BIOP Research Program
Programmable optical tweezers
The recent development of optical tweezers represents an extremely interesting and useful application of optics to the field of cell biology. Optical tweezers use the radiation pressure effect from a highly focused laser beam to trap and manipulate micron-sized cells and particles with pico-newton sized forces. This technique offers a hitherto unprecedented level of control and has been extensively applied in the study of the dynamics of biological systems.1-4
Follow this link to learn more about the principle of operation of optical tweezers.
Programmable tweezers
It is often desirable to simultaneously operate a number of optical tweezers to independently control the relative movement or placement of cell or molecules whilst not increasing the number of laser sources and a number of different techniques for achieving this have been suggested. We propose an alternative approach for the generation of reconfigurable arrays for optical tweezers and demonstrate the effectiveness of this technique at generating numerous, high intensity, beams suitable for optical trapping and manipulation.5,6 One of the principle aims of the programmable optical tweezer project is to minimize the number of components in a tweezer system and improve the flexibility of the system allowing multiple beam tweezer configurations as required.
We wish to apply the generalised phase contrast (GPC) technique7 in conjunction with a phase-only spatial light modulator to generate a dynamic, reconfigurable and computer controlled multiple beam tweezer system. In such a system, the number, shape and position of tweezer beams could be modified to best suit the trapping task at hand. A phase-only liquid-crystal spatial light modulator (SLM) encodes an image directly in the phase component of the collimated monochromatic wavefront of an expanded laser beam. This phase-encoded information serves as the input for a phase-contrast system, in which a phase-contrast filter (PCF) generates a high-contrast amplitude pattern that corresponds directly to the phase perturbation in the input wavefront. This amplitude pattern can then be focused down using a microscope objective in order to produce a suitable wavefront for microscopic optical particle trapping. The schematic layout for the optical system is shown in Figure 1. A key component in our dynamic optical tweezers system will be a spatial light modulator, that makes it possible to control the phase of the laser light over the entire 2p domain. This unique property of the SLM when combined with a phase contrast filter (PCF), makes it possible to control the intensity of each resolution cell in the output plane of the system. Thus we can shape individual optical tweezers, both in space and intensity, and in addition dynamically manipulate the individual tweezer beams.
Figure 1. The generic system layout proposed for a phase contrast based optical tweezer system. A phase-only modulation in the object plane of a 4-f imaging system can produce a high-contrast intensity distribution in the image plane when a suitably matched phase contrast filter (PCF) is placed in the Fourier plane. An objective lens reduces and focuses this image in the desired tweezer plane.
We have decided to work with a compact 200 mW laser diode working at the wavelength 830 nm. The reason for chosen an IR wavelength, instead of visible wavelength, is to reduce laser damage by light absorption in biological materials. From Figure 2, it can be seen that there is a window of transparency in the near infrared region, this is due to reduced absorption in both water (H2O) and hemoglobin (Hb and HbO2).
Figure 2. A graph illustrating the relative transparency of biological material (such as haemoglobin) and water as a function of wavelength which shows that the near infra-red region of the spectrum is attractive from a point of view of view of minimal absorption and availability of lasers. (figure reproduced from Svoboda and Block 4).
Referring to the transmission curves it can be seen that a wavelength of 830nm is centered within the region of relative transparency for both water and hemoglobin. Currently, many tweezer systems operate at a wavelength of 1064nm. Standard microscope objectives are optimized for visible light imaging and transmission, and as such working at 830 nm compared with 1064 nm results in an improvement of the transmission efficiency.
Current project status
A basic single beam optical tweezer system has been set up, a photograph of which is seen in figure 3. This is based around an 830 nm laser diode, and a high-resolution microscope. The output beam profile of the laser diode is circularized to have a Gaussian beam profile. Light is coupled through the objective lens onto the sample focusing the trapping beam in the focal plane of the microscope. In this basic tweezer system, beam steering is achieved by displacing a lens in the beam steering optics.
Figure 3. Photograph of the Optical tweezer system showing the inverted microscope with the objective beneath the sample stage. The laser system and focussing optics can be seen directly behind the microscope and the trapping and manipulation of samples is observed with a CCD camera.
The image sequence in Figure 4 shows the trapping and manipulation of biological object, in this case a single yeast cell, is carried out relatively easily with laser powers in the 5-20 mW range. The laser light is reflected and scattered from both the cover slip and the yeast cell during the trapping process. When the plane of focus is away from the glass interfaces and nothing is held in the trap, the beam can not be seen.
Figure 4. A set of images showing the trapping and manipulation of an individual yeast cell. (1) shows the introduction laser beam into the sample it is initially moved to the left (2) where it traps an irregularly shaped yeast cell, this is then manipulated (3)-(6). As the trapped cell is manipulated, it twists and moves in the trap changing the cross section of the cell that is in focus. The typical size variation of the cells of approximately 5-10 mm.
Laser tweezers - the movie
Below you may see a video clip of the above Figure 4.
View the animation by clicking on the window.
Acknowledgements
This project is funded partly by Center for Biomedical Optics and New Laser Systems (BIOP).
We would like to thank the Optical tweezer group at the Niels Bohr Institute, University of Copenhagen for advice in the choice of components for a tweezer system and practical instruction in optical particle trapping.
Online article on optical tweezers
A. Ashkin, "Optical trapping and manipulation of neutral particles using lasers" Proc. Natl. Acad. Sci. USA, vol. 94, pp. 4853-4860, May 1997.
References
- A. Ashkin, Phys. Rev. Lett. 24, 156 (1970).
- A. Ashkin, J. M. Dziedic, J. E. Bjorkholm and S. Chu, Opt. Lett. 11, 288 (1986).
- A. Ashkin, J. M. Dziedic and T. Yamane, Nature 330, 769 (1987).
- K. Svoboda and Block S. M, Ann. Rev. Biophys. Biomol. Struct. 23, 247 (1994).
- J. Glückstad and P. C. Mogensen, Opt. Comm. 173, 169 (2000).
- P. C. Mogensen and J. Glückstad, Opt. Comm. 175, 75 (2000).
- J. Glückstad and P. C. Mogensen, Appl. Opt. 40, 268 (2001).
Contact person
Follow this link to learn more about our optical tweezer activities.