D. G. Kotsifaki

Optical tweezers with enhanced efficiency based on laser-structured substrates

We present an optical nanotrapping setup that exhibits enhanced efficiency, based on localized plasmonic fields around sharp metallic features. The substrates consist of laser-structured silicon wafers with quasi-ordered microspikes on the surface, coated with a thin silver layer. The resulting optical traps show orders of magnitude enhancement of the trapping force and the effective quality factor.

Biophotonics for imaging and cell manipulation: quo vadis?

As one of the major health problems for mankind is cancer, any development for the early detection and effective treatment of cancer is crucial to saving lives. Worldwide, the dream for the anti-cancer procedure of attack is the development of a safe and efficient early diagnosis technique, the so called “optical biopsy”. As early diagnosis of cancer is associated with improved prognosis, several laser based optical diagnostic methods were developed to enable earlier, non-invasive detection of human cancer, as Laser Induced Fluorescence spectroscopy (LIFs), Diffuse Reflectance spectroscopy (DRs), confocal microscopy, and Optical Coherence Tomography (OCT). Among them, Optical Coherence Tomography (OCT) imaging is considered to be a useful tool to differentiate healthy from malignant (e.g. basal cell carcinoma, squamous cell carcinoma) skin tissue. If the demand is to perform imaging in sub-tissular or even sub-cellular level, optical tweezers and atomic force microscopy have enabled the visualization of molecular events underlying cellular processes in live cells, as well as the manipulation and characterization of microscale or even nanoscale biostructures. In this work, we will present the latest advances in the field of laser imaging and manipulation techniques, discussing some representative experimental data focusing on the 21th century biophotonics roadmap of novel diagnostic and therapeutical approaches. As an example of a recently discussed health and environmental problem, we studied both experimentally and theoretically the optical trapping forces exerted on yeast cells and modified with estrogen-like acting compounds yeast cells, suspended in various buffer media.

Geometrical effect characterization of femtosecond-laser manufactured glass microfluidic chips based on optical manipulation of submicroparticles

Abstract. Microfluidic devices provide a platform with wide ranging applications from environmental monitoring to disease diagnosis. They offer substantive advantages but are often not optimized or designed to be used by nonexpert researchers. Microchannels of a microanalysis platform and their geometrical characterization are of eminent importance when designing such devices. We present a method that is used to optimize each microchannel within a device using high-throughput particle manipulation. For this purpose, glass-based microfluidic devices, with three-dimensional channel networks of several geometrical sizes, were fabricated by employing laser fabrication techniques. The effect of channel geometry was investigated by employing an optical tweezer. The optical trapping force depends on the flow velocity that is associated with the dimensions of the microchannel. We observe a linear dependence of the trapping efficiency and of the fluid flow velocity, with the channel dimensions. We determined that the highest trapping efficiency was achieved for microchannels with aspect ratio equal to one. Numerical simulation validated the impact of the device design dimensions on the trapping efficiency. This investigation indicates that the geometrical characteristics, the flow velocity, and trapping efficiency are crucial and should be considered when fabricating microfluidic devices for cell studies.

Enhanced Optical Forces in Plasmonic Microstructures

Micromanipulation of dielectric objects, from polystyrene spheres to living cells, is achieved when radiation pressure forces create stable trapping by highly focused laser beams through microscopes. However, the impressive history of optical trapping is shadowed by the light diffraction limit, as research currently has focused on materials below the micron scale, requiring stronger optical confinement and higher intensities than can be provided by the conventional optical tweezers. Recently, plasmonic nanostructures have entered the field, either to assist or enhance it. In this study, we present experimental results on using localized fields of metallic structures for efficient trapping, with various patterns (dots, fringes and squares). The patterns were produced by laser interferometry on almost continuous Ag or Au films on glass and glass covered by an amorphous Al\(_{2}\)O\(_{3}\) layer (10 nm thick) respectively. We have calculated the optical forces by measuring the particle’s escape velocity. The results show that the effective quality factor Q in the patterned metal film is enhanced by a factor \(>\)10, with respect to the unpatterned metal film and a factor \(>\)100, with respect to an uncoated glass. In addition, mathematical simulation of plasmonic fields is investigated to confirm and explain theoretically, the experimentally observed plasmonic enhancement.

Near infrared optical tweezers and nanosecond ablation on yeast and algae cells

In recent years, lasers for optical trapping and micromanipulation of microscopic particles or cells and sub cellular structures, both in vivo and in vitro, have gained remarkable interest in biomedical research and applications. Although the principles and the mechanisms of pulsed laser ablation have been well described for macroscopic interventions, the microbeam operation under microscopic guidance necessitates further investigation. In this work, we present the research and development efforts towards a pulsed ultraviolet microbeam laser system, the design and realization efforts towards a near infrared laser trapping device and the results obtained on yeast cells and algae by the combined system. We investigated the optical dissection of the cells versus the presence of optical trapping forces and the presence of rhodamine dye. We characterized the optical ablation of the cell walls and resulting cavitation as plasma formation effects which create shock waves due to their occurrence only in nanosecond pulse irradiation mode. We estimated the minimum energy of the microbeam for optical dissection of yeast cell, under the influence of optical trapping forces, as lower as 3 μJ, while in the presence of rhodamine as lower as 2 μJ. Lastly, using the techniques of optical microsurgery we demonstrated the minimum energy value for sub cellular dissection on an algae cell equal to 27 μJ.

Micromanipulation of cells and microparticles using optical fibers

Optical tweezers is a powerful tool which is used to capture and manipulate microscopic particles such as dielectric microspheres and cells. In the single optical trap the beam is strongly focused to a diffraction limited spot by a high numerical aperture objective. Resently a new version of optical trap was demonstrated with optical fibers. Compared with the common optical tweezers which required high power microscope objective and carefully adjusted optical path, the fiber optical tweezers are compact in size and less expensive. Moreover, they have also a working distance not necessarily close to the objective as for a typical optical tweezers. In this work we present the development of a single beam optical fiber trapping system integrated with an optical fiber ablation system for micromanipulation of biological objects. The fiber trap was formed using a continuous wave He-Ne laser operating at 632.8 nm. The fiber ablation system was formed using a free-running Er:YAG laser operating at 2.94 μm with pulse duration of 80 μm. The ablation beam was coupled into the front end of a fluoride glass optical fiber via a focusing lens of 100 mm and a pinhole of 50 μm. We evaluated the fluoride glass optical fiber as far as attenuation and as far as the spatial distribution of the energy output is concerned. We verified that optical trapping and the micromanipulation of micro objects were easily achieved, by a focused laser beam, emerging from optical fiber inclined at 42 degrees to the sample.

Paper surface modification by lasers

Lasers can provide a precious tool to conservation process due to their accuracy and the controlled energy they deliver, especially to fragile organic material such as paper. The current study concerns laser modification such as paper cleaning, initially of test papers artificially soiled and then of an original book of the early 20th Century. The test objects were A4 copier paper, newspaper, and paper Whatman No.1056. During the experiments, ink of a pen, pencil and ink from a stamp was mechanically employed on each paper surface. Laser cleaning was applied using a Q-switched Nd:YAG operating at 532 nm and CO2 laser at 10.6 μm for various fluences. The experimental results were presented by using optical microscopy. Eventually, laser cleaning of ink was performed to a book of 1934, by choosing the best conditions and parameters from cleaning the test samples, like Nd:YAG laser operating at 532 nm.

Fibers and fiber end sealing caps for Er:YAG laser ablation

A considerable interest, in the recent years, has been allocated in the mid-infrared Er:YAG laser surgery and microsurgery. This interest has been increased after the development of optical fibers and waveguides, for safe and efficient transmission of the ~3.0 μm wavelength beams. On the other hand, a laser delivery system based on common silica glass fibers and caps are not applicable for delivery of the Er:YAG laser light, due to high absorption losses at the mid-infrared wavelengths. Thus, fluoride glass fibers and sealing quartz caps is a promising combination for laser delivery due to their low transmission loss. In this study, we investigated the properties of three sealing quartz caps, suitable for fluoride glass optical fibers, with various distal end geometries, in order to evaluate the attenuation and the spatial and temporal energy distribution of the transmitted laser radiation. Moreover, we evaluated the experimental beam divergence of the sealing caps. As a transmission medium, three fluoride glass optical fibers were used. As a laser source we used a Q-switched Er:YAG laser with a pulse duration of 190 ns and a repetition rate of 1 Hz. The mean value of the energy loss for dome geometry was found (0.73 ± 0.03), for planoconvex geometry was found (0.76 ± 0.03) and for ball geometry was found (0.73 ± 0.05). The beam divergence was found (62.4 ± 0.1) mrad, (156.2 ± 0.3) mrad and (37.5 ± 0.5) mrad for dome, ball and plano-convex geometry, respectively.

Optical tweezers and manipulation of PMMA beads in various conditions

Laser optical trapping and micromanipulation of microparticles or cells and subcellular structures have gained remarkable interest in biomedical research and applications. Several laser sources are employed for the combination of a laser scalpel with an optical tweezers device, under microscopic control. However, although the principles and the mechanisms of pulsed laser ablation have been well described for macroscopic interventions, the microbeam operation, under microscopic guidance, necessitates further experiments and investigations. We present experimental results of controlled micro-ablation of PMMA beads of 3-8 μm diameters, trapped by laser tweezers in various media e.g. solutes of different index of refraction. An optical tweezers system, based on a continuous wave He-Ne laser emitting at 632.8 nm, was tested on beads and, despite the low power of the He-Ne laser, the optical trap was stable. Another optical system, based on a cw Nd:YAG laser emitting at 1.06 μm, was tested on microspheres too. Successful beads ablation was carried out by irradiation with multiple, or even a single nitrogen laser pulse of 7 ns pulse duration at a wavelength of 337 nm. The ablative perforation of the microspheres was estimated by controlling the laser fluence. Moreover, shape deformations of PMMA microspheres were observed. The experimentally obtained results are theoretically explained via the spatial intensity distribution based on Mie light scattering theory. Furthermore, the appearance of laser ablation holes in the back side of microspheres is explained by the ablation triggered shock waves propagation. The role of the stretching forces action is also discussed. Additionally, we report experimental results on measuring the optical trap force of PMMA beads. A powerful optical tweezers system based on a continuous wave Nd:YAG laser was used in order to estimate the trapping efficiency for several beads diameter.

Radiation pressure effects in diamond structure and III-V semiconductors

The photon drag effect has been observed in several semiconductors. It arises from the transfer of momentum from laser radiation to mobile electrons or holes in the material. The sign and the magnitude of the effect depend on the combination of optical, transport and band structure properties of the semiconductor as well as the magnitude of the radiation momentum. The optical rectification is a second order phenomenon arising from the generation of polarization in a non-linear medium at the passage of an intense optical beam. Both effects are generally referred as radiation pressure effects. The intention of this work is to present and discuss new experimental evidence of photon drag-effect in diamond structure and photon drag-optical rectification in III-V semiconductors using Er:YAG laser emitting at 2.94 μm, CO2 laser emitting at 10.6 μm and at 9.6 μm, Nd:YAG laser emitting at 1.06 μm, Er:Tm:Ho:YLF laser emitting at 2.06 μm and Cr:Tm:Ho:YAG laser emitting at 2.08 μm. No saturation effects were found indicating that detectors based on these effects can be used as recording devices of pulses down to 0.1 ns. Measurements have been made on the response of the photon drag and the optical rectification detectors of Ge, Si, GaAs, GaP of several orientations. The responsivity results are converted, using the relevant theoretical equations, in to S, P and D coefficients. The experimentally obtained results are theoretically explained and are compared with previous results of other wavelengths in the literature.