A team of researchers from Los Alamos National Laboratory (LANL) has overcome a key challenge in the field of high-intensity light-emitting devices based on colloidal quantum dot technology, resulting in dual-function devices that can act as both photoexcited lasers and high-brightness electrically driven light-emitting diodes (leds).
The development, published in the journal Advanced Materials, represents a key milestone for electrically pumped colloidal quantum-dot lasers, or laser diodes. According to the presentation, this is a new type of device whose impact will span a multitude of technologies, including integrated electronics and photonics, optical interconnection, lab-on-a-chip platforms, wearables and medical diagnostics.
Victor Klimov, scientist at LANL Chemistry Department and leader of the research team, noted, "The exploration of colloidal quantum-dot laser diodes represents part of a worldwide effort to achieve electrically pumped lasers and amplifiers based on soluble-processable materials. These devices have been sought by the scientific community for compatibility with virtually any substrate, scalability, and ease of integration with on-chip electronics and photonics, including traditional silicon-based circuits."
Like standard leds, in the team's new device, the quantum dot layer acts as an electrically-driven light emitter. However, due to extremely high current density (over 500 amps/per square centimeter), these devices display unprecedented levels of luminance - over 1 million candelas/per square meter (candelas measure the power of light emitted in a given direction). This brightness also makes them ideal for applications such as daylight displays, projectors and traffic lights.
This special quantum-dot layer also presents as an efficient waveguide amplifier with large net optical gain. The LANL team achieved the narrow-band laser with a fully functional LED-type device stack that contains all the charge transfer layers and other elements needed for electric pumping. This advance opens the door to highly anticipated demonstrations of electropumped lasers, an effect that would allow colloidal quantum dot laser technology to be fully realized.
"Tame" colloidal quantum dots
Semiconductor nanocrystals or colloidal quantum dots are an attractive material for the realization of laser devices, including laser diodes. They can be prepared at the atomic precision level using mesothermal chemistry techniques.
Furthermore, because quantum dots are small in size, comparable to the natural range of the electron wave function, they exhibit discrete atom-like electron states whose energy depends directly on the size of the particle. The results of this so-called "quantum-size" effect can be used to adjust laser lines to specific wavelengths, or to design multicolor gain media that support multiple wavelengths of laser light. Due to its low optical gain threshold and laser suppression properties, special atom-like spectra from the electronic states of quantum dots can also achieve sensitivity to device temperature changes.
Innovative design to solve the challenge of electric pumping
While most quantum-dot laser research uses short pulses of light to excite an optical gain medium, implementing lasers that drive quantum dots electrically is a more challenging task. With their new device, the LANL team will achieve a functional quantum dot laser diode.
"Electrical and optical device design is a critical point," says Namyoung Ahn, a postdoctoral fellow at LANL's laboratory Director and lead device expert on the quantum dot team. "The charge-injection architecture of the device must be able to generate and maintain the very high current densities required for laser action. Similarly, it must exhibit low optical losses so as not to inhibit the gains generated in thin quantum-dot active media."
To improve the optical gain, the team developed new nanocrystals, which they call "compact composition-graded quantum dots." To promote light amplification, the researchers also reduced the optical loss of the device. Among other things, they redesigned the charge-injection structure to remove the optically lossless metal-like material and replace it with a suitably optimized, low-absorptivity organic layer. They also designed a device cross section profile to reduce the intensity of the light field in the highly absorbed charge transport layer while enhancing the intensity of the light field in the quantum dot gain medium.
Finally, to achieve laser oscillations, the device they developed is supplemented by an optical cavity prepared with a periodic grating, which is integrated into one of the device's electrodes. The grating acts as a so-called distributed feedback resonator, allowing light to circulate in the horizontal plane of the quantum dot layer, enabling multi-channel amplification.
Source: OFweek
