The scientists achieved light amplification of stimulated emission by electrically driven colloidal quantum dots

Decades in the making, Los Alamos scientists have achieved optical amplification - the tiny specifications of semiconductor material created through chemical synthesis, often called colloidal quantum dots - with electrically-driven devices based on solution-cast semiconductor nanocrystals. This demonstration, reported in the scientific journal Nature, opens the door to an entirely new class of electrically pumped laser devices - highly flexible, solutionable laser diodes that can be prepared on any crystalline or non-crystalline substrate without the need for complex vacuum-based growth techniques or a highly controlled clean room environment.

"The ability to achieve light amplification with electrically driven colloidal quantum dots has emerged decades before us from research into nanocrystal synthesis, their photophysical properties, and the optical and electrical design of quantum dot devices," said lab researcher Victor Klimov. "Our novel 'composition-graded' quantum dots have long optical gain lifetimes, large gain coefficients, and low laser thresholds, properties that make them perfect laser materials. The developed method for electrically driven light amplification using solution-cast nanocrystals may help solve the longstanding challenge of integrating photon and electronic circuits on the same silicon chip, and is expected to advance many other fields, from lighting and displays to quantum information, medical diagnostics and chemical sensing.

For more than two decades, research has attempted to achieve colloidal quantum-dot lasers by electrical pumping, which is a prerequisite for their widespread use in practical technologies. Traditional laser diodes are ubiquitous in modern technology, producing highly monochromatic coherent light under electrical excitation. But they also have drawbacks: scalability challenges, gaps in accessible wavelength ranges, and importantly incompatibility with silicon technology limit their use in microelectronics. These problems have prompted a search for alternatives in the area of highly flexible and easily scalable solutions for processable materials.

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

However, multiple challenges hinder the development of the technology, including rapid auger recombination of gain-active multi-carrier states, poor stability of nanocrystal films at the high current densities required by lasers, and the difficulty of obtaining net optical gain in complex electrodriven devices, where thin electroluminescent nanocrystalline layers are combined with charge conducting layers of various optical losses, They tend to absorb light from nanocrystals.

There are a number of technical challenges that need to be solved to realize electrically driven colloidal quantum-dot lasers. Quantum dots not only need to emit light, but also need to reproduce the resulting photons through stimulated emission. This effect can be converted into laser oscillations or lasers by combining quantum dots with optical resonators that will circulate the emitted light through the gain medium. Solve that problem, and you have an electrically powered quantum-dot laser.

In quantum dots, stimulated emission competes with very fast non-radiative auger recombination, which is a major obstacle to lasers in these materials. The Los Alamos team developed a very effective way to suppress non-radiative auger decay by introducing a carefully designed compositional gradient inside the quantum dot.

The laser state also requires a very high current density. However, this current can cause the device to fail.

"Typical quantum-dot leds operate at current densities of no more than about 1 amp per square centimeter," said Namyoung Ahn, Director Postdoctoral Fellow at Los Alamos and lead device design specialist on the project. "However, laser implementation requires tens to hundreds of amperes per square centimeter, which often leads to equipment failure due to overheating. This is the key problem that hinders the realization of electric pump 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 incorporated insulated interfilms with small current focusing apertures into the device stack and used short electrical pulses (about 1 microsecond duration) to drive the leds.

The developed devices are capable of achieving unprecedented current densities up to approximately 2,000 amperes per square centimeter, sufficient to generate powerful broadband optical gain in multiple quantum dot optical transitions.

"Another challenge is to achieve a favorable balance between optical gain and optical loss in a complete LED device stack containing various charge conduction layers that can exhibit strong light absorption," said Clement Livache, a postdoctoral researcher in the lab who coordinated the spectral components of the project. "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 and shape the spatial distribution of the electric field throughout the device, thereby reducing the field strength in the conductive layer of the optical loss charge and enhancing the field 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, "seed photons" produced by spontaneous emission emit "photon avalanches" driven by stimulated emission of excited quantum dots. This increases the intensity of the emitted light, increases its directivity and enhances coherence. ASE can be thought of as a precursor to laser, which is the effect that occurs when an ASE capable medium is combined with an optical resonator.

ASE quantum-dot leds as highly directional narrow-band light sources have considerable utility in consumer products (such as displays and projectors), metrological, imaging and scientific instruments. Interesting opportunities are also related to the anticipated applications of these structures in both conventional and quantum electronics and photonics, where they can help achieve spectrally tunable optical amplifiers integrated with various types of optical interconnection and photonics.

Currently, the team is working on laser oscillations of electrically pumped quantum dots. In one approach, they integrate into the device what's called a "distributed feedback grating," a periodic structure that acts as an optical resonator that circulates light in the quantum dot medium. The team also aims to expand the spectral coverage of its device, with a focus on demonstrating electrically-driven light amplification in the infrared wavelength range.

Infrared, solutionable optical gain devices may have great utility in silicon technology, communications, imaging, and sensing.

Source: Laser Net