The first time! Scientists have succeeded in electrically-driven laser light amplification of colloidal quantum dots

A team of scientists from Los Alamos National Laboratory (LANL) announced that they have successfully achieved light amplification with an electrically-driven device based on solution casting semiconductor nanocrystals, also known as "colloidal quantum dots". The demonstration opens the door to an entirely new class of electrically-pumped laser devices -- highly flexible, solution-processable laser diodes that can henceforth be prepared on any crystalline or amorphous substrate without the need for complex vacuum growth techniques or a highly controlled clean room environment, commented Nature.

Victor Klimov, lab researcher and leader of the quantum dot research Initiative, said LANL Lab has discovered the ability to achieve optical amplification by electrically driven colloidal quantum dots in previous decades of research into nanocrystal synthesis, their photophysical properties, and the optical and electrical design of quantum dot devices.

The new "composition-gradient" quantum dots synthesized by the lab exhibit a long optical gain lifetime, a large gain coefficient and a low laser threshold, which make them perfect laser materials.

He said the approach to electrically-driven light amplification using solution-cast nanocrystals could help solve the longstanding challenge of integrating photons and electronic circuits on the same silicon chip, and was expected to advance applications in many other areas - from lighting and display to quantum information, medical diagnostics and chemical sensing.

The result of more than 20 years of research

For more than two decades, researchers have been seeking to realize colloidal quantum-dot lasers by electric pumping, which is a prerequisite for their widespread application in practical technology.

Traditional laser diodes, which produce highly monochromatic coherent light under electrical excitation, are ubiquitous in modern technology. But they also have drawbacks: scalability challenges, gaps in the range of wavelengths they can carry, and incompatibility with silicon technology limit their use in microelectronics. These problems are driving the search for alternatives in the area of highly flexible and easily scalable solutions -- processable materials.

Chemically prepared colloidal quantum dots are particularly attractive for creating solution-processable laser diodes. In addition to compatibility with inexpensive and easily scalable chemical technologies, they have the advantages of tunable emission wavelengths, low optical gain thresholds, and high-temperature stability of laser characteristics.

However, several challenges hinder the development of the technology, including: 1) fast auger recombination of gain-active multi-carrier states; 2) The stability of nanocrystal films is poor at the high current density required by laser; 3) Net optical gain is difficult to obtain in complex electrodriven devices, where thin electroluminescent nanocrystal layers are combined with various optically lossless charge-conducting layers, which tend to absorb light emitted by nanocrystals.

Colloidal quantum dot laser scheme breakthrough

Many technical problems need to be solved to realize electrically driven colloidal quantum dot laser. Quantum dots need not only to emit light, but also to multiply the photons produced by stimulated emission. By combining a quantum dot with an optical resonator that circulates the emitted light through the gain medium, this effect can be converted into laser oscillations or lasers. If that's done, it could lead to electrically driven quantum-dot lasers.

In quantum dots, stimulated emission against very fast non-radiative auger recombination is a major obstacle to lasers in these materials. To overcome these obstacles, the Los Alamos Lab team developed a very effective way to suppress non-radiative auger decay by introducing a carefully designed compositive gradient inside the quantum dot.

Very high current densities (tens to hundreds of amps per square centimeter) are required to achieve stable laser output states. However, this current can often lead to equipment failure or even obsolescence due to overheating. This has always been the key problem that hinders the realization of electric-laser.

To solve the overheating problem, the team confined the current to the space and time domains, ultimately reducing the amount of heat generated while improving the heat exchange with the surrounding medium. To implement these ideas, the researchers added an insulating intermediate layer with a small current focusing aperture to the device stack and used short electrical pulses (about 1 microsecond in duration) to drive the LED.

The results show that the device they developed is capable of achieving unprecedented current densities, up to about 2000 amps per square centimeter, which is sufficient to generate powerful broadband optical gains between multiple quantum dot optical transitions.

"The further challenge is to achieve a favorable balance between optical gain and optical loss in a complete stack of LED devices containing various charge conducting layers that exhibit strong light absorption," said Clement Livache, a postdoctoral researcher at the lab. To solve this problem, we added a bunch of dielectric bilayer to form what's called a distributed Bragg reflector."

Using the Bragg reflector as the underlying substrate, the researchers were able to control the spatial distribution of the electric field across the device and shape it to reduce the field strength in the optically lossible charge conducting layer and enhance the field strength in the quantum dot gain medium.

Through these innovations, the team demonstrated the effect that the research community has been pursuing for decades: bright amplified spontaneous emission (ASE) with electrically pumped colloidal quantum dots. During ASE, the "seed photons" produced by spontaneous emission shoot "photon avalanche" driven by the stimulated radiation of the excited quantum dot. This increases the intensity of the emitted light, increases its directivity and enhances coherence.

ASE type quantum-dot leds have considerable utility as highly oriented narrow-band light sources in consumer products (such as displays and projectors), metrological, imaging and scientific instruments. The potential applications of these structures in electronics and photonics, conventional and quantum fields also open up numerous opportunities to help achieve spectrotunable optical amplifiers integrated with various types of optical interconnects and photonics.

Source: OFweek