Lasers and the Space Elevator Adaleena Mookerjee August 16, 2006 Center for Structures in Extreme Environments, Rutgers University

What are Lasers? LASER (light amplification by stimulated emission of radiation) A device that controls the way energized atoms release photons

Laser History Conceptually developed by Albert Einstein in 1917 Actually built by Theodore Maiman in the 1960s (Ruby laser) Previously, was not important at all Today, it is all around us from printers to barcode readers at the bookstore or mall

Basic Phenomena of Light Wave duality of matter: light is capable of behaving like a wave and particle

Light – The Wave Any electromagnetic radiation with a wavelength visible to the eye 3 properties of light: –Intensity (amplitude) – how bright something appears to the human eye –Frequency (wavelength) – color of light produced –Polarization – angle of vibration of light

Light the Photon Photons – quanta of electromagnetic radiation which can be light Bohr Model: –Made up of 3 subatomic particles Protons (positively charged subatomic particle) Neutrons (neutrally charged subatomic particle) Electrons (negatively charged subatomic particle) –Protons and neutrons are located in nucleus –Electrons are located in a hypothetical region surrounding the nucleus electrons neutrons protons

Producing Light Step 1: Light is emitted and photons are released, colliding with the orbiting electron Step 2: Collision causes change in velocity and position, making the electron absorb photon’s energy Step 3: Electron moves to a higher energy level or position around nucleus (excited!) Step 4: To return, electron releases energy from photon, producing light

Wave and Photon Correlation Speed of light (c) = 300,000 km/s Relationship between photon and wave: Where λ = wavelength and f = frequency

Terminology – Photons Population inversion: when a system of either a group of molecules or molecules exist in a state where more electrons are in an excited state than in the lowest energy level possible Absorption: photon with a particular frequency hits an atom at rest & excites it to a higher energy level while photon is absorbed Spontaneous emission: atom in an excited state emits a photon with a particular frequency & returns to ground state Stimulated emission: photon with a certain frequency hits excited atom & releases two photons of same frequency while electron returns to ground state Population inversion: when a system of either a group of molecules or molecules exist in a state where more electrons are in an excited state than in the lowest energy level possible Absorption: photon with a particular frequency hits an atom at rest & excites it to a higher energy level while photon is absorbed Spontaneous emission: atom in an excited state emits a photon with a particular frequency & returns to ground state Stimulated emission: photon with a certain frequency hits excited atom & releases two photons of same frequency while electron returns to ground state

Terminology -- Waves Scattering: when atoms of a transparent material is not smoothly distributed over distances greater than the length of a light wave Reflection: light normally collides with the boundary of 2 materials –Objects contain free electrons which jump from one atom to another within it –Energized electrons vibrate  sends back of object as light wave with same frequency of incoming wave –Does not deeply pierce material Refraction: bending of light when it passes from one kind of material to another –Frequency of incoming light matches natural frequency in electrons –Penetrates deeply into material causing vibrations in electrons –Waves slow down and outside it maintains original frequency

Examples scattering reflection refraction

Goals of Presentation Discuss what exactly a laser is Discuss how a laser works and how to build your own Discuss the types of lasers available today (solid, gas, liquid, semiconductor, excimer, free electron) Propose the best laser for the space elevator

What is a Laser? (in words) Did you know? Actually, it is an acronym: LASER (light amplification by stimulated emission of radiation) A device that controls the way energized atoms release photons Laser light is very intense, highly directional & pure in color Not very safe to look directly at laser light 4 types of lasers: 1) solid state lasers 2) gas lasers 3) liquid lasers 4) semiconductor lasers **Other forms also are excimer and free electron lasers, but they don’t fall into any of these categories. Classified based on gain medium (that will be defined in one slide…) used

Parts of a Laser Energy source: --begins the lasing process --examples include: electrical discharges, flashlamps, arclamps, lights from another laser, lights from chemical reactions, lights from explosions Gain Medium (excitation mechanism): --transfers external energy to beam --excites particles --keeps laser at desired wavelength --absorbs energy in the laser --maintains the laser Optical Resonator (feedback): arrangement of optical components allowing beam to circulate Output Coupler: --where light is allowed to come out --semitransparent mirrors --controls effective output of light produced

Laser Function & Construction As you noticed, light is commonly used as the energy source and the gain medium Energy source applied to system Gain medium transfers energy to beam, energizing electrons Electrons give off light energy to return back to its original energy level Resonators produce more laser light Need to have knowledge in glassblowing, fabricating small parts, operating a vacuum Use a solid, liquid or gas medium (best gas is nitrogen) Two resonating mirrors are used to reflect light formed Energy source applied to the system Emission of photons will result in light

Solid-State Lasers Uses a gain medium which is a solid (not semiconductor) Energy source: flashlamp or arclamp Optical resonator: two mirrors in parallel Produces power ranging from milliwatts to kilowatts  light lasts for short durations

The Ruby Laser (a solid state laser) 1 st laser by Theodore Maiman Used a synthetic ruby and made it in shape of cylinder Wrapped it around a high intensity lamp The blue and green wavelengths from the white light triggered an excitement within electrons of chromium atom When returned to stable state, they released energy in form of ruby light Phenomena continued until critical level reached and pulse released

Implementation on the Space Elevator High powered solid state lasers may be successful with providing power Obtaining solids may be a problem Most acquirable item today is Nd:YAG (neodymium doped yttrium aluminum garnet) Produces very limited power

Gas Lasers Active Medium: pure gas, mixture of gases, metal vapor Energy Source: electrical discharge, flashlamp, arclamp, light from other laser, chemical reaction or explosion Optical Resonator: 2 mirrors in parallel to each other Power Generated: 50 watts to 4 kilowatts of power

CO 2, N 2 and HeCd Lasers CO 2 Lasers: --uses CO 2 to begin lasing process --Active medium: 1 carbon dioxide, 1 nitrogen gas, 1 helium --Process: 1) nitrogen added, exciting carbon dioxide 2) helium added to remove electrons from lowest energy level (population inversion) 3) tube sealed and voltage added exciting system N 2 Lasers: --uses N 2 as active medium --high voltage power added to system --creates electrical discharge & population inversion --laser acts for short time --good for scientific research, pumping other lasers --minimal damages HeCd Lasers: --metal is cooked and vaporized --helium excited by collisions with excited electrons --pass on to cadmium atoms --cadmium heated and added to helium gas --helium fills cavity while cadmium goes to cathode

Implementation on the Space Elevator Possibly a good idea Could utilize gases available on Earth in excess, reducing pollution or its harmful effects Negative effect: too much gas or some gases could corrode elevator, cause possible explosions and potentially damage the Earth Moderate amounts of gas needed for use

Liquid Lasers Active Medium: liquid Energy Source: light from another laser Optical Resonator: 2 mirrors in parallel with one another Power Generated: few watts covering radius of 20 micrometers Tunable over a wide range and produces a broad range of colors in visible spectrum

Dye Lasers Uses organic liquid dyes as active medium 1 cm long quartz glass tube Dye cell: inside of the tube which consists of partially reflective mirrors on the front & diffraction grating on rear Laser Action: –Energy added by a light source such as flashlamp/laser –Dye absorbs wavelength of light shorter than what it emits and input energy in forms of energy & heat –Absorbed energy creates a population inversion (electrons excited) –Vibrational energy loss causes dye molecules to go into lowest energy state –Emission occurs when vibrational levels reach ground state.

Implementation on the Space Elevator Not practical Provides too little power Not feasible to use organic dye (harder to maintain) Dye could get stale over time (requires replacement every few days) Could cause malfunction of the laser if dye is not replaced Dependent on another laser for starting, so two lasers would be necessary

Semiconductor Lasers Active Medium: semiconductor solid (solid which conducts electricity) – needs to confine carrier & take up small volume Energy Source: electrical impulse 1 st semiconductor: 1962 –Coherent electromagnetic radiation produced by a p-n junction using GaAs

Semiconductor Terminology p-type semiconductor: semiconductor in which electrical conduction is due chiefly to the movement of positive holes n-type semiconductor: electrical conduction due chiefly to the movement of electrons p-n junction: where p-type semiconductor is adjoined with the n-type semiconductor Valence band: where highest energy level has full number of electrons Conduction band: where lowest energy level has no electrons Band gap (energy gap): space between valence band and conduction band Minority carrier: contains few mobile electrons in p- type semiconductor region & free holes in n-type semiconductor region Majority carrier: free holes in p-type region & electrons in n-type region

Semiconductor Lasing Population Inversion: charge carriers (electrons) cross p-n junctions  minority carriers Minority carriers mix with majority carriers Photon is absorbed by electrons (gives energy to jump from valence to conduction) Leads to stimulated emission, releasing a photon Optical resonator reflects the light out & sometimes back into the solid

Implementation on the Space Elevator Power Generated: few milliwatts Will not produce enough power Small in size & won’t create enough light Option: use a system of semiconductor lasers  pretty costly

Excimer Lasers Active Medium: noble gas (argon, krypton, xenon) + halogen (fluorine, chlorine, bromine, iodine) –Exists for 10 nanoseconds during excited state –In ground state, exists as separate atoms Energy Source: UV light Optical Resonator: 2 mirrors in parallel to each other

Excimer Lasers Chemical Composition: – % halogen –Little noble gas –90% of helium or neon Laser Action: –When electrical discharge or energy is added to noble gas, can bind to halogen (excited) –Gives up additional energy through stimulated emission, forming ground state molecule –Within picoseconds, can separate into 2 atoms  population inversion

Implementation on the Space Elevator FACT: sun is composed of hydrogen, helium, oxygen, carbon, iron, neon, nitrogen, silicon, magnesium & sulfur Use solar radiation as energy source for the laser (excess sunlight) Lasing process is for few nanoseconds, but power generated: few watts to few hundreds of watts Radiation exposure will be minimal, but effective for the space elevator

Free Electron Lasers Best laser according to Edwards & Westling Device which emits high powered electromagnetic radiation at any wavelength Contains an array of magnets in magnetic field to excite free unbound electrons Tunable over broad range of wavelengths Class IV lasers: capable of starting fires, burn flesh and cause eye damage

Free Electron Lasers Beam of electrons accelerated to relativistic speeds (electron accelerator) Electrons pass through periodic, transverse magnetic field Magnetic field causes electrons to travel at a sinusoidal path Electrons move at higher speeds, releasing photons Optical mirrors lengthen process

Implementation on the Space Elevator Efficiency of 65% Can emit radiation at any wavelength (tunable) Accelerated electrons release x-rays at hazardous levels Produces high quantities of power Electron accelerator is big Very expensive

Conclusions Best Laser: gas or excimer laser Why? – can use gases in the solar system or atmosphere Other lasers would require further research Other Factors to Study: –Threshold of maximum gas needed –Overall harmful effects of lasers for necessary protection –Some type of radiation shielding –What should be done if gas runs out? –Process of the laser –Strength to endure effects of nature

Acknowledgements At this time, I would like to thank: Professor Benaroya for giving me this opportunity to learn about lasers Yuriy Gulak for setting me up here and familiarizing me with the technology available here Everyone of you for teaching me about your research & making me feel comfortable here