Los Alamos scientists have achieved a significant breakthrough after decades of research. They have demonstrated light amplification with electrically driven devices that use solution-cast semiconductor nanocrystals, also known as colloidal quantum dots. This achievement has been reported in the journal Nature and marks the beginning of a new era for highly flexible, solution-processable laser diodes that can be prepared on any substrate without the need for complex vacuum-based growth techniques or controlled clean-room environments.
According to Victor Klimov, Laboratory Fellow and leader of the quantum dot research initiative, the team’s ability to attain light amplification with electrically driven colloidal quantum dots is the result of many years of research into nanocrystal syntheses, their photophysical properties, and the design of quantum dot devices. Their new ‘compositionally graded’ quantum dots have properties that make them an excellent lasing material, including long optical gain lifetimes, large gain coefficients, and low lasing thresholds. The techniques developed by the team for achieving electrically driven light amplification with solution-cast nanocrystals could help solve the challenge of integrating photonic and electronic circuits on the same silicon chip, as well as advancing fields such as lighting and displays, quantum information, medical diagnostics, and chemical sensing.
More than two decades of research
For more than two decades, researchers have been working on achieving colloidal quantum dot lasing with electrical pumping, which is essential for its practical application. While traditional laser diodes are commonly used in modern technologies to produce highly coherent light under electrical excitation, they have limitations, such as scalability challenges, limited range of accessible wavelengths, and incompatibility with silicon technologies, which restricts their use in microelectronics. As a result, there has been a growing search for alternatives using highly flexible and easily scalable solution-processable materials.
Colloidal quantum dots prepared chemically are particularly promising for developing solution-processable laser diodes. They are compatible with inexpensive and scalable chemical techniques and offer benefits like size-tunable emission wavelength, low-optical gain thresholds, and high-temperature stability of lasing characteristics.
Despite these advantages, the technology’s development has been hindered by various challenges. These include fast Auger recombination of gain-active multicarrier states, poor stability of nanocrystal films at high current densities needed for lasing, and the difficulty of obtaining net optical gain in a complex electrically driven device where a thin electroluminescent nanocrystal layer is combined with various optically-lossy, charge-conducting layers that tend to absorb light emitted by the nanocrystals.
Solutions for colloidal quantum dot laser diode challenges
Achieving electrically driven colloidal quantum dot lasing required overcoming several technical challenges. In addition to emitting light, quantum dots must multiply generated photons through stimulated emission, which can be turned into laser oscillations by combining the quantum dots with an optical resonator that circulates the emitted light through the gain medium.
However, stimulated emission in quantum dots is hindered by fast nonradiative Auger recombination. To suppress this phenomenon, the Los Alamos team introduced compositional gradients into the quantum dot interior. Furthermore, achieving lasing requires high current densities, which can lead to device breakdown due to overheating. Typically, quantum dot light-emitting diodes operate at current densities of around 1 ampere per square centimeter, while lasing requires tens to hundreds of amperes per square centimeter.
To resolve the overheating issue, the team confined the electric current in spatial and temporal domains, reducing the amount of generated heat and improving heat exchange with the surrounding medium. They achieved this by incorporating an insulating interlayer with a small, current-focusing aperture into a device stack and using short electrical pulses (approximately 1 microsecond in duration) to drive the LEDs.
The Los Alamos team successfully tackled a number of technical challenges to achieve electrically driven colloidal quantum dot lasing. One major hurdle was achieving the balance between optical gain and loss in a device stack with various charge conducting layers that can absorb emitted light. To overcome this issue, the team added a stack of dielectric bi-layers to form a distributed Bragg reflector, which allowed them to control the spatial distribution of the electric field across the device and reduce field intensity in optically lossy charge conductive layers while enhancing the field in the quantum-dot gain medium.
The team also introduced a highly effective approach to suppress nonradiative Auger decay, which competes with stimulated emission, by engineering compositional gradients into the quantum dot interior. They were then able to reach unprecedented current densities of up to approximately 2,000 amperes per square centimeter, which generated strong, broad-band optical gain spanning multiple quantum dot optical transitions. They achieved bright amplified spontaneous emission (ASE), which can be considered a precursor to lasing and represents a significant achievement pursued by the research community for decades.
These ASE-type quantum dot LEDs have significant practical applications, including as sources of highly directional, narrow-band light for consumer products, metrology, imaging, and scientific instrumentation. They also offer exciting opportunities for use in electronics and photonics, where they can help realize spectrally tunable on-chip optical amplifiers integrated with various types of optical interconnects and photonic structures.
The Los Alamos team’s next goal is to achieve laser oscillations using electrically pumped quantum dots. To achieve this, they plan to incorporate a “distributed feedback grating” into their devices, a periodic structure that acts as an optical resonator, circulating light in the quantum dot medium. They also aim to expand the spectral range of their devices, with a particular focus on demonstrating electrically driven light amplification in the infrared wavelength range.
Solution-processable, infrared optical-gain devices could prove highly beneficial in various fields, including silicon technologies, communications, imaging, and sensing.