosecond pumping. The laser emission was circularly polarized, exhibiting a spatially narrow far-field profile. The free-standing film maintained lasing operation even when submerged in water, with minimal changes in threshold or spectrum. The lasing wavelength exhibited predictable shifts under pressure (blue shift) and temperature (red shift with ~0.5 nm/°C). The device showed exceptional long-term stability, operating for over 32 million pulses without significant degradation. The lasing wavelength was not affected by the optical properties of the surrounding media, making it suitable for sensing applications.

The successful demonstration of a flexible, waterproof, and stable QDCLC laser addresses the challenges of integrating QDs into practical, adaptable optical devices. The stability and long operational lifetime are crucial for real-world applications. The sensitivity of the lasing wavelength to external factors like pressure and temperature opens avenues for sensing applications in diverse environments. The device's resilience to water immersion makes it particularly relevant for biological and chemical sensing. The findings underscore the synergistic potential of combining inorganic QD gain materials with the tunability and self-organizing properties of CLCs.

This research successfully demonstrated a novel flexible, waterproof, and highly stable QDCLC laser. Its long operational lifetime, sensitivity to pressure and temperature, and resilience to aqueous environments highlight its potential for various sensor applications. Future work could explore optimizing the device design for improved efficiency and exploring new applications, particularly in biosensing and flexible optoelectronics.

The current study focuses primarily on the proof-of-concept demonstration of the device. Further research is needed to fully characterize the device's performance under various operating conditions and explore its limitations in real-world scenarios. While long-term stability was demonstrated, exploring the device's ultimate lifetime and degradation mechanisms would enhance its practical applicability.