Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime.

We calculate the forces of single-beam gradient radiation pressure laser traps, also called "optical tweezers," on micron-sized dielectric spheres in the ray optics regime. This serves as a simple model system for describing laser trapping and manipulation of living cells and organelles within cells. The gradient and scattering forces are defined for beams of complex shape in the ray-optics limit. Forces are calculated over the entire cross-section of the sphere using TEM00 and TEM*00 mode input intensity profiles and spheres of varying index of refraction. Strong uniform traps are possible with force variations less than a factor of 2 over the sphere cross-section. For a laser power of 10 mW and a relative index of refraction of 1.2, we compute trapping forces as high as approximately 1.2 x 10(-6) dynes in the weakest (backward) direction of the gradient trap. It is shown that good trapping requires high convergence beams from a high numerical aperture objective. A comparison is given of traps made using bright field or differential interference contrast optics and phase contrast optics.

Optical trapping and manipulation of neutral particles using lasers.

The techniques of optical trapping and manipulation of neutral particles by lasers provide unique means to control the dynamics of small particles. These new experimental methods have played a revolutionary role in areas of the physical and biological sciences. This paper reviews the early developments in the field leading to the demonstration of cooling and trapping of neutral atoms in atomic physics and to the first use of optical tweezers traps in biology. Some further major achievements of these rapidly developing methods also are considered.

Fertilization of bovine oocytes induced solely with combined laser microbeam and optical tweezers.

PURPOSE: Our purpose was to show that fertilization of oocytes can be obtained solely by laser light-mediated manipulation of gametes. METHOD: A small channel was drilled into the zona pellucida of bovine oocytes using an ultraviolet (UV)-laser microbeam. Highly diluted cattle sperm were not able to fertilize the laser drilled oocytes. RESULTS: Fertilization was achieved only when three to five cattle sperm were trapped with optical tweezers and inserted directly through the laser drilled hole into the perivitelline space. After 20 hr, 3 of 79 (3.8%) oocytes revealed two pronuclei and a sperm tail within their cytoplasm. Cattle sperm are difficult to catch. Therefore, the gametes had to remain for about 20 min in room atmosphere, which might be the reason for the low fertilization results. CONCLUSIONS: The results indicate that a combined UV-laser microbeam and optical tweezers trap can be used successfully for "noncontact" microinsemination procedures.

Zona drilling and sperm insertion with combined laser microbeam and optical tweezers.

A combined UV-laser microbeam and optical-tweezers trap was used to perform laser zona drilling and subzonal insemination in cattle. Using a precisely focused UV-laser microbeam, a small channel of about 10 microns in diameter was drilled into the zona pellucida. With a three-dimensional optical-tweezers trap, a single sperm was caught and transported through the laser-drilled hole directly into the perivitelline space. Furthermore, the sperm was brought into close contact with the oolemma to facilitate sperm-oocyte fusion. Using the laser-microscope system, noncontact, entirely sterile, and highly selective micromanipulation of gametes can be achieved with no need for mechanical microtools. Laser micromanipulation seems to be less detrimental to the gametes and is comparatively is easy to perform. Thus, the combined UV-laser microbeam and optical tweezers trap may be a helpful tool for IVF procedures.

Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime.

We calculate the forces of single-beam gradient radiation pressure laser traps, also called "optical tweezers," on micron-sized dielectric spheres in the ray optics regime. This serves as a simple model system for describing laser trapping and manipulation of living cells and organelles within cells. The gradient and scattering forces are defined for beams of complex shape in the ray-optics limit. Forces are calculated over the entire cross-section of the sphere using TEM(00) and TEM(01) (*) mode input intensity profiles and spheres of varying index of refraction. Strong uniform traps are possible with force variations less than a factor of 2 over the sphere cross-section. For a laser power of 10 mW and a relative index of refraction of 1.2 we compute trapping forces as high as approximately 1.2 x 10(-6) dynes in the weakest (backward) direction of the gradient trap. It is shown that good trapping requires high convergence beams from a high numerical aperture objective. A comparison is given of traps made using bright field or differential interference contrast optics and phase contrast optics.

The study of cells by optical trapping and manipulation of living cells using infrared laser beams.

The new technique of optical trapping and manipulation of living cells using the forces of radiation pressure from infrared single beam gradient laser traps is reviewed. These traps, also referred to as "optical tweezers," are capable of stably trapping transparent micron and submicron biological particles free of optical damage. Entire cells or organelles within the interior of living cells can be manipulated without damaging the cell wall. The trap is built into a high resolution microscope for combined trapping and high resolution viewing. Experiments demonstrating reproduction of motile bacteria and yeast cells within infrared traps and manipulations of plant and animal cells are discussed. Applications to the study of the mechanical properties of cell cytoplasm, study of cell function, and cell separation and orientation are considered. The ability to apply controlled light forces on cells of magnitude comparable to or often much greater than gravity suggests that these optical techniques might have relevance to experiments showing the influence of gravity on cells.

Force generation of organelle transport measured in vivo by an infrared laser trap.

Organelle transport along microtubules is believed to be mediated by organelle-associated force-generating molecules. Two classes of microtubule-based organelle motors have been identified: kinesin and cytoplasmic dynein. To correlate the mechanochemical basis of force generation with the in vivo behaviour of organelles, it is important to quantify the force needed to propel an organelle along microtubules and to determine the force generated by a single motor molecule. Measurements of force generation are possible under selected conditions in vitro, but are much more difficult using intact or reactivated cells. Here we combine a useful model system for the study of organelle transport, the giant amoeba Reticulomyxa, with a novel technique for the non-invasive manipulation of and force application to subcellular components, which is based on a gradient-force optical trap, also referred to as 'optical tweezers'. We demonstrate the feasibility of using controlled manipulation of actively translocating organelles to measure direct force. We have determined the force driving a single organelle along microtubules, allowing us to estimate the force generated by a single motor to be 2.6 x 10(-7) dynes.

Internal cell manipulation using infrared laser traps.

The ability of infrared laser traps to apply controlled forces inside of living cells is utilized in a study of the mechanical properties of the cytoplasm of plant cells. It was discovered that infrared traps are capable of plucking out long filaments of cytoplasm inside cells. These filaments exhibit the viscoelastic properties of plastic flow, necking, stress relaxation, and set, thus providing a unique way to probe the local rheological properties of essentially unperturbed living cells. A form of internal cell surgery was devised that is capable of making gross changes in location of such relatively large organelles as chloroplasts and nuclei. The utility of this technique for the study of cytoplasmic streaming, internal cell membranes, and organelle attachment was demonstrated.

Optical trapping and manipulation of single cells using infrared laser beams.

Use of optical traps for the manipulation of biological particles was recently proposed, and initial observations of laser trapping of bacteria and viruses with visible argon-laser light were reported. We report here the use of infrared (IR) light to make much improved laser traps with significantly less optical damage to a variety of living cells. Using IR light we have observed the reproduction of Escherichia coli within optical traps at power levels sufficient to give manipulation at velocities up to approximately 500 micron s-1. Reproduction of yeast cells by budding was also achieved in IR traps capable of manipulating individual cells and clumps of cells at velocities of approximately micron s-1. Damage-free trapping and manipulation of suspensions of red blood cells of humans and of organelles located within individual living cells of spirogyra was also achieved, largely as a result of the reduced absorption of haemoglobin and chlorophyll in the IR. Trapping of many types of small protozoa and manipulation of organelles within protozoa is also possible. The manipulative capabilities of optical techniques were exploited in experiments showing separation of individual bacteria from one sample and their introduction into another sample. Optical orientation of individual bacterial cells in space was also achieved using a pair of laser-beam traps. These new manipulative techniques using IR light are capable of producing large forces under damage-free conditions and improve the prospects for wider use of optical manipulation techniques in microbiology.

Optical trapping and manipulation of viruses and bacteria.

Optical trapping and manipulation of viruses and bacteria by laser radiation pressure were demonstrated with single-beam gradient traps. Individual tobacco mosaic viruses and dense oriented arrays of viruses were trapped in aqueous solution with no apparent damage using approximately 120 milliwatts of argon laser power. Trapping and manipulation of single live motile bacteria and Escherichia coli bacteria were also demonstrated in a high-resolution microscope at powers of a few milliwatts.

Polarization-selective 3-dB fiber directional coupler.

Measurements on a 3-dB polarization-selective fiber directional coupler demonstrate that the polarization selectivity arises from different mismatches for the propagation constants of the two polarizations.

Studies of self-focusing bistable devices using liquid suspensions of dielectric particles.

We describe the results of experiments on bistable and multistable self-focusing devices using liquid suspensions of dielectric particles as the nonlinear medium. The nonlinear parameters for this medium are calculable from first principles. A simple model predicts regions of bistability and multistability that are in good agreement with the experimental observations.

In-line fiber-polarization-rocking rotator and filter.

An in-line polarization rotator has been built into a single-mode birefringent fiber. The rotator utilizes periodic twists of the fiber's principal axes, which were formed by rocking the preform as the fiber was drawn. The polarization conversion between the principal axes is wavelength dependent, with a bandwidth inversely proportional to the number of twist periods. The bandwidth of the present rotator was 4.8 nm for 100% conversion in a fiber length of 170 cm.

Stable radiation-pressure particle traps using alternating light beams.

A new type of stable alternating-beam light trap is proposed for confinement of neutral atoms and macroscopic dielectric particles. This trap, based only on the scattering force of radiation pressure, overcomes the limitations of the optical Earnshaw theorem. Trapping of ~10(7) sodium atoms in large volumes (100 cm(3)) seems possible with well depths >1 K and with optical cooling close to the Purcell limit of ~10(-4) K. Trapping at points of zero light intensity is considered.

Simultaneous determination of refractive index and size of spherical dielectric particles from light scattering data.

The diameter and refractive index of micrometer sized spherical dielectric particles are simultaneously deduced using the wavelength dependence of backscattering data from optically levitated particles. The accuracy of the results is set by experimental errors in the determination of the wavelength of backscatter resonance peaks and the ratio of slopes of specified peaks. At present the refractive index and diameter can be deduced with relative errors of 5 x 10(-5). This represents the most accurate determination of absolute size and refractive index yet made by light scattering. A reduction of these errors by an order of magnitude is possible. We assume a priori knowledge of diameter and refractive index with accuracy of 10(-1) and 5 x 10(-3), respectively.