1. No category

Devil’s lens optical tweezers
Jixiong Pu1 and P. H. Jones( 2,∗ )
1 Fujian Provincial Key Laboratory of Light Propagation and Transformation, College of
Information Science and Engineering, Huaqiao University, Xiamen, Fujian361021, China
2 Department of Physics and Astronomy, University College London, Gower Street, London,
WC1E 6BT, United Kingdom
∗
philip.jones@ucl.ac.uk
Abstract:
We demonstrate an optical tweezers using a laser beam on
which is imprinted a focusing phase profile generated by a Devil’s staircase
fractal structure (Cantor set). We show that a beam shaped in this way
is capable of stably trapping a variety of micron- and submicron-sized
particles and calibrate the optical trap as a function of the control parameters
of the fractal structure, and explain the observed variation as arising from
radiation pressure exerted by unfocused parts of the beam in the region of
the optical trap. Experimental results are complemented by calculation of
the structure of the focus in the regime of high numerical aperture.
© 2015 Optical Society of America
OCIS codes: (140.7010) Laser trapping; (050.0050) Diffraction and gratings.
References and links
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6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008190 | OPTICS EXPRESS 8190
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1.
Introduction
Optical tweezers [1] are well-established as a powerful and versatile tool for the study of matter
on the micron and sub-micron scale [2, 3, 4]. In the most common implementation an optical
tweezers uses a strongly focused gaussian beam as a three-dimensional trap which can hold,
move, guide and exert a precisely calibrated force on a wide range of micro- and nanoscopic objects ranging from single biomolecules [5], through nanomaterials such as carbon nanotubes [6]
and graphene flakes [7] to cells [8]. More advanced trapping schemes can be realised by controlling the phase [9] or polarization [10] structure of the trapping beam to enable, for example,
trapping of low-index particles in the core of a phase vortex beam [11], enhanced trapping of
anisotropic particles with a predominantly longitudinally polarised beam [12], or control of the
trap geometry for spherical particles by shaping the focal volume [13].
An exact fractal [14] is defined as “an object which appears self-similar under varying degrees of magnification, in effect, possessing symmetry across scale, with each small part replicating the structure of the whole” [15]. Several structures exhibiting such self-similarity over
length scales have been considered for use as optical elements. Saavedra et al [16] analysed the
focusing properties of a fractal zone plate (FZP), finding that the axial irradiance also exhibited
self-similar properties. Such a FZP was realised experimentally by Davis et al [17] using a liquid crystal display, and further modified by Tao et al [18] with the inclusion of a spiral phase
which thereby produced a sequence of focused optical vortices. High efficiency fractal zone
plates have also been fabricated by femtosecond laser two-photon photopolymerization [19].
Fractal structures have also been used to design the phase profile of conical [20] and spherical
lenses [21] and spherical spiral phase lenses [22], which again can be realised experimentally
using a liquid crystal spatial light modulator [23]. The application of fractal structures to the
#232584 - $15.00 USD
(C) 2015 OSA
Received 19 Jan 2015; revised 9 Mar 2015; accepted 9 Mar 2015; published 23 Mar 2015
6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008190 | OPTICS EXPRESS 8191
manipulation of light has even been extended to plasmonics, where a two-dimensional “Sierpinski nanocarpet” structure was shown to produce sub-diffraction limit confinement of the
optical field [24].
In this paper we report on the experimental demonstration of a fractal structure lens, the
so-called “Devil’s Lens”, and its use in optical tweezers. While the use of a Devil’s Lens for
controlling the structure of optical traps has been suggested previously [25] it is not straightforward that a stable trap will be formed in this situation, since radiation pressure from rays
originating in different zones of the lens may act to push particles away from the intended trap
location. We first calculate the field distribution of the beam formed by such a lens in the focal
region of a high numerical aperture microscope objective, showing how the structure of the
focus is determined by the parameters of the fractal generating function. We then implement
a Devil’s Lens using a spatial light modulator in an optical tweezers set-up, and show that the
Devil’s Lens focus can indeed be used to stably trap a variety of microscopic particles. We
characterise the strength of the Devil’s Lens trap by determining the trap spring constants, and
measure these for several different values of the fractal generating function parameters.
2.
Theory
A Devil’s Lens is one whose phase profile is derived from a devil’s staircase function. The
exemplar devil’s staircase is the Cantor function FS (x) which can be generated from a Cantor
set [26] as illustrated in Fig. 1. At the first stage (S = 0) the initiator, namely a straight line
of unit length is defined. The generator of the set is then applied, namely the line is divided
into m equal parts and some of them removed (stage S = 1), illustrated in Fig. 1(a) for the
triadic Cantor set where m = 3. The process is repeated to leave 2S line segments of length 3−S
remaining, shown in Fig. 1(a) up to the stage S = 3. The gaps between segments are specified by
the start and end-points (pS,l , qS,l ), with l = 1 . . . 2S − 1. The resulting devil’s staircase Cantor
function FS (x) increases linearly in the regions where line segments remain, and is constant in
the gaps as plotted in Fig. 1(b) for S = 3, so is specified by:
( l
if pS,l ≤ x ≤ qS,l
2S
(1)
FS (x) =
x−qS,l
l
1
if qS,l ≤ x ≤ qS,l+1 .
+
−q
2S p
2S
S,l+1
S,l
The phase profile of a Devil’s Lens of order S is then given by
ΦS = 2S+1 π FS (ξ )
(2)
where ξ = (r/a)2 is the square of the radial co-ordinate in the lens normalised by the lens radius.
The equivalent Devil’s lens phase profile is shown in Fig. 1(c). With the addition of a phase
which increases with azimuthal angle, φ such that the net phase retardation is Φ = ΦS + mφ ,
where m is an integer known as the topological charge, the resulting element is known as a
Devil’s vortex lens. Examples of the calculated two-dimensional phase retardation patterns used
in the subsequent experiments are shown in Fig. 1(d) Fig. 1(e) for Devil’s lenses with S = 2 and
S = 3 respectively, and in Fig. 1(f) for a Devil’s vortex lens with S = 3 and m = 3. We note here
that the measure of the Cantor set, i.e. the length of the remaining line segments at each stage
of generation, tends to zero as S → ∞, and so in this limit the beam will not be modulated by a
Devil’s lens function.
The field distribution around the focus of a high numerical aperture objective lens is calculated following the method of Richards and Wolf [27]. Calculations are performed using a
wavelength of λ = 1.064 µ m (Nd:YAG laser wavelength) and a numerical aperture of NA =
1.48 for a beam initially polarised in the x−direction. Example results are shown in Fig. 2 for
focusing by a Devil’s Lens for increasing stage S = 0...3, and Fig. 3 for a Devil’s Vortex Lens
#232584 - $15.00 USD
(C) 2015 OSA
Received 19 Jan 2015; revised 9 Mar 2015; accepted 9 Mar 2015; published 23 Mar 2015
6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008190 | OPTICS EXPRESS 8192
(a)
(b)
1
0
0
(c)
1/3
2/3
1
2π
0
(d)
(e)
(f)
Fig. 1. (a) Illustration of the generation of the triadic Cantor set, starting from the initiator,
S = 0 to the stage S = 3; (b) the Cantor function FS (x) for S = 3; (c) the phase profile of a
“Devil’s Lens”, ΦS = 2S+1 π FS (ξ ), ξ = (r/a)2 ; (d) phase retardation pattern for a Devil’s
Lens with S = 2; (e) phase retardation pattern for a Devil’s Lens with S = 3; (f) phase
retardation pattern for a Devil’s Vortex Lens with S = 3, m = 3. In the experiments an
additional linear phase term is included to separate the phase retarded beam from the zero
order.
with m = 1 and increasing stage S = 0...3. In both cases for S > 0 multiple foci can be seen to
along the optic axis. The existence of multiple foci can be understood as arising from rays originating from different radius intervals in the Devil’s lens where the phase profile has different
curvatures (focal lengths) are brought to a focus at different distances from the Devil’s Lens.
At the stage of generation S = 0, where the beam profile is unmodified by the Devil’s lens
function the principal focus is close to z = 0, as shown in Figs. 2(a) and 3(a). At the stage S = 1
a large fraction of the beam area is modulated by the Devil’s lens which produces a focus at an
axially shifted location, seen in Figs. 2(b) and 3(b). Due to the decreasing measure of the Cantor
set at increasing stages of generation for S > 1 an increasing area of the beam is not modulated
by the Devil’s lens, producing a shift in the focus back towards z = 0, as seen in Figs. 2(b)–(c)
and 3(b)–(c). We note here, however, that the location of the principal focus is slightly shifted
from z = 0 by aberration introduced by focusing through a planar interface which is included
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Received 19 Jan 2015; revised 9 Mar 2015; accepted 9 Mar 2015; published 23 Mar 2015
6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008190 | OPTICS EXPRESS 8193
(a)
(b)
(c)
(d)
Fig. 2. Calculated intensity distributions in the z = 0 plane transverse to the optic axis (left
column), and in a longitudinal (y = 0) plane (right column) for Devil’s lenses of order
(a) S = 0; (b) S = 1; (c) S = 2; (d) S = 3. In each plot the intensity is normalised to the
maximum intensity in the plot.
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6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008190 | OPTICS EXPRESS 8194
in the calculation to reproduce the effect of the microscope cover slip.
While the extent of the principal focus appears largely unchanged by increasing Devil’s Lens
stage S, it is not necessarily obvious that a stable optical tweezers will still be formed. Stable
three-dimensional trapping requires that the backward-directed gradient force be sufficient to
overcome the forward directed scattering force (radiation pressure) to confine a particle at (or
near) the beam waist. This condition is usually met by strong (high numerical aperture) focusing
of the beam. In the case of Devil’s lens focusing however, near the principal focus, which is
formed by rays emerging from a particular annular region of the Devil’s Lens and where a trap
may be expected to occur, a particle is also subject to additional scattering forces from rays that
emerge from other annuli and focused to a different axial position. These additional forwarddirected forces may be high enough to destabilise the optical trapping potential of the principal
focus leading to an inability to form an optical tweezers with these beams. Our experiments,
therefore, set out to determine whether a stable optical tweezers could be formed using a Devil’s
Lens or Devil’s Vortex Lens.
(a)
(b)
(c)
(d)
Fig. 3. Calculated intensity distributions in the z = 0 plane transverse to the optic axis (left
column), and in a longitudinal (y = 0) plane (right column) for Devil’s vortex lenses with
topological charge m = 1 of order (a) S = 0; (b) S = 1; (c) S = 2; (d) S = 3. In each plot the
intensity is normalised to the maximum intensity in the plot.
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Received 19 Jan 2015; revised 9 Mar 2015; accepted 9 Mar 2015; published 23 Mar 2015
6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008190 | OPTICS EXPRESS 8195
3.
Experiment
Our experimental apparatus is described in detail elsewhere [13], but briefly consists of an
inverted fluorescence microscope equipped with a ×100, NA = 1.3 oil immersion objective lens.
The trapping laser beam is derived from a single mode Nd:YAG laser with maximum output
power of 3 W. The Devil’s lens is implemented by displaying the appropriate phase retardation
pattern, such as those shown in Fig. 1(d) - Fig. 1(f) on a spatial light modulator, which is then
expanded and relayed into the microscope to form the optical tweezers. A 5 mW helium-neon
laser beam is used as a probe for particle tracking in the optical tweezers and trap calibration.
This is combined with the trapping beam at a dichroic mirror just below the objective which
also separates the probe laser light that is backscattered from the trapped particle, and directs
it toward a quadrant photodiode (QPD). Scattered light from an additional Gaussian probe
laser beam is used for particle tracking rather than the trapping beam to avoid difficulties in
interpreting the relationship between the scattered light distribution and particle position when
using a phase and amplitude structured beam. A diagram of the complete optical set-up is shown
in Fig. 4.
Fig. 4. Diagram of the optical tweezers experimental apparatus. The Spatial Light Modulator (SLM) is used to display the Devil’s lens phase retardation pattern which is applied
to the Nd:YAG trapping laser beam before injecting into the inverted microscope optical
tweezers. Quantitative measurements are made by observing a trapped particle’s Brownian
motion using the light backscattered from a helium-neon probe laser beam onto a quadrant
photodiode (QPD).
We have tested the ability of the Devils lens optical tweezers to confine micron- and
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Received 19 Jan 2015; revised 9 Mar 2015; accepted 9 Mar 2015; published 23 Mar 2015
6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008190 | OPTICS EXPRESS 8196
submicron-sized particles with a number of different samples. Figure 5 shows images from
a number of experiment that clear demonstrate stable trapping for a variety of sample materials which are suspended in a water-filled sample cell. These include silica microspheres with
diameters 1.5 µ m and 1.0 µ m, and an aggregate of gold nanorods (individual nanorods are
10 nm × 45 nm) [28] shown in Figs. 5(a) – 5(c) respectively in an optical tweezers made using
a Devil’s lens with S = 2. Furthermore, stable trapping in a Devil’s vortex lens optical tweezers
with S = 2, m = 1 is shown to be possible as illustrated in Fig. 5(d) for the aggregate of gold
nanorods.
Calibration experiments on the Devil’s lens optical tweezers are performed by particle tracking using the backscattered light from the probe beam as detected on the QPD. For these experiments only 1.5 µ m diameter silica microspheres have been used. Single particles are trapped
in the optical tweezers and steered to a location distant from the fixed boundary of the microscope cover slip and any other particles to minimise the effects of hydrodynamic coupling. The
signals from the segments of the QPD are combined via analog electronics to produce tracking
signals that are proportional to the trapped particle’s displacements from equilibrium in x, y and
z, and these signals are sampled at 50 kHz by a data acquisition board. The autocorrelations of
the position tracking signals are calculated, and are found to be well-fitted by an exponential decay, as is characteristic of a particle in an overdamped harmonic potential performing Brownian
motion. We extract the spring constant of the optical potential in each direction, κi , (i = x, y, z),
from the time constant of the corresponding autocorrelation signal.
(a)
(b)
(c)
(d)
Fig. 5. Demonstration of optical trapping in a Devil’s lens optical tweezers. (a) A 1.5 µ m
diameter silica sphere, S = 2 (see also Media 1); (b) A 1.0 µ m diameter silica sphere, S = 2
(see also Media 2); (c) An aggregate of gold nanorods, S = 2 (see also Media 3); (d) An
aggregate of gold nanorods in a Devil’s vortex lens optical trap. S = 2, m = 1 (see also
Media 4). In each panel the trapped particle is circled.
Results of the measurement of spring constant are shown in Fig. 6(a) and Fig. 6(b) for the
transverse (the average of κx and κy ) and longitudinal (κz ) directions respectively. In both cases
the spring constant for a Devil’s lens (m = 0) and Devil’s vortex lens (with m = 1) were measured as a function of the Cantor set stage S from S = 0...6, and normalised by the optical power.
In both cases a similar behavior with the order of the Cantor set, S, is observed, namely that the
stiffness of the optical trap (in both directions) slightly increases with increasing S in the case
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6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008190 | OPTICS EXPRESS 8197
of the Devil’s lens (m = 0), and decreases with increasing S in the case of the Devil’s vortex
lens (m = 1). One possible interpretation of these results is the destabilising effects of radiation
pressure from the parts of the beam that arise from different focusing annuli in the Devil’s lens
and do not contribute to the principal focus, but to the secondary foci. For lower orders of the
Cantor set, as seen from Fig. 2 and Fig. 3 these foci are closer to the principal focus, thus the radiation pressure from the unfocused rays is greater, pushing the particle from the region where
the optical trap is stiffest. For the Devil’s vortex lens the additional amplitude variation of the
Laguerre-Gaussian beam profile plays a role, as the lack of on-axis rays in the dark core of the
beam mitigates the destabilising radiation pressure. We also note that the decreasing measure
of the Cantor set with increasing S decreases the area of the annuli that contribute to the Devil’s
lens focusing and also changes the radii at which the annuli are located, thus contributing an
additional apodization of the beam in the radial direction.
(a)
(b)
Fig. 6. Measured optical trap parameters for topological charges m = 0 and m = 1 with
increasing order of the Cantor function, S. (a) transverse spring constant; (b) axial spring
constant. Data are an average of between 2 and 4 repeats with a fractional uncertainty of
better than 0.001.
4.
Conclusion
We have calculated the focusing of a beam on which is imprinted a lensing function governed
by a devil’s staircase fractal structure (Cantor set) in the limit of high numerical aperture. In the
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6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008190 | OPTICS EXPRESS 8198
calculated focal volume we observe a series of subsidiary foci along the optical axis, although
we note that this in itself is not sufficient to ensure stable optical trapping. We have further
demonstrated that an optical tweezers that is formed by a beam focused by a fractal structure
(Devil’s lens) can be used to stably trap micron-sized particles. We have investigated the dependence of the optical trap strength on the parameters of the fractal lens (order of Cantor set and
topological charge), and suggest that the observed variation in trap stiffness may be due to the
action of unfocused rays emanating from different annular regions of the fractal lens. Exact calculation of optical forces using a full electromagnetic scattering (T-matrix) theory [29] would in
future permit verification of this suggestion. Having successfully demonstrated that the Devil’s
Lens Optical Tweezers can stably trap micron and sub-micron sized particles this suggests that
it may find application the simultaneous manipulation of particles at different aial positions in
the subsidiary foci, where the self-similarity of the foci [21] may prove advantageous.
Acknowledgments
This research is supported by the National Natural Science Foundations of China (NSFC) under
grant numbers 61178015 and 11304104.
#232584 - $15.00 USD
(C) 2015 OSA
Received 19 Jan 2015; revised 9 Mar 2015; accepted 9 Mar 2015; published 23 Mar 2015
6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008190 | OPTICS EXPRESS 8199