Controlled micromanipulation of human sperm in three dimensions with an infrared laser optical trap: effect on sperm velocity.

Individual human sperm can be micromanipulated in three dimensions using a 1.06 microns Nd:YAG laser trap. Single sperm swimming with velocities in the range of 65 to 85 microns/sec can be trapped with 40 mW of power through 120 seconds without a deleterious effect on velocity. Even though it will be necessary to further evaluate the effects of laser light on specific functions of sperm, our data suggest that decreasing the time of manipulation to a minimum will increase the safety of the micromanipulation procedures. Laser traps may play a role in assisted reproductive technology by facilitating the selective transport of individual sperm.

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.

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.

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.

Tension-adjusted in-line optical-fiber attenuator.

We describe an in-line, single-mode fiber attenuator using a combination of polarization-preserving and single-polarization fibers. The attenuation is adjustable over a 30-dB range by varying the birefringence of a short lengthof fiber with tension. The insertion loss is 1.3 dB. The tension birefringence arises from the difference in Poisson's ratio between the noncircular stress cladding and the silica substrate glass.

Continuous-wave self-focusing and self-trapping of light in artificial Kerr media.

Artificial Kerr media made from liquid suspensions of submicrometer particles were used as a new type of nonlinear medium for observing cw self-focusing and self-trapping of laser beams. Self-trapping of TEM(00)-mode beams and higher-order TEM(01)- and TEM(01)* -mode beams were investigated. Saturation-free operation down to filament diameters of ~2 microm was observed. The independence of the critical power for self-trapping on the beam diameter in the unsaturated regime was confirmed for the first time to our knowledge. Values of the nonlinear coefficient were determined for a range of particle diameters from 0.038 to 0.234 microm.

Optical Kerr effect in long fibers.

Optical Kerr modulation of >100% is demonstrated in long birefringent optical fibers using low laser powers of ~1 W. We have experimentally investigated the effects of group-velocity dispersion, thermal stability of polarization, and fiber birefringence. Use of fiber Kerr modulation as a fast optical shutter is studied, and it is concluded that resolution times shorter than 1 psec are possible in principle.

Observation of optical resonances of dielectric spheres by light scattering.

Use of the wavelength and size dependence of light scattering from optically levitated liquid drops is demonstrated as a sensitive means of detecting optical resonances of dielectric spheres. High resolution spectra are presented of the radiation pressure, far- and near-field backscatter, and 90 degrees scatter. Excellent agreement is found between experimental spectra and high resolution Mie calculations of Chylek et al. Strong evidence supporting the van de Hulst dielectric surface-wave model for these resonances is presented. Use of resonances for high precision measurement of sphere size and sphere distortion, index of refraction, temperature, and vapor pressure is discussed.

Outer diameter measurement of low birefringence optical fibers by a new resonant backscatter technique.

A new, highly precise, optical fiber outer diameter measuring technique based on near-field resonant backscattered light is described. Relative accuracies of +/-10(-2) microm were achieved in diameter measurements using Fabry-Perot resonances and +/-10(-3) microm in average diameter measurements using dielectric surface-wave resonances for ~100-microm fibers. The shape of a new type of low birefringence spun fiber, made by rapidly spinning a near circular preform in the pulling oven, was measured. We observed a small ellipticity which rotated helically along the fiber. The possibility of making absolute and real-time outer diameter measurements is discussed.

Observation of light scattering from nonspherical particles using optical levitation.

The optical levitation technique has been extended to the study of light scattering from several basic types of nonspherical particles such as spheroids; spherical doublets, triplets, and quadruplets; and spheres within spheres. Individual particles were assembled in air using two independently maneuverable laser beams and then held at rest in the beam in fixed orientation. Observations were made of the light scattering in the near and far field with high resolution. Other applications of the technique are envisaged.

Optical levitation of liquid drops by radiation pressure.

Charged and neutral liquid drops in the diameter range from 1 to 40 microns can be stably levitated and manipulated with laser beams. The levitation technique has been extended toward smaller particles (about 1 micron), lower laser power (less than 1 milliwatt), and deeper traps (greater than ten times the particle's weight). The techniques developed here have particular importance in cloud physics, aerosol science, fluid dynamics, and optics. The interactions of the drops with light, the electric field, the surrounding gas, and one another can be observed with high precision.