Krishnan Venkatakrishnan

Synthesis of three-dimensional calcium carbonate nanofibrous structure from eggshell using femtosecond laser ablation

BackgroundNatural biomaterials from bone-like minerals derived from avian eggshells have been considered as promising bone substitutes owing to their biodegradability, abundance, and lower price in comparison with synthetic biomaterials. However, cell adhesion to bulk biomaterials is poor and surface modifications are required to improve biomaterial-cell interaction. Three-dimensional (3D) nanostructures are preferred to act as growth support platforms for bone and stem cells. Although there have been several studies on generating nanoparticles from eggshells, no research has been reported on synthesizing 3D nanofibrous structures.ResultsIn this study, we propose a novel technique to synthesize 3D calcium carbonate interwoven nanofibrous platforms from eggshells using high repetition femtosecond laser irradiation. The eggshell waste is value engineered to calcium carbonate nanofibrous layer in a single step under ambient conditions. Our striking results demonstrate that by controlling the laser pulse repetition, nanostructures with different nanofiber density can be achieved. This approach presents an important step towards synthesizing 3D interwoven nanofibrous platforms from natural biomaterials.ConclusionThe synthesized 3D nanofibrous structures can promote biomaterial interfacial properties to improve cell-platform surface interaction and develop new functional biomaterials for a variety of biomedical applications.

A femtosecond laser-induced periodical surface structure on crystalline silicon

A laser-induced periodic surface structure (LIPSS) has attracted research interest for its promising potential in micromachining for microelectronics and microelectromechanical systems. A femtosecond laser-induced periodical surface structure was investigated for polished crystalline silicon. The observed structure is similar to the classical ripples that are characterized by long, nearly parallel lines extending over the entire irradiated area on the metal and silicon surface after continuous or pulsed laser irradiation. The spacing of the ripples nearly equals the irradiation wavelength. The depth of these ripples increases nonlinearly with the fluence of irradiation. The orientation of these periodic structures is perpendicular to the vector of the electric field of the laser beam. It seemed that the pattern formed by a femtosecond laser complies well with conventional models. Unlike the patterns formed by a continuous or nanosecond pulsed laser, however, the spacing of the ripple formed by femtosecond pulses is not influenced by the incident angle of the laser beam. The formula used to predict the ripple spacing in the conventional model does not apply to the femtosecond laser induced ripple structure. A plausible explanation to this phenomenon is proposed. The effect of the pulse repetition rate was studied and it was found that a femtosecond laser oscillator generates the same periodic structure as the amplified laser system.

Phase-shifting interferometry immune to vibration

A modification of phase-shifting interferometry is proposed for microsurface profiling of flat surfaces under vibrating conditions. With this technique the required phase shift, achieved by quarter-wave plates and polarizers, is free of errors associated with motion. A nearly common optical-path configuration is achieved, and the effect of environment is reduced. The effect of environment on the optical system is also studied. Moreover, the measurement of phase is instantaneous, which increases the versatility of this technique to measure vibrating objects. Experiments were carried out on a smooth mirror surface excited with high-frequency vibrations, and the technique was found to be immune to vibrations of both high and low frequency.

Synthesis of bio-functionalized three-dimensional titania nanofibrous structures using femtosecond laser ablation.

The primary objective of current tissue regeneration research is to synthesize nano-based platforms that can induce guided, controlled, and rapid healing. Titanium nanotubes have been extensively considered as a new biomaterial for biosensors, implants, cell growth, tissue engineering, and drug delivery systems. However, due to their one-dimensional structure and chemical inertness, cell adhesion to nanotubes is poor. Therefore, further surface modification is required to enhance nanotube-cell interaction. Although there have been a considerable number of studies on growing titanium nanotubes, synthesizing a three-dimensional (3-D) nano-architecture which can act as a growth support platform for bone and stem cells has not been reported so far. Therefore, we present a novel technique to synthesize and grow 3-D titania interwoven nanofibrous structures on a titanium substrate using femtosecond laser irradiation under ambient conditions. This surface architecture incorporate the functions of 3-D nano-scaled topography and modified chemical properties to improve osseointegration while at the same time leaving space to deliver other functional agents. The results indicate that laser pulse repetition can control the density and pore size of engineered nanofibrous structures. In vitro experiments reveal that the titania nanofibrous architecture possesses excellent bioactivity and can induce rapid, uniform, and controllable bone-like apatite precipitation once immersed in simulated body fluid (SBF). This approach to synthesizing 3-D titania nanofibrous structures suggests considerable promise for the promotion of Ti interfacial properties to develop new functional biomaterials for various biomedical applications.

Synthesis of fibrous nanoparticle aggregates by femtosecond laser ablation in air.

In this article we report the synthesis of nanoparticles using femtosecond laser ablation with MHz pulse frequency at room temperature in air. Nanoparticles agglomerate by fusion, and form interweaving fibrous structures that show certain degree of self-assembly. It is found that there is a threshold-like pulse frequency at which fibrous nanoparticle aggregates start to form. The growth mechanism can be explained by existing theories regarding nanoparticle formation through femtosecond laser ablation. The threshold pulse frequency is in good agreement with the time to start nanoparticle formation, which has been derived numerically by previous analyses.

Angular dispersion compensation for acousto-optic devices used for ultrashort-pulsed laser micromachining

Ultrashort pulsed laser material processing is a new micromachining method that is gaining interest. Its capability of submicrometer machining has been proved. To obtain high speed and highly flexible beam steering, a two-axis acousto-optic deflector is employed. However, dispersion associated with acoustic-optic interaction will cause serious spatial deformation on the writing spot. The compensation for dispersion is proposed and studied. Experiments show promising results. An additional advantage of the proposed compensation method is that it can also precisely control the pulse number, and, hence improve the quality of ablation.

Noncontact acoustic optic scanning laser vibrometer for determining the difference between an object and a reference surface

A technique and apparatus for non-contact scanning measuring of the dynamic parameters of micro and macro devices using an acousto optic scanning laser vibrometer are disclosed. The system includes an acousto optic deflector to induce scanning in the laser beam. The apparatus also includes either a heterodyne or homodyne system for laser scanning. The heterodyne detection technique involves two acousto optic deflectors driven by a common signal generator. The invention may include an interference technique in which the measuring scanning beam emitted by the acousto optic deflector interferes with the reference-scanning beam. For some applications, this acts as a second measuring beam. With this technique, the frequency shift induced in the laser beam on scanning with the acousto optic deflector is canceled due to fact that the two acousto optic deflector are of same specification and driven by a common driver. The invention may also include an apparatus and technique for homodyne detection. A method adopting single and double acousto optic deflectors in the optical layout is also disclosed for homodyne detection system. The technique also incorporates two axes scanning. The invention may include a computer controller to control the scanning parameters and the data capturing parameters simultaneously. This technique incorporates a parallel scanning beam of small spot size for the purpose of micro device inspection in optical head, hard disk, micro components, etc. The use of the invention for measurement of least fly-height information in hard disk by double axis scanning is also disclosed. The method and apparatus for applying the invention to measuring the dynamic parameters of rotating targets and eliminating the error induced due to pseudo vibration noise is also disclosed.

Interconnect microvia drilling with a radially polarized laser beam

High-density interconnection via drilling using a diode-pumped solid-state laser is one of the industrially accepted techniques for 3D packaging and backside contacting applications. The advancement in microelectronic systems requires this technique to be improved constantly. In this paper, we introduce the technique of applying a radially polarized UV laser beam for interconnect via drilling. Microvias drilled with a p-polarized beam were compared to those drilled with a radially polarized beam. Results revealed that a radially polarized laser beam significantly improves the performance of laser drilling in all aspects, including feature size, efficiency, the cross-section profile and cleanness of the finishing.

The effect of polarization on ultrashort pulsed laser ablation of thin metal films

Ultrashort pulse lasers have proven to have superior advantages over conventional continuous wave and long pulse lasers for ablation of thin metal films. Though several investigations have been carried out to understand the phenomena of ultrashort pulse laser machining, the effect of the beam polarization on ablation of thin metal films has been seldom investigated. In this article, we report our recent observations on how the shape of the machined feature and also the damage threshold of the material varies according to the polarization of the ultrashort pulse laser beam. Based on this we have explained how the polarization of the beam controls the laser cutting rate, kerf width, edge quality, and ablation depth of the ablated feature.

Femtosecond pulsed laser ablation of thin gold film

Abstract Laser micromachining on 1000 nm -thick gold film using femtosecond laser has been studied. The laser pulses that are used for this study are 400 nm in central wavelength, 150 fs in pulse duration, and the repetition rate is 1 kHz . Plano-concave lens with a focal length of 19 mm focuses the laser beam into a spot of 3 μm (1/e2 diameter). The sample was translated at a linear speed of 400 μm / s during machining. Grooves were cut on gold thin film with laser pulses of various energies. The ablation depths were measured and plotted. There are two ablation regimes. In the first regime, the cutting is very shallow and the edges are free of molten material. While in the second regime, molten material appears and the cutting edges are contaminated. The results suggest that clean and precise microstructuring can be achieved with femtosecond pulsed laser by controlling the pulse energy in the first ablation regime.