ADVANCE ANALYTICAL CHEMISTRY

ASSIGNMENT # 2

Nitrogen , carbondioxide and dye laser,applications of laser in chemistry spectroscopy,environmental science medicine and industry.

Laser means amplification of light by stimulated emission of radiation. It is a device which gives us intense light in specific direction.stimulated emission is the process behind it which is responsible for the intensity of light.casual light sources are totally different from laser in four ways.

Directionality
Monochromaticity
Coherence
High intensity.

Coherence.

coherence can easily understood by the following picture

Monochromaticity

Ordinary light sources have light waves with many wave lengths .the emission of photons by ordinary light sources are out of phase.Thus, ordinary light does not show any coherence. Photons emitted by laser light are in phase show coherency .so the laser is monochromatic because all the waves in laser light hav single wave length.

Directional.

the laser beam have ability to concentrate on localized and small area due to its narrowness. It makes laser light directional.In space the laser light can spread in small region , so energy is concentrated in that region .

High intensity

Therefore, laser light has greater intensity than the ordinary light.

Types of Emission

There are TWO Types of emission

Spontaneous Emission 2. Stimulated Emission

Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level.

Spontaneous emission

Atoms ions or molecules in asystem, when absorb energy they would get excited and then drop down at its ground state after emission of light photon .the light we see all around us is due to spontaneous emission. Many processes are involve in thislike,phosphorescence,luminescence,fluorescence, light,chemiluminescence.

Spontaneous Emission

As there are more than one

– Absorbing photons of different energies

– Large number and types of transitons_

_Excited energy levels.

Hence we get emitted photons of

– Different direction.

– Different wavelenths

– Different frequency.

– Different energy.

Types of lasers

Lasers are classified into 4 types based on the type of laser medium used:

Solid-state laser
Gas laser
Liquid laser
Semiconductor laser
Excimer laser

Solid-state laser

In solid state laser , laser medium is solid.laser medium may be crystalline or glass. . In these lasers, glass or crystalline materials are used.Host material could be glass or crystalline.in the host material we can introduce impurities by the process of dopping .rare earth metals like cerium,terbium,erbium,are usually used as dopants .

Neodymium doped glass,Sapphire,neodymium-doppedyttrium ,aluminium garnet,are used as host material.A solid-state laser is a laser that uses solid as a laser medium. In these lasers, glass or crystalline materials are used.host material could be glass or crystalline.in the host material we can introduce impurities by the process of dopping .rare earth metals like cerium,terbium,erbium,are usually used as dopants .neodymium doped glass, yatterium-doped glass are used as host materials.

Ruby laser is the first solid state .Crystal of ruby is used as laser medium.it is capable of applications.Pumping source is light in laser medium.lamps ,flash lamps,flash tubes,laser(diode)are example of pumping source. Semiconductor laser cant be catagorised in this section,different electrical phenomena is involve in semiconductor laser.

Gas laser

A gas laser is different type of laser in which electric current is passed through the gas which causes the production of laser light. Laser medium should used in the form of gaseous state.when we require beam of very high quality and long coherent lengths in different applications we can use gas laser.laser medium can be called as gain medium.Gain medium Could be mixture of gases.Glass tube is used to pack the gases.which is used as active medium or laser medium.this is a first laser in which electrical energy is converted into light energy and produces laser beam in infra red region at 1.15um. examples ;neon(Ne),argon (Ar), cabonmonoxide(CO), carbondioxideCO2, nitrogenN2,hydrogenH2 , helium(He),excimer .

Efficiency and activity can be changed by using different types of medium.which is the basis to produce different types of laser.

Liquid laser In liquid lasergain medium or laser medium would be liquid .Energy would provided to the laser medium in the form of light.

A dye laser falls in the category of liquid laser.Organic dye in the form of solution used as a gain medium.

In dye laser ,dye is dissolves in a solvent. laser light generated from the excited states of organic dye which is dissolve in the solvent. It produces laser light in the region from UV to IR spectrum

Semiconductor laser

In our daily lifelaser play a very important role.These lasers are not so costly ,small in size,and use very small power.laser diodes and semiconductor laser are same lasers.

Solid state laser is contrasting to the semiconductor laser.They are non identical to each other.in semiconductor laser ,electric energy is used as driving force and in solid state laser light energy is used as driving force.

In semiconductor laser,the active medium is p-n junction of semi conductor .The amplifications is prodced in semi conductor material.

Excimer lasers

(the name is derived from the terms excited and dimers)

In this laser we use reactive gases, such as flourine and chlorine, mixed with noble gases such as xenon, krypton or argon. When electrically stimulated, dimera( pseudo molecule ) is produced. When lased, the dimer produces light in the ultraviolet region of spectrum.

Discussion about three basic types of laser

Nitrogen laser ( N2 Laser)

1.nitrogen laser invented in 1963,and used commercially in 1972.nitrogen gas is a laser medium or gain medium. Liquid nitrogen or nitrogen gas cylinders used can supply nitrogen in nitrogen laser.nitrogen laser depend upon the discharge of electricity through nitrogen gas.it emits light in UV region with great intensity and short pulse width.

2.Nitrogen laser has high input power aligned with vacuum pumps . the function of vacuum pump is to draw current of 20A at 110V,orto draw a small amperes at 220V .this whole procedure is done for cooling forced air system or liquid(water) coolants.

3.Electricity is used in nitrogen laser for excitation .when electric spark passess through spark gap in the laser, colliding of electrons with nitrogen atoms occurs which causes the excitation into meta stable stage . laser state is generated ,when photon having wavelength of 337 nm proceeds

the excited state ,stimulated emission occurs.

4.the gases has very restricted energy storage capacity .it begins due to the short upper state life time.N2 +,the single ionized species has line at 428nm which cannot be produced by the N2(STANDARD) laser .standard nitrogen is capable of giving and emitting line at 337.11 nm. It can also emit line at 357.60 nm in near IR , but other lines are not prominent in standard model.

Spatial coherence is small. The coherence length with this bandwidth can be considered in order of 1.0 mm. The pulsed N2 laser having efficiency of 0.11nm and the spectral band width is of 0.1 nm.

Laser type gas Pump source, electrical discharge ,Operating wavelength 337.1 nm ,

Pulse energy 170 micro j , Bandwidth 1.1nm

Construction and Operation

As in all lasers, nitrogen lasers typically consist of three basic parts:

1.an energy source (or pump),

a laser medium (also known as a gain medium),

3.and an optical resonator.

As in most gas lasers, power supply provide the energy sourceis an electrical discharge .

The gain medium or laser mediumis concentration of N₂ The laser medium is typically either nitrogen-helium mixture, simply air or Pure nitrogen.

Unlike all other lasers, nitrogen lasers can be operated without an optical resonator(the series of mirrors and windows used to amplify and direct emitted radiation) .
This is due to the fact that stimulation of nitrogen atoms results in amplified spontaneous emission (SAE) — also called as superluminescent light— by achieving population inversion within the gain medium.
Population inversionrefers to a state where more atoms exist in an excited state than in a low energy state. Nitrogen lasers may still include a single reflective mirror at the back of the laser to confirm correct output.

In nitrogen laser the building materials are cheaper and simple as compared to other powerfull gas lasers like excimers and cabondioxide lasers.

Nitrogen lasers use simpler and cheaper building materials when compared to more powerful gas lasers such as excimer and CO₂ lasers. the gas is often keep within a simple plastic (acrylic glass) chamber. The image that is shown below is a simple home assembled (non-commercial) nitrogen laser reflecting the simplicity of the parts involved. In this design, the spark gap at right provides the electrical discharge which is transmitted through the plates and into the gas chamber. The chamber itself is made of Plexiglas , while the sole optical components are a microscope slide and covers positioned at the output point.

Image credit: Nu Energy

Nitrogen lasers are exclusively ultraviolet (UV) devices which predominantly emit at 337.1 nm. Their operation consists solely of extremely short, powerful pulses; for more about pulsing and laser power, see the Laser Power section below.

Levels and Population Inversion

As stated above, nitrogen lasers achieve superluminescent emission by using population inversion. In order to accomplish this, they use three different energy levels (and are therefore classified as three-level lasers). It is important to note that only two of the energy levels are specifically important, as the third level represents the unexcited ground state.

A nitrogen laser’s upper energy level is directly pumped by a high voltage electrical discharge. This pumping is powerful enough to cause population inversion and stimulates the nitrogen atoms to immediately emit at 337.1 nm; the atoms remain at this level for approximately 20 ns. As the energy level drops the atoms fall into the second level and remain there for much longer (approximately 10 microseconds) than the first level. The effect of this time difference is that the atoms rapidly cease emission, resulting in a fast pulse of radiation. When considering this effect, it is clear that the pulse length is directly determined by the lifetime of the upper level, which in turn is dependent upon the gas medium’s pressure: the higher the pressure, the shorter the lifetime of the level (and pulse).

The graph below shows the three energy levels (as well as a fourth “meta-stable” level) of a typical nitrogen laser. The purple line represents the lasing action, which is incited by the pumping of the upper level and terminated at the second level.

Image credit: World of Lasers

Gas Pressure and TEA Lasers

Nitrogen lasers are generally constructed using one of two designs. The first is a “traditional” low-pressure design involving a vacuum pump; this one results in pulses 5-10 ns in length and is most suitable for pumping other lasers.

The second design is a higher-pressure one known as a TEA (transverse electrical discharge at atmospheric pressure) nitrogen laser. (It is important to note that, while TEA lasers are frequently nitrogen types, TEA designs for excimer and carbon dioxide lasers are common as well.) Nitrogen TEA lasers use plain “open” air at atmospheric pressure as their lasing medium, as air is 78% nitrogen. While TEA lasers require no vacuum pump to operate, they require a much faster electrical discharge to achieve effective pulses when compared to lower pressure designs. The home-built laser diagram above represents a TEA laser.

Applications

Compared to other gas lasers, nitrogen lasers are used in a relatively narrow range of applications. Because of their simple construction and inexpensive components, they are highly valued by beginning laser hobbyists. Practical nitrogen laser uses include:

Nondestructive testing (NDT)
Measurement of rapid processes such as time of flight (TOF), due to fast pulsing
Pumping of dye lasers
UV spectroscopy
Fluorescence measurement

Transverse optical pumping of dye lasers.} Treatment of nonhealing wounds, pulmonary tuberculosis, etc } Measurement of air pollution } Other major applications of nitrogen lasers include: } Nitrogen lasers can be used for a wide range of applications in the UV-visible region. They can be easily coupled to a microscope for carrying out experiments in life science laboratories. They are also efficient sources for laser-induced fluorescence and photochemistry and general spectroscopy. }

Laser Power

Because nitrogen lasers produce pulsed output, manufacturers specify their power as average power. Average power is calculated by multiplying the energy within each pulse (in joules) by the number of pulses per minute . Nitrogen lasers in particular are capable of very powerful pulses (several megawatts per pulse)

CO2 Laser – Principle, Construction And Working

Introduction

The carbon dioxide laser (CO2 laser) was one of the earliest gas lasers to be developed

(invented by Kumar Patel of Bell Labs in 1964[1]), and is still one of the most useful.

Carbon dioxide lasers are the highest-power continuous wave lasers that are currently

available. They are also quite efficient: the ratio of output power to pump power can be

as large as 20%.

The CO2 laser produces a beam of infrared light with the principal wavelength bands

centering around 9.4 and 10.6 micrometers.

Amplification

The active laser medium is a gas discharge which is air-cooled (water-cooled in higher

power applications). The filling gas within the discharge tube consists primarily of:

Carbon dioxide (CO2) (around 10–20%)

Nitrogen (N2) (around 10–20%)

Hydrogen (H2) and/or xenon (Xe) (a few percent; usually only used in a sealed tube.)

Helium (He) (The remainder of the gas mixture)

The specific proportions vary according to the particular laser.

The population inversion in the laser is achieved by the following sequence:

1) Electron impact excites vibrational motion of the nitrogen. Because nitrogen is a

homonuclear molecule, it cannot lose this energy by photon emission, and its excited

vibrational levels are therefore metastable and live for a long time.

2) Collisional energy transfer between the nitrogen and the carbon dioxide molecule

causes vibrational excitation of the carbon dioxide, with sufficient efficiency to lead to

the desired population inversion necessary for laser operation.

3) The nitrogen molecules are left in a lower excited state. Their transition to ground

state takes place by collision with cold helium atoms. The resulting hot helium atoms

must be cooled in order to sustain the ability to produce a population inversion in the

carbon dioxide molecules. In sealed lasers, this takes place as the helium atoms strike

the walls of the container. In flow-through lasers, a continuous stream of CO2 and

nitrogen is excited by the plasma discharge and the hot gas mixture is exhausted from

the resonator by pumps.

Construction

Because CO2 lasers operate in the infrared, special materials are necessary for their

construction. Typically, the mirrors are silvered, while windows and lenses are made of

either germanium or zinc selenide. For high power applications, gold mirrors and zinc

selenide windows and lenses are preferred. There are also diamond windows and even

lenses in use. Diamond windows are extremely expensive, but their high thermal

conductivity and hardness make them useful in high-power applications and in dirty

environments. Optical elements made of diamond can even be sand blasted without

losing their optical properties. Historically, lenses and windows were made out of salt

(either sodium chloride or potassium chloride). While the material was inexpensive, the

lenses and windows degraded slowly with exposure to atmospheric moisture.

The most basic form of a CO2 laser consists of a gas discharge (with a mix close to that

specified above) with a total reflector at one end, and an output coupler (usually a semireflective

coated zinc selenide mirror) at the output end. The reflectivity of the output

coupler is typically around 5–15%. The laser output may also be edge-coupled in higher

power systems to reduce optical heating problems.

The CO2 laser can be constructed to have CW powers between milliwatts (mW) and

hundreds of kilowatts (kW).[2] It is also very easy to actively Q-switch a CO2 laser by

means of a rotating mirror or an electro-optic switch, giving rise to Q-switched peak

powers up to gigawatts (GW) of peak power.

Because the laser transitions are actually on vibration-rotation bands of a linear

triatomic molecule, the rotational structure of the P and R bands can be selected by a

tuning element in the laser cavity. Because transmissive materials in the infrared are

rather lossy, the frequency tuning element is almost always a diffraction grating. By

rotating the diffraction grating, a particular rotational line of the vibrational transition

can be selected.

DYE Laser

A dye laser uses a gain medium consisting of an organic dye, which is a carbon-based, soluble stain that is often fluorescent, such as the dye in a highlighter pen. The dye is mixed with a compatible solvent, allowing the molecules to diffuse evenly throughout the liquid

APPLICATIONS

Lasers have become so much a part of daily life. Laser applications are so numerous that it would be fruitless to try to list them all; however; there are some illustrative examples lasers are used today.

SCIENTIFIC APPLICATIONS

Lasers are used extensively in the scientific laboratory for awide variety of spectroscopic and analytic tasks. Two interesting

examples are confocal scanning microscopy and time-resolved spectroscopy.

Time-resolved spectroscopy

Time-resolved spectroscopy is a technique used to observe phenomena

that occur on a very short time scale. This technique has been used extensively to understand biological processes such a

photosynthesis, which occur in picoseconds (10412 seconds) or less. A fluorescing sample is excited by a laser whose pulse length is much shorter than the time duration of the effect being observed.

Then, using conventional fluorescence spectroscopy measurement techniques, the time domain of the fluorescence decay process can be analyzed. Because of the speed of the processes, mode-locked lasers are used as the exciting source, often with pulse compressions chemes, to generate pulses of the femtosecond (10415 sec) time scale, very much faster than can be generated by electronic circuitry.

Confocal scanning microscopy

Scanning microscopy is used to build up a three-dimensional image of a biological sample. In standard light microscopy, a relatively large volume of the sample is illuminated, and the resultant light gathered by the objective lens comes not only from the plane in focus itself, but also from below and above the focal plane. This results in an image that contains not only the in-focus light, but also the haze or blur resulting from the light from the out-of-focus planes. The basic principle of confocal microscopy is to eliminate the out-of-focus light, thus producing a very accurate, sharp, and high-resolution image

TIR and Fluorescence Correlation Spectroscopy

Fluorescence correlation spectroscopy measures the variation in fluorescence emission at the molecular level as fluorochromes

travel through a defined field. The data can then used to determine binding and fusion constants for various molecular interactions. Because the measured volumes are so small, measurements are typically made using single-photon or two-photon confocal microscopy.

Microarray scanning

In DNA research, a microarray is a matrix of individual DNA molecules attached, in ordered sets of known sequence, to a substrate which is approximately the size of a microscope slide. A single array can contain thousands of molecules each tagged with a specific fluorochrome. The array is then put into a microarray reader where each individual site of the matrix is individually probed by a variety of laser wavelengths at, or near, the excitation band of specific protein tags. The resulting fluorescence is measured and the fluorescence, position, and sequence data are stored in a computer database for later analysis.

. Laser pointing stabilityis important as the microarray wells are quite small and repeatability is needed to relocate cells. Power stability and low noise are also extremely important due to the small sample size and the resulting weak fluorescence signal. The most common lasers in use today for excitation are the blue

solid-state (473–488 nm), green solid-state (532 nm) and red diode (650–690 nm) lasers. Solid-state and semiconductor laser technology is chosen primarily for its compact size, reliability, and power efficiency. Other wavelengths, including violet (405 nm) and ultraviolet (375 nm) from diode lasers, are currently being tested forapplication in microarray-reading applications

INDUSTRIAL APPLICATIONS

High-power lasers have long been used for cutting and welding materials. Today the frames of automobiles are assembled using laser welding robots, complex cardboard boxes are made with laser-cut dies, and lasers are routinely used to engrave numbers and codes on a wide variety of products. Some less well-known applications include three-dimensional stereolithography and photolithography.

Three-Dimensional Stereolithography

Often a designer, having created a complex part on a CAD machine, needs to make a prototype component to check out the dimensions and fit. In many cases, it is not necessary for the prototype to be made of the specified (final) material for this checking step, but having a part to check quickly is important. This is where rapid prototyping, i.e., three-dimensional stereolithography, comes in. The stereolithography machine consists of a bath of liquid photopolymer, an ultraviolet laser, beam-handling optics, and computer control . When the laser beam is absorbed in the photopolymer, the polymer solidifies at the focal point of the beam. The component design is fed directly from the CAD program to the stereolithography computer. The laser is scanned through the polymer, creating, layer by layer, a solid, threedimensional model of the part.

Photolithography

Lasers are used throughout the manufacture of semiconductor devices, but nowhere are they more important than in exposing photoresist through the masks used for creating the circuits themselves. Originally, ultraviolet mercury lamps were used as the light sources to expose the photoresist, but as features became smaller and more complex devices were put on a single wafer, the mercury lamp’s wavelengths were too long to create the features. Approximately ten years ago, manufactures started to switch to ultraviolet lasers operating at approximately 300 nm to expose the photoresist.

Marking and Scribing

Lasers are used extensively in production to apply indelible, human and machine-readable marks and codes to a wide variety of products and packaging. marking semiconductor wafers for identification and lot control, removing the black overlay on numeric display pads, engraving gift items, and scribing solar cells and semiconductor wafers.

The basic marking system consists of a laser, a scanning head, a flat-field focusing lens, and computer control. The computer turns the laser beam on and off (either directly of through a modulator) as it is scanned over the surface to make the mark. Depending upon the application, scanning may occur in a raster pattern (typical for making dot-matrix marks) or in a cursive pattern, with the beam creating letters one at a time. The mark itself results either from ablation of the surface of the material, or by a photochemically induced change in the color of the material. High-energy pulsed CO2 and excimer lasers, is to shine the light through a mask containing the marking pattern and focusing the resulting image onto the marking surface.

Laser scribing is similar to laser marking, except that the scan pattern is typically rectilinear, and the goal is to create microscoring along the scan lines so that the substrate can be easily broken

apart. A wide variety of materials, including metal, wood, glass, silicon, and rubber, are amenable to laser marking and scribing. Each material has different absorption and thermal characteristic, and some even have directional preferences due to crystalline structure.Consequently, the type of laser used depends, to some extent, on them material to be marked (e.g., glass transmits the 1.06 mm output from a YAG laser but absorbs the 10.6 mm output from aCO2 laser). Other considerations are the size of the pattern, the speed of the scan, cosmetic quality, and cost. Currently, most volume marking applications are performed

with lamp-pumped YAG-based pulsed or Q-switched lasers. Pulsed and cw CO2 lasers make up the bulk of the remainder.

Noncontact measurement

There are many types of laser-based noncontact measurement techniques in use today including scatter measurement, polarimetry and ellipsometry, and interferometric measurement.

Scatter Measurement: In the semiconductor industry, patterns of material are deposited on a wafer substrate using photolithographic processes. Defects on the wafer can result in poor reliability, disconnects in circuitry, or complete circuit failure. Consequently manufacturers need to map the wafer to determine the defects’ location and size so that they can either be eliminated or avoided. To do this, they scan the wafer with a laser and measure backscatter with a

very sensitive photodetector array. Lasers used in this application have to have excellent pointing stability, constant wavelength and power stability to calculate the correct size of the defects through complex algorithms, and low noise so the little scatter the defect makes can be distinguished from the background laser light. Blue 488-nm argon ion lasers have been

the laser of choice for many years. However; as lithography has shifted to shorter and shorter ultraviolet wavelengths, however, we are beginning to see the metrologic techniques for wafer defect measurement also moving to shorter wavelengths. Ultraviolet diodeand solid-state lasers are likely to replace the ion laser in the next generation of instruments.

Polarimetry and Ellipsometry:

The optical phasethickness of a thin film can be carefully measured using polarimetry or ellipsometry. A beam of known polarization and phase state enters the thin film layer at an angle. The thin film has a known index of refraction

Surface film thickness measurement

The measured phase change in the reflected beam is then correlated to an optical phase thickness for that layer using the known

index of refraction. This technique can also be used with a thicker transparent media, such as glass, where changes in the polarization and phase state of a beam scanned across the substrate indicate

variations in index of refraction due to inclusions or stress-induced birefringence. The most common lasers used in these

applications are violet, red and near infrared single-emitter laser diodes and mid-visible diode-pumped solid-state lasers owing to their cw output, low noise, and compact sizes.

Interferometric Measurement: Interferometric measurement can be used for high-resolution position measurement as well as for

measuring waveform deformation of optical beams as they pass through a component or system.

The technique uses the wave periodicity of the light beam as a very fine ruler. The position of an object in the path of the beam is computed from the phase of the light reflected from it. Interference between the object beam and a reference beam provides measureable intensity variations which yield this phase information. Distance and velocity measurement can be performed for moving objects as long as the fringe-recording mechanism is paced with it.

Typical applications of this technique include positioning of masks for the lithography process, mirror distance correlation within an FTIR spectrometer, optical feedback in many high-resolution positioning systems, and determining the alignment and flatness of hard disk drive heads.

. In these cases, frequency-stabilized helium neon lasers or a solid-state lasers with frequency selective elements are used.

.

Industrial (cutting and welding)

Because of the high power levels available (combined with reasonable cost for the laser), CO₂ lasers are frequently used in industrial applications for cutting and welding, while lower power level lasers are used for engraving. It is also used in the additive manufacturing process of Selective laser sintering (SLS).

The common plastic poly (methyl methacrylate) (PMMA) absorbs IR light in the 2.8–25 μm wavelength band, so CO₂ lasers have been used in recent years for fabricating microfluidic devices from it, with channel widths of a few hundred micrometers. Because the atmosphere is quite transparent to infrared light, CO₂ lasers are also used for military rangefinding using LIDAR techniques

CLINICAL AND MEDICAL APPLICATIONS

One of the earliest applications of lasers in medicine was photocoagulation,

using an argon-ion laser to seal off ruptured blood vessels on the retina of the eye. The laser beam passed through the

lens and vitreous humor in the eye and focused on the retina, creating scar tissue that effectively sealed the rupture and staunched

the bleeding. Today, lasers are used extensively in analytical instrumentation, ophthalmology, cellular sorting, and of course, to correct vision. Many types of lasers are used in clinical applications including CO2 , solid state, and diode lasers, as well as array of gas lasers covering the spectrum from the ultraviolet to the infrared.

Flow cytometry

Flow cytometry is a technique used for measuring single cells.

it a key research tool for 1)cancer and immunoassay disease

research, 2)food industry for monitoring natural beverage drinks for disease-causing microbes.

In a basic cytometer, the cells flow, one at a time, through a capillary or flow cell where they are exposed to a focused beam of laser light . The cell then scatters the light energy onto a detector or array of detectors. The pattern and intensity of the scattered

energy helps to determine the cell size, and shape. In many cases the cells are tagged with a variety of fluorochromes designed to

selectively adhere to cells or cell components with specific characteristics. When exposed to the laser light, only those with the tag fluoresce. This is used in many systems to assist with separation or

sorting of cells or cellular components. The most popular lasers used in flow cytometry are the 488-nm

(blue) argon-ion laser and the 632-nm (red) and 594-nm (yellow) HeNe lasers. However, new violet, blue and red diode lasers and a variety of new DPSS lasers are entering the field.

Surgical Applications

Lasers are used in a variety of surgical and dental procedures from cutting tissue, vaporizing tumors, removing tattoos, removing

plaque, removing cavities, removing hair and follicles, resurfacing

of skin and of course, correcting vision. In many ways, medical applications are like materials processing applications. In

some cases material is ablated. In others tissue is cut or welded, and in yet others, photochemical changes are caused in blood vessels to encourage shrinkage and absorption. Understanding tissue absorption characteristics and reaction to wavelength and power are key.

Ultraviolet excimer lasers are used for vision correction because they can ablate material from the lens of the eye without causing

thermal damage which could blur vision or make the lens opaque. Ruby lasers are used for tattoo removal because many of the dyes break down when exposed to 694-nm radiation, yet the skin tissue is left undamaged. Cosmetic treatment of wrinkles, moles, warts, and discolorations (birth marks) is often accomplished with near infrared and infrared

lasers. These procedures are often assisted by topical or injected photosensitive chemicals that assist with selective absorption at

specific sites. Lasers are also used to treat macular degeneration, an overgrowth of veins and scar tissue in the retinal region, a condition associated with advancing age. In this procedure, the patient is injected with a selective dye, which enhances the absorption of laser light by the blood in the blood vessels. When the blood vessels absorb laser energy, they wither in size, uncovering the active retina. A multiwatt green DPSS laser is most commonly used for this application

because the green wavelength is not absorbed by the lens or aqueous portion of the eye, which allows the laser to affect only

the targeted veins.

Some examples of medical carbondioxide laser uses are laser surgery, skin

resurfacing (“laser facelifts”) (which essentially consist of burning the skin to promote

collagen formation), and dermabrasion. Also, it could be used to treat certain skin

conditions such as hirsuties papillaris genitalis by removing embarrassing or annoying

bumps, podules, etc

Medical (soft-tissue surgery)

Carbon dioxide lasers have become useful in surgical procedures because water (which makes up most biological tissue) absorbs this frequency of light very well. Some examples of medical uses are laser surgery and skin resurfacing (“laser facelifts“, which essentially consist of vaporizing the skin to promote collagen formation). CO₂ lasers may be used to treat certain skin conditions such as hirsuties papillaris genitalis by removing bumps or podules. CO₂ lasers can be used to remove vocal fold lesions,^[9] such as vocal fold cysts. Researchers in Israel are experimenting with using CO₂ lasers to weld human tissue, as an alternative to traditional sutures.^[10]

The 10.6 μm CO₂ laser remains the best surgical laser for the soft tissue where both cutting and hemostasis are achieved photo-thermally (radiantly). CO₂ lasers can be used in place of a scalpel for most procedures, and are even used in places a scalpel would not be used, in delicate areas where mechanical trauma could damage the surgical site. CO₂ lasers are the best suited for soft tissue procedures in human and animal specialties, as compared to other laser wavelengths. Advantages include less bleeding, shorter surgery time, less risk of infection, and less post-op swelling. Applications include gynecology, dentistry, oral and maxillofacial surgery, and many others.

The CO₂ laser at the 9.25 – 9.6 μm wavelength is sometimes used in dentistry for hard-tissue ablation. The hard-tissue is ablated at temperatures as high as 5,000 °C, producing bright thermal radiation.

Enviromental applications.

Climate change is one of our most immediate challenges. Deforestation and the burning of fossil fuels are the likely causes for the increased concentrations of greenhouse gases in the Earth’s atmosphere.

Lasers can be used to analyse the concentrations of these gases and even to monitor their effects on ecosystems.

Lasers to monitor and understand climate change

Climate change and poor air quality are major challenges in a world of increasing industrialisation and urbanisation. Understanding and monitoring the Earth’s atmosphere is of crucial importance. Laser Heterodyne Radiometer (LHR) has involved the development of a prototype instrument that could potentially be the forerunner of a new generation of satellite-based infrared monitoring instruments. The LHR has been developed to provide a unique combination of high spatial resolution (to locate gaseous emission sources and observe between clouds), high spectral resolution (to discriminate between types of gases), and high sensitivity to detect the tiniest concentrations of atmospheric constituents.

.Global cooling

Clouds are considered to have a substantial impact on climate change because of their role in both absorbing and reflecting heat transferred through the atmosphere. Pollutants such as the organic compounds produced when fossil fuels are burnt, are believed to affect the formation and growth of water droplets in clouds.

. Typical experiments involve spraying a mist of particles into a model ‘cloud chamber’ and using a microscope to focus a laser beam into it. One of the droplets eventually finds its way into the centre of the laser beam and is held in position via the intense light-field pressure of the laser, which acts like ‘optical tweezers’. Analysis of laser light scattered by the droplet underpins the development of complex computer models of the atmosphere and contributes to improving our understanding of climate change.

Monitoring forests with lasers

Important properties of ecosystems such as those found in tropical forests can be measured using a system of lasers attached to the underside of aircraft. Often covered with dense vegetation, these forests tend to be difficult to study on the ground or even with satellites.

A combination of advanced spectroscopic imaging and laser remote-sensing technologies can measure differences in ground elevation to an accuracy of within a few inches. This technology can provide insights into how changes in climate and land use can affect the structure, composition and functioning of ecosystems.

Using lasers to tackle oil spills

Oil spills represent a major environmental risk, often taking many years to clean up. But, for some bacteria, oil is the perfect meal! By introducing colonies of hydrocarbon-digesting bacteria, it may be possible to lessen the impact of oil spills and clean them more efficiently.

By capturing individual bacteria in laser optical tweezers and analysing their spectra, scientists can establish whether the bacteria have broken down particular chemicals.

Potential future applications of this research range from cleaning oil spills at sea, to the contaminated land under petrol stations.

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