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abstract: The most frequently used mechanism of photon energy conversion in laser medicine is heating. Average heating of irradiated samples occurs with all methods of tissue destruction (cutting, vaporization, coagulation, ablation) At low light intensities the photochemical conversion of the energy absorbed by a photoacceptor prevails. This type of reaction is well known for specialized photoacceptors such as rhodopsin or chlorophyll. In medicine light absorption by non specialized photoacceptor molecules (that is, molecules that can absorb light at certain wavelengths, but that are not integral to specialized light reception organs) is used rather extensively. The absorbing molecule can transfer the energy to another molecule, and this activated molecule can then cause chemical reactions in the surrounding tissue. This type of reaction is successfully used in photodynamic therapy (PDT) of tumors. Alternatively, the absorbing molecule in a light activated form can take part in chemical reactions, as occurs in treatment of skin diseases with psoralens and UVA radiation (PUVA) Importantly, in both PDT and PUVA therapy the photoabsorbing molecules are artificially introduced into a tissue before irradiation. Irradiation of cells at certain wavelengths can also activate some of the native components. In this way specific biochemical reactions as well as whole cellular metabolism can be altered. This type of reaction is believed to form the basis for low power laser effects [1 41. One should note that light therapy methods based on photochemical conversion of photoabsorbing molecules are not laser specific methods [1] conventional sources generating the appropriate wavelength can also be used (as is done in PUVA and UV therapy) Laser sources are just handy tools providing many practical advantages (e.g. efficient fiber optic coupling to irradiate interior body parts, high monochromaticity and easy wavelength tunability, simplicity in use and electrical safety in the case of semiconductor lasers) It was shown that coherent effects in the light cells interaction occur at intensities 2x1015 W/m2 [1] Bear in mind that typical intensities used in low power laser therapy are in the range 1OlO2 W/m2. The successful use of light emitting diodes in low power laser therapy in the last years [5] proves that the coherence of light is of no importance in low power laser clinical effects. However, one should not forget about coherent laser light speckles which can cause local heating of inhomogeneous tissues [6] This possible mechanism forms a basis of laser thermotherapy [7] and supposedly can be taken into account at certain laser light parameters in low power laser therapy [8] when irradiating inhomogeneously absorbing tissues. For cell culture, the localized transient heating of absorbing chromophores is considered as one ofpossible primary mechanisms (Section 2.2) A photobiological reaction involves the absorption of a specific wavelength of light by the functioning photoreceptor (jhotoacceptor) molecule. To distinguish specialized photoreceptor molecules such as rhodopsin, phytochrome, bacteriorhodopsin and chlorophyll's from nonspecialized chromophores (molecules capable of absorbing the wavelength used for irradiation resulting in a photobiological response) below we will use the term photoacceptors to refer to the nonspecialized photoabsorbers. The photoacceptors take part in a metabolic reaction in a cell that is not connected with a light response. After absorbing the light of the wavelength used for irradiation this molecule assumes an electronically excited state from which primary molecular processes can lead to a measurable biological effect in certain circumstances. To work as a photoacceptor taking part in photobioregulation, this molecule must be part of a key structure that can regulate a metabolic pathway. Redox chains are example of this type of key structures which suit to these requirements. Possible photoacceptors (respiratory chain components) as well as chemical reactions occurring under illumination (primary reactions) or those following after the end of the irradiation (secondary reactions) are considered in Section 2:
author_names: Tiina I Karu
title: Mechanisms of low power laser light action on cellular level
venue: European Conference on Biomedical Optics
year: 2000

abstract: A strip of a liquid crystal elastomer doped with a near infrared dye with one side crosslinked monodomain and the other crosslinked polydomain along the thickness behaves like a multifunctional photoactuator without the need for a support. A flat strip with two ends fixed on substrate surface forms a moving bump under laser scanning, which can be used as light fueled conveyor to transport an object. Cutting off and laser scanning the bump with two free ends makes a soft and flexible millimeter scale crawler that can not only move straight and climb an inclined surface, but also undergo light guided turning to right or left as a result of combined out of plane and in plane actuation. Based on the self shadowing mechanism, with one end of the strip fixed on substrate surface, it can execute a variety of autonomous arm like movements under constant laser illumination, such as bending unbending and twisting, depending on the laser incident angles with respect to the strip actuator.
author_names: Feijie Ge

Rong Yang

Xia Tong

Franck Camerel

Yuer Zhao
title: A Multifunctional Dye doped Liquid Crystal Polymer Actuator: Light Guided Transportation, Turning in Locomotion, and Autonomous Motion.
venue: Angewandte Chemie
year: 2018

abstract: 1. The Basic Principles. 1.1 The Cutting Process. 1.1.1 Cutting Parameters. 1.2 How Materials Respond to Laser Light. 1.3 How CO2 Lasers Work. 1.3.1 The Storage and Release of Energy from a CO2 Molecule. 1.3.2 Lasing. 1.3.3 The Role of Nitrogen. 1.3.4 The Role of Helium. 1.3.5 Gas Mixture Cooling. 1.3.6 Gas Recirculation. 1.3.7 A Summary of the Basic Energy Exchanges in a CO2 Laser. 1.4 Laser Modes. 1.4.1 Mode Types. 1.4.2 A Quick Guide to TEM Nomenclature. 1.4.3 The Effect of Laser Design on Mode and the Implications for Materials Processing. 2. Laser Cutting Steels. 2.1 Mild steels. 2.1.1 Introduction. 2.1.2 The Cutting Mechanism for Mild Steels. 2.1.3 Cutting Speeds and Cut Quality. 2.1.4 Cut Initiation and Termination. 2.1.5 Techniques to Avoid Unwanted Material Burning. 2.1.6 The Effect of Sheet Surface Finish on the Cutting Process. 2.1.7 The Importance of Axial Symmetry of the Energy Input to the Cutting Zone. 2.1.8 Pulsed Laser Cutting of Mild Steels. 2.2 Alloy Steels. 2.2.1 Introduction. 2.2.2 Stainless Steels. 2.2.3 Low Alloy Steels. 2.2.4 Silicon Iron. 2.2.5 Free Cutting Steels. 2.2.6 Carbon Steels. 3. Cutting Non ferrous Metals. 3.1 Introduction. 3.2 Titanium Alloys. 3.2.1 Laser Oxidation Cutting. 3.2.2 Laser Inert Gas Cutting. 3.3 Aluminium Alloys. 3.3.1 Introduction. 3.3.2 The Effect of Surface Condition on Cutting (Anodising etc. 3.3.3 Pulsed Laser Cutting. 3.3.4 Piercing a High Reflectivity Metal. 3.4 Nickel Alloys. 3.5 Copper Alloys. 3.6 General Comments. 4. Cutting Non metals. 4.1 Introduction. 4.1.1 Melt Shearing. 4.1.2 Vaporisation. 4.1.3 Chemical Degradation. 4.2 Polymers. 4.2.1 Cutting by Melt Shearing. 4.2.2 Cutting by Vaporisation. 4.2.3 Cutting by Chemical Degradation. 4.2.4 Cutting Speeds for Polymers. 4.2.5 Cut Speed Forecasting for Polymers Using a Standard Processing Curve. 4.3 Wood Based Products. 4.4 Ceramics and Glasses. 4.4.1 Ceramics. 4.4.2 Glasses. 4.5 Composites and Miscellaneous Materials. 4.5.1 Composite Materials. 4.5.2 Miscellaneous Materials. 5. Setting Up for Cutting. 5.1 Introduction. 5.2 Beam Alignment. 5.2.1 Introduction. 5.2.2 Taking a Beam Print. 5.2.3 Alignment by the Beam Print Method. 5.2.4 Alignment by use of a HeNe Laser. 5.3 Finding the Focus Position. 5.3.1 Introduction. 5.3.2 The Blue Flash Test. 5.3.3 The Drilling Test. 5.4 Nozzles and Nozzle Alignment. 5.4.1 General Comments. 5.4.2 Practical Considerations. 5.4.3 Nozzle Alignment. 5.4.4 The Gas Dynamics of Nozzles. 5.5 Jigging. 5.5.1 Methods of Supporting the Workpiece. 5.5.2 Laser Cut Supports. 5.5.3 Chutes. 5.5.4 Overlay Jigging for Flimsy Materials. 5.5.5 Clamps and Vices. 5.6 Tuning the Laser. 5.6.1 Fine Tuning. 5.6.2 Mode Prints. 5.6.3 Use of a Laser Beam Analyser. 5.6.4 Laser Alignment. 6. Troubleshooting. 6.1 Introduction and Checklist. 6.2 Notes on Checklist. 6.2.1 Nozzle Contamination. 6.2.2 Laser Power Level and Type. 6.2.3 Cutting Speed. 6.2.4 Cutting Gas Type, Pressure and/or Flow Rate. 6.2.5 Nozzle Material Standoff. 6.2.6 Nozzle Type, Condition and Alignment. 6.2.7 Material Specification. 6.2.8 Lens Type, Condition and Alignment. 6.2.9 Laser Mode Quality. 6.2.10 External Mirror Condition and Alignment. 7. Safety Guidelines. 7.1 Introduction. 7.2 Beam Exposure. 7.2.1 Skin Damage. 7.2.2 Eye Safety. 7.3 Fumes. 7.3.1 Metals. 7.3.2 Non metals. 7.4 Electrocution. 7.5 Fires. 8. Alternative Cutting Methods. 8.1 Nd:YAG Laser Cutting. 8.1.1 The Principle of Operation of Nd: YAG Lasers. 8.2 Plasma Arc Cutting. 8.3 Abrasive Water Jet Cutting. 8.3.1 Abrasive Free Fluid Jets. 8.4 Oxygen Flame Cutting. 8.5 A Summary of the Strengths and Weaknesses of Each Process Compared with CO2 Laser Cutting. 9. The Physics and Design of CO2 Lasers. 9.1 Introduction. 9.2 The Physics of CO2 Lasers. 9.2.1 The CO2 Energy Spectrum. 9.2.2 Interactions in a Pure CO2 Laser. 9.2.3 The Role of Nitrogen. 9.2.4 The Role of Helium. 9.2.5 The Energy Cycle of Mixed Gas CO2 Lasers. 9.2.6 Population Inversion. 9.2.7 Laser Modes and TEM Nomenclature. 9.3 Aspects of the Design of CO2 Lasers. 9.3.1 Introduction. 9.3.2 Methods of Exciting the Lasing Gas Mixture. 9.3.3 Methods of Cooling the Lasing Gas Mixture. 9.3.4 Designs of Optical Cavity. 10. Some Aspects of the Physics and Chemistry of Laser Cutting. 10.1 Introduction. 10.2 The Energy Balance in the Cut Zone and Its Relationship to the Efficiency of the Process. 10.2.1 The Effect of Decreasing the Material Thickness on Cutting Speeds. 10.2.2 The Limits on Material Thickness. 10.2.3 The Implications of the Energy Balance Argument to Changes in Laser Power. 10.3 The Role of Oxidation when Cutting Steels. 10.3.1 Mild Steel. 10.3.2 Stainless Steel. 10.4 Conductive Losses Experienced when Cutting Steels. 10.5 Notes on Reflected, Transmitted, Radiated and Convective Losses from the Cut Zone. 10.5.1 Reflected and Transmitted Losses. 10.5.2 Radiation and Convective Losses. 10.6 Notes on the Focusing Characteristics of CO2 Lasers. 10.6.1 Theoretical Focused Spot Size and Depth of Focus. 10.6.2 The Discrepancy Between Theory and Practice. 10.6.3 Concluding Comments. 11. Bibliography and Further Reading. 11.1 Conference Proceedings and Journals. 11.1.1 Conference Proceedings. 11.1.2 Journals. 11.2 Books. 11.3 Specific Papers. 11.3.1 General Reviews of Laser Processing and Cutting. 11.3.2 Cutting Metals. 11.3.3 Cutting Non metals. 11.3.4 Theoretical Analyses of the Cutting Process. 11.3.5 CO2 Lasers. 11.3.6 Nozzle Design and Gas Dynamics. 11.3.7 Laser Beam Analysis. 11.3.8 Alternative Cutting Methods. 11.4 Miscellaneous References. 11.5 Further reading update for the second edition of this book.
author_names: John Powell
title: CO[2] laser cutting
year: 1993

abstract: Abstract The mechanisms of melt ejection and striation formation in continuous wave laser cutting of mild steel are discussed. Melt ejection from the cutting front is shown to be a cyclic phenomenon. Striation formation is strongly affected by the oscillatory characteristic of the thin liquid film on the cutting front during melt ejection, together with the oxidation and heat transfer process. Cutting speed determines whether the liquid film will rupture or generate waves on the cutting front. Theoretical explanations are given according to the instability theory of a thin liquid film in a high velocity gas jet and the diffusion controlled oxidation theory. Striation frequency and depth are predicted according to the above theories. Experimental investigations were carried out and the results are consistent with the calculations. The better understanding has shed light on further investigations and optimal process development.
author_names: Kai Chen

Y Lawrence Yao
title: Striation Formation and Melt Removal in the Laser Cutting Process
year: 1999

abstract: Abstract The present study examines the combined effects of chemical reactions taking place between a gas jet and molten metal, the cooling effect of the jet and the evaporation of metal, during a CO 2 laser cutting process. A laminar boundary layer approach was used to develop a theoretical model for the oxygen gas jet laser cutting mechanism. An experiment was carried out to monitor the keyhole formation using a video recorder and detect the light emitted from the entrance and exist surfaces of the workpiece using a fibre optic probe during the cutting process. The experimental study was extended to employ two different workpiece materials (stainless steel and mild steel) at two thicknesses, and varying oxygen assisting gas pressures. It is found that the theoretical model developed in the present study is valid for a cutting speed of about 30 mm s 1 and all jet velocities up to sonic, since the effect of shock is excluded in the model.
author_names: Bekir Sami Yilbas

Ahmet Z Sahin
title: Oxygen assisted laser cutting mechanism a laminar boundary layer approach including the combustion process
year: 1995

abstract: Abstract The objective of this study is to experimentally investigate the cutting mechanisms of polycrystalline diamond compacts (PDC) using two different lasers: (a) a near infrared Nd:YAG laser (Neodymium doped Yttrium Aluminum Garnet) of 1064 nm wavelength and 100 ms pulse width; and (b) a green light KTP (Potassium Titanyl Phosphate)/Nd:YAG laser of 532 nm wavelength and 120 ns pulse width. To realize the objective, the study applies polishing, lapping and etching processes to the cut surfaces of the PDC samples. It further observes and analyzes the processed cut surfaces with scanning electron microscopy (SEM) and Raman spectroscopy. A discussion is provided to reveal the underlying physics of the laser cutting mechanisms, and a conclusion is drawn based on the outcomes from the experimental investigation and the discussion.
author_names: G F Zhang

Bi Zhang

Zhaohui Deng

J F Chen
title: An Experimental Study on Laser Cutting Mechanisms of Polycrystalline Diamond Compacts
year: 2007

abstract: Abstract The relationship between fiber dislocations and fiber cutting during the enzymatic hydrolysis of lignin free fibers was investigated. Dislocations are morphological changes in the microfibril direction of cellulose. It was hypothesized that enzymatic activity is concentrated at dislocation sites, resulting in fiber cutting. Bleached softwood kraft pulp was analyzed during enzymatic hydrolysis (Novozynme 188 and Celluclast 1.5L) to confirm fiber cutting and explore its relationship with dislocation sites via microscopy, fiber length, and sugar determination. Results reveal that fibers were quickly cut through during enzymatic hydrolysis, resulting in shorter fiber fragments during the initial 60% of hydrolysis. Polarized light microscopy images show a relationship between dislocations and fiber cutting during enzymatic hydrolysis. Images show fibers partially cut at dislocation sites, and that there are no dislocations on cut fibers after 6 h hydrolysis. PLM and FQA data revealed that there were about seven major dislocations per fiber and each fiber was cut through about 6, and 10 times after 4 and 6 h of enzymatic hydrolysis, respectively. In combination, this data strongly implicates dislocation sites as the location of fiber cutting.
author_names: Kim Gail Clarke

Xinping Li

Kecheng Li
title: The mechanism of fiber cutting during enzymatic hydrolysis of wood biomass
year: 2011

abstract: Optical coherence imaging can measure hole depth in real time >20 kHz) during laser drilling without being blinded by intense machining light or incoherent plasma emissions. Rapid measurement of etch rate and stochastic melt relaxation makes these images useful for process development and quality control in a variety of materials including metals, semiconductors, and dielectrics. The ability to image through the ablation crater in materials transparent to imaging light allows the guidance of blind hole cutting even with limited a priori knowledge of the sample. Significant improvement in hole depth accuracy with the application of manual feedback from this imaging has been previously demonstrated [P. J. L. Webster et al. Opt. Lett. 35, 646 (2010) However, the large quantity of raw data and computing overhead are obstacles for the application of coherent imaging as a truly automatic feedback mechanism. Additionally, the high performance components of coherent imaging systems designed for their traditio.
author_names: Paul J L Webster

Logan G Wright

Kevin D Mortimer

Ben Y C Leung

Joe X Z Yu

James M Fraser
title: Automatic real time guidance of laser machining with inline coherent imaging
year: 2011

abstract: Abstract An experimental investigation of carbon fiber reinforced plastic (CFRP) composite processing with a high power pulsed fiber laser was conducted. A CFRP plate was irradiated with laser light from a pulsed fiber laser with an average power of 125 W, a repetition rate of 167 kHz and a pulse width of 10 ns. The wavelength of the laser light was 1064 nm. A galvanometer scanner was used as the processing head for high speed scanning of the pulsed laser light. A hatching distance was introduced, and the processing rates were measured according to the parameters of hatching distance and scanning speed. The walls at the grooves irradiated by laser light were observed using scanning electron microscopy (SEM) and cross sectional profiles of the processed CFRP were measured using confocal laser scanning microscopy (CLSM) The kerf width was measured by optical microscopy observation of the CFRP sample surface processed by laser irradiation. The growth mechanism of the kerf and heat affected zone (HAZ) structures was investigated based on cross sectional SEM micrographs of the kerfs. The optimal hatching distance for the target groove depth is discussed, together with the importance of the hatching distance for high speed and high quality processing of CFRP. The results indicate that adjustment of the hatching distance and the scanning speed are important for obtaining both good cutting speed and quality.
author_names: Kunitoshi Takahashi

Masahiro Tsukamoto

Shin-ichiro Masuno

Yuji Sato

Hidetsugu Yoshida

Koji Tsubakimoto

Hisanori Fujita

N Miyanaga

Masayuki Fujita

Hidenari Ogata
title: Influence of laser scanning conditions on CFRP processing with a pulsed fiber laser
year: 2015

abstract: Abstract BaGd 2 ZnO 5 Dy 3+ Yb 3+ was successfully synthesized by the sol gel method. The crystal structure and morphology of the samples were characterized by X ray powder diffraction (XRD) and scanning electron microscope. Under 3 W 971 nm near infrared laser excitation, the near infrared emission of phosphors was intense. The dependence of the up conversion emission intensities of the phosphors on the working power of laser diode was obtained as an evidence for the energy transfer from Yb 3+ to Dy 3+ In comparison, under excitation of 354 nm light, the near infrared quantum cutting for the BaGd 2 ZnO 5 Dy 3+ Yb 3+ samples is proved by the emission spectra. The corresponding quantum cutting mechanisms are discussed through the energy level. The maximum quantum efficiency calculated approaches 158.0% This study has the prospect to be applied in silicon based solar cells to promote the conversion efficiency.
author_names: Yanmin Yang

Lin-lin Liu

Shuzhen Cai

Fuyun Jiao

Chao Mi

Xian-yuan Su

Jiao Zhang

Fang Yu

Xiaodong Li

Ziqiang Li
title: Up conversion luminescence and near infrared quantum cutting in Dy3+ Yb3+ co doped BaGd2ZnO5 nanocrystal
year: 2014