17 dec 2010 - Publication in Physical Review B: Picking a needle in a haystack of light

Picking a needle in a haystack of light

Optical cavities are widely used for their ability to trap light in tiny volumes of space or to filter a single color of light from a multicolor ensemble. Of particular interest for science and applications are on-chip micropillar cavities made from semiconductors (Fig. 1, lower panel). Usually, such cavities support many light modes of different colors that reside at different – a priori unknown – positions in the cavity, like needles in a haystack.

A team of scientists from the University of Twente and the Amolf Institute in the Netherlands, and the Institute for Nanoscience and Cryogeny (CEA/INAC) in Grenoble, France has managed to identify the positions where different colors of light oscillate in the cavity. To this end, they employed a very powerful microscope with a focus smaller than the cavity. By placing the microscope lens at a predetermined position, the researchers are now able to pick a single mode of oscillation (Fig. 1, upper panel).

The results are being published in the leading American journal Physical Review B, and are anticipated to lead to tiny on-chip light sources and lasers, ultrafast optical data communication, and perhaps even to future quantum computers.

Figure 1: (upper panel) Cartoon of a cavity that confines light with different colors at different positions. By placing a magnification glass at the desired position, we can select to see a particular color from inside the cavity.
(lower panel) A micropillar cavity viewed at high magnification with an electron microscope. In the fabrication process two different semiconductor materials were alternatingly put on each other. The arrow denotes the position of the cavity the scale bar is 0.002 mm long.

Background:

Semiconductor micropillar cavities have attracted considerable attention both in fundamental physics and in applications due to their ability to confine light a tiny volume for a long period of time. The majority of experiments on such micropillars to date are conducted using internal light sources. Since such sources emit light in all directions simultaneously, it was impossible to map the spatial extension of the different modes of oscillation of the light in the cavity.

To get this important information a look from the outside of the micropillar is necessary. Surprisingly, to map out the different modes has proven to be a very hard task up to now.

The researchers have made an important step by demonstrating that they could pick out a specific color of the light and map the mode of oscillation from the outside. They achieved this by designing a new experimental method that consists of three steps.

First, high quality micropillar cavities of varius diameters were fabricated. An example of one pillar is shown in the electron microscopy image of Fig. 1. It has a diameter of only 1µm, that is a hundred time thinner than a human hair. Second, a novel white light laser beam was focused to such an extent that the focus became smaller than the micropillar diameter (CCD images in Fig. 2). This helped to overcome constraints that prohibited earlier measurements. Third, the beam was accurately positioned at a point on the surface, which allowed to pick up the desired mode. Figure 2 shows such an experimental result where the recorded mode intensity is plotted versus the frequency (y-axis) and the position where the light beam was positioned on the micropillar’s top (x-axis). The circles indicate how the modes extend along the pillar. To pick now the needle out of the haystack, one has to position the light beam in one of the circles, i.e., choosing the right position in color and space.

The proposed method is fast and convenient and it leads to a direct access and understanding of the nature of light trapped inside a cavity. It therefore opens prospects to on-chip light sources and ultrafast optical data communication.

Figure 2:
The microscope images at the top of the panel show the position of the laser beam with respect to the top facet of the micropillar, as taken with a CCD camera.
(main panel) Intensity of the different modes of oscillation of light in a 6 µm diameter micropillar (given colorscale) versus frequency (on the y-axis) and position of the beam on the pillar (x-axis). The circles indicate the different modes of light and how they extend in color and along the pillar. Symbols indicate the scientific names of the modes of the light. By choosing color and position of a particular circle, we can select a single mode of oscillation.

The team:

The research has been performed by Dr. Georgios Ctistis, Dr. Alex Hartsuiker, Edwin van der Pol, and Prof. Dr. Willem Vos from the Complex Photonic Systems (COPS) Chair, MESA+ Institute for Nanotechnology, University of Twente, The Netherlands, in close collaboration with Dr. Julien Claudon and Prof. Dr. Jean-Michel Gérard from the Institute for Nanoscience and Cryogeny (CEA/INAC), Grenoble, France. Drs. Ctistis and Hartsuiker were also affiliated with the FOM Institute Amolf in Amsterdam, the Netherlands.

Further information can be obtained from:

Prof. Dr. Willem Vos, University of Twente, Enschede, The Netherlands, email: w.l.vos@utwente.nl phone: +31-53-489 5390.

Prof. Dr. Jean-Michel Gérard, CEA/INAC Grenoble, France, email: jean-michel.gerard@cea.fr, phone: +33-(0)438-783134

Information on the world wide web:

The paper is entitled “Optical characterization and selective addressing of resonant modes of a micropillar cavity with a white light beam” and is being published in Physical Review B: http://link.aps.org/doi/10.1103/PhysRevB.82.195330.

A preprint of the paper can also be found at www.photonicbandgaps.com