New quantum dots can amplify light for tunable lasers

Materials World magazine
1 Jul 2007

Researchers at Los Alamos National Laboratory, New Mexico, USA, have engineered a new version of quantum dots (semiconductor nanocrystals) that can amplify light for cheaper and tunable lasers.

Unlike conventional quantum well diode lasers, nanocrystal-based devices could potentially emit light at a range of wavelengths by adjusting the size of the crystal’s inorganic core. These lasers could also be manufactured using less expensive colloidal organometallic chemistry rather than, for example, chemical vapour deposition. Formed in solution, the nanocrystals, which are surrounded by a layer of organic ligand molecules, are flexible substrates that can be sprayed, incorporated into optical fibres or sensors, and mixed with a range of other materials.

To date, nanocrystal-based lasing devices have been limited due to the difficulties in amplifying light.

Team leader Victor Klimov explains, ‘Because of the almost exact balance between absorption and simulated emission in nanoparticles excited with single electron-hole pairs (excitons), optical gain can only occur in nanocrystals that contain at least two excitons. The complication with this is fast optical-gain decay.’

In nanocrystals, there are only 50 picoseconds of amplified light before the two excitons interact, and one recombines and transfers its energy to another in a process called non-radiative Auger recombination. Amplification, thereafter, cannot be maintained with one exciton.

‘Because of these short optical gain lifetimes, lasing action in nanocrystals requires pumping by short and intense optical pulses from another laser, which is impractical,’ says Klimov. ‘We need to solve Auger recombination.’

The team at Los Alamos have created nanocrystals with cadmium-sulphide cores of two to four nanometers and zinc-selenide shells of up to 1.5nm in thickness. An intermediate layer of cadmium zinc selenide is used to limit defects at the interface of the two materials which cause carrier trapping. These engineered nanocrystals spatially separate the charges.

Klimov explains, ‘The lowest energy state for electrons is in the core, while the lowest energy state for holes is in the shell. Upon photoexcitation, the generated exciton “splits” across the core-shell interface, producing a charge-separated state.’

He adds, ‘The resulting imbalance between negative and positive charges produces a strong local electric field, which induces a giant transient Stark shift of the absorption spectrum. This breaks the exact balance between absorption and simulated emission and allows for optical amplification from single excitons.’

The optical gain lifetime, in principle, can be longer than 100 nanoseconds.

Further research aims to increase emission quantum yields above 50% and improve the stability of the colloidal nanocrystals, possibly by encapsulating them into sol-gel glasses. Eventually, the team hope to pump the nanocrystals electrically for use as optical amplifiers in fibre circuits, or as micro- or thin-film lasers.