Experimental observation of the trapped rainbow

Abstract

We report on the first experimental demonstration of the broadband "trapped rainbow" in the visible frequency range using an adiabatically tapered waveguide. Being a distinct case of the slow light phenomenon, the trapped rainbow effect could be applied to optical computing and signal processing, and to providing enhanced light-matter interactions. Comment: 2 pages, 1 figure

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Experimental Observation of the Trapped Rainbow

V.N. Smolyaninova 1), I.I. Smolyaninov, A.V. Kildishev 3), V. M. Shalaev 3)
1) Department of Physics Astronomy and Geosciences, Towson University, 8000 York Rd.,
Towson, MD 21252 USA
3) Birck Nanotechnology Centre, School of Electrical and Computer Engineering, Purdue
University, IN 47907, USA

Abstract: We report on the first experimental demonstration of the broadband “trapped rainbow” in the
visible frequency range using an adiabatically tapered waveguide. Being a distinct case of the slow light
phenomenon, the trapped rainbow effect could be applied to optical computing and signal processing, and to
providing enhanced light-matter interactions.


The concept of a “trapped rainbow” has attracted considerable recent attention. According to various theoretical
models, a specially designed metamaterial [1] or plasmonic [2,3] waveguide has the ability to slow down and stop
light of different wavelengths at different spatial locations along the waveguide, which is extremely attractive for
such applications as spectroscopy on a chip. In addition, being a special case of the slow light phenomenon [4], the
trapped rainbow effect may be used in applications such as optical signal processing and enhanced light-matter
interactions [5]. On the other hand, unlike the typical slow light schemes, the proposed theoretical trapped rainbow
arrangements are extremely broadband, and can trap a true rainbow ranging from violet to red in the visible
spectrum. Unfortunately, due to the necessity of complicated nanofabrication and the difficulty of producing
broadband metamaterials, the trapped rainbow schemes had previously remained in the theoretical domain only.

In this communication we demonstrate an experimental realization of the broadband trapped rainbow effect which
spans the 457-633 nm range of the visible spectrum. Similar to our recent demonstration of broadband cloaking [6],
the metamaterial properties necessary for device fabrication were emulated using an adiabatically tapered waveguide
geometry. A 4.5-mm diameter double convex glass lens was coated on one side with a 30-nm gold film. The lens
was placed with the gold-coated side down on top of a flat glass slide coated with a 70-nm gold film (Fig.1A). The
air gap between these surfaces has been used as an adiabatically changing waveguide. Light from a multi-
wavelength argon ion laser (operating at λ=457 nm, 465 nm, 476 nm, 488 nm and 514 nm) and 633-nm light from a
He-Ne laser were coupled to the waveguide via side illumination. This multi-line illumination produced the
appearance of white light illuminating the waveguide (Fig.1B). Light propagation through the waveguide was
imaged from the top using an optical microscope (Fig. 1C). Since the waveguide width at the entrance point is large,
the air gap waveguide starts as a multi-mode waveguide. Gradual tapering of the waveguide leads to mode number
reduction: the colored rings around the central circular dark area each represent a location where the group velocity

of the n-th waveguide mode becomes zero. These locations are defined by
λ
λ
R
Rnrn
) 2/ 1( +=
, where R is the
lens radius [6]. Finally, the light in the waveguide is completely stopped at a distance
of contact between the gold surfaces. The group velocity of the only remaining waveguide mode at this point is zero.
This is consistent with the fact that the area around the point of contact appears dark in Fig. 1C. Since the stop radius
depends on the light wavelength, different light colors stop at different locations inside the waveguide, which is
quite obvious from Fig.1C. Thus, the visible light rainbow has been stopped and “trapped.” To our knowledge, this
is the first experimental demonstration of the broadband trapped rainbow effect in the visible frequency range. The
same principle can be applied to any spectral range of interest.
2/
r =
from the point
References
[1] K. L Tsakmakidis et al., Nature 450, 397 (2007).
[2] M. I. Stockman, Phys. Rev. Letters 93, 137404 (2004).
[3] Q. Gan et al., Phys. Rev. Letters 102, 056801 (2009).
[4] L. V. Hau et al., Nature 397, 594 (1999).
[5] Y. A. Vlasov et al., Nature 438, 65 (2005).
[6] I. I. Smolyaninov et al., Phys. Rev. Letters 103, 213901 (2009).

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Figure 1: (A) Experimental geometry of the trapped rainbow experiment: a glass lens was coated on one side with a gold film. The lens was
placed with the gold-coated side down on top of a flat glass slide also coated with a gold film. The air gap between these surfaces formed an
adiabatically changing waveguide. (B) Photo of the trapped rainbow experiment: HeNe and Ar:Ion laser light is coupled into the waveguide. (C)
Optical microscope image of the trapped rainbow.