Biomedical Engineering Reference
In-Depth Information
Fig. 4
Optical tweezer system and trapping schemes. Dual optical trap system using 1064 nm and
830 nm lasers.
M
, mirrors;
M3
, steering mirror;
BFP O
, backfocal plane objective;
DM
, dichroic
mirror;
TL
, telescope lens;
f
, focal length;
QPD
, quadrant photodiode;
L
,lens;
BFP C
, backfocal
plane condenser;
DIC
, differential interference contrast microscopy;
S
, sample;
LED
, light emitting
diode;
CCD
, charge-coupled device camera
photodiodes to monitor two probe displacements simultaneously (Fig.
4
). Passing
through only one microscope objective, the lasers are combined with a “split tele-
scope system” using three lenses in tandem. The manipulating beam is a crystal
laser, 700 mW intense, and has a wavelength of 1064 nm, to avoid unnecessary
heating of the samples (in water) [
36
]; but with enough power to trap colloidal par-
ticles with radii from hundreds of nm to about 10 micrometers. The monitoring
beam is an 830 nm He-Ne low intensity laser. The monitoring beam (830 nm) has a
fixed position, whereas the trapping beam (1064 nm) can be steered by using a ro-
tating mirror, which is expected to be mechanically linear until about 100 Hz (beam
steering inset, Fig.
4
).
12 Discussion and Limitations
One particle microrheology is implemented using this dual-wavelength optical
tweezers system by overlapping the trap (1064 nm) and monitoring (830 nm) beams
(Fig.
5
A). Two-point microrheology is to be performed by separating the beams
(Fig.
5
B). The optical trap system described here is expected to be optimum in both
one particle and two-point microrheology for biopolymer networks or on the surface
of bulk materials (e.g., living cells). Active microrheology for either one particle or
two-point microrheology is performed with the use of lock-in amplification to mon-
itor displacement responses (measured with the monitoring beam) at the specific
frequency of oscillatory mechanical deformation (implemented with the trapping
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