A miniature laser-like device for surface plasmons

October 17, 2017 by Oliver Morsch, ETH Zurich
Electron microscope image of the spaser. In between the two micrometre-sized silver blocks, a layer of quantum dots (red) provides amplification for the surface plasmons. Credit: ETH Zurich / David Norris

Researchers at ETH Zurich have developed a miniature device capable of producing laser-like beams of a particular kind of electromagnetic wave called a surface plasmon. Surface plasmons can be focused much more tightly than light waves, making them useful for applications such as sensing.

When light is confined between two partially reflecting mirrors and amplified by some material in between them, the resulting beam can be extremely bright and of a single colour. This is the working principle of the laser, a tool used in all areas of modern life from the DVD player to the operating theatre.

Researchers at ETH Zurich led by David Norris, professor at the Optical Materials Engineering Laboratory, and Prof. Dimos Poulikakos, professor at the Laboratory of Thermodynamics in Emerging Technologies, have developed a miniature device that applies the same principle to so-called surface plasmons. The created by such a surface plasmon laser, or "spaser", can be focused much more tightly than light, which makes them interesting both for fundamental research and for technical applications such as sensing.

A tiny cavity for surface plasmons

In contrast to ordinary , which propagate freely inside a transparent material, surface plasmons consist of electromagnetic waves that are tightly bound to ripples in the distribution of electrons on the surface of a metal. The optical effects of surface plasmons can be admired, for instance, in the stained glass windows of medieval cathedrals. There, plasmons generated on metallic nanoparticles inside the glass by the incoming light give the windows their peculiar and vibrant colours.

The ETH team has now created the equivalent of a laser for surface plasmons by engineering extremely smooth silver surfaces, on top of which two slightly curved silver blocks, a few micrometres in length and just half a micrometre in height, are placed. These micro-blocks act as the equivalent of the mirrors in a laser. Between the blocks surface plasmons can bounce back and forth many times. Finally, the amplification necessary to obtain a spaser beam is provided by that are placed inside the cavity. Quantum dots are tiny semiconductor particles that behave similarly to single atoms (they are sometimes called "artificial atoms") and can be produced to amplify electromagnetic waves at a desired frequency.

The researchers injected the quantum dots into the spaser cavity by dissolving them in a liquid which was then printed with nanometer precision onto the silver surface through a tiny nozzle, using a technique developed in Poulikakos's lab. Once the cavity and quantum dots were in place, surface plasmons could be injected into the spaser by shining laser light onto the quantum dots.

Further amplification possible

"In our work we have tried to integrate the basic elements of a spaser in a single small device", explains Jian Cui, a senior postdoctoral researcher in Norris's group and author of the study recently published in the scientific journal Science Advances. In addition to the spaser cavity and the gain material, the researchers also included an amplifier that uses quantum dots to further increase the brightness of the beam once it leaves the cavity.

The amplifier has a triangular shape, such that the plasmons are not just amplified, but also focused onto a nanometre-sized tip. There, the electromagnetic waves are concentrated in a volume that is much smaller than the smallest size to which ordinary light could be focused. This feature could be used in the future, for instance, for the highly sensitive detection of biological molecules.

Towards integrated circuits with spasers

Now that they have demonstrated that their miniature spaser works, the ETH researchers are already working on the next logical step. "Our fabrication methods are very reproducible and versatile, so we can now think about creating integrated circuits with multiple elements: spasers, amplifiers, sensing regions, and so forth", says professor Norris.

The new approach has several advantages compared to previous attempts at realizing spasers. Earlier techniques used a metallic particle as the cavity, which did not allow extraction of the beam. The procedure developed at ETH uses a planar film with integrated mirrors, which gives the researchers more freedom of choice regarding the size and geometry of the cavity, while also allowing them to study the directly.

Explore further: Plasmons in an open box create miniature laser

More information: Stephan J. P. Kress et al. A customizable class of colloidal-quantum-dot spasers and plasmonic amplifiers, Science Advances (2017). DOI: 10.1126/sciadv.1700688

Related Stories

Plasmons in an open box create miniature laser

October 13, 2017

Scientists at the National Institute of Standards and Technology (NIST) have developed the first miniature laser in which the light is guided along the floor of an open metallic trench. The laser could act as a nanoscale ...

'Sniffer plasmons' could detect explosives

August 16, 2016

Physicists from the Moscow Institute of Physics and Technology (MIPT) have found that graphene might be the ideal material for manufacturing plasmonic devices capable of detecting explosive materials, toxic chemicals, and ...

Could computers reach light speed?

May 28, 2015

Light waves trapped on a metal's surface travel nearly as fast as light through the air, and new research at Pacific Northwest National Laboratory shows these waves, called surface plasmons, travel far enough to possibly ...

The 'Spaser' heats up laser technology

January 13, 2011

Lasers have revolutionized the communications and medical industries. They focus light to zap tumors and send digital TV signals and telephone communications around the world.

Recommended for you

ATLAS experiment observes light scattering off light

March 20, 2019

Light-by-light scattering is a very rare phenomenon in which two photons interact, producing another pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of ...

How heavy elements come about in the universe

March 19, 2019

Heavy elements are produced during stellar explosion or on the surfaces of neutron stars through the capture of hydrogen nuclei (protons). This occurs at extremely high temperatures, but at relatively low energies. An international ...

Trembling aspen leaves could save future Mars rovers

March 18, 2019

Researchers at the University of Warwick have been inspired by the unique movement of trembling aspen leaves, to devise an energy harvesting mechanism that could power weather sensors in hostile environments and could even ...

Quantum sensing method measures minuscule magnetic fields

March 15, 2019

A new way of measuring atomic-scale magnetic fields with great precision, not only up and down but sideways as well, has been developed by researchers at MIT. The new tool could be useful in applications as diverse as mapping ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.