Lasers//Section 3

Absorption Spectrum

A continuous spectrum interrupted by dark bands, observed when light passing through a substance is absorbed at specific frequencies. -Created by passing a continuous spectrum of electromagnetic waves through a collection of atoms. -Solar atomic spectroscopy revealed helium to be a new element almost three decades before it was first found on Earth.

Laser

Device that emits a narrow beam of single-wavelength, coherent light as a result of stimulated emission. -Acronym for "Light Amplification by Stimulated Emission of Radiation." -The output can very from a few thousandths of a watt (laser pointers) to several thousand watts (industrial laser cutters).

Spectral Lines

Differently colored lines forming a distinct image on a screen from a spectroscope.

Pauli Exclusion Principle

Each electron in an atom must occupy a unique quantum state. -Each electron in an atom must have a unique set of quantum numbers as well. -Each shell can contain two or more electrons, but each electron must have a unique set of quantum numbers. -In the simplest case of two-electron shells, the two electrons spin in opposite directions.

Atomic Number

Each element in the periodic table has this, which is equal to the number of protons in the nucleus of an atom of that element.

Emission Spectrum

The characteristic set of wavelengths emitted by a collection of excited atoms. -Bands of light at specific frequencies, emitted by a source of electromagnetic radiation.

Ions

Atoms can gain or lose electrons to become this, which has an overall negative or positive charge.

Lyman Series

Another set of hydrogen emission lines outside the visible spectrum that involves transitions to n = 1.

Paschen Series

Another set of hydrogen emission lines outside the visible spectrum that involves transitions to n = 3.

Photodiode

Another type of diode that produces an electric current when light is incident upon it, which is essential to the operation of solar cells.

Gas-Discharge Lamp

A light source consisting of a glass tube filled with a noble gas that is excited by an electric discharge between two electrodes. -The color depends on which gas is contained within the tube. -At the end of each tube are electrodes that are connected to a voltage source; the voltage causes electrons to leave one of the electrodes and vibrate back and forth within the tube, colliding with millions of atoms as they do so. -The collisions with the electrons transfer energy to the atoms, exciting their orbital electrons to higher energy levels.

Semiconductor

A material that can be made to conduct electricity under some conditions but not others. -One layer (the "n-type") contains excess electrons, whereas the other layer (the "p-type") contains "holes" where electrons could be accepted. -A barrier prevents electrons from moving across the boundary. -If a voltage is placed across the two semiconductor layers, the electrons will be energetic enough to cross the barrier and fill the holes in the adjoining layer. -The electrons lose energy; this energy is released in the form of a photon of visible light. -The color of the emitted light is determined by the depth of the energy "holes," which is a property of the elements used to create the semiconductor layers. -A larger drop in energy will correspond to a higher energy photon (blue), and and lower drop will correspond to a lower energy photon (red).

Population Inversion

A necessary condition for laser operation; occurs when more atoms within a system occupy a higher energy state than occupy a lower energy state. -Stimulated emission will continue as long as this exists within the active medium, but it will slow down and eventually stop if a majority of the atoms are no longer in the higher-energy state. -The excited state must be metastable to ensure that atoms will remain in that state long enough to sustain this.

Quantum Number

A number that indexes a particular quantum state of an atom. -Could take whole number values such as n = 1, n = 2, n = 3, and so on.

Doppler Effect

A perceived change in frequency of a wave due to relative motion between the source and receiver. -Because the exact frequencies of spectral lines due to measurements here on Earth are known, they can be compared with spectral lines from distant objects to determine the relative shift. -If the spectral lines are shifted toward the red side of the spectrum (lower frequency), then the star or galaxy is moving away from Earth.

Photons

A quantum of electromagnetic radiation that has energy proportional to its frequency. -Have zero mass, but they have energy and momentum.

Element

A substance composed of only one kind of atom.

Light-Emitting Diodes (LEDs)

A type of diode, consisting of a junction between two semiconductor layers, that emits light when an electric current is passed through it. -Like photodiodes in reverse. -Used in traffic lights, automobile brake lights, and electronic billboards and displays. -Have lifetimes approximately five times that of a CFL bulb, and forty times that of an incandescent bulb. -Consume less energy and do not have disposable risks.

Four-Energy Level Lasers

Adds an additional energy level above the ground state, which becomes the lower level for the laser emission transition. -The lower level of the laser transition is not the ground state, which makes it easier to maintain a population inversion between the laser transition states. -The lower level of the laser transition starts out nearly empty because it has a higher energy level than the ground state. -Only exciting a small fraction of the ground state atoms to the metastable state is sufficient to establish a population inversion.

Thermal Radiation

All matter that has a temperature above 0 K (absolute zero) radiates electromagnetic waves. -At room temperature, objects radiate mostly infrared waves, which cannot be detected by the human eye, but can be captured by an infrared-sensitive camera. -Emitted as a continuous range of wavelengths instead of a discrete set of spectral lines.

Optical Cavity

An arrangement of precisely aligned parallel mirrors that sustains stimulated emission, amplifies the laser light, and constrains the photons emitted during laser operation to a narrow beam. -One mirror is coated to be completely reflective (100 percent chance of reflection), whereas the other is coated to partially reflective (95 chance of reflection). -Photons that leak from the partially reflective mirror form the laser beam. -Also known as a laser cavity or optical resonator.

Diode

An electric device that allows electric charge to pass through it in only one direction. -These are commonly used in power supplies to convert alternating current (AC) to direct current (DC).

Atoms

Building blocks of the world. -Made up of a positively charged nucleus surrounded by electrons.

Incandescent Lamps

Consists of a glass globe enclosing a thin tungsten filament. -An electric current heats the filament to a temperature between 2000 K and 3300 K, at which point it emits a continuous spectrum of light and with an intensity peak in the infrared. -Argon is commonly used to fill these sources of light. -If oxygen reached the hot tungsten filament, it would undergo rapid oxidation and be destroyed. -Over time, the filament gradually evaporates until it breaks and no longer conducts electricity. -Halogen can slow the rate of evaporation of the tungsten filament and extend the lifetime of the bulb. -However, these are not particularly efficient because they convert less than 5 percent of the energy they consume into visible light-the rest is emitted as heat.

Semiconductor Diode Laser

Consists of two semiconductor layers, an n-type with an excess of electrons and a p-type with electron "holes" to be filled. -The semiconductor layers are separated by the active region. -Configured with reflective ends to form an optical resonator in the region between the semiconductor layers. -Stimulated emission occurs when a photon emitted by one electron transition triggers another electron to fill a hole, and so on, resulting in a coherent beam of light that emerges from one side of the diode. -Also known as laser diodes. -Operate at much higher currents than LEDS, typically ten times greater. -Most compact and easy to mass-produce; most common type of laser. -Their small size makes them well suited for use in low-power applications such as laser pointers, laser printers, and CD/DVD players. -They can also be operated at lower voltages than other types of lasers. -In many cases, an external lens is used to correct the shape of the beam, which contributes to the overall fragility of the laser since damage to the lens could render in non-functional. -The delicate nature of the semiconductors makes these more sensitive to static discharges and current.

Nucleus

Dense core that comprises most of the atom's mass. -Made up of positively charged particles called protons, as well as, neutrons, which have no electric charge. -Surrounded by negatively charged electrons; these electrons occupy "shells" that have different spatial distributions.

Collimated

Describes a beam of light with very little divergence, or spread. -This is a direct result of the precise alignment of the parallel mirrors that form the optical cavity. -As the light waves reflect back and forth many times within the cavity, the mirrors constrain the waves to an axis perpendicular to the surface of both mirrors. -Due to this property, lasers can be used as sharp scalpels in medicine; in industry, they can serve as fast, powerful, and computer-controllable cutting tools.

Metastable

Describes an energy transition with a relatively long lifetime. -Certain excited states can have lifetimes on the order of microseconds, or even milliseconds, as opposed to nanoseconds. -These states exist because certain electron transitions are less probable than others under the rules of quantum mechanics.

Monochromatic

Describes light that is made up of a single frequency. -However, this is some "spread" due to Doppler shifts from the motion of atoms or molecules within the same active medium. -The line width, or bandwidth of a laser describes the spread of its spectrum of emitted frequencies; this spread is extremely narrow compared to the spectrum of frequencies emitted by an incandescent light bulb.

Coherent

Describes waves that are in-phase; that is, the peaks of each wave are aligned with one another. -In laser light, this property is a direct consequence of stimulated emission.

Spectroscope

Emission spectra can be viewed with a measuring device that uses a prism or grating to separate light emitted by a collection of excited atoms into component wavelength. -The emitted light is passed through a thin slit and projected onto a viewing screen. -Each component color is refracted to a definite position on the screen, forming a distinct image of the slit as a narrow line.

Excited State

Energy levels that are higher than the ground state. -Electrons must accept energy, for example by absorbing a photon, in order to move from one energy level to a higher one. -Only certain frequencies of photons can be absorbed by an individual atomic system, and therefore each energy level diagram is quantized by the allowable transitions between states, which is unique for each type of atom.

Relationship between Peak Wavelength and Temperature

For any given temperature, there is a "peak" wavelength that is emitted with the greatest intensity. -Experimental evidence has shown that the peak wavelength is inversely proportional to the temperature of the object. -If the kelvin temperature of an incandescent object is doubled, the peak wavelength of emitted electromagnetic waves will be halved. -Allows for comparing the surface temperature of distant starts based on their distributions of emitted light. -Since the wavelength of violet light is just about half of red light, a violet-hot star has approximately twice the surface temperature of a red-hot star. -As the distribution peak overlaps the entire visible spectrum, the object will appear "white hot."

Quantum/Quantization

In 1900, a German physicist named Max Planck proposed that energy could only be emitted or absorbed in discrete bundles that are multiples of a fundamental unit of energy. -This relationship can be expressed as E = nhf. -h = 6.63 x 10^-34 Js is called Planck's constant. -f is the frequency of oscillation. -n is a positive integer. -Energy can be exchanged in amounts hf, but not anything in between. Because the energy can only take certain specific values, it is "quantized;" the quantized packets of energy are too small to be individually perceived by the senses.

Albert Einstein's Contribution to the Theory of Light Emission

In 1917, Albert Einstein proposed the concept of stimulated emission in a paper he published.

Willis Lamb and R.C. Retherford

In 1947, these scientists were the first to experimentally demonstrate stimulated emission.

Alfred Kastler

In 1950, this French physicist proposed the technique of optical pumping, whereby a collection of atoms could be excited to a higher energy state through absorption of incident light. -He was awarded the 1966 Nobel Prize in Physics for this contribution.

Charles Townes

In 1951, a professor at Columbia University devised a method for producing stimulated emission at microwave frequencies. -The idea was to pass a beam of excited ammonia molecules through a reflective cavity, where they would emit microwaves. -The cavity would contain and amplify the waves as they were emitted, thereby establishing a stimulated emission condition. -A small hole in one side of the cavity would permit the coherent beam of microwaves to escape. -This device was dubbed by the professor the "maser," which stood for "Microwave Amplification by Stimulated Emission of Radiation." -This professor began work on the maser with graduate students at Columbia named James P. Gordon and Herbert J. Zeiger; in 1954, his team demonstrated the first working maser using excited ammonia molecules. -Shared a 1964 Nobel Prize in Physics with Nikolay Basov and Aleksandr Prokhorov "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle."

Charles Townes and Arthur Schawlow

In 1957, these scientists explored the possibility of infrared light amplifiers but eventually moved on to visible wavelengths, which they called "optical masers." -They published a paper in 1958 confirming that masers operating at visible wavelengths were a theoretical possibility.

Electrical Pumping

Involves the use of an electric discharge or current to cause atomic excitation.

Lasers in the 1970s

Laser technology began to enter people's daily lives with a steady stream of practical applications. -On June 26, 1974, a pack of chewing gum became the first product ever scanned by a laser barcode reader. -In 1978, LaserDisc technology, which used HeNe lasers to read optical discs, became commercially available for the first time; this technology led to the introduction of audio CD players in 1982. -The first commercial laser printer was released by IBM in 1976. -In 1984 laser printing entered mainstream availability with Hewlett-Packard's introduction of the desktop-size HP Laserjet.

Fluorescent Lamps

Long, cylindrical glass tube filled with a low-pressure mercury vapor. -Electrodes connected to a voltage source discharge energetic electrons into the tube, where they collide with mercury atoms and excite them to a higher energy level. -The excited mercury atoms emit photons primarily with ultraviolet frequencies. -Upon absorbing an ultraviolet photon, the phosphors fluoresce, thereby emitting an assortment of low-frequency photons that are collectively perceived as white light. -However, they all produce an environmental and health hazard due to their high mercury content.

Fluorescence

Occurs when a material absorbs a photon and then de-excites by emitting a photon with a lower frequency (energy). -An atom excited by high-frequency ultraviolet light can take smaller steps to a lower energy state by emitting lower-frequency visible light as it returns to the ground state. -Fluorescence allows objects to glow vibrant colors when exposed to an artificial source of ultraviolet light. -Natural fluorescence occurs in various minerals responding to ultraviolet light. -The atoms in the mineral structures are excited by high-energy ultraviolet photons; as the excited electrons cascade down to lower energy levels, they emit photons with frequencies corresponding to the tiny energy-level spacings between each step. -Every excited atom emits a characteristic set of frequencies, and no two different minerals emit light of exactly the same color.

Theodore Maiman

On May 16, 1960 this scientist succeeded in demonstrating the first functional laser at Hughes Research Laboratories in California. -This laser used a synthetic ruby crystal the size of a fingertip to produce 694 nm red laser light. -He used a flash lamp to deliver an intense optical pulse that would excite a sufficient number of atoms. -Shortly after this demonstration, many other notable scientists succeeded in producing laser light.

Gordon Gould

Partnered with Charles Townes to work on optical pumping. -On November 13, 1957, this graduate student wrote an entry in his lab notebook wherein he used the term "laser," which is believed to be the first usage of the term. -He filed a patent application in April 1959, but it was rejected by the U.S. Patent Office; instead, the patent was awarded to Schawlow and Townes on behalf of Bell Labs, who had applied nine months earlier. -This scientist sued, and after thirty years of legal proceedings, he was awarded forty-eight patents, including optical pumping and a variety of laser applications.

Stimulated Emission

Photon emission that is triggered by the oscillating electric field from a passing photon with the same frequency. -The emitted photon is identical in frequency, phase, and direction to the first photon. -These identical photons are what make up a laser beam.

Phosphors

Powdery materials coating the inside of the tube. -The emitted ultraviolet light interacts with them. -Different ones can be used to coat the inside of the tube in order to produce different shades of white light.

Spontaneous Emission

Process in which the atom will return to a lower energy state by emitting a photon. -The random emission of a photon due to an atomic electron moving to a lower energy state. -Impossible to predict exactly when a specific excited atom will emit a proton. -The direction of the emitted photon is also random.

Incandescence

Production of light due to an object being at a high temperature.

Albert Einstein's Explanation of the Photoelectric Effect

Proposed that light itself is quantized into discrete energy packets that exhibit properties of particles. -This scientist argued that the photoelectric effect could only be explained as packets of light colliding with electrons, thereby transferring their energy and ejecting the electrons. -An individual photon from a higher-frequency beam of light carries more energy than a photon from a lower-frequency beam of light.

Compact Fluorescent Lamp (CFL)

Small fluorescent tube lamp that is twisted into a helical shape and attached to a screw-in base like that of an incandescent bulb. -More efficient than incandescent lamps and have average lifetimes around ten times longer. -However, General Electric announced that they would be phasing out the production of these in favor of LED bulbs.

Nikolay Basov and Aleksandr Prokhorov

Soviet physicists that suggested that optical pumping could be used to create a population inversion in a three-level quantum system. -This model correctly anticipated the configuration that would ultimately be used in the first laser. -They both shared a 1964 Nobel Prize in Physics with Charles Townes "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle."

Fraunhofer Lines

The Sun is an incandescent source of light, yet if we zoom in to the emission spectrum of sunlight, it is found that it is not perfectly continuous-instead, there are thin lines where certain frequencies of light are missing. -These were named after Joseph von Fraunhofer who first discovered them and determined their wavelengths. -Occur because the light emitted from the body of a star is absorbed by the atmosphere of cooler gases that surround it.

Photoelectric Effect

The emission of electrons from the surface of a material when it is illuminated with light of certain frequencies. -J.J. Thompson helped to determine this phenomenon. -Physicists originally attempted to use Maxwell's electromagnetic wave model to explain this, however, it later turned out to be incorrect due to the fact that electrons were emitted almost instantaneously, whereas the wave model predicted a time delay, as well as the idea that light below a certain frequency was not observed to eject any electrons, regardless of the intensity of light. -According to the existing theory, any frequency of light would be expected to eject electrons if the incident light was sufficiently intense. -Increasing the frequency of the incident light results in higher-energy electrons being emitted.

Phosphorescence

The emission of light from a metastable state; the emission process takes place over a longer time scale than fluorescence due to the lower probability of de-excitation. -"Glow in the dark" paint, toys, stickers, or light switches are examples. -Many such materials can be made to emit light by first exposing them to daylight; the visible light excites the atoms within the material to a higher energy level, where they remain in a metastable state while gradually undergoing de-excitation. -Unlike fluorescent materials, which stop glowing immediately after the external source of radiation is turned off, phosphorescent materials can glow for a considerably long time.

Balmer Series

The emission spectrum for hydrogen contains four distinct lines in the visible spectrum, which occur as a result of a hydrogen atom transitioning to the n = 2 state from a higher energy level.

Pump Source

The energy source for a laser; it excites atoms in the active medium to the metastable state.

The Bohr Model

in 1913, Niels Bohr suggested that the electron's energy was only allowed to take on particular values and could not be anything in between those values. -These energy levels correspond to specific fixed orbits around the nucleus.

Ground State

The lowest allowable energy level of the atom. -n = 1. -The electron is closest to the nucleus.

Wave-Particle Duality

The observation that lights exhibits properties of both particles and waves. -The photoelectric effect is a demonstration of the particle nature of light, whereas double-slit interference is a wave phenomenon.

Stacking Property of Electrons

The primary quantum number of the last ground state electron corresponds to the row the element occupies on the periodic table. -Helps explain some of the chemical properties observed within families of elements in the periodic table.

Excitation

The process of an electron moving from a lower to a higher energy level. -Occurs when the atom absorbs energy corresponding to a particular energy level transition. -This energy input can result from absorbing a photon, but can also come from kinetic energy due to collisions with other particles or thermal energy from being heated. -Once they are in an excited state, electrons can drop to lower energy levels by emitting a photon with energy equal to the difference in energy between the two levels. -Aurora borealis and aurora australis are examples.

Atomic Spectroscopy

The process of analyzing spectral lines to identify the chemical makeup of an excited sample. -Originated as a chemical analysis technique in the late 1800s.

Active Layer

The semiconductor layers in laser diodes are separated by a microscopic region that serves as the optical resonator.

Active Medium

The source of atoms that will undergo cycles of excitation and de-excitation to release photons that will form the laser beam. -Can be solid, liquid, or gas.

Interferometry

Use of superimposed waves to make extremely fine measurements of small displacements, and surface irregularities, or changes in refractive index. -This tool uses a beam splitter and mirrors to overlap beams of light from a coherent source such as a laser. -By slightly adjusting the path length difference between the two beams, shifts in the observed interference pattern will occur as the relative phases of the two beams change.

Optical Pumping

The use of an intense light source, such as a flash lamp, arc lamp, or external laser to excite atoms through photon absorption.

Excimer Laser

Type of gas laser that relies on the excitation of "dimer" molecules, such as argon fluoride, that are stable only in the excited state. -Were first demonstrated in the 1970s, and are capable of removing extremely fine layers of surface material by breaking molecular bonds without burning or heating the surrounding area. -Suited for delicate eye surgery such as LASIK.

Gas Laser

Uses a low-pressure gas mixture as an active medium. -Most of these are excited by passing an electric current through the gas, delivered by electrodes placed at opposite ends of the tube. -The helium-neon (HeNe) laser is common; it produces light in the visible spectrum at a wavelength of 632.8 nm. -The bright red output and relatively low cost of HeNe lasers make them well suited for many low-power applications in educational and research laboratories. -Carbon dioxide lasers are commonly used for high-power industrial applications such as welding and cutting. -The gas medium tends to be both relatively inexpensive and largely resistant to damage. -Still, these are typically larger than other types of lasers due to the low density of the medium.

Solid-State Laser

Uses an active medium consisting of a solid crystalline or glass rod (known as the host) containing light-emitting atoms (the active species). -The active species is identified first (typically by its chemical symbol) followed by the host material. -For instance, Ti:sapphire laser consists of titanium atoms in sapphire crystal. -The active species determines the laser transition, but its interactions with the host may shift the wavelength slightly. -The Nd:YAG (neodymium-yttrium aluminum garnet) is one of the most common types of laser (that emits infrared light), with applications in research, medicine, etc. -The host material is selected based on its optical, thermal, and mechanical properties.

Absorption Lines

What emerges from atoms is a continuous spectrum except for black lines, where specific wavelengths of light have been absorbed.

Thermodynamic Equilibrium

When a collection of atoms is not exchanging energy with its surroundings, the vast majority of the atoms tend to be in the lowest possible energy state.