Electrically-Driven Colloidal Quantum Dots Successfully Demonstrate Light Amplification through Stimulated Emission.

Decades after making them, the Los Alamos scientists have achieved light amplification using electrically driven devices based on solution-molded semiconductor nanocrystals — tiny specs of semiconductor material made through chemical synthesis and often called colloidal quantum dots.

This event was reported by the newspaper natureopens the door to an entirely new class of electrically pumped lasers — highly flexible and processable diode lasers that can be prepared on any crystalline or amorphous substrate without the need for sophisticated vacuum-based growth techniques or a highly controlled clean-chamber environment.

said Victor Klimov, lab fellow and leader of the Quantum Dot Research Initiative.

“The new ‘synthetically labeled’ quantum dots exhibit long optical gain lifetimes, large gain coefficients, and low laser limits—characteristics that make them an ideal laser material. Methods developed to achieve electrically driven light amplification using solution-cast nanocrystals may help present a long-standing challenge of integrating Optical and electronic circuits on the same silicon chip are poised for advances in many other areas from lighting and displays to quantum information, medical diagnostics and chemical sensing.”

More than two decades of research

Research for more than two decades has pursued the realization of colloidal dot laser technology using electrical pumping, which is a prerequisite for its widespread use in practical technologies. Conventional laser diodes, which are ubiquitous in modern technologies, produce highly monochromatic and coherent light under electrical excitation. But they do have shortcomings: challenges with scalability, gaps in the range of accessible wavelengths, and most importantly, incompatibilities with silicon technologies that limit their use in microelectronics. These problems have stimulated the search for alternatives in the field of highly flexible and easily scalable materials and processing solutions.

Chemically prepared colloidal quantum dots are particularly attractive for the implementation of solution-addressable laser diodes. In addition to being compatible with inexpensive and easily scalable chemical techniques, they offer the advantages of size-tunable emission wavelength, low optical gain thresholds, and high-temperature stability of laser properties.

However, multiple challenges have hampered the development of the technology, including rapid Auger recombination of the multi-active carrier states, poor stability of the nanocrystal membranes at the high current densities required for the lasing process, and the difficulty in obtaining a net optical gain in a complex electrically driven device where a layer is fused. Electrically thin nanocrystals with different charge-conducting layers and optically lossy tend to absorb the light emitted by the nanocrystals.

Solutions to the challenges of a colloidal quantum dot laser diode

There are a number of technical challenges that need to be resolved to achieve an electrically driven colloidal dot laser. Quantum dots not only need to emit light, they also need to multiply the photons generated by stimulated emission. This effect can be converted into laser oscillations, or lasers, by integrating the quantum dots with an optical resonator that spins the emitted light through the gain medium. Solve that, and you have an electrically driven quantum dot laser.

In quantum dots, stimulated emission competes with very fast non-radiative Auger recombination, which is the primary drawback to lasing in these materials. The Los Alamos team has developed a highly effective method for suppressing non-radiative Auger decay by introducing carefully designed compositional gradients into the interior of a quantum dot.

Very high current densities are also required to realize the laser system. This current, though, can doom the device.

“A typical quantum dot LED operates at current densities of only about 1 ampere per square centimeter,” said Namyoung Ahn, a postdoctoral fellow to the Los Alamos director and principal device design expert for the project. “However, realizing lasers requires tens to hundreds of amperes per square centimeter, which usually results in device failure due to overheating. This has been a major problem hindering laser realization using electrical pumping.”

To solve the overheating problem, the team confined the electrical current to the spatial and temporal domains, ultimately reducing the amount of heat generated and simultaneously improving heat exchange with the surrounding medium. To implement these ideas, the researchers integrated a current-focused micro-hole dielectric inner layer into a device beam and used short electrical pulses (about 1 microsecond in duration) to power the LEDs.

The developed devices have been able to reach unprecedented current densities of nearly 2,000 amperes per square centimeter, which is sufficient to generate robust, broadband optical gain that spans multipoint optical transitions.

“Another challenge is to achieve an adequate balance between optical gain and optical loss in a complete LED device array that contains different charge-conducting layers that can exhibit strong light absorption,” said postdoctoral laboratory researcher Clement Levach, who coordinated the spectral component of this project. . “To address this problem, we added a stack of dielectric bilayers, forming what is called a distributed Prague inverter.”

Using a Bragg reflector as a substrate, the researchers were able to control and shape the spatial distribution of the electric field across the device to optically reduce the field strength in the charge-losing conducting layers and to enhance the field in the quantum dot. Medium gain.

With these innovations, the team demonstrated an effect that the research community has followed for decades: amplified bright spontaneous emission (ASE) achieved using electrically pumped colloidal quantum dots. In the ASE process, “seed photons” generated by spontaneous emission trigger a “photon avalanche” driven by a stimulated emission from excited quantum dots. This enhances the intensity of the emitted light, increases its direction, and enhances coherence. ASE can be thought of as a precursor to lasers, an effect that emerges when an ASE-capable medium is combined with an optical resonator.

ASE quantum dot LEDs represent great practical utility as highly directional narrow light sources for applications in consumer products (eg, displays and projectors), measurement, imaging, and scientific instrumentation. Interesting opportunities are also related to the prospective use of these structures in electronics and photonics, both conventional and quantum, where they can help achieve spectrally tunable optical amplifiers combined with different types of optical couplings and photonic structures.

What then?

For now, the team is working on achieving laser oscillations with electrically pumped quantum dots. In one approach, they have incorporated into the devices a so-called “distributed feedback network,” a periodic structure that acts as an optical resonator that spins light at the center of a quantum dot. The team also aims to broaden the spectral coverage of their device, with a focus on demonstrating electrically driven light amplification in the infrared wavelength range.

Solution-addressable infrared optical gain devices can be of great use in silicon, communications, imaging and sensing technologies.