Thermal Lens Micro Optical Systems

Realization of a high power optical trapping setup free from thermal lensing effects

Transmission of high power laser beams through partially absorbing materials modifies the light propagation via a thermally-induced effect known as thermal lensing. This may cause changes in the beam waist position and degrade the beam quality. Here we characterize the effect of thermal lensing associated with the different elements typically employed in an optical trapping setup for cold atoms experiments. We find that the only relevant thermal lens is represented by the TeO₂ crystal of the acousto-optic modulator exploited to adjust the laser power on the atomic sample. We then devise a simple and totally passive scheme that enables to realize an inexpensive optical trapping apparatus essentially free from thermal lensing effects.

An Optical Configuration of Crossed-Beam Photothermal Lens Spectrometer Operating at High Flow Velocities and Its Application for Cysteine Determination in Human Serum and Saliva

Photothermal lens spectrometry (TLS) is a high sensitive technique for trace determination of nonfluorescent materials. Previous photothermal lens spectrometers suffer from operating limitations at high flow velocities, arising from taking the heated element off the probe beam direction, which results in a decrease in the thermal lens (TL) signal. Herein, we describe an optical configuration of the crossed-beam photothermal lens in transversal flow mode in which the propagating direction of the probe beam and liquid sample flow azimuth are concentric (CBTC). The system consists of a microfluidic cell with a volume of lower than 3 μL. In the current optical configuration, using 1-(2-pyridylazo)-2-naphthol (PAN) in ethanol as a test solution, by increasing the sample flow velocity and without increasing chopping frequency, the reduction in sensitivity is less pronounced. Under a 15 Hz chopping frequency, the optimum sample flow velocity is about 2 cm s^-1, which is among the highest reported values achieved to date for photothermal lens spectrometers. Although the system operates at higher flow velocities and lower chopping frequencies compared to the collinear configuration, it provides a comparable analytical limit of detection. This optical configuration has been successfully employed for highly sensitive and selective determination of cysteine in human serum and saliva samples through a competitive complexation reaction with Cu-PAN as a colorimetric probe. The detection limit of this method (9.5 nM) shows a significant enhancement (726-times) in comparison to UV-vis measurements.

Thermal lens detection device

A thermal lens detection device was developed to realize an easy-to-use, portable and sensitive detector for nonfluorescent molecules. Two laser diodes (658 nm for excitation and 785 nm for probe) were made coaxial in an optical unit and were coupled to a single-mode optical fiber. On a microfluidic chip, a small holder for the optical fiber was fixed, and micro-lenses (numerical aperture of 0.2) were also integrated inside the holder. The micro-lenses were designed to realize an adequate chromatic aberration (50 μm), which was essential for sensitive thermal lens detection. Compared with conventional thermal lens detection systems which required very laborious and accurate optical alignment with the microchannel, the new device needed just attachment-detachment of the optical fiber, which was important for practical application. The lower limit of detection was 10 nM for nickel(II) phthalocyaninetetrasulfonic acid tetrasodium salt solutions (model sample), and the absorbance was 9 × 10(-6) AU. The absolute number of molecules detected was less than 200 zmol. The coefficient of variance for 5-time attachment-detachment of the optical probe was as small as 3.6%. The technical development allowed integration of the thermal lens detection devices inside a microsystem (e.g. enzyme-linked immuno-sorbent assay system), and practical microsystems were realized with sensitivities several-orders higher than absorptiometry.

Parallel multiphase microflows: fundamental physics, stabilization methods and applications

Parallel multiphase microflows, which can integrate unit operations in a microchip under continuous flow conditions, are discussed. Fundamental physics, stabilization methods and some applications are shown.

Microchip-based cell analysis and clinical diagnosis system

Cell analysis and clinical diagnosis systems are now becoming the largest field of application for microchip-based analytical systems. Technological advantages include: small volume, fast analysis time, highly integrated analytical functions, easy operation and small size. For these purposes, basic methodologies for general micro-integration and basic technologies, including fluidic control and ultrasensitive detection, are required. In this review, we introduce our approach to the general integration of various analytical functions and the application of cell analysis systems with cultured cells in microchannels, as well as practical analytical systems for clinical diagnosis utilizing human serum samples.

Simultaneous coaxial thermal lens spectroscopy and retro-reflected beam interference detection for capillary electrophoresis

A novel optical scheme, which accomplished simultaneous coaxial thermal lens spectroscopy and retro-reflected beam interference detection for capillary electrophoresis, has been described. By a special design, an adjustable pump laser waist relative to the probe laser waist was implemented, while some key elements for both detection modes were optimized. In either detection modes, certain preponderance compared with former reports was indicated. With both coaxial thermal lens spectroscopy and retro-reflected beam interference detection, the reported detection scheme combined high sensitivity and universal property for capillary electrophoresis detection.

micro-Hotplate enhanced optical heating by infrared light for single cell treatment

In this study we present a simple approach for fast and localised heating that relies on the strong absorbance of infrared light by microsized patterned surfaces ("micro-hotplates"). Two different materials, micro-arrays of carbon and gold, were tested with respect to their absorbance of the 830 nm diode laser light and their applicability. Both materials were found to be suitable for inducing controlled heating to a temperature increase of more than 10 degrees C within less than a second. The effect of optical heating on living cells (colon cancer cell line SW 480) was investigated with a modified fluorescence microscope. The temperature was controlled by varying the intensity and the exposure time of the laser light. Depending on temperature, induced death of cells in direct contact with the absorbent material was observed, or otherwise cells were kept alive. Cells survive the direct exposure of IR light without the use of the micro-hotplates. In contrast to common heating systems, the optical heating does not need direct contact to a temperature control device. Therefore, it is a very flexible method that can easily be implemented within any microchip. We believe that it will be a versatile tool for initiation and modulation of biochemical or cellular reactions, reversible cell membrane opening, and for control of cell growth.

Optofluidic integration for microanalysis

This review describes recent research in the application of optical techniques to microfluidic systems for chemical and biochemical analysis. The "lab-on-a-chip" presents great benefits in terms of reagent and sample consumption, speed, precision, and automation of analysis, and thus cost and ease of use, resulting in rapidly escalating adoption of microfluidic approaches. The use of light for detection of particles and chemical species within these systems is widespread because of the sensitivity and specificity which can be achieved, and optical trapping, manipulation and sorting of particles show significant benefits in terms of discrimination and reconfigurability. Nonetheless, the full integration of optical functions within microfluidic chips is in its infancy, and this review aims to highlight approaches, which may contribute to further miniaturisation and integration.

Remote in vivo imaging of human skin corneocytes by means of an optical fiber bundle

Human corneocytes forming the outermost layer of the epidermis (stratum corneum) were imaged in vivo by epifluorescence through a coherent optical fiber bundle. A very simple and rapid method to remotely visualize the cells forming this protective layer of the skin is presented. After the topical application of fluorescein, the distal face of an optical fiber bundle is gently applied perpendicularly onto the labeled skin (contact mode). Remote fluorescence images of the corneocytes are acquired in 50 ms through the bundle comprising 30 000 individually cladded 3.5 microm diameter optical fibers. The very short focal distance which is an intrinsic characteristic of such bundles, allows visualizing only the most superficial monolayer of cells in contact with the external environment. An image displays about 400-500 cells directly on the human body. The size and the arrangement of the corneocytes can thus be acquired and analyzed in a very simple and easy way. The method is flexible and can be used for any location on the human body. Using a gradient-index lens objective (magnification 2.8x) fused to the distal face of the bundle allows the shape of the corneocytes to be better resolved. In addition, the working distance is 300 microm and hence this second approach works in a noncontact imaging mode. Both approaches are complementary and allow providing instantaneously either a global view of the cells with a possible statistical determination of their area or morphological information, which are essential for dermatology and cosmetic sciences. Finally, to improve the quality and the contrast of the recorded images, we tested silica nanoparticles containing fluorescein. In brief, this diagnostic method is nontoxic, painless, easy to use, noninvasive, and nondestructive.

Unconventional detection methods for microfluidic devices

The direction of modern analytical techniques is to push for lower detection limits, improved selectivity and sensitivity, faster analysis time, higher throughput, and more inexpensive analysis systems with ever-decreasing sample volumes. These very ambitious goals are exacerbated by the need to reduce the overall size of the device and the instrumentation - the quest for functional micrototal analysis systems epitomizes this. Microfluidic devices fabricated in glass, and more recently, in a variety of polymers, brings us a step closer to being able to achieve these stringent goals and to realize the economical fabrication of sophisticated instrumentation. However, this places a significant burden on the detection systems associated with microchip-based analysis systems. There is a need for a universal detector that can efficiently detect sample analytes in real time and with minimal sample manipulation steps, such as lengthy labeling protocols. This review highlights the advances in uncommon or less frequently used detection methods associated with microfluidic devices. As a result, the three most common methods - LIF, electrochemical, and mass spectrometric techniques - are omitted in order to focus on the more esoteric detection methods reported in the literature over the last 2 years.

UV Excitation Thermal Lens Microscope for Sensitive and Nonlabeled Detection of Nonfluorescent Molecules

An ultrasensitive and nonlabeled detection method of nonfluorescent molecules on a microchip was developed by realizing a thermal lens microscope (TLM) with a 266-nm UV pulsed laser as an excitation light source (UV-TLM). Pulsed laser sources have advantages over continuous-wave laser sources in more compact size and better wavelength tuning, which are important for microchip-based analytical systems. Their disadvantage is difficulty in applying a lock-in amplifier due to the high (>10(4)) duty ratio of pulse oscillation. To overcome this problem, we realized a quasi-continuous-wave excitation by modulating the pulse trains at approximately 1 kHz and detecting the synchronous signal with a lock-in amplifier. The optimum pulse repetition frequency was obtained at 80 kHz, which was reasonable considering thermal equilibrium time. Furthermore, a permissible flow velocity in the range of 6.6-19.8 mm/s was found to avoid sensitivity decrease due to photochemical reactions and thermal energy dissipation. Under these conditions, we detected adenine aqueous solutions on a fused-silica microchip without labeling and obtained a sensitivity that was 350 times higher than that in a spectrophotometric method. The sensitivity was enough for detection on a microchip with an optical path length that was 2-3 orders shorter than that in conventional cuvettes. Finally, the UV-TLM method was applied to liquid chromatography detection. Fluorene and pyrene were separated in a microcolumn and detected in a capillary (50-microm inner diameter) with 150 times higher sensitivity than a spectrophotometric method. Our method provides highly sensitive and widely applicable detections for various analytical procedures and chemical syntheses on microchips.

Miniaturized thermal lens and fluorescence detection system for microchemical chips

We have developed a miniaturized two-way detection system using thermal lens and fluorescence spectroscopies for microchip chemistry. The system was composed of laser diode (LD) modules, fiber-based optics combined with a gradient index lens, and miniaturized detection units for thermal lens and fluorescence signals. The detection limits in the thermal lens and fluorescence spectroscopies were 6.3 x 10(-9)M for Ni(II) phthalocyanine tetrasulfonic acid and 3.0 x 10(-9)M for cy5, respectively. The performance of the system with the miniaturized thermal lens was equivalent to that of a conventional thermal lens microscope. The fluorescence sensitivity was comparable to sensitivities offered by conventional miniaturized systems.

Reflective thermal lens detection device

A reflective thermal lens detection device was developed for realizing a portable and sensitive detector for a microsystem. An aluminum mirror was formed on the main plate of a microchip, and a reflected probe beam was detected with a single pick-up unit. The background signal due to light absorption of the aluminum mirror was 60 times reduced when the microchannel and the mirror were separated with an interval of 600 microm. The tilt angle of the microchip significantly affected the precision of the measurement. Then a quadrant photodiode was used to detect the center of gravity of the reflected probe beam to regulate the tilt angle within +/-0.05 degrees , and this value was enough to achieve 1% CV (coefficient of variance) precision in the measurements. The limit of detection (LOD) was 60 nM for xylene cyanol solution, and the absorbance was 9.4 x 10(-6) AU. About 40 times higher sensitivity was obtained in comparison with a spectrophotometer.