Scientists at Los Alamos have studied quantum dots, often colloidal, created through chemical synthesis. They developed electrically powered devices based on solution-cast semiconductor nanocrystals, which are microscopic particles of semiconductor material. This demonstration, published in the academic journal Nature, paves the way for a whole new category of electrically pumped lasing devices: a highly flexible, solution-processable laser that can be performed on any crystalline or non-crystalline substrate without the use of advanced vacuum-based growth methods or a tightly regulated clean room environment. diodes.
Laboratory Researcher and head of the quantum dot research initiative, Victor Klimov, said decades of research into the synthesis of nanocrystals, their photophysical properties, and the optical and electrical design of quantum dot devices led to the development of their ability to achieve light amplification with electrically powered colloidal quantum dots. “Our new, 'compositionally graded' quantum dots have long optical gain lifetimes, high gain coefficients and low lasing thresholds – these properties make them ideal lasing materials,” the researchers write.
There are longstanding problems with integrating photonics and electronic circuits on the same silicon chip. In addition to helping with this problem, methods developed for electrically driven light amplification with solution-cast nanocrystals are expected to help a wide variety of other fields, including lighting and displays, quantum information, medical diagnostics, and chemical sensing.
Twenty years of research
Colloidal quantum dot radiation with electrical pumping has been the target of research for over 20 years. This is a requirement for its widespread application in real-world technology. Modern technologies often use conventional laser diodes that produce highly consistent, monochromatic light when electrically excited. However, they have disadvantages that hinder their use in microelectronics, including scaling issues, gaps in the available wavelength range, and most importantly their incompatibility with silicon technologies. The search for alternatives in the field of highly flexible and simply scalable solution-machinable materials stems from these problems.
Chemically formed colloidal quantum dots are particularly attractive for use in solution-processable laser diodes. In addition to being affordable and compatible with easily scalable chemical processes, they have advantages such as tunable emission wavelength, low optical gain thresholds, and high temperature stability of lasing properties.
The fast Auger recombination of gain-active multicarrier states and the poor stability of nanocrystalline films at the high current densities required for lasing are two issues that hinder the development of the technology.
Problem solving techniques for colloidal quantum dot laser diodes
Electrically driven colloidal quantum dot radiation required solving a number of technological problems. In addition to emitting light, quantum dots also need the ability to induce more photon emission. By combining the quantum dots with an optical resonator that will rotate the emitted light through the gain medium, this process can be converted into laser oscillations or lasing. If you can figure this out, you can have electrically powered quantum dot radiation.
The main obstacle to radiation in quantum dots is stimulated emission, which competes with the extremely fast non-radiative Auger recombination. By carefully designing the compositional gradients inside the quantum dot, Los Alamos scientists have created a highly effective method for suppressing non-radiative Auger decay.
Extremely high current densities are also required to achieve the lasing regime. However, a device can be destroyed by this current.
"A typical quantum dot light-emitting diode operates at current densities not exceeding about 1 amp per square centimeter," said Namyoung Ahn, Los Alamos Director's Postdoctoral Fellow and the project's principal device design expert. However, tens to hundreds of amperes per square centimeter are required to produce the radiation, which would normally cause the device to malfunction due to overheating. This has been a major problem hindering the realization of electrical laser pumping.
To address the overheating problem, the researchers restricted the electric current in spatial and temporal domains. This reduced the amount of heat produced while also increasing heat exchange with a surrounding material. To put these concepts into practice, the researchers built a device stack that contains an insulating interlayer with a small, current-focusing aperture and drives the LEDs with short electrical pulses (roughly 1 microsecond long).
The devices created were able to provide robust, broadband optical gain in a few quantum dot optical passes at previously unheard of current densities of up to about 2.000 amps per square centimeter.
"Another challenge is to achieve an appropriate balance between optical gain and optical losses in a complete LED device stack containing several charge-conducting layers that can exhibit strong light absorption," said Clément Livache, Lab postdoctoral researcher who led the spectroscopic component of the project. To address this issue, we created a dielectric double layer stack called the distributed Bragg reflector.
Using a Bragg reflector as the supporting substrate, the researchers were able to manipulate the spatial distribution of an electric field throughout the device and shape it to weaken the field in optically lossy charge-conducting layers and strengthen it in the quantum dot gain environment.
The researchers used these developments to demonstrate a phenomenon that has been studied by the scientific community for many years: bright amplified spontaneous emission (ASE), made possible by electrically pumped colloidal quantum dots. The ASE technique uses "seed photons" produced by spontaneous emission to initiate a "photon avalanche" propelled by stimulated emission from excited quantum dots. As a result, the intensity of the light increases, its directionality increases, and its consistency improves. When an ASE-specific material is paired with an optical resonator, the resulting effect, known as lasing, can be considered an initiation of ASE.
The team is currently focused on using electrically pumped quantum dots to generate laser oscillations. A "distributed feedback grating", a periodic structure that acts as an optical resonator and circulates light in a quantum dot medium, is a strategy they use to assemble devices. The team is focusing on proving how to expand the spectrum coverage of their device to demonstrate electrically driven light amplification in the infrared wavelength region.
Devices with infrared optical gains that can be processed in a solution can be very useful in the fields of silicon technologies, communications, imaging and sensing.
Source: discover.lanl.gov/news – nature.com/articles/s41586-023-05855-6
Compiled by: Hasan Ongan
📩 05/05/2023 13:00