wavefront correction: trapping through turbid media
Post on 29-Jul-2016
218 Views
Preview:
TRANSCRIPT
NATURE PHOTONICS | VOL 4 | JUNE 2010 | www.nature.com/naturephotonics 349
news & views
require the quantum information contained in a fl ying photon qubit to be converted into a stationary qubit. Entanglement could then be created so that the quantum information is stored and at some point transmitted by reconversion into a photon. Th e spin of an electron may provide such a stationary memory qubit. Th e next challenge
in constructing a semiconductor-based quantum repeater will therefore be to establish entanglement between the qubits. ❐
Manfred Bayer is at the Institute of Physics, Dortmund University of Technology, Otto-Hahn-Strasse 4, Dortmund, D-44227, Germany. e-mail: manfred.bayer@physik.uni-dortmund.de
References
1. Berezovsky, J., Mikkelsen, M. H., Stoltz, N. G., Coldren, L. A. & Awschalom, D. D. Science 320, 349–352 (2008).
2. Press, D., Ladd, T. D., Zhang, B. & Yamamoto, Y. Nature 456, 218–221 (2008).
3. Greilich, A. et al. Nature Phys. 5, 262–266 (2009).4. Press, D. et al. Nature Photon. 4, 367–370 (2010).5. Greilich, A. et al. Science 313, 341–345 (2006).6. Hahn, E. L. Phys. Rev. 80, 580–594 (1950).
Optical tweezers rely on strongly focused laser beams to trap and move micro- and nanoscale
particles. To guarantee stable trapping, the gradient component of the light force must be greater than the scattering component, and thus high numerical aperture microscope objectives are used to bring the beam into sharp focus. Such optics are very well corrected, and form small diff raction-limited optical traps. However, heterogeneities in the refractive index of the surrounding sample are a source of signifi cant optical aberration and so greatly diminish the ability to form a sharp focal spot, thus making it diffi cult to achieve trapping in scattering media and thick biological tissue. Now, writing in Nature Photonics, Tomáš Čižmár and colleagues from the University of St Andrews in Scotland present an answer to this problem: an in situ method that compensates for strong aberrations, allowing optical trapping in a highly turbid medium for the fi rst time1.
Unwanted optical aberrations are a common problem in the deep imaging of biological specimens and in astronomy2,3. In the latter case, atmospheric turbulence causes images of stars captured by ground-based telescopes to be severely blurred. Th e common solution is to monitor the distortion and use an adaptive optical element, typically a deformable membrane mirror, to compensate for it. Th ese sample-induced optical aberrations not only deform the incoming wavefront, but also scatter light outside the region of interest. When attempting to achieve optical tweezing in a heterogeneous medium, the incident laser light suff ers from rapid spreading and dissipates to form a random
speckle pattern with low intensity. Stable trapping thus requires wavefront sensing prior to compensation to be performed very close to the target position. So far, most of the work on aberration correction in optical tweezers either has been concentrated on addressing the spherical aberration associated with oil-immersion
objectives, or is inappropriate for in situ correction in deep, complex samples4,5.
Th e research of Čižmár et al. follows from the work of Ivo Vellekoop and colleagues6–8, which demonstrated how to focus coherent light in strongly scattering media. Th e method uses a spatial light modulator as an adaptive optical element, and is based on the decomposition of the light fi eld into orthogonal modes. Th e active area of the spatial light modulator is divided into non-overlapping sub-domains with independent control; a probe senses the intensity at a target point, which is maximized by optimizing the amplitude and phase modulation at each sub-domain for the constructive interference of all modes. In the scheme of Čižmár et al., optimization is achieved by isolating two single modes at a time on the fi rst diff raction order, thus enhancing the contrast of interference signals. Th is enables the user to unlock the full benefi t of the detector’s dynamic range, and is especially advantageous when dealing with small aberrations. Čižmár et al. achieve improved results over Vellekoop and colleagues by performing complex compensation, rather than simply correcting the phase distortions. Care must be taken, however, in cases where many modes with small amplitudes may severely reduce the light effi ciency of the optical system.
Čižmár et al. demonstrate the potential of their correction method for optical trapping experiments1 (Fig. 1). Light from a fl uorescent or scattering metallic nanoparticle (both 200 nm in diameter) is used as a probe within the sample for optimizing aberration correction. Th ey stably trap 1 μm polystyrene beads at
WAVEFRONT CORRECTION
Trapping through turbid mediaThe aberrations induced by strongly scattering and turbid samples make optical trapping in such media impossible.
Now, researchers in Scotland have overcome the problem using in situ aberration correction.
Estela Martín-Badosa
Figure 1 | Optical aberrations in the laser
wavefront are compensated for by the spatial light
modulator, thus enabling stable trapping even after
propagation through a strongly scattering medium.
Spatial light
modulator
Microscope
objective
Scattering layerTrapped particle
run.indd 349run.indd 349 10.5.14 3:38:58 PM10.5.14 3:38:58 PM
© 20 Macmillan Publishers Limited. All rights reserved10
350 NATURE PHOTONICS | VOL 4 | JUNE 2010 | www.nature.com/naturephotonics
news & views
low power (0.2 mW) and with excellent trapping effi ciencies (trap stiff ness per unit laser power) of ~2.5 pN μm–1 mW–1 in the lateral direction and 0.5 pN μm–1 mW–1 in the axial direction, close to the theoretical values. Minimizing laser exposure is important in live biology experiments to reduce optical damage to the specimen.
Interestingly, the authors were also able to use the scheme to allow trapping using a ‘non-corrected’ high aperture condenser lens, instead of the usual well-corrected microscope objective, thus broadening the design possibilities of optical tweezers.
Th e most exciting and promising result, however, is successful trapping in a highly turbid medium. In this case, light propagating through a strongly scattering 40-μm-thick layer of PDMS (polydimethylsiloxane) fi lled with polystyrene beads (5 μm in diameter) is used to trap a 1.6 μm polystyrene bead suspended in water behind the scattering layer. A supplementary laser holds an identical bead, to be used as the intensity probe, at the exact trapping position. It is important to note that the aberration correction is site-specifi c, and must be repeated and modifi ed for every target point of the sample.
Th is constraint currently hinders the application of the method to real biological specimens, as individual particles functioning as sensing probes must be injected and immobilized at controlled sites within the sample, and this may not always be practical. More conveniently, the reference for optimization could be the target to be trapped itself, which should be static and labelled with a fl uorophore compatible with near-infrared, non-harmful optical manipulation. Quantum dots would probably be required, which, owing to their improved photostability and brightness over conventional organic dyes, can help in the detection of light from the probe and its diff erentiation from the background. Furthermore, owing to the long refresh time of the spatial light modulator technology, the optimization procedure is currently too slow to follow time-varying distortions (occurring in ex vivo or in vivo experiments). It is encouraging to hear that the authors are already considering an alternative design to achieve sub-second correction.
Th ere are other less demanding (yet still interesting) applications in which in situ aberration correction should yield signifi cant progress, such as in colloidal
studies, data storage and micromachining. For optical tweezers, trapping inside living cells has already been demonstrated9 in single-cell environments with small aberrations. Th e prospect of intact deep manipulation within complex tissues is both challenging and stimulating. Many other biological techniques, including nanosurgery, tissue engineering and advanced microscopy, would also benefi t from advances in in situ aberration correction. ❐
Estela Martín-Badosa is at the Department of Applied Physics and Optics, University of Barcelona, Martí i Franquès 1, E-08028 Barcelona, Spain. e-mail: estela.martinb@ub.edu
References
1. Čižmár, T., Mazilu, M. & Dholakia, K. Nature Photon. 4, 388–394 (2010).
2. Booth, M. J. Phil. Trans. R. Soc. A 365, 2829–2843 (2007).3. Girkin, J. M., Poland, S. & Wright, A. J. Curr. Opin. Biotechnol.
20, 106–110 (2009).4. Wulf, K. D. et al. Opt. Express 14, 4170–4175 (2006).5. Jesacher, A. et al. Opt. Express 15, 5801–5808 (2007).6. Vellekoop, I. M. & Mosk, A. P. Opt. Lett. 32, 2309–2311 (2007).7. Vellekoop, I. M., van Putten, E. G., Lagendijk, A. & Mosk, A. P.
Opt. Express 16, 67–80 (2008).8. Vellekoop, I. M. & Mosk, A. P. Phys. Rev. Lett. 101, 120601 (2008).9. Ashkin, A., Schütze, K., Dziedzic, J. M., Euteneuer, U. &
Schliwa, M. Nature 348, 346–348 (1990).
Although liquids typically fl ow downwards,
the capillary eff ect — a gravity-defying
process exploited by the roots and
stems of plants — also allows them to
be drawn up through narrow tubes. The
phenomenon relies on the adhesion of
liquid to the walls of a narrow vessel,
pulling the edges of the liquid up to form
a concave meniscus. Surface tension then
causes the entire column of liquid to be
drawn upwards — a process commonly
called wicking.
Now, A. Y. Vorobyev and Chunlei Guo
from the Institute of Optics at the
University of Rochester, USA, have
demonstrated that laser pulses can be
used to treat the surface of silicon so that
it becomes superhydrophilic and exhibits
the capillary eff ect (Opt. Express 18, 6455–
6460; 2010). The pulsed beam from an
amplifi ed Ti:Sapphire femtosecond laser
was used to create a 22 mm × 11 mm
array of parallel microgrooves on a
25 mm × 25 mm × 0.65 mm single-
crystal phosphorus-doped silicon sample.
Making silicon superhydrophilic
LASER FABRICATION
100 μm
The laser generated 65 fs pulses at a
central wavelength of 800 nm and with
1.5 mJ of energy per pulse, fabricating
each microgroove at a scanning speed
of 1 mm s–1 and with a focused laser spot
diameter of 100 μm. The resulting grooves
had an average depth of 40 μm.
The researchers tested the wetting
properties of the samples by applying
1–5 μL drops of various liquids, including
distilled water, acetone and methanol, to
their silicon surfaces. Using a camera to
capture the spreading dynamics of the
liquids at a speed of fi ve frames per
second, they were able to compare the
wicking behaviours of liquids on both
laser-treated and untreated samples
of silicon. The results showed that the
laser treatment caused the silicon to
become superhydrophilic, and that
the spreading distance of the liquids,
regardless of chemical composition,
followed the classical square root of time
dependence — a characteristic of the
Washburn equation, which governs the
motion of liquids in a closed capillary.
The same results were achieved when the
silicon samples were stood vertically, with
the liquid rising upwards against gravity.
Vorobyev and Guo believe that the
capillary properties of their ultrafast-
laser-treated silicon may fi nd applications
in a number of areas, including in
microfl uidics, lab-on-a-chip technology,
biomedicine, and chemical and
biological sensors.
SONIA SHAHI
UN
IVE
RS
ITY
OF
RO
CH
ES
TE
R
run.indd 350run.indd 350 10.5.14 3:38:58 PM10.5.14 3:38:58 PM
© 20 Macmillan Publishers Limited. All rights reserved10
top related