Naoki Murazawa

Freestanding and movable photonic microstructures fabricated by photopolymerization with femtosecond laser pulses

We present a method to fabricate freestanding and movable photonic microstructures by photopolymerization in a zirconium-based sol–gel hybrid resist with femtosecond laser three-dimensional (3D) direct writing. The freestanding microstructures exhibit much less distortion than the conventional structures attached to substrates, especially when made at close to the photopolymerization threshold. A few percent shrinkage which exists rescales the structure and it can lead to the shift of the photonic stop band toward shorter wavelengths. Fabrication of freestanding and freely movable (in micro-fluidic environment) 3D microstructures realizes a simple approach to quantitatively study the shrinkage of photopolymers and to improve the resolution. 3D-movable micro-optical elements with designed photonic properties and micro-devices have potential in opto-fluidics and opto-micro-mechanical applications based on laser trapping and manipulation.

Coupled laser molecular trapping, cluster assembly, and deposition fed by laser-induced Marangoni convection.

A coupled mechanism for molecular aggregation in a thin water solution film by laser-tweezers is suggested based on (i) simulation of light intensity distribution and (ii) order of magnitude analysis of heat and mass transport induced by Marangoni convection. The analysis suggests that the laser induced temperature distribution develops within 1 ms and Marangoni convection flow commences within 0.01-1 s, which increases by 1-2 orders of magnitude the mass transfer of dissolved molecules into the laser focus where they are trapped and aggregate by attractive van der Waals forces. This mechanism, considered for the particular case of polymer assembly, suggests that it can also be successfully applied for assembling other types of clusters and molecular aggregates from solutions.

Laser manipulation based on a light-induced molecular reordering

We report on a novel principle of actuation of micrometer-sized liquid crystal droplets. It is based on a light-induced reordering of liquid crystal molecules inside the droplets. Polariscope imaging allowed to evaluate the birefringence change inside the micro-droplets. Directional actuation of the trapped droplet was achieved by cycling laser power with the direction defined by the polarization of the tweezing beam. Micro-actuation resulted from optically-induced birefringence; i.e., a nonlinear optical effect was utilized for mechanical manipulation of the micro-droplet. This principle of actuation can be used to induce molecular flows in sub-micrometer volumes.

Quantitative measurement of the near-field enhancement of nanostructures by two-photon polymerization

The quantitative determination of the strength of the near-field enhancement in and around nanostructures is essential for optimizing and using these structures for applications. We combine the gaussian intensity distribution of a laser profile and two-photon-polymerization of SU-8 to a suitable tool for the quantitative experimental measurement of the near-field enhancement of a nanostructure. Our results give a feedback to the results obtained by finite-difference time-domain (FDTD) simulations. The structures under investigation are gold nanotriangles on a glass substrate with 85 nm side length and a thickness of 40 nm. We compare the threshold fluence for polymerization for areas of the gaussian intensity profile with and without the near-field enhancement of the nanostructures. The experimentally obtained value of the near-field intensity enhancement is 600 ± 140, independent of the laser power, irradiation time, and spot size. The FDTD simulation shows a pointlike maximum of 2600 at the tip. In a more extended area with an approximate size close to the smallest polymerized structure of 25 nm in diameter, we find a value between 800 and 600. Using our novel approach, we determine the threshold fluence for polymerization of the commercially available photopolymerizable resin SU-8 by a femtosecond laser working at a wavelength of 795 nm and a repetition rate of 82 MHz to be 0.25 J/cm(2) almost independent of the irradiation time and the laser power used. This finding is important for future applications of the method because it enables one to use varying laser systems.

Pulse duration dependent nonlinear propagation of a focused femtosecond laser pulse in fused silica.

The nonlinear propagation of a single focused femtosecond laser pulse in fused silica has been investigated both experimentally and by numerical simulations. In particular, the filamentation behavior was systematically studied by varying pulse duration. At low pulse energy, the peak plasma density inside the filament first increases to a maximum value with increasing pulse duration and then begins to decrease. At relatively high pulse energy, denser plasma can be induced around the geometrical focus with a certain longer pulse duration, where the peak power is already below the self-focusing critical power and no filament is formed. This pulse duration dependent behavior can be explained by different ionization mechanisms.

Laser trapping of deformable objects

We report the trapping and manipulation of bubbles in viscous glass melts through the use of a laser. This phenomenon is observed in bubbles tens of micrometers in diameter under illumination by low numerical aperture (NA = 0.55). Once the bubble was centered on the optical axis, it was trapped and followed a lateral relocation of the laser beam. This phenomenon is explained by modifications of the bubbles shape induced by axial heating and a decrease in surface tension. It is shown that formation of a concave dimple on the bubbles front surface explains the observed laser trapping and manipulation. This mechanism of laser trapping is expected to take place in other deformable materials and can also be used to remove bubbles from melts or liquids. For this technique to be effective, the alteration of the bubbles shape should be faster than its expulsion out of the lasers point of focus.

Rheology Measurement at Liquid-Crystal Water Interface Using Laser Tweezers

We demonstrated that viscosity measurement using a spinning laser-trapped microsphere depends on the hydrophobicity or hydrophylicity of a spherical material. Reduction in viscous drag was observed in hydrophobic liquid crystal droplets. Rheology on a liquid crystal surface with D2O was demonstrated by spinning and stopping a laser tweezed droplet. The importance of nonslipping boundary conditions in viscosity measurements is discussed.

Femtosecond laser photopolymerization of photonic and free-movable microstructures in sol-gel hybrid resist

We present the fabrication of microstructures for photonic and micro-/opto-fluidic applications using femtosecond laser 3D direct writing technique in zirconium-based sol-gel hybrid resist. The advantages and mechanism of photo-polymerization of this new material under fs-pulsed laser exposure are discussed. We suggest and achieve a novel method to fabricate free-standing and movable photonic microstructures, which exhibit much less distortion than the conventional structures attached to substrates, especially when made at close to the photopolymerization threshold. Fabrication of free-movable structures allows us to quantitatively study the shrinkage of photoresist and to improve the resolution. It also contributes to tuning the stop band position of photonic crystals to shorter wavelength. Furthermore, the demonstrated freely-movable property makes it possible to laser trap and manipulate photonic microstructures and have potential application in optofluidics and bio-applications.

Ultrafast laser processing and metrology for consumer applications

High-intensity, blue LEDs have attracted interest because of their wide applications. Dicing method using tightly focused ultra-fast laser beam inside the sapphire substrate is one of the remarkable processing method in terms of high yield and LED performance. In this paper, we would like to introduce the polarization controlled laser processing technique in which laser beam is focused tightly inside the sapphire substrate. The morphology of a cut line can be controlled by changing the direction of laser polarization to an orientation at of the substrate. We, moreover, found that the crack inside the sapphire substrate can be elongated by using the double pulse train for 20 % larger than the single pulse when the pulse interval was 20 nsec. The crack was fabricated effectively by shorter pulse interval.

Effect of drying process on photon-polymerized microstructures in resists

We present the fabrication of photonic microstructures using femtosecond laser direct writing technique in resist. The effect of drying process on photo-polymerized microstructures is investigated. We find that shrinkage mostly arises during the drying process.