Nanomanipulation Using Silicon Photonic
Crystal Resonators
Sudeep Mandal,
†
Xavier Serey,
†
and David Erickson*
,‡
†
School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853 and
‡
Sibley School of
Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York 14853
ABSTRACT Optical tweezers
1
have enabled a number of microscale processes such as single cell handling
2
, flow-cytometry,
3
directed
assembly,
4,5
and optical chromatography.
6,7
To extend this functionality to the nanoscale, a number of near-field approaches have
been developed that yield much higher optical forces by confining light to subwavelength volumes.
8-10
At present, these techniques
are limited in both the complexity and precision with which handling can be performed. Here, we present a new class of nanoscale
optical trap exploiting optical resonance in one-dimensional silicon photonic crystals. The trapping of 48 nm and 62 nm dielectric
nanoparticles is demonstrated along with the ability to transport, trap, andmanipulate larger nanoparticles by simultaneously exploiting
the propagating nature of the light in a coupling waveguide
11
and its stationary nature within the resonator. Field amplification within
the resonator is shown to produce a trap several orders of magnitude stronger than conventional tweezers and an order of magnitude
stiffer than other near-field techniques. Our approach lays the groundwork for a new class of optical trapping platforms that could
eventually enable complex all-optical single molecule manipulation and directed assembly of nanoscale material.
KEYWORDS Optical trapping, photonic crystal, resonator, nanomanipulation
S
ince Arthur Ashkin’s pioneering work
12
on laser-
induced optical trapping and the manipulation of
micrometer-sized dielectric microspheres, optical twee-
zers have developed into an invaluable tool for a variety of
applications such as flow cytometry,
2,3
single-molecule
studies,
13,14
and optical chromatography.
6,7
The interest in
optical tweezers lies in their ability to precisely and nonin-
vasively manipulate particles and to decouple their motion
from that of the ambient background (for example a flow
within a microfluidic environment). Recently demonstrated
indirect optical methods
15
have enabled a similar level of
control with significantly lower optical power requirements.
Researchers have also demonstrated easy fabrication of well-
calibrated optical traps in an integrated, microfluidic system
by incorporating Fresnel zone plates.
16
However, in all these
methods diffraction limits how tightly light can be focused
which in turn limits the ultimate strength of the optical trap
and by extension the size of the matter which can be
manipulated.
Recently, researchers have demonstrated the ability to
surpass the limits imposed by free-space diffraction by tailoring
the optical and structural properties of a medium.
10,17,18
For
example, Grigorenko et al.
8
utilized the strongly enhanced and
localized optical near-fields of closely spaced metallic nano-
structures. Similarly Yang et al.
9
were able to demonstrate
optical trapping and transport of dielectric nanoparticles by
exploiting the strong field confinement within slot
waveguides.
19
While the strength of optical traps can be
enhanced by the strong confinement of the optical field, it can
also be improved by exploiting the field amplification within
an optical resonator. Recently, Arnold et al.
20
demonstrated the
trapping and transport of polystyrene nanoparticles as small
as 280 nm in diameter in a circular orbit around whispering
gallery mode (WGM) resonators possessing Q-factors as high
as 10
6
.
Here, we present a new class of resonant optical traps
that are capable of generating extremely strong optical field
gradients in three dimensions while simultaneously enhanc-
ing the trap stiffness due to the amplification of the optical
field within the resonator and enabling advanced particle
handling functionalities. As illustrated in Figure 1a, our
optical trap consists of a one-dimensional silicon photonic
crystal resonator that is evanescently coupled to a single
mode waveguide. The standing wave nature of the resonant
optical field within the resonator enables a true static point
trap with strong field confinement in all three dimensions.
An SEM image of a typical resonator is shown in supple-
mentary Figure S1 and the fabrication procedure is de-
scribed in the Supporting Information.
When light at the resonant wavelength is coupled into the
bus-waveguide, a stationary interference pattern is formed
within the photonic crystal resonator resulting in a tight
confinement of the optical field in an extremely small
volume as illustrated in Figure 1b. These strong field gradi-
ents coupled with the resonant amplification of the optical
field within the resonator enables the stable trapping of
particles ranging in size from 50 to 500 nm. Figure 2 (also
see Movie 1in Supporting Information) shows the trapping
and release of a 62 nm polystyrene nanoparticle (refractive
index n) 1.59). As can be seen in the supplementarymovie,
we were also able to trap polystyrene nanoparticles that
*To whom correspondence should be addressed, E-mail: de54@cornell.edu.
Received for review: 09/4/2009
Published on Web: 12/02/2009
pubs.acs.org/NanoLett
? 2010 American Chemical Society 99 DOI: 10.1021/nl9029225 | Nano Lett. 2010, 10, 99-104

were 48 nm in diameter. We note that both of these targets
are well below the size limit of what could be trapped using
previous approaches.
19
A tunable infrared laser was used to
couple TE polarized light at the resonant wavelength of
1548.15 nm into the input end of the waveguide using a
lensed fiber. The output power at the waveguide exit was
measured to be 1.7 mW. In the experiment a microfluidic
flow convects the particles along the channel and toward the
resonator. If a particle passes within close proximity of the
resonator surface and the resonant optical field lobes, it
experiences a tweezing force due to the strong local field
gradient resulting in the particle getting trapped at the
resonator surface. The trapped particle is subsequently
released by turning the laser power off (as is shown for the
62 nm case above). Trapped particles can also be released
either by detuning the input wavelength away from reso-
nance or by switching the polarization of light from TE to
TM.
One interesting aspect of our optical trap design is that
the guided optical mode within the waveguide possesses a
forward momentum which enables the simultaneous trap-
ping and propulsion of nanoparticles along its surface.
11,21,22
In contrast, at resonance, the field within the optical resona-
tor consists of a tightly confined standing wave with no
propagation component. By tailoring the microfluidic flow
and exploiting this contrasting nature of the optical field
within the waveguide and the one-dimensional resonator,
we demonstrate a novel technique for performing particle
manipulations.
Figure 3 (see also supplementary movie 2) illustrates a
series of time-lapse images demonstrating the trapping and
manipulation of 500 nm polystyrene microspheres. In the
top panel of Figure 3 the flow in the microfluidic channel is
from left to right. A 500 nm polystyrene microsphere is
trapped and transported along the waveguide by the eva-
nescent field of the guided optical mode. The input light is
initially tuned to the resonant wavelength. As a result, when
the particle moves up along the waveguide and approaches
the resonator, it experiences a lateral tweezing force to-
ward the resonator center. Due to the field amplification
within the resonator and the stronger field gradients in the
photonic crystal structure, the lateral tweezing force expe-
rienced by the particle is much stronger than the trapping
force exerted by the waveguide. This results in the particle
hopping from the waveguide to the resonator center. Once
trapped, the particle is held stationary on the resonator. To
release the particle back onto the waveguide, the input
wavelength is tuned away from resonance. This releases the
particle from the resonator trap, and it is convected with the
fluid flow toward the waveguide. Since the waveguide is not
FIGURE 1. Photonic crystal resonator for enhanced optical trapping. (a) 3D schematic of the one-dimensional photonic crystal resonator
optical trapping architecture (b) 3D FEM simulation illustrating the strong field confinement and amplification within the one-dimensional
resonator cavity. The black arrows indicate the direction and magnitude of the local optical forces.
FIGURE 2. Trapping of nanoparticles on photonic crystal resonator. Movie showing the capture and subsequent release of 62 and 48 nm
diameter polystyrene nanoparticles is included in the Supporting Information (supplementary movie 1). In this figure the waveguide is excited
at the resonant wavelength while 62 nm particles flow through a microfluidic channel running over the resonator and the waveguide. The
sequence of images was captured by a Hamamatsu CCD camera with contrast and brightness adjustments applied to the entire image. The
yellow circle tracks the 62 nm particle which is trapped (indicated by orange circles) on the resonator. Turning the laser power off releases
the particle from the trap.
? 2010 American Chemical Society
100
DOI: 10.1021/nl9029225 | Nano Lett. 2010, 10, 99-104