Laser manipulation of a smectic liquid-crystal droplet

The laser trapping of a smectic-A liquid-crystal micro-droplet was spatially traced during its transient into the trapped position. The lateral and angular orientation of the droplet were determined and followed in time during the axial descent of the micro-droplet into the stationary trapped position using the analysis of polarization changes of the light passed through the droplet with temporal resolution of a video refresh rate of 30 ms. The spatial resolution of 0.1-1μm has been achieved for typical laser trapping powers of 2-600 mW. The axial profile of a laser trapping force (an ellipticity of the focal spot) has been determined. The laser trapping mechanism of smectic micro-droplets is discussed in terms of minimization of a light-droplet interaction.     

Theoretical Demonstration of a Newly Designed Micro-Rotator Driven by Optical Pressure on a Light Incident Surface

A newly designed cylindrical optical micro-rotator which has slopes for trapping and rotation on its upper surface is proposed. The cylindrical shape is effective in decreasing viscous drag force (damping factor) in the medium. A ray-tracing method considering the beam waist is employed to analyze the radiation pressure exerted on the upper surface of the rotator. We have demonstrated optical trapping and high speed rotation for various optical beam parameters such as the lens numerical aperture and the Gaussian mode profile as well as rotator shape parameters including oblique angle, height and diameter.     

Optical vortex beams for trapping and transport of particles in air

In this paper we show that laser beams containing phase singularity can be used for trapping and guiding light-absorbing particles in air. The experiments were performed with agglomerates of carbon nanoparticles with the size in the range 0.1–10 μm; the typical cw laser power was of a few mW. The stability of open-air three-dimensional trapping was within ±2 μm in both the transverse and the longitudinal directions. The particle position on the beams axis within the trap can be controlled by changing the relative intensity of two beams. The distinguishing feature of the trapping strategy is that particles are trapped at the intensity minimum of the beam, thus with minimum heating and intervention into the particle properties, which is important for direct studies of particle properties and for air-trapping of living cells.     

Femtosecond-laser-induced-crystallization and simultaneous formation of light trapping microstructures in thin a-Si:H films

We report the observation of crystallization and simultaneous formation of surface microstructures in hydrogenated amorphous silicon (a-Si:H) thin films as one step laser processing. Light trapping microstructures of around 300 nm in height were formed on a-Si:H films of thickness in the range of 1.5 μm to 2 μm deposited on soda lime glass after exposure to femtosecond laser pulses. Scanning electron microscope (SEM) images show the formation of spikes that are around 1 μm part and their heights could be controlled by the laser fluences. Atomic force microscope (AFM) images were taken to study the roughness created on the surface. The mean roughness of the textured surface increases with laser fluence at smaller power densities, and for power densities beyond 0.5 J/cm² the film removal deteriorates the texturing. X-ray diffraction results indicate the formation of a nano-crystalline structure with (111) and (311) crystal orientation after the laser treatment. The observed black color and enhanced optical absorption in the near infrared region in laser treated films may be due to a combined effect of light trapping in the micro-structured silicon surface because of multiple total internal reflections, phase change in the film, possible defect sites induced after laser treatment and formation of SiO_x. Demonstration of light trapping microstructures in thin a-Si:H films and simultaneous crystallization could provide new opportunities for optoelectronic devices. PACS 42.55.Px; 42.62.Cf; 81.05.Ge     

Near Field Observation of a Refractive Index Grating and a Topographical Grating by an Optically-Trapped Gold Particle

We observed the near field for a refractive index grating fabricated on a planar light waveguide circuit (PLC) by scanning an optically-trapped 100 nm diameter gold particle. We demonstrate that stable trapping and scanning occur with a Gaussian laser beam at the scan velocity of 1.6 m/s and Nd:YAG laser power of 25mW. The scattered Ar laser light from the gold particle is strong at high refractive indexes of the grating with periods of 1.06 m and 0.53 m both by s and p polarized illumination. In addition, we observed the surface profile of the optical disk tracking groove with and without the gold particle. © 2004 The Optical Society of Japan     

Processing of micro-particles by UV laser irradiation in a field cage

An electric cage-laser micro-turning lathe was realised and applied to contact-free handling and mechanical processing of micro particles. Since particles with diameters of several micrometers cannot be fixed in mechanical chucks, an octode field cage was used to trap and rotate a single particle in a fluid without any mechanical surface contact. A pulsed nitrogen laser of high beam quality focused to about 1 μm in diameter could be adjusted independently of the cage position. The trapping forces (negative dielectrophoresis) acting on a bead of 5 to 15 μm are up to several hundred pN. This and the surrounding fluid damp down the effect of the laser pulses during bead processing. Examples demonstrating the possibilities of this technique are shown. Microsystems with high optical quality were fabricated photolithographically or by laser direct-write chemical vapor deposition (LCVD). Technical and biotechnological applications are discussed. Received: 20 October 1999 / Accepted: 27 October 1999 / Published online: 10 November 1999     

采用超连续谱激光的双光束光纤光阱实验

以超连续谱激光器作为捕获光源,首次提出并搭建了超连续谱双光束光纤光阱实验系统,实现了聚苯乙烯微球的捕获和操控。通过改变光纤端面间隔和调整捕获光功率的方式精确控制微球的位置,采用CCD图像分析方法实现了微球位置的精确测量。对微球受限布朗运动下的位置变化进行傅里叶变换,计算得到功率谱,与理论功率谱函数拟合后求出了其光阱刚度。结果表明,捕获光束的功率为28 mW时,光阱刚度达到1.3×10-6N/m,高于相同实验条件下单波长光纤光阱的刚度。与传统采用单色光作为捕获光源的光镊系统不同,超连续谱双光束光阱系统利用其宽谱优势,通过研究被捕获微粒的散射光谱信息可获取其尺寸、折射率等物理特征参数。     

Optically-driven red blood cell rotor in linearly polarized laser tweezers

We have constructed a dual trap optical tweezers set-up around an inverted microscope where both the traps can be independently controlled and manipulated in all the three dimensions. Here we report our observations on rotation of red blood cells (RBCs) in a linearly polarized optical trap. Red blood cells deform and become twisted in hypertonic phosphate buffer saline and when trapped, experience an unbalanced radiation pressure force. The torque generated from the unbalanced force causes the trapped RBC to rotate. Addition of Ca⁺⁺ ions in the solution, keeping the osmolarity same, makes the cell membranes stiffer and the cells deform less. Thus the speed of rotation of the red blood cells can be controlled, as less deformation and in turn less asymmetry in shape produces less torque under the radiation pressure resulting in slower rotation at the same laser power     

A vision chip with column-level amplification of optical data signals for indoor optical wireless local area networks

We present a vision chip architecture with column-level photo-amplification of optical data signals for optical wireless local area networks (LANs) to reduce the pixel area. Based on the architecture, we have fabricated a prototype vision chip in a standard 0.8 μm bipolar complementary metal-oxide-semiconductor (BiCMOS) technology. The device offers position detection of other optical transceivers in the LAN and 4-ch concurrent high-speed optical data acquisition. A data rate of 400 Mbps was demonstrated. The pixel size was 125 μm square, which can be shrunk to smaller than around 60 μm square in 0.35 μm or more advanced BiCMOS or CMOS technologies.     

Ring beam shaping optics fabricated with ultra-precision cutting for YAG laser processing

In this study, a method for generating ring intensity distribution at a refraction-type lens with an aspheric element was proposed, and the beam shaping optical element was finished using only ultra-precision cutting. The shape of the optical element and its irradiance pattern were determined from numerical calculation based on its geometrical and physical optics. An ultra-precision lathe was employed to fabricate beam shaping optical elements, and acrylic resin was used as the material. The transmittance of an optical element (a rotationally symmetrical body) with an aspheric surface fabricated using a single-crystal diamond tool was over 98%, and its surface roughness was 9.6 nm Ra. The method enabled the formation of a circular melting zone on a piece of stainless steel with a thickness of 300 μm through pulse YAG laser (λ 1:06 μm) processing such that the average radius was 610 μm and the width was 100–200 μm. Circular processing using a ring beam shaping optical element can be realized by single-pulse beam irradiation without beam scanning.     

Optical manipulation of gold nanoparticles using an optical nanofiber

Gold nanoparticles are gaining increasing attention due to their biological and medical applications.In this letter,we experimentally demonstrate the optical manipulation of 250-nm-diameter gold nanoparticles along an optical nanofiber(550 nm in diameter) injected by an 808-nm laser light.The nanoparticles situated in the evanescent optical field are trapped by optical gradient force and move along the direction of light propagation due to optical scattering force.The velocities reach as high as 132 μm/s at an optical power of 80 mW.     

Antireflection structures written by excimer laser on CVD diamond

Two-dimensional antireflective periodical microstructures for the IR range are fabricated on the surface of CVD diamond films. These structures are created using an ArF excimer laser (λ=193 nm) and a direct writing scheme consisting of a beam collimator and a microscope objective to focus the beam onto the sample. Two different arrays are investigated. One has a spacing of 3 μm and is produced with single shots and the other one has a spacing of 4 μm and is produced with three shots per spot. The hole depth and shape are measured with an atomic force microscope (AFM). The optical transmittance and the scattering properties of the structure at 10.6 μm are reported for a CO₂ laser beam. With a spectrometer further transmission measurements in the range of 5 to 20 μm are performed. Received: 16 September 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

Three-dimensional dynamic optical manipulation by combining a diffractive optical element and a spatial light modulator

Since the introduction of computer-controlled spatial light modulators (SLMs), holographic optical tweezers have become an important tool for dynamic parallel optical manipulation. In this paper we clarify the usefulness of a new configuration for optical trapping that creates light patterns using the combination of a diffractive optical element (DOE) and an SLM. This configuration not only enables the use of the higher part of the SLM’s diffraction efficiency curve, because a simple hologram can be chosen for the SLM, but also achieves three-dimensional dynamic optical manipulation over a large spatial range. By switching blaze-like holograms displayed on the SLM, we demonstrated simultaneous transportation of three 6-μm-diameter polystyrene beads over a range of 90 μm in the vertical direction and 37.5 μm in the horizontal direction. Compared with the same manipulation executed using only the SLM, the range of this method is extended four-fold in the vertical direction and three-fold in the horizontal direction.     

Yb-doped strictly all-fiber laser actively Q-switched by intermodal acousto-optic modulation

We show an actively Q-switched ytterbium-doped strictly all-fiber laser. Cavity loss modulation is achieved in a tapered optical fiber by core-to-cladding mode-coupling induced by travelling flexural acoustic waves. When the acoustical signal is switched-off, the optical power losses within the cavity are reduced, and then a laser pulse is emitted. Trains of Q-switched pulses were successfully obtained at repetition rates in the range 1–10 kHz, with pump powers between 59 and 88 mW, at the optical wavelength of 1064.1 nm. Best results were for laser pulses of 118 mW peak power, 1.8 μs of time width, with a pump power of 79 mW, at 7 kHz repetition rate.     

Amplification assisted optical parametric oscillator in the mid-infrared region

We demonstrate a high efficiency mid-infrared laser source based on optical parametric oscillator (OPO) assisted by an intracavity optical parametric amplification (OPA). The OPA-assisted-OPO scheme was realized in one piece of commensurable dual-periodic superlattice in which the signal light generated from the OPO process serves as the pump light for the OPA process. A maximum output power of 508 mW at 3.92 μm was achieved under a pump power of 2.85 W at 1.064 μm. The pump-to-idler conversion efficiency is 17.8% and the slope efficiency is 23.8%, and the enhancements of them are 58.9% and 67.6%, respectively, comparing with the standard OPO scheme.     

Optical travelator: transport and dynamic sorting of colloidal microspheres with an asymmetrical line optical tweezers

We report the transport, funnelling and dynamic sorting of colloidal microspheres in an aqueous suspension using line optical tweezers with asymmetrical intensity profiles. The line tweezers readily trapped and propelled the microspheres along the length of the line tweezers. Using this simple technique, transporting and funnelling of microspheres within a microscopic region were demonstrated. To illustrate the dynamic particle-sorting capability of the line tweezers, a binary colloidal system comprising of microspheres with diameters of 1.1 μm and 3.2 μm were driven past the line tweezers by electrophoresis. As the optical trapping force is dependent on the size of the microspheres, the line tweezers was able to change the path of the larger spheres while exerting little influence on the smaller spheres thus sorting the two types of microspheres. At optimized laser power and flow rate of microspheres, sorting efficiency greater than 90% has been achieved. PACS 42.15.Eq; 87.80.Cc; 87.80.Fe; 82.70.Dd     

Self-organized structuring of W/C multilayers on Si substrate

After aging at room temperature for several months W/C multilayers (20 periods, single layer thicknesses in the nanometer range) grown on Si-(111) substrates by pulsed laser deposition (PLD) developed homogeneously wrinkled surfaces. Their structures were studied by optical microscopy, atomic force microscopy and X-ray diffractometry. Typical dimensions of debonded areas are some 100 μm in length, about 40 μm in width and 2–3 μm in height. The formation of wrinkles is accompanied by an increase in the free surface by 1–2%. Stress relaxation is considered the driving force of this phenomenon. Received: 26 July 1999 / Accepted: 29 July 1999 / Published online: 16 September 1999     

Low-threshold mid-infrared optical parametric oscillation in periodically poled LiNbO3 synchronously pumped by a Ti:sapphire laser

We report the generation of tunable high-repetition-rate optical pulses in the mid-infrared using synchronously pumped parametric oscillation in periodically poled LiNbO₃ (PPLN). Using a Kerr-lens-mode-locked Ti:sapphire laser as the pump source and a PPLN crystal incorporating grating periods of 21.0–22.4 μm, we have achieved wavelength conversion in the -–4 6μm spectral range in the mid-infrared. The use of a semi-monolithic cavity design and hemispherical focusing has permitted pulse generation in the strong idler absorption region of PPLN, resulting in a simple, compact, all-solid-state configuration with a pump power threshold as low as 17 mW and mid-infrared idler powers of up to 64 mW at 9% extraction efficiency. Signal output powers of up to 280 mW at 35% extraction efficiency are available over the -–1.004 1.140μm spectral range at 80.5 MHz and pulse repetition rates at harmonics of the fundamental frequency up to 322 MHz have also been obtained. Received: 5 December 2000 / Revised version: 23 January 2001 / Published online: 27 April 2001     

Designing a coupler for the intersatellite optical communication system

Coupling a single-mode laser diode with 200 mW to a single-mode fiber (SMF) through an orthonormal aspherical cylindrical lens and a GRIN lens for the intersatellite optical communication system is proposed and demonstrated. We experimentally studied how the coupling efficiency changes with the SMF's position displacement and axial angle variation, and obtained 80 mW output power at the end of the SMF, which shows that the coupling units have satisfied the designed request.     

A cw room-temperature Ho,Tm:YLF laser pumped at 1.682 μm

We present the results obtained with a Ho,Tm:YLF crystal grown at a new crystal growth facility in Pisa. The optical quality of the sample has been tested by studying its performance as the active medium of a laser operating at 2.06 μm. We employed three different pump laser sources: a Ti:sapphire, a diode (both tuned at 793 nm) and, for the first time, a continuous-wave Co:MgF₂ laser, tuned at 1.682 μm. At room temperature the best slope efficiency was 30 % in the case of “red” pumping, and 59 % in the case of “infrared” excitation. The typical lasing threshold is about 100 mW. Received: 14 March 2001 / Revised version: 15 June 2001 / Published online: 19 September 2001