Lasers, Chemistry tutorial

Introduction

Lasers are machines that create intense beams of light that are monochromatic, coherent, and highly collimated. The wavelength (colour) of laser light is tremendously pure monochromatic) when compared to other sources of light, and all of the photons (energy) that make up the laser beam have a fixed phase relationship (coherence) by respect to one another. Light from a laser typically has extremely low divergence. It can travel over huge distances or can be focused to an extremely small spot through a brightness which exceeds that of the sun. Since of such properties, lasers are utilized in a broad variety of applications in all walks of life. 

The basic operating principles of the laser were put forth via Charles Townes and Arthur Schalow from the Bell Telephone Laboratories in the year 1958, and the 1st actual laser, depend on a pink ruby crystal, and was revealed in the year 1960 via Theodor Maiman at Hughes Research Laboratories. Since that time, literally thousands of lasers have been discovered (including the edible "Jello" laser), but only a much smaller number have originate practical applications in scientific, industrial, commercial, and military applications. The helium neon laser (the 1st continuous-wave laser), the semiconductor diode laser, and air-cooled ion lasers have originate broad OEM application. In recent years utilize of diode-pumped solid-state (DPSS) lasers in OEM applications has been producing speedily. The term 'laser' is an acronym for (L)ight (A)mplification via (S)timulated (E)mission of (R)adiation.

The creation of the LASER is one of the most important developments in science and engineering. A thorough understanding and appreciation of the operation of this exclusive machine needs an understanding of the behaviour and the properties of light itself. The term 'LASER' is an acronym for Light Amplification via Stimulated Emission of Radiation, even though general usage nowadays is to utilize the word as a noun - laser - rather than as an acronym - LASER. A laser is a machine that generates and amplifies a narrow, intense beam of coherent light. To comprehend the procedures of amplification of light requires familiarity through the nature of light and absorption and emission of light. Atoms release radiation. For instance, 'excited' neon atoms in a neon sign release light that is usually radiated in random directions at random times. The consequence is incoherent light (a technical term for a jumble of photons going in all directions). To produce a coherent light (nearly monochromatic and going in one exact direction); it is imperative to discover the right atoms and an atmosphere that would build the all light given up to go in one direction.

There are many dissimilar kinds of lasers, through output wavelengths ranging from ultraviolet to far infra-red. Lasers are generally classified into solid, liquid and gas lasers. Most solid and gas lasers are only able to manufacture photons at a fixed wavelength, and this tends to limit their utility for chemical applications. Liquid lasers (that have dye solutions and nearly activate mostly in the visible and near UV) on the other hand can be adjusted (tuned) over a indeed wavelength range. A main benefit of lasers over other light sources is that they are capable of producing extremely vigorous and nearly monochromatic photons. The procedures, which go on inside them differing greatly from one laser type to another, hence, it is easy to become distracted by detail that might relate to one kind of laser only. The features explained here are those that lasers have in general.

Characteristics of Laser radiation 

The light released via lasers is dissimilar from that produced by more common light sources such as incandescent bulbs, fluorescent lamps, and high-intensity arc lamps. An understanding of the unique properties of laser light might be attained via contrasting it through the light generated via other, less exclusive sources.

Monochromaticity

Laser light consists of basically one wavelength, having its origin in stimulated emission from one set of atomic energy levels.  All light consists of waves travelling through space. The colour of the light is computed via the length of those waves, as demonstrated in Figure. 

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Figure: Comparison of the wavelengths of red and blue light

Wavelength is the distance over that the wave recurs itself and is symbolized via the Greek letter ? (lambda). Each colour of visible light has its own trait wavelength. White light consists of a mixture of many different wavelengths. A prism can be employed to disperse white light into its component wavelengths (colours), as in Figure. 

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Figure: Dispersion of white light by a prism 

All general light sources release light of many dissimilar wavelengths. White light encloses all, or most, of the colours of the visible spectrum. Ordinary coloured light consists of a wide range of wavelengths covering an exacting portion of the visible-light spectrum. A green traffic light, for instance, releases the whole green portion of the spectrum, in addition to several wavelengths in the neighbouring yellow and blue regions. The beam of a helium-neon gas laser, on the other hand, is a extremely pure red colour. It consists of a tremendously narrow range of wavelengths within the red portion of the spectrum. It is called to be nearly 'monochromatic,' or almost "single-coloured."  Near-monochromaticity is a unique property of laser light, meaning that it consists of light of approximately a single wavelength.  Perfectly monochromatic light can't be generated even by a laser, but laser light is many times more monochromatic than the light from any other source. In several applications, particular methods are employed to further narrow the range of wavelengths enclosed in the laser output and, therefore, to raise the monochromaticity. 

Directionality

Figure shows light being emitted from a light bulb in all directions. All conventional light sources release light in this manner. Devices such as automobile headlights and spotlights contain optical systems that collimate the emitted light these that it leaves the machine in a directional beam; though, the beam produced always diverges (spreads) more quickly than the beam produced via a laser.

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Figure: Conventional source 

Figure demonstrates the extremely directional nature of light generated through a laser. 'Directionality' is the trait of laser light that causes it to travel in a single direction inside a narrow cone of divergence.

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Figure: Directionality of laser light 

But again, perfectly parallel beams of directional light-which we terms to as collimated light-can't be generated. All light beams eventually spread (diverge) as they move through space. But laser light is more highly collimated, that is, it is far more directional than the light from any conventional source and therefore less divergent. In several applications, optical systems are employed via lasers to progress the directionality of the output beam. One system of this kind can create a spot of laser light only one-half mile in diameter on the moon (a distance of 250,000 miles).

Coherence 

Figure: depicts a parallel beam of light waves from an ordinary source traveling through space. None of these waves has any fixed relationship to any of the other waves within the beam. This light is said to be 'incoherent,' meaning that the light beam has no internal order. 

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Figure: Incoherent light waves 

Figure: illustrates the light waves inside an extremely collimated laser beam. All of such individual waves are in step, or "in phase," through one another at every point. 'Coherence' is the term utilized to explain the in-phase property of light waves inside a beam.

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Figure: Coherent light waves 

Just as laser light can't be totally monochromatic or perfectly directional, it can't have ideal coherence; yet laser light is far more coherent than light from any other source. Methods currently in utilize greatly develop the coherence of light from many kinds of lasers.

Coherence is the most essential property of laser light and distinguishes it from the light from other sources. Therefore, a laser might be described as a source of coherent light. The full significance of coherence can't be comprehended until other ideas have been introduced, but evidence of the coherence of laser light can be examined merely. 

In Figure, the beam of a low-powered laser strikes a rough surface, these as paper or wood, and is revealed in all directions. A portion of this light attains the eye of an observer several meters away. The observer will see a bright spot that shows to be stippled through many bright and dark points. This 'speckled' appearance is trait of coherent light, and is caused via a procedure said 'interference,' that will be discussed in a later module.

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Figure

High Irradiance Intensity  

Irradiance is the power of electromagnetic radiation per unit area (radiative flux) incident on a surface. 

Emission and Absorption of Light

A laser generates coherent light through a procedure termed 'stimulated emission.' The word 'LASER' is an acronym for "Light Amplification via Stimulated Emission of Radiation." A short conversation of the contact of light through atoms is necessary before stimulated emission can be explained.

Energy Levels in Atoms

An atom is the smallest element of an element, which maintains the traits of the component. An atom consists of a positive nucleus surrounded via a "cloud" of negative electrons. All neutral atoms of a specified element have the similar number of positive charges (protons) in the nucleus and negative charges (electrons) in the cloud. The energy content of atoms of a meticulous kind might fluctuate, though, depending on the energies enclosed via the electrons inside the cloud.  Each kind of atom can enclose only indeed amounts of energy. When an atom encloses the lowest amount of energy that is available to it, the atom is said to be in its "atomic ground state." If the atom encloses extra energy over and above its ground state, it is to be in an 'excited atomic state.' 

Figure is a simplified energy-level diagram of an atom, which has three energy levels. This atom can enclose 3 distinct amounts of energy and no others. If the atom has an energy content of El, it is in the atomic ground state and is incapable of releasing energy. If it encloses energy content E2 or E3, it is in an excited state and can liberate its excess energy, thereby dropping to a lower energy state. Real atoms might contain hundreds or even thousands of probable distinct energy states. The three-level mode is utilized here for purposes of clarity. 

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Figure: Atomic energy-level diagram 

Spontaneous Emission of Light

An atom in an excited state is unstable and will liberate spontaneously its surplus energy and return to the ground state. This energy liberate might take place in a single transition or in a series of transitions that involve intermediate energy levels. For instance, an atom in state E3 of Figure could reach the ground state via means of a particular evolution from E3 to El, or via 2 transitions, 1st from E3 to E2 and then from E2 to E1. In any downward atomic transition, an amount of energy equal to the difference in energy content of the 2 levels must be liberated through the atom. In many cases, this overload energy shows as a photon of light. A photon is a quantum of light having a trait wavelength and energy content; actually, the wavelength of the photon is computed via its energy. A photon of longer wavelength (these as that for red light) possesses less energy than one of shorter wavelength (these as that for blue light), as demonstrated in Figure. 

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Figure: Spontaneous emission 

In common light sources, individual atoms liberate photons at random. Neither the direction nor the phase of the consequential photons is controlled in any method, and many wavelengths generally are present. This procedure is termed to as 'spontaneous emission' since the atoms emit light impulsively, quite independent of any exterior influence. The light generated is neither monochromatic, directional, nor coherent. 

Stimulated Emission of Light

The coherent light of the laser is generated via a 'stimulated-emission' procedure (Figure). In this case, the excited atom is excited via an outside influence to release its energy (photon) in a particular way.

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Figure: Stimulated emission 

The stimulating agent is a photon whose energy (E3-E2) is exactly equal to the energy difference between the present energy state of the atom, E3 and some lower energy state, E2. This photon stimulates the atom to make a downward transition and emit, in phase, a photon identical to the stimulating photon. The emitted photon has the same energy, same wavelength, and same direction of travel as the stimulating photon; and the two are exactly in phase. Thus, stimulated emission produces light that is monochromatic, directional, and coherent. This light shows as the output beam of the laser.

Absorption of Light

Figure demonstrates another procedure that happens inside a laser. Here, a photon strikes an atom in energy state E2 and is absorbed via that atom. The photon ceases to exist; and its energy shows as enhanced energy in the atom that shifts to the E3 energy level. The procedure of absorption eliminates energy from the laser beam and decreases laser output. 

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Figure: Absorption of light 

Population Inversion

In order for a laser to generate an output, more light must be created via stimulated emission than is vanished through absorption. For this procedure to take place, more atoms must be in energy level E3 than in level E2, that doesn't occur under normal circumstances. In any huge collection of atoms in matter at any temperature T, most of the atoms will be in the ground state at an exacting moment, and the population of each higher energy state will be lower than that of any of the lower energy states. This is said a "normal population distribution." Under "normal" circumstances, each energy level encloses many more atoms than the energy level just above it, and so on up the energy lever ladder. For instance, at room temperature, if there are No atoms in the ground state of Neon (He-Ne laser) there are only 10-33No atoms in the 1st excited state, even fewer in the 2nd excited state and so forth. The population of the ascending energy levels reduces exponentially. Therefore, in any huge collection of atoms in matter at any temperature T, most of the atoms will be in the ground state at a particular instant, and the population of each superior energy state will be lower than that of any of the lower energy states. This is said a 'normal population distribution.' 

A population inversion survives whenever more atoms are in an excited atomic state than in several lower energy states. The lower state might be the ground state, but in most cases it is an excited state of lower energy. Lasers can generate coherent light via stimulated release only if a population inversion is present. And a population inversion can be attained merely through exterior excitation of the atomic population.

Elements of a Laser

All lasers have 3 essential components namely: energy source, the active medium and the feedback method. Energy source (Energizer): frequently electricity, but an incredibly intense light, flash tube, chemical reaction or even another laser can as well be utilized. The active medium (Amplifying medium): Can be a solid, a liquid or a gas. Whatever its physical form, the amplifying medium must enclose atoms, molecules or ions, an elevated proportion of that can store energy that is afterward liberated as light. The feedback mechanism: This consists of 2 mirrors or other highly reflective surfaces situated at each end of the active medium. Energizing the active medium the procedure of energizing an active medium is recognized in laser terminology as 'pumping' Pumping an amplifying medium via irradiating it through intense light is termed to as optical pumping. 

Amplification

A laser consists of a pumped active medium positioned between 2 mirrors (Figure). The purpose of the mirrors is to give what is explained as 'positive feedback'. This means basically that several of the light, which emerges from the active medium is revealed back into it for amplification. One of the mirrors is a total reflector, and reflects almost all of the light that falls upon it. The other mirror, known as the output coupler, is only partly reflective. The light that isn't reflected being transmitted through the mirror, and constitutes the production beam of the laser.

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Figure: Light amplification in the LASER cavity

The procedure of light amplification could be accounted for via a deliberation of the interaction of light through individual atoms inside the active medium. Light absorption is a resonant procedure, and so there will be no absorption if there is no pair of energy states these that the photon energy can raise the system from the lower to the upper state, for instance, there must be an energy matching between the photon and the division of the 2 electronic states. If an electron is previously in an excited state (an upper energy level, in contrast to its lowest possible level or 'ground state'), then an incoming photon for that the quantum energy is equivalent to the energy difference between its present level and a lower level can kindle a evolution to that lower level, producing a second photon of the similar energy. This procedure is said "stimulated emission". When a sizable population of electrons resides in upper levels, this situation is said a "population inversion", and it situates the stage for stimulated emission of multiple photons. This is the precondition for the light amplification that happens in a laser, and because the emitted photons have a specific time and phase relation to each other, the light has a high degree of coherence. The stimulated emission of light is the vital quantum procedure needed for the operation of a laser.  

Creating a population inversion

Population inversion is an indispensable prerequisite for laser action. Electrons generally reside in the lowest available energy state. This population inversion is the situation needed for stimulated emission to overcome absorption and so provide increase to the amplification of light. Easy absorption and spontaneous emission can't provide increase to amplification of light. A population inversion can't be attained through just 'two levels' (Figure) since the probabilities of absorption and spontaneous emission are precisely the similar. There should be a non-radiative evolution to a metastable excited level having a comparatively long lifetime. This long lifetime permits an elevated proportion of the active medium in the metastable level so that a population inversion can take place. In order to found a population inversion, the upper state must be populated via pump energy, and the lower state must be depopulated. This could be attained via using either the 3-level laser or the 4-level laser.

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Figure: A two-level system. Population inversion not achievable

In the 3-level scheme, the particles are 1st excited to an excited state higher in energy than the upper laser state (Figure). The particles then rapidly decay down into the upper laser state. It is significant for the pumped state to have a small lifetime for spontaneous production evaluated to the upper laser state. The upper laser state should have as long a lifetime (for spontaneous emission) as possible, so that the particles live long enough to be stimulated and therefore donate to the achieve. The gain is the factor via which the intensity of the light is increased via the active (amplifying) medium. In this 3-level laser system, the lower state is the ground state, so in order to depopulate it; a huge amount of pump energy must be put in so that the ground state is actually in lower concentration than the excited state. The only way to depopulate the ground state is to put in more and more pump energy. 

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Figure: 3-level and 4-level LASER systems

In the case of the "4-level" method (Figure), only modest pump energy might be adequate to found a population inversion between higher laser state (3) and lower laser state (4) if the upper state is comparatively long lived (metastable) and lower state is comparatively short lived (unstable). Laser transition takes place between the states (3) and (4) and so a quick depopulation of state (3) to the ground state is needed to make sure that the upper level is always complete and the lower level always (almost) empty (Figure).

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Figure: Population inversion in LASER systems

Applications of Lasers

Soon after its invention over 4 decades ago, the laser was spoken of as a 'solution in search of a problem'. Though, this status quickly vanished as the laser earned itself a reputation as a precious tool in many scientific, medical and industrial applications. The expansion of laser instruments that control at ultraviolet wavelengths and those depend on nonlinear substances has even broadened the spectrum of laser applications.

 The subsequent are several significant areas where lasers discover huge application:

 Scientific

  • Laser spectroscopy
  • Photochemistry
  • Photo physics
  • Raman spectroscopy

Medical

  • Cosmetic surgery (tattoo, hair, scar, stretch mark, sunspot, wrinkle & birthmark removal)
  • Eye surgery (LASIK, and so on...)
  • Laser scalpel (gynecological, urology, laparoscopic)
  • Dental procedures
  • Imaging

Military

  • 'Death ray': A hand-held device that might replace the gun as a weapon for infantry
  • "Laser cannon": Able to destroy tanks, ships and aircraft
  • Laser sight (to enhance the targeting of other weapons systems)
  • Saber 203 Laser Illuminator (U.S. Air Force)
  • laser target designator utilized to specify a target for a laser guided munition, for instance, missile

Industrial and Commercial

  • Cutting, welding, marking,
  • Isotope separation
  • Rangefinder / surveying,
  • LIDAR / pollution monitoring,
  • CD/DVD player,
  • Laser printing, scanners,
  • Laser engraving of printing plates,
  • Laser pointers, holography, laser light displays
  • Photolithography,
  • Optical communications.
  • Optical tweezers

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