Simultaneous rotation, orientation and displacement control of birefringent microparticles in holographic optical tweezers.

We report the experimental implementation of a new method for generating multiple dynamical optical tweezers, where each one of them is generated with an independent linear polarization state with arbitrary orientation. This also allows an independent simultaneous polarization-rotation control. The laser beam, both for generating multiple traps and polarization control, has been modulated using a single reflective nematic liquid crystal with parallel alignment. We present experimental results of controlled displacement, orientation and rotation of birefringent particles. In addition, a simple method for estimating and canceling out the primary astigmatism present in the system is presented.

Cavity optomechanical magnetometer.

A cavity optomechanical magnetometer is demonstrated. The magnetic-field-induced expansion of a magnetostrictive material is resonantly transduced onto the physical structure of a highly compliant optical microresonator and read out optically with ultrahigh sensitivity. A peak magnetic field sensitivity of 400 nT Hz(-1/2) is achieved, with theoretical modeling predicting the possibility of sensitivities below 1 pT Hz(-1/2). This chip-based magnetometer combines high sensitivity and large dynamic range with small size and room temperature operation.

Phase-transition-like properties of double-beam optical tweezers.

We report on double-beam optical tweezers that undergo previously unknown phase-transition-like behavior resulting in the formation of more optical traps than the number of beams used to create them. We classify the optical force fields which produce multiple traps for a double-beam system including the critical behavior. This effect is demonstrated experimentally in orthogonally polarized (noninterfering) dual-beam optical tweezers for a silica particle of 2.32 µm diameter. Phase transitions of multiple beam trapping systems have implications for hopping rates between traps and detection of forces between biomolecules using dual-beam optical tweezers. It is an example of a novel dynamic system with multiple states where force fields undergo a series of sign inversions as a function of parameters such as size and beam separation.

Versatile two-dimensional potentials for ultra-cold atoms.

We propose and investigate a technique for generating smooth two-dimensional potentials for ultra-cold atoms based on the rapid scanning of a far-detuned laser beam using a two-dimensional acousto-optical modulator (AOM). We demonstrate the implementation of a feed-forward mechanism for fast and accurate control of the spatial intensity of the laser beam, resulting in improved homogeneity for the atom trap. This technique could be used to generate a smooth toroidal trap that would be useful for static and dynamic experiments on superfluidity and persistent currents with ultra-cold atoms.

Linewidth reduction in a large-smile laser diode array.

We present theory and simulations for a spectral narrowing scheme for laser diode arrays (LDAs) that employs optical feedback from a diffraction grating. We calculate the effect of the so-called smile of the LDA and show that it is possible to reduce the effect by using a cylindrical lens set at an angle to the beam. The scheme is implemented on a 19-element LDA with smile of 7.6 microm and yields frequency narrowing from a free-running width of 2 to 0.15 nm. The experimental results are in good agreement with the theory.

Laser frequency locking by direct measurement of detuning.

We present a new method of laser frequency locking in which the feedback signal is directly proportional to the detuning from an atomic transition, even at detunings many times the natural linewidth of the transition. Our method is a form of sub-Doppler polarization spectroscopy, based on measuring two Stokes parameters (I2 and I3) of light transmitted through a vapor cell. It extends the linear capture range of the lock loop by as much as an order of magnitude and provides frequency discrimination equivalent to or better than those of other commonly used locking techniques.

Absorption and fluorescence spectroscopy of rhodamine 6G in titanium dioxide nanocomposites.

A comparison has been made between the spectroscopic properties of the laser dye rhodamine 6G (R6G) in mesostructured titanium dioxide (TiO(2)) and in ethanol. Steady-state excitation and emission techniques have been used to probe the dye-matrix interactions. We show that the TiO(2)-nanocomposite studied is a good host for R6G, as it allows high dye concentrations, while keeping dye molecules isolated, and preventing aggregation. Our findings have important implications in the context of solid state dye-lasers and microphotonic device applications.

Micromanipulation of chloroplasts using optical tweezers.

This paper describes experiments using optical tweezers to probe chloroplast arrangement, shape and consistency in cells of living leaf tissue and in suspension. Dual optical tweezers provided two-point contact on a single chloroplast or two-point contact on two adhered chloroplasts for manipulation in suspension. Alternatively, a microstirrer consisting of a birefringent particle trapped in an elliptically polarized laser trap was used to induce motion and tumbling of a selected chloroplast suspended in a solution. We demonstrate that displacement of chloroplasts inside the cell is extremely difficult, presumably due to chloroplast adhesion to the cytoskeleton and connections between organelles. The study also confirms that the chloroplasts are very thin and extremely cup-shaped with a concave inner surface and a convex outer surface.

Dynamical tunnelling of ultracold atoms.

The divergence of quantum and classical descriptions of particle motion is clearly apparent in quantum tunnelling between two regions of classically stable motion. An archetype of such non-classical motion is tunnelling through an energy barrier. In the 1980s, a new process, 'dynamical' tunnelling, was predicted, involving no potential energy barrier; however, a constant of the motion (other than energy) still forbids classically the quantum-allowed motion. This process should occur, for example, in periodically driven, nonlinear hamiltonian systems with one degree of freedom. Such systems may be chaotic, consisting of regions in phase space of stable, regular motion embedded in a sea of chaos. Previous studies predicted dynamical tunnelling between these stable regions. Here we observe dynamical tunnelling of ultracold atoms from a Bose-Einstein condensate in an amplitude-modulated optical standing wave. Atoms coherently tunnel back and forth between their initial state of oscillatory motion (corresponding to an island of regular motion) and the state oscillating 180 degrees out of phase with the initial state.

Distributed laser refrigeration.

A 250-mum-diameter fiber of ytterbium-doped ZBLAN (fluorine combined with Zr, Ba, La, Al, and Na) has been cooled from room temperature. We coupled 1.0 W of laser light from a 1013-nm diode laser into the fiber. We measured the temperature of the fiber by using both fluorescence techniques and a microthermocouple. These microthermocouple measurements show that the cooled fiber can be used to refrigerate materials brought into contact with it. This, in conjunction with the use of a diode laser as the light source, demonstrates that practical solid-state laser coolers can be realized.

Coherent atomic beam splitter using transients of a chaotic system

A coherent atomic beam splitter can be realized using the transient dynamics of a chaotic system. We have experimentally observed such an effect using ultracold rubidium atoms. Our experimental results are in good agreement with numerical simulations of the Schrodinger equation for the system.

Three-dimensional imaging with optical tweezers.

We demonstrate a three-dimensional scanning probe microscope in which the extremely soft spring of an optical tweezers trap is used. Feedback control of the instrument based on backscattered light levels allows three-dimensional imaging of microscopic samples in an aqueous environment. Preliminary results with a 2-microm-diameter spherical probe indicate that features of approximately 200 nm can be resolved, with a sensitivity of 5 nm in the height measurement. The theoretical resolution is limited by the probe dimensions.

Optical torque controlled by elliptical polarization.

We show theoretically and demonstrate experimentally that highly absorbing particles can be trapped and manipulated in a single highly focused Gaussian beam. Our studies of the effects of polarized light on such particles show that they can be set into rotation by elliptically polarized light and that both the sense and the speed of their rotation can be smoothly controlled.

Simultaneous two-wavelength holographic interferometry in a superorbital expansion tube facility.

A new variation of holographic interferometry has been utilized to perform simultaneous two-wavelength measurements, allowing quantitative analysis of the heavy particle and electron densities in a superorbital facility. An air test gas accelerated to 12 km/s was passed over a cylindrical model, simulating reentry conditions encountered by a space vehicle on a superorbital mission. Laser beams with two different wavelengths have been overlapped, passed through the test section, and simultaneously recorded on a single holographic plate. Reconstruction of the hologram generated two separate interferograms at different angles from which the quantitative measurements were made. With this technique, a peak electron concentration of (5.5 +/- 0.5) x 10(23) m(-3) was found behind a bow shock on a cylinder.

Determination of the force constant of a single-beam gradient trap by measurement of backscattered light.

A single-beam gradient trap could potentially be used to hold a stylus for scanning force microscopy. With a view to development of this technique, we modeled the optical trap as a harmonic oscillator and therefore characterized it by its force constant. We measured force constants and resonant frequencies for 1-4-µm-diameter polystyrene spheres in a single-beam gradient trap using measurements of backscattered light. Force constants were determined with both Gaussian and doughnut laser modes, with powers of 3 and 1 mW, respectively. Typical values for spring constants were measured to be between 10(-6) and 4 × 10(-6) N/m. The resonant frequencies of trapped particles were measured to be between 1 and 10 kHz, and the rms amplitudes of oscillations were estimated to be around 40 nm. Our results confirm that the use of the doughnut mode for single-beam trapping is more efficient in the axial direction.

Resonant degenerate four-wave mixing in I(2): effect of buffer gas pressure.

The influence of molecular collisions on the production of the degenerate four-wave mixing signal in I(2) is presented. Measurements were performed on gaseous molecular iodine, I(2), contained in a glass cell in which pressure, temperature, and species concentration are easily and independently varied. Frequencydoubled outputs from a seeded Nd:YAG laser and an excimer-pumped dye laser were used as excitation sources. We have studied the dependence of signal strength versus buffer gas pressure, with pump intensity as a third parameter. It is evident from our results that, for pump intensities of less than 1 MW/cm(2), the pressure dependence of the signal follows that given by a simple two-level model in the homogeneously broadened regime. In this regime collisional deexcitation becomes significant, leading to changes in saturation intensity. This is evidenced by a reduction in the signal with an increase in buffer gas pressure. This behavior is similar to that seen in laser-induced fluorescence. At higher pump intensities, the signal is seen to increase with pressure; this behavior cannot be described by the simple two-level model.

Detection of trace amounts of Cr by two laser-based spectroscopic techniques: laser-enhanced ionization in flames and laser-induced fluorescence in graphite furnace.

The detectability of Cr in water solutions by two-step laser-enhanced ionization (LEI) in flames and two-step excitation laser-induced fluorescence in graphite furnace (LIF-GF) with nonresonant detection is investigated for what is, to our knowledge, the first time. A thorough investigation of possible excitation and detection routes for Cr for both techniques is given. The detection limit of Cr in water by the LEI technique was found to be 2 ng/mL, while the LIF-GF technique showed a detection limit of 1.4 pg (which corresponds to 0.3 ng/mL, with a 5-microL sample volume), both of which are limited by contamination (from the burner head-nebulizer unit in the flame and from the graphite material in the furnace). A more sensitive two-step LEI excitation scheme than that used here is also proposed. A new technique for reducing fluctuations from blackbody radiation by using sequential detection of the blackbody radiation from one photomultiplier by two boxcar integrators is presented. A possible means of increasing the nonresonant signal in two-step excitation LIF-GF by adding small amounts of quenching enhancing N(2) to the Ar atmosphere gives no positive results. The influence of large amounts of Na on the detectability of Cr by LEI is investigated.

Detection of traces in semiconductor materials by two-color laser-enhanced ionization spectroscopy in flames.

In this paper we demonstrate that two-color laser-enhanced ionization (LEI) spectroscopy in flamescan be used for detecting impurities at a sub-ppm level in GaAs. Six elements were investigated(Co, Cr, Fe, In, Mn, Ni), and the content could be determined for three of them (Cr, Fe, Ni), while only an upperlimit below ppm could be given for the other three. The analysis was performed on the bulk material whichwas dissolved in acid and further diluted. In the diluted solution, the detection limits were of the same orderas in pure water solutions. Spectral interferences have been corrected for by using a background correctionmethod, which is performed without scanning the lasers. Furthermore, we discuss the applicability of LEI inflame and graphite furnace to samples with various impurity elements, matrices, and total amounts of sampleavailable for analysis.