A microfluidic device consisting of a 3D network of buried microchannels and integrated waveguides has been fabricated and used to controllably manipulate particles within the micro channels. The channels and waveguides were made using the direct laser writing technique of ultrafast laser inscription, followed by selective chemical etching to fabricate channels.
Particles flowing through the device undergo hydrodynamic flow focusing into a narrow stream within a main channel due to the geometry of the channel network. 3D hydrodynamic focusing performance was validated using polystyrene microspheres, coloured dye and cells by visualizing the focusing within the device. A focusing width of 4 um was achieved, reducing the risk of particles sticking to walls, clogging the channel and ensuring all particles pass the beam. Particles are irradiated by 1064 nm light, in a direction perpendicular to the flow, from the embedded waveguide, causing a lateral displacement of the particle due to the optical scattering force. 5 and 10 micron beads in water were focused to a narrow stream. Lateral displacement was evaluated for 5 different laser powers for particles flowing at a constant velocity >1 mm/s. A linear increase in displacement of the particles with laser power was observed. Bacteria, yeast, microalgae and mammalian cells have been flow-focused and optically manipulated within the device. The device is capable of both passive and active separation of particle species, and the routing of particles to required outlets demonstrates potential for cell sorting.
Silver nanoparticles (Ag-NP) with Surface Enhanced Raman Scattering (SERS) activity were fabricated on a fused silica substrate by ultrafast femtosecond laser photoreduction of a silver salt solution. The SERS effectiveness of the Ag-NP increased with laser writing power and number of scans. SEM images show that the Ag-NP have a more uniform density distribution when using a multi-scan writing technique. A number of different laser parameters were compared, including scan speed, laser power, and number of scans. Overall, it was found that the most effective laser parameters were: 20 µms-1 scan speed, 10 mW laser power and 200 scans. The Ag-NP substrates have been used to detect single bacteria and hold promise to give fast, accurate and specific spectra according to the cell specimen present.
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.
We describe a variety of technological advances in the development of efficient, powerful, and continuously tunable Cr:ZnSe lasers operating in the 2.3-2.7 μm spectral region. This includes the development of compact "single chip" waveguide Cr:ZnSe lasers, waveguide mode-locked Cr:ZnSe lasers, and the creation of homogeneously broadened laser material.
We report on efforts to improve the lifetime of biological lasers through the use of ascorbic acid (also commonly known as vitamin C). Fluorescent proteins and dyes, used in biological lasers, suffer from photobleaching due to the build-up of reactive oxygen species (ROS) which causes damage leading to a decrease in emission over time. This is an issue both for laser lifetime and cell health. It has previously been shown that ascorbic acid can be effective in reducing ROS levels in a variety of applications. For our experiments human embryonic kidney cells (HEK293), containing the fluorescent dye Calcein AM, were placed between two dielectric plane mirrors to form a laser cavity. The cells were pumped using the output of a Ti:Sapphire femtosecond OPO system, frequency doubled twice in BBO crystals, giving an output of 474 nm. Initial results have shown an increase in laser lifetime when ascorbic acid is added to cells indicating a reduction in the build-up of ROS.
Optical micromanipulation techniques and microfluidic techniques can be used in same platform for manipulating biological samples at single cell level. Novel microfluidic devices with integrated channels and waveguides fabricated using ultrafast laser inscription combined with selective chemical etching can be used to enable sorting and isolation of biological cells. In this paper we report the design and fabrication of a three dimensional chip that can be used to manipulate single cells in principle with a higher throughput than is possible using optical tweezers. The capability of ultrafast laser inscription followed by selective chemical etching to fabricate microstructures and waveguides have been utilised to fabricate the device presented in this paper. The complex three dimensional microfluidic structures within the device allow the injected cell population to focus in a hydrodynamic flow. A 1064 nm cw laser source, coupled to the integrated waveguide, is used to exert radiation pressure on the cells to be manipulated. As the cells in the focussed stream flow past the waveguide, optical scattering force induced by the laser beam pushes the cell from out of the focussed stream to the sheath fluid, which can be then collected at the outlet. Thus cells can be controllably deflected from the focussed flow to the side channel for downstream analysis or culture.
The present study discusses the photosensitivity of GeS2 chalcogenide glass in response to irradiation with femtosecond pulses at 1047 nm. Bulk GeS2 glasses are prepared by conventional melt quenching technique and the amorphous nature of the glass is confirmed using X-ray diffraction. Ultrafast laser inscription technique is used to fabricate the straight channel waveguides in the glass. Single scan and multi scan waveguides are inscribed in GeS2 glasses of length 0.65 cm using a master oscillator power amplifier Yb doped fiber laser (IMRA μjewel D400) with different pulse energy and translation speed. Diameters of the inscribed waveguides are measured and its dependence on the inscription parameters such as translation speed and pulse energy is studied. Butt coupling method is used to characterize the loss measurement of the inscribed optical waveguides. The mode field image of the waveguides is captured using CCD camera and compared with the mode field image of a standard SMF-28 fibers.
Development of efficient methods for isolation and manipulation of microorganisms is essential to study unidentified and yet-to-be cultured microbes originating from a variety of environments. The discovery of novel microbes and their products have the potential to contribute to the development of new medicines and other industrially important bioactive compounds. In this paper we describe the design, fabrication and validation of an optofluidic device capable of redirecting microbes within a flow using optical forces. The device holds promise to enable the high throughput isolation of single microbes for downstream culture and analysis. Optofluidic devices are widely used in clinical research, cell biology and biomedical engineering as they are capable of performing analytical functions such as controlled transportation, compact and rapid processing of nanolitres to millilitres of clinical or biological samples. We have designed and fabricated a three dimensional optofluidic device to control and manipulate microorganisms within a microfluidic channel. The device was fabricated in fused silica by ultrafast laser inscription (ULI) followed by selective chemical etching. The unique three-dimensional capability of ULI is utilized to integrate microfluidic channels and waveguides within the same substrate. The main microfluidic channel in the device constitutes the path of the sample. Optical waveguides are fabricated at right angles to the main microfluidic channel. The potential of the optical scattering force to control and manipulate microorganisms is discussed in this paper. A 980 nm continuous wave (CW) laser source, coupled to the waveguide, is used to exert radiation pressure on the particle and particle migrations at different flow velocities are recorded. As a first demonstration, device functionality is validated using fluorescent microbeads and initial trials with microalgae are presented.
We report the first demonstration of a gain-switched chromium-doped zinc selenide channel waveguide laser. The guided-wave structure was produced by ultrafast laser inscription and exhibited output pulse energies up to 12 μJ . The laser exhibited narrow spectral output with a linewidth less than 1 nm. The beam quality was measured to be M2 ≤ 7 with a highly multimode output profile. The laser had a maximum slope efficiency of 9.8% and no deleterious thermal effects were observed up to an average pump power of 3.3 W .
Tellurite glass photonic crystal fibers (PCF) offer a large potential for broadband supercontinuum generation with bandwidths of 4000 nm demonstrated in suspended-core tellurite PCFs under pumping at 1500-1600 nm. We fabricated a hexagonal-lattice, tellurite PCF with lattice constant Λ = 2 μm, linear filling factor d/Λ=0.75 μm, and a solid core with 2.7 μm diameter. Dispersion, calculated from SEM image of drawn fiber, has ZDW at 1500 nm and 4350 nm with a maximum of 193 ps/nm/km at 2900 nm. Under pumping with 150 fs / 36 nJ / 1580 nm pulses, supercontinuum in a bandwidth from 800 nm to over 2500 nm was measured in a 2 cm long PCF sample. Measured coupling efficiency was 8%. Dispersive and nonlinear length scales are 52 cm and 0.2 mm respectively, yielding nonlinearity-dominant propagation regime in the fiber. Numerical analysis of measured supercontinuum spectrum using NLSE, enabled identification of soliton fission and their subsequent red-shifting, dispersive wave generation across first ZDW, as well as FWM among the red-shifted spectral components. FWM phase-matching condition in the fiber is satisfied in a broad range from 1500 nm to 4000 nm with roughly 900 nm bandwidth around the signal wavelength. Developed model is in good agreement with experimental results. Model is used to estimate supercontinuum bandwidth for other experimental conditions with pump pulse lengths up to 1 ps and PCF lengths up to 10 cm.
B. del Rosal, P. Haro-González, W. Ramsay, L. Maestro, K. Santacruz-Gómez, M. C. Iglesias-de la Cruz, F. Sanz-Rodríguez, J. Y. Chooi, P. Rodríguez-Sevilla, D. Choudhury, A. Kar, J. García Solé, L. Paterson, D. Jaque
Laser-induced thermal effects in optically trapped microspheres and single cells have been investigated by Luminescence
Thermometry. Thermal spectroscopy has revealed a non-localized temperature distribution around the trap that extends
over tens of microns, in agreement with previous theoretical models. Solvent absorption has been identified as the key
parameter to determine laser-induced heating, which can be reduced by establishing a continuous fluid flow of the
sample. Our experimental results of thermal loading at a variety of wavelengths reveal that an optimum trapping
wavelength exists for biological applications close to 820 nm. This has been corroborated by a simultaneous analysis of
the spectral dependence of cellular heating and damage in human lymphocytes during optical trapping. Minimum
intracellular heating, well below the cytotoxic level (43 °C), has been demonstrated to occur for optical trapping with 820 nm laser radiation, thus avoiding cell damage.
In this work, we synthesized bulk amorphous GeGaS glass by conventional melt quenching technique. Amorphous nature of
the glass is confirmed using X-ray diffraction. We fabricated the channel waveguides on this glass using the ultrafast laser
inscription technique. The waveguides are written on this glass 100 μm below the surface of the glass with a separation of 50
μm by focusing the laser beam into the material using 0.67 NA lens. The laser parameters are set to 350 fs pulse duration at
100 KHz repetition rate. A range of writing energies with translation speeds 1 mm/s, 2 mm/s, 3 mm/s and 4 mm/s were
investigated. After fabrication the waveguides facets were ground and polished to the optical quality to remove any tapering
of the waveguide close to the edges. We characterized the loss measurement by butt coupling method and the mode field
image of the waveguides has been captured to compare with the mode field image of fibers. Also we compared the
asymmetry in the shape of the waveguide and its photo structural change using Raman spectra.
We report here, a finite difference thermal diffusion (FDTD) model for controlling the cross-section and the guiding nature of the buried channel waveguides fabricated on GeGaS bulk glasses using the direct laser writing technique. Optimization of the laser parameters for guiding at wavelength 1550 nm is done experimentally and compared with the theoretical values estimated by FDTD model. The mode field diameter (MFD) between 5.294 μm and 24.706 μm were attained by suitable selection of writing speed (1mm/s to 4 mm/s) and pulse energy (623 nJ to 806 nJ) of the laser at a fixed repletion rate of 100 kHz. Transition from single-mode to multi-mode waveguide is observed at pulse energy 806nJ as a consequence of heat accumulation. The thermal diffusion model fits well for single-mode waveguides with the exception of multi-mode waveguides.
Recent results from our work using ultrafast laser writing to fabricate waveguides and on-chip devices inside sulphide chalcogenide glasses are presented in this paper. Low loss single-mode (SM) and multi-mode (MM) waveguide arrays were successfully laser fabricated, for the first time to our knowledge, for operation in the whole near-IR (NIR) to mid- IR (MIR) range (1 to 11 μm wavelengths). These waveguides are demonstrated to have numerical apertures (NA) which can exceed NA=0.2, therefore also allowing for low bend losses as well as direct coupling to QC lasers. We also demonstrate the control over the waveguide mode field diameters (MFDs) (at 1/e2) by changing the waveguide core sizes and index contrasts, achieving typical values of 44 μm at 10.6 μm, down to 6 μm for telecom 1.55 μm light. The optical nonlinear properties of these waveguides have also been preliminarily investigated. Using a femtosecond (fs) optical parametric amplifier system, the optical nonlinearity of bulk gallium lanthanum sulphide (GLS) glass was first measured at 2.5 μm. The upper limits for the nonlinear properties of the laser modified material could be estimated based upon the nonlinear spectral broadening of a 2.5 μm fs pulse train coupled into SM waveguides. Further work includes the demonstration of on-chip three dimensional (3D) beam combiners for the MIR range (10.6 μm in this work), for near future implementation in astronomical observatories for stellar interferometry.
This article reports the advances on the development of mid-infrared integrated optics for stellar interferometry.
The devices are fabricated by laser writing techniques on chalcogenide glasses. Laboratory characterizaton is
reported and analyzed.
In this paper we report a two octave spanning supercontinuum generation in the range 750-3000 nm with a newly
developed photonic crystal fiber. The fibre is fabricated using an in-house synthesized lead-bismuth-galate glass PBG08
with optimised rheological and transmission properties in the range 500-4800 nm. The photonic cladding consists of 8
rings of air holes with a fibre core diameter of 3 μm and a lattice constant of 2.2 μm. The dispersion characteristic is
determined mainly by the material dispersion and the first ring of holes in the cladding with a filling factor of 0.68. The
filling factor of the remaining 7 rings is 0.45 which allows single mode performance of the fibre in the infrared range.
The fibre has a zero dispersion wavelength of 1490 nm which allows the use of 1550 nm wavelength as an efficient
pump in the anomalous dispersion regime. The 2 cm long sample of photonic crystal fiber is pumped in the femtosecond
regime with a pulse energy of 10 nJ at a wavelength of 1550 nm. A flatness of 5 dB is observed in the spectral range 950-2500 nm.
Recent advances in the field of ultrafast laser inscription provide ample evidence underscoring the
potential of this technique in fabricating novel and previously unthinkable 2D and 3D photonic and
optofluidic platforms enabling current sensor, diagnostics, monitoring and biochemical research to
scale new heights. In addition to meeting the demands for compact, active waveguide devices
designed for diverse applications such as optical metrology, non-linear microscopy and
astrophotonics, this technology facilitates the integration of microfluidics with integrated optics
which creates a powerful technology for the manufacture of custom lab-on-chip devices with
advanced functionality. This paper highlights the capabilities of ultrafast laser inscription in
fabricating novel 3D microfluidic devices aimed for biomedical applications.
A continuous flow microfluidic cell separation platform has been designed and fabricated using femtosecond laser
inscription. The device is a scalable and non-invasive cell separation mechanism aimed at separating human embryonic
stem cells from differentiated cells based on the dissimilarities in their cytoskeletal elasticity. Successful demonstration
of the device has been achieved using human leukemia cells the elasticity of which is similar to that of human embryonic
stem cells.
Astrophotonics offers a solution to some of the problems of building instruments for the next generation of telescopes
through the use of photonic devices to miniaturise and simplify instruments. It has already proved its worth in
interferometry over the last decade and is now being applied to nightsky background suppression. Astrophotonics offers
a radically different approach to highly-multiplexed spectroscopy to the benefit of galaxy surveys such as are required to
determine the evolution of the cosmic equation of state. The Astrophotonica Europa partnership funded by the EU via
OPTICON is undertaking a wide-ranging survey of the technological opportunities and their applicability to high-priority
astrophysical goals of the next generation of observatories. Here we summarise some of the conclusions.
Strain and thermal sensitivities of germanate and tellurite glass fibres were measured using a fibre Fabry-Perot (FFP)
interferometer and fibre Bragg gratings (FBG). The strain phase sensitivity for germanate and tellurite fibre were
5900×103 rad/m and 5600×103 rad/m respectively at a central wavelength of 1540nm using FFP interferometer, which is
consistent with the value of 1.22pm/με obtained for a germanate fibre FBG. The Young's modulus for germanate and
tellurite fibre were also measured to be 58GPa and 37GPa. The thermal responses of germanate fibre were examined as
24.71 and 16.80 pm/°C at 1540nm and 1033nm wavelength using the FBG.
The invited paper explains the transmission properties of a range of near-, mid-, and far-IR optical fibres for their
applications in chemical and biological sensing. Methods for the fabrication of single and multiple-core mid-IR fibres are
discussed in view of controlling the thermal and viscosity properties for fibre drawing. In particular, the need for
removing impurity bands in the 5000 to 1000 cm-1 range is explained. The importance of engineering multi-core fibres
is also discussed for simultaneous measurements of Raman, IR and surface plasmon enhanced modes together with say,
temperature using a mid-IR transmitting tellurite fibre e.g. in a chemical process. The paper explains the principles and
advantages of evanescent wave coupling of light at the resonant frequency bands for chemical sensing using a fibre
evanescent wave spectroscopic sensor having a GeTeSe chalcogenide fibre. Using fibre based techniques, measurements
for Cr6+ ions in solution and As3+ and As5+ in solids have been characterized at visible and mid-IR regions, respectively.
In this paper we also explain the importance of using mid-IR fibres for engineering novel laser and broadband sources
for chemical sensing.
This work investigates the influence of the pulse duration and the wavelength on the laser cleaning of thin silver plating
layers found in late Roman coins. Comparative cleaning tests were performed using Nd:YAG (1064 nm and 532 nm - 6
ns), GaAlAs diode (780 nm - 90 ps) and Ti-Sapphire regenerative amplifier (800 nm - 100 fs) laser systems. The
cleaning results on the plated areas were characterised by high resolution optical microscopy, SEM-EDX, XRF and
micro-profilometry.
The authors report on the fabrication of buried waveguides in both lithium niobate and periodically poled lithium
niobate. First a low insertion loss waveguide is fabricated in z-cut lithium niobate using femtosecond laser
waveguide inscription. To fabricate a waveguide exhibiting both low propagation and coupling losses, we used the
multiscan fabrication technique to control the size of the waveguide cross section. We measured coupling losses of
1.1 dB/facet and propagation losses as low as 0.6 dBcm-1. Optical waveguides have been also inscribed in
periodically poled lithium niobate by femtosecond laser pulses with the same multiscan technique. Second harmonic
generation experiments from a fundamental wavelength of 1567 nm demonstrate that the nonlinear optical
coefficient in the waveguides is preserved, yielding a conversion efficiency of 18% W-1.
We have measured the phase sensitivity to temperature of a fibre Fabry-Perot (FFP) formed by splicing multimode
tellurite (TeO2) glass fibre to singlemode silica fibre. The free spectral ranges of two FFPs of different lengths were
consistent with the values expected from independent determinations of the core refractive indices. The phase sensitivity
was 89.3 ± 0.3rad m-1 K-1 at a central wavelength of 1536 nm, compared with 99.8 rad m-1 K-1 for silica fibre.
Er-doped silicate thin films were deposited by the pulsed laser deposition technique, starting from an Er-doped silicate glass of composition: 65%SiO2 - 3%Al2O3 - 11%Na2O - 10%PbF3 - 10%LaF3 - 1%ErF3. The irradiations were performed with an ArF excimer laser (pulse length ~ 30 ns) in a dynamic flow of oxygen at a pressure of 5 Pa. The laser fluence at the target surface was about 2 J/cm2. The films were deposited on pure silica substrates, either at room temperature or heated to 200°C. The morphology of the films was studied by using optical microscopy, scanning electron microscopy and atomic force microscopy. The optical transmission of the films in the NIR-visible-UV regions (200-2500 nm) was recorded by using a double beam spectrophotometer. The optical spectra were analyzed by a computer code to evaluate the refraction index n and the extinction coefficient k along with the film thickness. The optical transmission was performed soon after the deposition and after one month to evaluate the aging effects. The films deposited at room temperature presented cracks over all the area of the film when submitted to SEM inspection or ion etching. Films deposited at 200°C remained undamaged. Optical waveguides were fabricated in the films deposited at 200°C by ion etching. Very low losses (down to 0.74 dB/cm) were measured by the prism coupling technique.
The fundamental characteristics of supercontinuum filament formation in condensed media are investigated along with several novel techniques for controlling their normally random distribution pattern. The utilisation of these supercontinuum filaments as light sources in remote sensing experiments is also discussed. In the first experiments a longitudinal imaging technique is used to scan through the filament and generate a plot of the intensity profile. From this profile an accurate measurement for the filament length and waist may be obtained. Using a similar set-up, the remarkably stable phase relationship is demonstrated between neighbouring filaments and the consequent interference pattern recorded. The second investigations present techniques for controlling both the radial distribution of filaments and the distance at which they form on the axis of propagation.
Three aspects of filament formation due to self-focusing are investigated. In the first the generation of a horizontal array of stable white light (super) continuum (WLC) filaments in water has been observed using a cylindrical plano-convex lens. Far field interference patterns are observed suggesting that the optical paths and phase stability between neighbouring filaments is remarkably constant. The pattern created by a filament pair is similar to that due to a pair of Young’s slits. The experimental results agree well with theoretical predictions based on the number of fringes and the fringe spacing. These observations suggest that regular arrays of WLC filaments may be treated as phased arrays to steer the beam in the far field. In the second investigation the effect of beam quality on self-focusing has been studied. The small intrinsic aberration of a high quality TEM00 beam is shown to cause hot-spots leading to multiple filaments. In the third investigation a circular aperture is used to create a Fresnel diffraction pattern. It is shown that self-focusing (a pre-requisite for filament formation) occurs in the presence of the aperture but that no formation is observed when the aperture is removed, even though the beam has higher power well above the threshold for critical power. An analytical solution to the Huygens-Fresnel diffraction integral shows that the axial intensity oscillates between maxima and minima as the distance from the aperture increases and that filament formation coincides with the presence of an axial maximum.
In this work, we present a study of the third-order non linearities of two derivatives of the organic molecule 2- amino-1,2,3 triazole-quinone. The absorptive and refractive contributions to the nonlinearity were resolved using the z- scan technique with a 88ps pulse source at 532nm. For one of the materials, the open aperture z-scan result show a nonlinearity arising from a combination of two-photon and saturable absorption, and the closed aperture the signature of a negative n2.
In this paper, we study the ablation behavior of polyetheretherketone polymer samples of 100 micrometers thickness when irradiated with visible laser light of 445 nm wavelength and 200 fs pulse width at 82 MHz pulse repetition rate. Etch depths corresponding to a particular laser power were measured at room temperature in open air, by counting the number of pulses required to perforate the sample. The laser focal spot size has been measured to be 0.03 mm2 and was kept constant during the experiment. The surface topology of the polymer samples was investigated by Atomic Force Microscopy in the continuous contact mode. The obtained Atomic Force Microscopy images revealed no mechanical damage in the inner ablation crater wall. Plots of the ablation rate as a function of the laser power, both in a linear and a logarithmic scale, show a large deviation from a straight line. These findings along with the high average power and suggest that Multiphoton Dissociation is the predominant mechanism responsible for ablation in this experiment.
Exciton interactions in ZnSe-based thin layers and quantum wells have been studied using ultrafast four-wave-mixing. We present results to show the influence of quantum confinement on the exciton-acoustic-phonon scattering rate, and compare the coherence lifetimes of excitons and biexcitons. Using a thin ZnSe layer, we reveal a new signature of quantum beating, namely polarization-state beating, which in turn reveals the role of excitation-induced dephasing in exciton interactions.
High-speed all-optical signal routing is achievable by designing new materials with fast response optical nonlinearities; in this context, third-order high nonlinear refractive index glasses are the best candidates. In this work, the characterization of the third-order nonlinear optical properties of a new chalcohalide glass and its application for the fabrication of high-efficiency nonlinear optical devices are presented. We report the measured values of high third-order optical nonlinear refractive index n2 and the two-photon absorption (beta) of a new ternary GeS2-Ga2S3-CsI chalcohalide glass. Improvements in the glass third-order optical nonlinearity were achieved by the inclusion of Ag-ions in the ternary glass composition. A theoretical analysis of signal propagation along a nonlinear directional coupler (NLDC) in twin-core fiber geometry and the interaction between the signal electromagnetic field and the waveguiding medium were considered in order to estimate the performance of the considered device for all-optical switching applications. All-optical switching, by controlling the optical pulse intensity in NLDC, has been numerically modeled in the second telecommunication window, and preliminary result support the suitability of this glass for these applications.
Polydiacetylene microcrystals in a water suspension or a polymer matrix were prepared by a simple precipitation procedure in order to obtain a thin samples with good optical quality maintaining the characteristics of crystalline materials. The nonlinear refraction, n2, and nonlinear absorption, (beta) , around the exciton peak were measured by z-scan method using a tunable picosecond laser. The signs of the n2 are positive/negative at wavelengths shorter/longer than the exciton peak and strong saturable absorption, which (beta) is negative, are observed around the peak. The largest n2 was obtained to be plus 0.024 cm2/GW (Re(chi(3) equals plus 1.4 multiplied by 10-9 esu) at 640 nm for a gelatin composite film containing 24 wt percent of polydiacetylene microcrystals.
Pulsed laser ablation is well established as a universal tool for surface processing of organic polymer materials. The effect of pulse duration on the ablation process is extended to ultrashort laser pulses, of picosecond and femtosecond pulse width. The main advantages of using such very short laser pulses for the ablation of polymers is that heat diffusion into the polymer material is negligible an the energy loss into the sample is minimized. As a results the ablation threshold is reduced and also high precision patterning of the sample without thermal damage of the surroundings becomes possible. For transparent materials, in the corresponding ablation wavelengths, the mechanism of ultrashort pulse laser ablation is complex involving a multiphoton photodissociation process. Ablation rates of polymers have been measured by irradiating the polymers with a variety of laser sources. The laser sources use were producing pulses of picosecond and subpicosecond duration, in order to investigate comparatively the ablation efficiency and the phenomenology of the etched patterns.
In this study, the interaction of picosecond and femtosecond pulsed laser radiation with human dental tissue was investigated experimentally, as this unexplored field is expected to be a potential alternative in powerful laser processing of biomedical structures. Dentin ablation rate experiments were performed by using teeth sections of different thickness. Dental tissue samples were irradiated in air with i) a regenerative amplifier laser at 1064 nm, pulse duration 110 ps, ii) the second harmonic laser at 532 nm, pulse duration 100 ps, and iii) a picosecond tunable dye amplifier at 595 nm, pulse width 800 fs. In all the experiments the pulse repetition rate was 10 Hz. The ablation rate per pulse at different energy fluence settings was calculated by measuring the time needed for the perforation of the whole dental sample thickness. Short laser pulses can confine thermal energy within the optical zone, which maximizes photothermal and photomechanical mechanisms of interaction. Tissue ablation rates were found to be comparable to or better than other nanosecond lasers, and left smooth surfaces, free of thermal damage.
The withdrawal technique is used to obtain the 9-BCMU polydiacetylene (PDA) in the red and blue phases. The ordered blue phase film has an exciton band edge which is shifted by 0.3 eV from that in the less ordered red phase film. The chiral 3-SMBU PDA is found to only exist in an amorphous form in the solid state and films of this polymer are obtained by casting onto a glass substrate. Its absorption edge is intermediate between those obtained in the 9-BCMU films. Refractive indices of 1.65 +/- 0.01 are obtained at 632.8 nm and 1.57 +/- 0.01 at 1.3 micrometers in the red phase 9-BCMU film using a prism coupling technique.
Polymeric composites containing microcrystals of 1:1 salts of tetracyano-p-quinodimethane, TCNQ, have been prepared. Re-crystallization of these organic one-dimensional semiconductors in-situ by either heat treatment or exposure to vapor yields uniform dispersions of crystallites of sub-optical dimensions. We report a strong dependence of the lowest energy electronic absorption band in the lithium and potassium TCNQ salt with crystallite size. The inter-molecular charge transfer band shows a shift of up to 0.5 eV between films where the crystallite size is a few microns and those where crystals cannot be observed optically. The non-linear refraction of a heat treated film containing TEA(direct sum)(TCNQ) (TEA(direct sum) - triethylammonium) is reported. (chi) (3) at 712 nm is 2.69 X 10-10 esu and is found to be a bleaching resonant non-linearity.
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