Researchers have created the first organic semiconductor laser that can be operated without the need for a separate light source

Researchers at the University of St. Andrews in Scotland have manufactured the first organic semiconductor laser to operate without the need for a separate light source - which has proven to be extremely challenging. The new all electric driven laser is more compact than previous devices and operates in the visible light region of the electromagnetic spectrum. Therefore, its developers stated that it may play a role in applications such as sensing and spectroscopy.

The working principle of a laser is that light is reflected back and forth in an optical cavity composed of a gain medium sandwiched between two mirrors. When light bounces back and forth between mirrors, the gain medium amplifies it, stimulates more light emission, and generates coherent beams with a very narrow spectral range.

In 1992, the first organic laser was introduced. However, the laser uses a separate light source to drive its gain medium, which makes its design complex and limits its application. Since then, researchers have been trying to find a way to manufacture an organic laser that only uses an electric field to drive it, but without success. Therefore, in the past 30 years, this has been a huge challenge in this field, "explained physicist Ifor Samuel, who led the new study together with his colleague Graham Turnbull from St. Andrews.

Firstly, breaking the world record
Samuel explained that there are two main strategies for designing electrically driven organic lasers. The first method is to place electrical contacts on organic laser gain media and inject charges through them. However, it is difficult to manufacture a laser in this way, as the injected charge absorbs light from the emission spectrum of the material through the so-called triplet state. The contacts themselves also absorb light. Due to the fact that lasers require gain to exceed loss, this light absorption is a huge obstacle, "Samuel said.

In the new study detailed in the journal Nature, researchers solved this problem in a second way: by keeping charges, triplets, and contacts at a distance from the laser gain medium in space. However, achieving this is not an easy task, as it means they need to manufacture a pulse blue organic light emitting diode with world record light output intensity to drive the gain medium. Then, they need to find a way to couple all the light from OLEDs into a laser made of a layer of semiconductor polymer that emits green light.
In order to manufacture this type of device, we initially manufactured OLEDs and laser cavities separately, and then transferred OLEDs onto a substrate with a thickness of only a few micrometers, onto the surface of the laser waveguide, "he said. The careful integration of these two parts is crucial for the gain medium to obtain strong electroluminescence generated inside OLEDs.

To complete the design, the team used diffraction gratings in thin film lasers to provide distributed feedback of stimulated emission on the thin film plane, while diffracting the output laser beam from the surface.

Slow technological acceleration
Organic semiconductor devices are widely considered a "slow" technology because the charge mobility in organic materials is usually several orders of magnitude lower than that of silicon or III-V group crystal semiconductors. However, Turnbull believes that the team's innovation may begin to change this perception. Our work is pushing these materials into a very fast and intensive operational solution, "he told Physical World.

As for applications, researchers say that the new all electric organic semiconductor laser will be directly integrated into real-time medical devices that use light based sensing and spectroscopy to diagnose diseases or monitor symptoms. Electric drives eliminate the need for individual light sources to pump them, which should expand potential applications, "Turnbull said.

However, further work needs to be done to optimize the output power and efficiency of the new laser and expand its light output in the visible spectrum. The next major challenge in this field will be to manufacture continuous wave organic semiconductor lasers, which will require further control of the troublesome triplet population, "concluded Turnbull.

Source: Laser Network