Plasmonics is a relatively new branch of photonics that exploits surface plasmon polaritons (SPPs). SPPs arise from the interaction of light with the electrons that oscillate at a metal's surface (plasmons). Metallic and metallodielectric nanostructures that exploit SPPs are alternatives to planar waveguides and photonic crystal structures for strongly guiding and manipulating light, and for making plasmonic circuits.

“As well as being able to very efficiently modulate the plasmons in InAs, we found that we could also do this using a very weak laser beam,” explains team member Martin Wagner. “This means that InAs is much more suited to ultrafast mid-infrared plasmonics than all other near- and mid-IR materials studied to date, including graphene.”

The team, led by Dmitri Basov, developed a technique that combines an ultrafast laser with sub-diffraction-limited mid-IR spectroscopy in a scanning near-field optical microscope (SNOM). “Such a unique combination allows us to image structures on the 10 nm scale and study processes that happen as fast as in just 200 femtoseconds,” Wagner tells nanotechweb.org. “This allows us to identify physical phenomena that occur on these very short timescales in the mid-IR – a part of the electromagnetic spectrum that is important for analysing materials because it is here that material excitations such as plasmons, lattice vibrations or vibrational modes of many chemical bonds are found.”

No more far-field techniques and complicated sample nanostructuring

Our method is a vital technical development, he adds. “Until now, such time-resolved mid-IR spectroscopy relied on far-field techniques which are fundamentally limited by the diffraction limit of light.”

In their experiments, the San Diego researchers exploited SNOM’s ability to couple to high momentum surface plasmons. Until recently this was only possible using far-field techniques , as mentioned, and using complicated nanostructuring of the samples themselves – something that can often completely change a sample’s properties.

Basov and colleagues studied the spectroscopic fingerprints of the semiconductor surface plasmons in a 2 µm-thick InAs film by shining a laser beam on the sample. The light photoexcites charge carriers to the conduction band and “turns on” the surface plasmons in the film. As the carrier density decays, these plasmons begin to emit more in the red part of the spectrum and the researchers analyse this shift.

Better than metal-based plasmonics

Compared to metal-based plasmonics, we are able to tune the frequency of the plasmons in semiconductors by modulating the carrier density, explains Wagner. “InAs is also better than other semiconductors in this respect because it has a small electronic band gap, which means that we can photoexcite its charge carriers from the valence to the conduction band using just a standard fibre laser operating at the telecoms wavelength of 1.56 µm. So the good thing is that our experiment does not require a complicated, high-power laser amplifier system.”

In their work, the researchers investigated how the spectroscopic signatures of semiconductor surface plasmons change in time. However, certain future applications, such as data transfer in plasmonic circuits, will require a better understanding of how they evolve in space too. “To this end, we are now planning to image propagating plasmons that have either been switched on or modified by ultrafast laser pulses,” says Wagner. “Doing this will be an important step towards making ultrafast, optically controlled plasmonic circuits.”

The team reports its work in Nano Letters.