Shortly after quantum dots (QDs) were synthesized, about 20 years ago, scientists suggested that the sharpness of the quantum-confined states in these structures would increase optical non-linearities, such as two-photon absorption (2PA). The QDs would thus be ideal for a variety of applications that required large 2PA – a prospect that excited optics researchers worldwide. Indeed, over the last decade, many groups have been searching for quantum confinement induced enhancement of 2PA.

However, the excitement gave way to disappointment recently when experiments by several groups revealed that the 2PA enhancement actually decreased with increasing quantum confinement. An unexpected result.

Things have now taken a more positive turn with the latest results from Edward Sargent, Eric Van Stryland and colleagues at the universities of Toronto and Central Florida. "We have shown that, by choosing material with favourable properties, QDs can live up to their promise of enhanced 2PA," team member Lazaro Padilha told "Our results reopen paths to new applications for semiconductor quantum dots in areas such as optical communications and 3D imaging."

Van Stryland's team was among those who previously found that quantum confinement does not necessarily enhance 2PA cross sections in most QD materials because of the electronic band structure in QDs, which leads to an effectively reduced density of states. "This understanding has now allowed us to search for materials with a specific set of properties – that is materials with a small bulk band gap, large exciton Bohr radius and symmetric band structure for which our calculations predict quantum confinement enhanced 2PA," said Padilha. "We found that lead sulphide QDs satisfy these requisites and so we have focused our attention on the non-linear properties of these structures,"

Two-photon absorption can occur in QDs when the light source employed is so bright that the probability of having two photons present is large and the energy between quantum levels matches the combined energy of the two photons. (In a typical 2PA process the energy of a single photon is too small to excite the transition.) "Just think of a stream of photons – brighter beams have a higher probability of simultaneously having two photons at any particular time and space in the beam," explained Padilha.

3D imaging
2PA-induced fluorescence can be used to image materials in 3D better than using ordinary one-photon absorption because the light can penetrate more deeply into a material. Another application is ultrafast all-optical switching devices that take advantage of the fact the 2PA is highly dependent on the light power/intensity. "In such configurations, we can think about 'optical fuses' that let through low power light but block out higher power light – something that could be effective in optical fibre communication lines."

And that's not all: the fact that the optimum photon enhancement in PbS QDs occurs around 1.55 µm (which is within the optical communications window) makes this material ideal for communication networks that exploit light. "Moreover, the large photoluminescence quantum yield shown by these quantum dots makes them good candidates for high-resolution 3D bio-labelling and also opens the door to making two-photon excited lasers," added Padilha.

The Toronto-Florida team will now concentrate on looking for, or engineering, other materials and alloys with similar or even more promising band structures – for further 2PA enhancement. It will also look for other materials that show the same enhancement in other parts of the optical spectrum, such as the visible.

The work was published in Nano Letters.