Yves Bellouard

Fabrication of high-aspect ratio, micro-fluidic channels and tunnels using femtosecond laser pulses and chemical etching.

We present novel results obtained in the fabrication of high-aspect ratio micro-fluidic microstructures chemically etched from fused silica substrates locally exposed to femtosecond laser radiation. A volume sampling method to generate three-dimensional patterns is proposed and a systematic SEM-based analysis of the microstructure is presented. The results obtained gives new insights toward a better understanding of the femtosecond laser interaction with fused silica glass (a-SiO(2)).

Integrating optics and micro-mechanics in a single substrate: a step toward monolithic integration in fused silica.

We present a novel optical sensor concept that merges integrated optics and micro-mechanics in a monolithic substrate. This concept pushes microsystems integration and defines a new class of monolithic optical microsystems where only optical signals are processed. As an illustration, we present a high-precision, monolithic, glass-based, micro-displacement sensor. Our displacement sensor is made out of a single piece of glass through a two-step process based on femtosecond laser illumination followed by chemical etching.

Optofluidic lab-on-a-chip for rapid algae population screening

The rapid identification of algae species is not only of practical importance when monitoring unwanted adverse effects such as eutrophication, but also when assessing the water quality of watersheds. Here, we demonstrate a lab-on-a-chip that functions as a compact robust tool for the fast screening, real-time monitoring, and initial classification of algae. The water-algae sample, flowing in a microfluidic channel, is side-illuminated by an integrated subsurface waveguide. The waveguide is curved to improve the device sensitivity. The changes in the transmitted optical signal are monitored using a quadrant-cell photo-detector. The signal-wavelets from the different quadrants are used to qualitatively distinguish different families of algae. The channel and waveguide are fabricated out of a monolithic fused-silica substrate using a femtosecond laser-writing process combined with chemical etching. This proof-of-concept device paves the way for more elaborate femtosecond laser-based optofluidic micro-instruments incorporating waveguide networks designed for the real-time field analysis of cells and microorganisms.

Adaptive Scanning Optical Microscope (ASOM): A multidisciplinary optical microscope design for large field of view and high resolution imaging

From micro-assembly to biological observation, the optical microscope remains one of the most important tools for observing below the threshold of the naked human eye. However, in its conventional form, it suffers from a trade-off between resolution and field of view. This paper presents a new optical microscope design that combines a high speed steering mirror, a custom designed scanner lens, a MEMS deformable mirror, and additional imaging optics to enlarge the field of view while preserving resolving power and operating at a high image acquisition rate. We describe the theory of operation and our design methodology, present a preliminary simulated design, and compare to existing technologies. A reduced functionality experimental prototype demonstrates both micro-assembly and biological observation tasks.

Optical classification of algae species with a glass lab-on-a-chip

The identification of submillimetre phytoplankton is important for monitoring environmental and climate changes, as well as evaluating water for health reasons. Current standard methods for phytoplankton species identification require sample collection and ex situ analysis, an expensive procedure which prevents the rapid identification of phytoplankton outbreaks. To address this, we use a glass-based microchip with a microchannel and waveguide included on a monolithic substrate, and demonstrate its use for identifying phytoplankton species. The microchannel and the specimens inside it are illuminated by laser light from the curved waveguide as algae-laden water is passed through the channel. The intensity distribution of the light collected from the biochip is monitored with an external photodetector. Here, we demonstrate that the characteristics of the photodiode signal from this simple and robust system can provide significant and useful information as to the contents of the channel. Specifically, we show first that the signals are correlated to the size of algae cells. Using a pattern-matching neural network, we demonstrate the successful classification of five algae species with an average 78% positive identification rate. Furthermore, as a proof-of-concept for field-operation, we show that the chip can be used to distinguish between detritus in field-collected water and the toxin-producing cyanobacterium Cyanothece.

Scanning thermal microscopy and Raman analysis of bulk fused silica exposed to low-energy femtosecond laser pulses

Low energy femtosecond laser pulses locally increase the refractive index and the hydro-fluoric acid etching rate of fused silica. These phenomena form the basis of a direct-write method to fabricate integrated glass devices that are of particular interest for optofluidics and optomechanical applications. Yet the underlying physical mechanism behind these effects remains elusive, especially the role of the laser polarization. Using Scanning Thermal Microscope and Raman spectrometer we observe in laser affected zones, a localized sharp decrease of the thermal conductivity correlated with an increased presence of low-number SiO(2) cycles. In addition, we find that a high correlation exists between the amount of structural changes and the decrease of thermal conductivity. Furthermore, sub-wavelength periodic patterns are detected for high peak power exposures. Finally, our findings indicate that, to date, the localized densification induced by femtosecond laser pulses remains well below the theoretical value achievable in mechanically densified silica.

Nanoindentation and birefringence measurements on fused silica specimen exposed to low-energy femtosecond pulses

Femtosecond laser pulses used in a regime below the ablation threshold have two noticeable effects on Fused Silica (a-SiO2): they locally increase the material refractive index and modify its HF etching selectivity. The nature of the structural changes induced by femtosecond laser pulses in fused silica is not fully understood. In this paper, we report on nanoindentation and birefringence measurements on fused silica exposed to low-energy femtosecond laser pulses. Our findings further back the hypothesis of localized densification effect even at low energy regime.

Manufacturing by laser direct-write of three-dimensional devices containing optical and microfluidic networks

The index of refraction of most glasses can be permanently changed by exposure to femtosecond laser pulses. This effect allows for the fabrication of various two-dimensional or three-dimensional light guiding structures. Passive and active optical devices have been manufactured using this femtosecond direct-write technique. A closely related technique has recently been demonstrated to manufacture three-dimensional microfluidic networks. We describe recent work at Translume and RPI in femtosecond direct write to produce devices which incorporate on a single glass chip optical network with microfluidic network.

Towards fast femtosecond laser micromachining of fused silica: The effect of deposited energy

Femtosecond laser micromachining of glass material using low-energy, sub-ablation threshold pulses find numerous applications in the fields of integrated optics, lab-on-a-chips and microsystems in general. In this paper, we study the influence of the laser-deposited energy on the performance of the micromachining process. In particular, we show that the energy deposited in the substrate affects its etching rate. Furthermore, we demonstrate the existence of an optimal energy deposition value. These results are not only important from an industrial point-of-view but also provide new evidences supporting the essential role of densification and consequently stress-generation as the main driving factor promoting enhanced etching rate following laser exposure.

Direct volume variation measurements in fused silica specimens exposed to femtosecond laser

We introduce a new method to investigate localized volume variations resulting from laser exposure. Our method is based on the measurement of fused silica cantilevers deflection from which we calculate the effective stress and density variation in laser-affected zones. Specifically, we investigate density variations in fused silica exposed to femtosecond laser exposure in the regime where nanogratings are found. We demonstrate that a volume expansion is taking place in that particular regime.