[3.19] demonstrated the optical trappingof a transparent mi-crosphere by a strongly focused laser beam.. Not only a solid laser but also an LD can be used as a light source for trapping.
Trang 1Stage velocity (mm/s)
Polystyrene Glass
0 2 4 6 8 10 12 14 16
Fig 3.37 Relationship between minimum trapping power obtained using solitary
optical fiber and stage velocity for microsheres of 10 micrometer in diameter
microspheres This is because the trappingforce is equivalent to Fg− Fs
for downward illumination, but to Fg+ Fs for upward illumination The-oretically, the minimum trappingpower ratios between the upward- and downward-directed beams are 1.8 for polystyrene sphere and 1.6 for glass sphere
2 The experimental minimum trapping powers are in fairly good agreement with the theoretical ones for axial trapping, but not in good agreement for transverse trapping This is because the trapping position for transverse trapping changes due to the large gravitational force, particularly for high-density and/or large particles
3 The minimum axial trappingpower increases as the trappingposition increases from the chamber surface This is because the spherical aberra-tion due to the refractive index difference between the immersion oil of the objective and the aqueous medium in which a microsphere is suspended
4 Brownian motion is active for microspheres less than about 1µm in diam-eter, which increases trappingpower
5 Optical fiber trappingis expected to improve both the operation and implementation
Example 3.6 Show that the force due to Brownian motion of a microsphere suspended in water is equivalent to F = 2kT /d where k is the Boltzman con-stant, T is the absolute temperature and d is the diameter of a microsphere.
of the optical trap when laser power is reduced below a certain level This
is due to the thermal energy driving the particle in the weakest direction of the optical trap, i.e., parallel to the beam axis To express the thermal effect
in force units we assume that the harmonic trap potential Kz2/2 (K is the
Trang 2optical springconstant) equals the thermal energy kT /2 (Brownian motion energy) [3.11], Therefore, K = kT /z2
At the moment of escape, z = d/2 because the maximum
trappingeffi-ciency is close to the surface of the sphere In this case, the equivalent force
of the Brownian motion is
F = Kz = kT
z2z = 2kT
d .
3.4 Applications of Optical Tweezers
Ashkin et al [3.19] demonstrated the optical trappingof a transparent mi-crosphere by a strongly focused laser beam A single-beam gradient-force optical trappingtechnique has been proved to be useful in the study of biological processes because of its noninvasive nature [3.20] Recently, op-tical tweezers have been applied in various scientific and engineering fields listed in Table 3.8 Inexpensive fiber manipulation is expected for easy implementation
Not only a solid laser but also an LD can be used as a light source for trapping The optical pressure force is very weak, nearly pN/mW, but can manipulate particles on the micrometer scale Since the gravitational force in-creases proportional to the third power of the particle radius and the Brownian effect increases inversely proportional to the radius, there exists an adequate objective size in trapping It corresponds to several micrometers, facilitating the manipulation of livingcells in its early developingstage 3-D trappingis possible for various particles ranging from 20 nm to tens of micrometers in-cludingbiological, dielectric and polymer particles which are transparent for the laser beam, as shown in Fig 3.38
Recently, materials have been wideningfor further applications For ex-ample, the 3-D trappingof metallic objects is possible due to a gradient force
of the light intensity in the Rayleigh regime where the size is much less than the wavelength, and also due to the diffractive effect of the light at the sur-face of the object with a size of several wavelengths [3.21] Gahagam et al of Wochester Polytechnic Institute demonstrated the 3-D trappingof low-index particles in the size range of 2–50µm usinga donut-shaped intensity pro-file beam [3.22] Higurashi et al of NTT trapped ringlike (hollow), low-index microobjects in a high-index liquid using upward bottom-surface radiation pressure [3.17] The ringlike microobject was made of fluorinated polyimide, with a refractive index of 1.53 and a surroundingliquid refractive index of 1.61 Followingare the actual applications of the optical tweezers classified in the field of basic research and industry
3.4.1 Basic Research
Biology
Livingcells of several micrometers in size, which are easy to trap, leads to optical tweezers were first used in biology [3.23] For example, results of the
Trang 3Table 3.8 Applications of optical tweezers
basic research 1 Physics: Measurement of optical pressure (1964)[3.1]
2 Biology: Measurement of swimming velocity of bacteria (1987)[3.23]
3 Biology: Measurement of compliance of bacterial flagella (1989)[3.24]
4 Chemistry: Microchemical conversion system (1994)[3.6]
5 Optics: Microsphere laser oscillation (1993)[3.29]
6 Biology: Kinesin stepping with 8 nm (1993)[3.25]
7 Mechanics: Measurement of particle rotation rate (1995)[3.34]
8 Mechanics: Measurement of the drag force on a bead (1995)[3.33]
9 Physics: Optically trapped gold particle near-field probe (1997)[3.31]
10 Biology: Single molecule observation (1998)[3.26]
industry 1 Space engineering: Solar sail flight [http://planetary.org]
2 Applied optics: Particle transport (1986)[3.19, 3.35]
3 Biological engineering: Living cell fusion (1991)[3.20]
4 Mechanical engineering: 3-D microfabrication (1992)[3.9]
5 Mechanical engineering: Shuttlecock type optical rotor (1994)[3.8, 1.62]
6 Applied optics: Optical fiber trapping (1995)[3.13], (1999)[3.15]
7 Mechanical engineering: Optical rotor with slopes(2003)[1.63]
8 Applied optics: Optically induced angular alignment (1999) [3.17]
9 Mechanical engineering: Gear type optical rotor (2001)[1.65]
10 Applied optics: Optical mixer (2002)[1.50], (2004)[1.66]
11 Applied chemistry: Patterning surfaces with nanoparticles (2002)[3.40, 3.41]
12 Applied optics: Microstructure formation and control (2004) [3.39]
manipulation of bacteria and the measurement of the swimmingspeed of mitochondria are shown in Fig 3.39 Furthermore, living cell fusion [3.20] by violet light exposure in the contact area of two cells trapped independently is shown in Fig 3.40
Another example is the compliance measurement of bacterial flagella The torque generated by the flagella motor of a bacterium tethered to a glass surface by a flagella filament was measured by balancing that generated by the optical pressure force The balance was realized by calibratingoptical power [3.24]
The direct observation of kinesin steppingwas performed by optical trappinginterferometry with a special and temporal sensitivity for resolving movement on the molecular scale, as shown in Fig 3.41 [3.25] Silica spheres carryingsingle molecules of the motor protein kinesin were deposited on mi-crotubules usingoptical tweezers and their motion was analyzed to determine whether kinesin moves in 8 nm steps
Trang 4Particle diameter (mm)
Semiconductor
Metal oxide
n = 1.16 - 2.2
k < 0.002 (l = 0.63 mm)
Living cell Dielectric Organic Metal
n = 0.28
k = 7.5
3.0 2.5 2.0 1.5 1.0 0.5 0
Fig 3.38 Reported materials and sizes possible for optical trapping by YAG laser
beam Various particles ranging from 20 nm to tens micrometer in size including biological, dielectric, polymer and metal particles are included
L
BS
BS
Objective
Sample
cell
CL
Bacteria
X, Y, Z stage SL
F E
X, Y, Z mount
BF BF VC
I
Fig 3.39 Example of bacterial manipulation and measurement of swimming speed
of mitocondria by optical tweezers [3.23]
Figure 3.42 shows the simultaneous measurement of individual ATPase and mechanical reactions of single one-headed myosin molecules [3.26] A single actin filament with beads attached to both ends was suspended in a solution
by YAG laser trapping The fluorescence was excited by the evanescent wave generated by the total reflection of the green laser shown in the figure The local illumination by the evanescent light greatly reduced the background luminescence
Trang 5Befor trapping
Trapping and contacting
Laser radiation
After 1 second
After 1 minute After 10 seconds
After 5 minutes
After 15 minutes
(e)
(f)
(g)
(h)
(a)
(b)
(c)
(d)
Fig 3.40 Living cell fusion by violet light exposure at contact area between two
cells trapped independently [3.20] Courtesy of S Sato, Tohoku University, Japan
Displacement and force due to actin–myosin interactions were determined
by measuringbead displacement with nanometer accuracy by a quadrant photodiode Individual ATPase reactions were monitored by an SIT camera as changes in fluorescence intensity due to association–(hydrolysis)–dissociation events of a fluorescent ATP (analoglabeled with Cy3-ATP) with the myosin head As a result, it was found that the myosin head produces several hun-dred of milliseconds after a bound nucleotide is released This suggests that myosin has hysteresis or memory state, and stores chemical energy from ATP hydrolysis [3.26]
Chemistry
Optical tweezers are used in the field of chemistry Figure 3.43 shows a mi-crochemical conversion system [3.6] for the studies of reaction kinetics that allows the selective excitation of optically manipulated particles in reaction en-vironments, which was prepared by micromachining Continuous wave YAG
Trang 6Polarization Photodetector A
Photodetector B
Volts = (A-B)/(A+B) Normalizing differential amplifier
diffraction-limited laser spots
Interferometer input-output relationship
Polarizing
beam-splitting
cube
l/4 plate
Wollaston
prism
Wollaston
prism
Polarized laser light
Volts
200
d (nm)
400
x-y piezo
stage
Lens
Lens
Specimen
and
Fig 3.41 Direct observation of kinesin stepping by optical trapping interferometry
[3.25]
Prism Halogen lamp
Objective
Green laser
Frosted glass filter
He-Ne laser
DM DM
Filter
SIT camera
APD YAG Laser PBS
DM
DM
Galvano scanner Stage
Fig 3.42 Simultaneous measurement of individual ATPase and mechanical
re-actions of single myosin molecules Reprinted from [3.26] with permission by
T Yanagida, Osaka University, Japan
Trang 7Dichromatic mirror (DM)
Objective
CCD camera
Spectroscopic data
Q-switch YAG Laser
CW YAG Laser Optical fiber
Electrochemical measurement
OH
-OH
-D
H y
z x
Fig 3.43 Microchemical conversion system for studies of chemical reaction process.
Reprinted from [3.6] with permission by H Masuhara, Osaka University, Japan
lasers (λ = 1, 064 nm) trap and close particles in contact with each other Q-switched YAG laser (λ = 350 nm) stimulates the photochemical reaction
between such particles Such a chemical reaction was studied by a picosecond time-resolved laser spectroscopy They expect that such approaches will make
it possible to study the chemical and physical properties of a single fine parti-cle as a function of its size, shape, surface morphology and to promote highly selective/efficient material conversion [3.27]
Optics
Micrometer-sized spherical particles can act as optical cavities in air or liquid [3.28] Resonant field is formed inside the surface of particles doped with laser dye such that the light propagates in a circumferential manner due to the total internal deflection at the interface [3.29] The optical characteristics of the microsphere laser oscillation, such as polarization of resonant modes and interaction between close particles, were studied Photon tunnelingfrom the lasingmicrosphere to an object was demonstrated as a marked change of
an emission spectrum dependingon microsphere-to-object distance Lasing microspheres have the advantage of high sensitivity due to the intracavity enhancement of tunnelingloss, i.e., a probe of a scanningnear-field optical microscopy (SNOM) [3.30] In addition, an optically trapped gold particle was demonstrated to be a useful near-field probe for the study of the surface characteristics beyond the diffraction limit resolution [3.31, 3.32]
Trang 8Laser scanningmanipulation was applied to measure the dragforce [3.33] acting on a glass bead moving in mineral oil between two glass plates The rotation rate of a small particle induced by optical pressure was measured by the cycle of the scattered light from optically trapped particles [3.34]
3.4.2 Industry
Particle Transport
The spatial patterningand directional transport of plural particles in wa-ter were shown to be possible by single-beam laser trapping For radioactive substance or nucleus materials, the optical trappingof metallic oxide parti-cles with various optical constants were performed to confine, position and transport without physical contact in water by Omori et al 3-D trappingwas possible for a ThO2 particle but only 2-D trappingwas observed for a UO2 particle in water usingan He–Ne laser light at 633 nm This is because a UO2 particle has a relatively large refractive index and a large extinction coefficient
in the visible region [3.35]
Figure 3.44 shows the relationship between optical constant (refractive
index n and extinction coefficient k) and the maximum trappingefficiency
Qmax for microspheres with a wavelength of 633 nm The objective’s NA is 1.3 and the microsphere diameters are 2µm (a) and 10 µm (b) In this
calcu-lation, absorption was considered, therefore decreasing Qmax with increasing the diameter It is also seen from the figure that 3-D trapping was possible for the metallic oxide havinga refractive index less than 2.4 by an He–Ne
laser light (Qmax < 0) They also demonstrated that laser trappingwas also
possible in air [3.36]
-1.5
-2
-2.5
-3
-4
-5
-4.5
1.4 1.5
0
0
0.1 0.2 0.3
0.1 0.2 0.3 0.4
0.1 0
0
0.2 0.3
0.4 0.2
0.1
1.6 1.7 1.8
n
1.9 2 2.1 2.2 2.3 2.4
-1.5 -2 -2.5 -3
-4
-5
-3.5
-4.5
1.4 1.5 1.6 1.7 1.8
n
1.9 2 2.1 2.2 2.3 2.4
Fig 3.44 Relationship between optical constant and maximum trapping efficiency
Q for microsphere with wavelength of 633 nm [3.35]
Trang 9Ar laser for assembly
(l = 514.5 nm)
YAG laser for adhesion
(l = 355 nm)
CCD camera (l = 355 nm) Filter
ND filter Filter
Fillter
Filter Iris Iris
Illuminator
Quater-wave plate Mirror Mirror
Mirror
Mirror Quater-wave plate Mirror
Mirror
Objective lens
Objective lens Lens
Lens
Expander Movement
Movement Expander
G.M.
G.M.
G.M.
G.M.
Pinhole Dichroic mirror
Light guide Axis alignment plates Half mirror
Half mirror
Dichroic mirror
Filter CCD camera
Eyeplece
Specimen plane
Fig 3.45 Micro assembly system using two laser beams, one is for trapping
(as-sembly)and the other is for ablation (adhesion)
Fabrication of 3-D Microstructures
The simultaneous manipulation and microfabrication of spatially arranged fine particles are attained usingoptical tweezers by introducingpulsed violet laser illumination [3.9] Figure 3.45 shows a microassembly system The trapping and ablation (adhesion) laser sources used are a 515-nm CW Ar+laser and a 355-nm pulsed YAG laser, respectively
Such systems mentioned earlier were limited to a small number of objects trapped in a single plane Recently, components can be designed to split a laser beam into many separate beams Holographic optical tweezers can trap objects
in different focal planes allowingmany objects to be simultaneously trapped [3.37] Crystal-like structures over a scale of tens micrometers were constructed using holographic optical tweezers [3.38] Eight 2-µm-diameter silica spheres were trapped through the multiple trapping function of the hologram at the corner of a cube [3.39] The real-time calculation of the required holographic pattern allows us to rotate the structure about an arbitrary axis
Patterning Surfaces with Nanoparticles
The 2-D arrangement of colloids on a substrate is of interest for photonics, electronics, magnetic, and sensor applications.Optical tweezers are used to
Trang 10bringparticles from a reservoir and pattern nanoparticles on the substrate Fixingwas carried out usingopposite charges [3.40] or local photopolymer-ization [3.41] around the nanoparticle assembly
Optical Rotor
Optical pressure can also rotate dissymmetrical microobjects Many types
of optical rotor have been proposed for future applications, which will be described in Chap 4
Problems
3.1 Explain the method of measuringan optical pressure force.
3.2 Explain the procedure how to simulate the trappingforce exerted on a
microsphere illuminated by a converging laser beam
3.3 Compare the axial trappingefficiencies for a microsphere predicted by a
straight ray with a parabolic ray
3.4 Calculate the transverse trappingefficiency for a microsphere when the
focus of the uniformly input laser beam is located alongthe transverse center line (perpendicular to the optical axis) of the sphere
3.5 Compare the transverse trappingefficiency for a microsphere predicted
by a straight ray with a parabolic ray
3.6 Calculate the total trappingefficiency for a microsphere when the focus
of the input laser beam is located at arbitral positions in the sphere
3.7 Consider the reasons for the transverse trappingpower discrepancy
be-tween the theoretical prediction and the experimental result Show the tra-jectory of the trapping(focus) position in the sphere