Colloidal quantum dot technology refers to the use of tiny particles called quantum dots, which are typically between 1-10 nanometers in size, for various applications such as solar cells, LEDs, and displays. Colloidal quantum dots are made from semiconductor materials and have unique optical and electrical properties that can be tuned by adjusting their size and composition. In particular, colloidal quantum dots are known for their ability to efficiently absorb and emit light, which makes them useful for applications that require precise control over the color and intensity of light, such as in displays and lighting. They are also highly tunable, meaning that their properties can be precisely engineered to suit specific applications. Colloidal quantum dot technology is still a relatively new field, but it has the potential to revolutionize a wide range of industries by enabling the development of highly efficient and low-cost devices that are capable of producing high-quality light.
quantum dot laser diode (QDLD) as a type of semiconductor laser that uses quantum dots as the active medium for light emission. Unlike traditional semiconductor lasers, which use bulk materials as the active medium, QDLDs utilize the unique properties of quantum dots to achieve high-performance characteristics such as high efficiency, low threshold current, and high temperature stability. Quantum dots are tiny nanoparticles with a size in the range of a few nanometers. They have a well-defined electronic structure, which allows them to emit light of a specific wavelength when excited. In QDLDs, quantum dots are embedded in a semiconductor material and form the active layer of the device. The operation of a QDLD involves the injection of current into the active layer, which excites the quantum dots and causes them to emit photons. Due to the size and electronic structure of quantum dots, they have a much narrower emission spectrum than traditional bulk materials, which allows QDLDs to produce high-quality and stable laser light. QDLDs have several advantages over traditional semiconductor lasers. They have a lower threshold current density, which means that they require less power to operate. They also have a higher temperature stability, which makes them more suitable for applications in harsh environments. Additionally, QDLDs can be fabricated using standard semiconductor processing techniques, which makes them relatively easy to manufacture and integrate into existing devices. A Los Alamos National Laboratory team has overcome key challenges toward technologically viable high-intensity light emitters based on colloidal quantum dot technology, resulting in dual-function devices that operate as both an optically excited laser and a high-brightness electrically driven light-emitting diode (LED). In a new paper published in the peer reviewed journal Advanced Materials, Dr. Victor Klimov and co-workers at Los Alamos’s Chemistry division developed an electrically pumped colloidal quantum dot laser or a laser diode, a new type of devices whose impact would span numerous technologies including integrated electronics and photonics, optical interconnects, lab-on-a-chip platforms, wearable devices and medical diagnostics. These devices have been pursued for their compatibility with virtually any substrate, scalability and ease of integration with on-chip electronics and photonics including traditional silicon-based circuits. As in a standard LED, in the team’s new devices, the quantum dot layer acted as an electrically actuated light emitter. However, due to extremely high current densities of more than 500 ampere per square centimeter, the devices demonstrated unprecedented levels of brightness of more than a million candela per square meter (candela measures luminous power emitted in a given direction). This brightness makes them well-suited for applications such as daylight displays, projectors and traffic lights. The quantum dot layer also behaved as an efficient waveguide amplifier with large net optical gain. The authors achieved narrow-band lasing with a fully functional LED-type device stack containing all charge transport layers and other elements required for electrical pumping. This advance opens the door to the highly anticipated demonstration of lasing with electrical pumping, the effect which will allow for full realization of the colloidal quantum dot lasing technology.
Semiconductor nanocrystals are attractive materials for implementing lasing devices, including laser diodes. They can be prepared with atomic precision via moderate-temperature chemical techniques. Additionally, because of their small dimensions, comparable to a natural extent of electronic wave functions, quantum dots exhibit discrete atomic-like electronic states whose energies directly depend on particle size. This consequence of a so-called “quantum-size” effect can be exploited to tune the lasing line to a desired wavelength or to design a multi-color gain medium that supports lasing at multiple wavelengths. Additional advantages derived from a peculiar atomic-like spectrum of quantum dot electronic states include low optical gain thresholds and suppressed sensitivity of lasing characteristics to changes in device temperature.
Most quantum dot lasing research has employed short optical pulses for exciting an optical gain medium. The realization of lasing with electrically driven quantum dots is a much more challenging task. With their new devices, the Los Alamos research team made an important step toward this objective. To boost optical gain, the team developed new nanocrystals that they dubbed “compact compositionally graded quantum dots. These novel quantum dots feature suppressed Auger recombination due to a built-in compositional gradient and simultaneously exhibit a large gain coefficient when assembled in a close-packed solid used as an optical gain medium. This helps realize net optical gain in a complex electroluminescent structure wherein a thin, light-amplifying quantum dot layer is combined with multiple light-absorbing charge-conducting layers.
To facilitate light amplification, the authors also reduced optical losses in their devices. In particular, they re-designed the charge injection architecture by removing optically lossy metal-like materials and replacing them with properly optimized low-absorptivity organic layers. In addition, they engineered a device cross-section profile so as to reduce the optical field intensity in highly absorptive charge transport layers and simultaneously to enhance it in the quantum dot gain medium. Finally, to enable laser oscillations, the developed devices were supplemented by an optical cavity prepared as a periodic grating that was integrated into one of the device electrodes. This grating acted as a so-called distributed feedback resonator that allowed for circulating light in the lateral plane of the quantum dot layer, allowing for multi-pass amplification. The lasing effect was attained employing optical excitation. Lasing using electrical pumping was not observed because of degradation of device performance caused by excessive heat generated by a passing current. This is the final challenge that needs to be addressed to demonstrate electrically driven laser oscillations. Just a few years ago, electrically pumped colloidal quantum dot lasers were widely deemed impossible due to problems such as ultrafast Auger decay, insufficient current densities in quantum dot LEDs, and difficulties in combining electroluminescent and lasing functions in the same device. The authors findings results demonstrate practical solutions to most of these problems, suggesting that a functional quantum dot laser diode is close at hand. Indeed, the quantum dot laser diode is a promising technology for a wide range of applications such as optical communications, sensing, and medical imaging. Its unique properties and advantages make it an attractive option for engineers looking to develop high-performance and low-cost laser devices.
Ahn N, Park YS, Livache C, Du J, Gungor K, Kim J, Klimov VI. Optically Excited Lasing in a Cavity-Based, High-Current-Density Quantum Dot Electroluminescent Device. Adv Mater. 2023 Mar;35(9):e2206613