Design of nanoslotted photonic crystal waveguide cavities for single nanoparticle trapping and detection.
ABSTRACT We design and numerically simulate an on-chip photonic device that integrates both optical manipulation and detection functionalities for a single nanoparticle or macromolecule. A unique combination of a photonic crystal waveguide cavity and a nanoslot structure leads to a approximately 1300 times enhancement of the optical gradient trapping force compared with a conventional waveguide trapping device. Numerical simulations indicate that the designed device is capable of stably trapping a single nanoparticle inside the nanoslot cavity, and thus provides an ideal platform for single particle detection and analysis using cavity-enhanced spectroscopic technologies.
Design of nanoslotted photonic crystal waveguide cavities
for single nanoparticle trapping and detection
Shiyun Lin et al. “Design of nanoslotted photonic crystal
waveguide cavities for single nanoparticle trapping.” Lasers and
Electro-Optics, 2009 and 2009 Conference on Quantum
electronics and Laser Science Conference. CLEO/QELS 2009.
Conference on. 2009. 1-2.©2009 IEEE.
Institute of Electrical and Electronics Engineers
Final published version
Sat Jan 07 07:03:21 EST 2012
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Design of Nanoslotted Photonic Crystal Waveguide Cavities
for Single Nanoparticle Trapping
Shiyun Lin1, Juejun Hu2, Lionel Kimerling2, and Kenneth Crozier1
1-School of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts 02138, USA
2-Microphotonics Center, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Abstract: We design and numerically simulate a photonic crystal waveguide cavity with a
nanoslot structure for single nanoparticle trapping. A 135x enhancement of optical gradient
trapping force compared to plain waveguide trapping devices has been achieved.
©2009 Optical Society of America
OCIS codes: (350.4855) Optical tweezers or optical manipulation; (350.4238) Nanophotonics and photonic crystals
Optical trapping is ideally suited to manipulating living cells, virus particles, and even DNA molecules as a
contactless and nondestructive tool. To date, most research on optical micromanipulation has focused on laser beam
free space trapping . However, it has been demonstrated that the optical force arising from an evanescent field
can also be used to trap particles at the interface between two media. For example, using an integrated planar
waveguide for on-chip particle manipulation dramatically decreases the system footprint, possibly allowing high-
volume mass production using mature planar processing technology and hence very low cost [2-4]. However,
waveguide optical trapping often requires large input power due to the low evanescent field intensity, a major
roadblock towards its applications. Moreover scattering forces in the near-field also propel particles along the
waveguide, making subsequent analysis of particles (e.g. confocal fluorescence/Raman microscopy) difficult, as
their positions are not fixed. Finally, trapping particles smaller than 100 nm in diameter is difficult either using free
space optics or evanescent trapping configurations with the limitation imposed by laser spot size and power.
In this paper, we propose and numerically verify a novel near-field trapping design utilizing nanoslotted
waveguide photonic crystal (PhC) cavities. Resonant enhancement of optical fields effectively reduces the power
required for stable trapping; trapping by standing wave inside the resonator also eliminates the traveling wave
scattering forces [5,6]. Further, introduction of a nanoslot structure in the PhC cavity takes advantage of the large
electric field discontinuity at dielectric boundaries and results in significant field concentration and enhancement.
We show that such a cavity-based trapping device is capable for single nanoparticle trapping, and that the nanoslot
cavity device setting is also ideal for single nanoparticle detection, given the strong cavity-enhancement effect and
the ultra-small interrogation volume.
2. Device Design & Simulation
A waveguide PhC structure is formed by etching 1-d hole arrays in an SOI photonic wire waveguide (Si on SiO2,
refractive indices n = 3.46 and 1.45, respectively), as shown in Fig.1. A PhC cavity with tapered boundary regions is
introduced by locally varying the hole spacing using a Gaussian envelope function. Compared to a conventional PhC
cavity design with abrupt disruption of hole periodicity, such a gradual change of hole spacing minimizes radiative
loss of the cavity mode and effectively improves the cavity Q-factor from 50 to 450 (Fig. 2) . Further
improvement of cavity Q-factor can be readily achieved by increasing the number of 1-d holes in the waveguide. 3-d
FDTD simulations are performed to determine the field distribution in this structure when the nanoparticle position
is varied. The power input to the waveguide is taken as 100mW. A polystyrene particle (refractive index n = 1.59)
with a radius of 10 nm and immersed in water (n = 1.33) is used in our simulation.
Fig.2 shows that the optical gradient force is increased 4 times compared to a plain SOI waveguide of the same
size without the PhC cavity structure. The gradient force along the y axis is calculated by integrating the Maxwell
stress tensor over the particle surface. A further 32-fold enhancement of trapping force is obtained by incorporating
the taper structure and a nanoslot due to the increased resonant cavity enhancement effect and the high field
intensity confined in the nanoslot.
© 2009 OSA/CLEO/IQEC 2009
978-1-55752-869-8/09/$25.00 ©2009 IEEE
Fig.1 Schematic illustration of a nanoslot waveguide photonic crystal
Fig.2 Cavity Q and corresponding optical gradient force in a plain
waveguide, a waveguide PhC cavity w/o taper structure and a
waveguide PhC cavity with a taper.
Figure 3(a) plots the Ez field component and gradient force distribution on the y-z plane across the nanoslot. High
field intensity has been confined in the nanoslot. In Fig. 3b, the gradient forces is calculated based on the dipole
approximation, applicable for Rayleigh particles with diameters much smaller than the wavelength (d<<λ) .
Figure 3(b) shows gradient force distribution around the nanoslot. The gradient trapping force (Fy) is enhanced 135
and 32 times, compare to a plain waveguide and a waveguide PhC cavity without the slot, respectively. As shown in
Fig. 3c, such a dramatic enhancement leads to a > 10 kT trapping potential even for particles with sizes down to 10
nm (comparable to that of single protein/DNA molecules), which would allow the stable trapping of a single
macromolecule inside the nanoslot cavity despite the presence of Brownian motion. Optical damage to the trapped
molecule should remains relatively low at such a power density, according to a previous study .
Fig.3 (a)Ez field distribution in the plane z=0, (b)gradient force around the nanoslot,(b) trapping potential well along y-axis normalized to kT. The
broken line indicates the criterion of stable trapping (10kT).
We quantitatively evaluate the field perturbation and resulting resonant modification due to trapping of a single
particle/molecule inside the nanoslot cavity for in-situ single particle detection . Our results indicate that the
optical trapping cavity also serves as a highly sensitive particle detection device suitable for rapid manipulation and
analysis of molecules. The resonant peak shift due to the particle could be used to detection. We find a 0.5 nm
resonance peak shift after trapping the nanoparticle in the center of slot. We will also discuss initial experimental
results to verify our design concepts.
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