2013-2-26 FLASHES OF BRILLIANCE THE HISTORY OF THE LASER “A splendid light has dawned on me” – Albert Einstein In 1917 Einstein published ideas on stimulated emission of radiation.The laser is credited as being invented in 1958 by Charles H. Townes andArthur L. Schawlow. Townes coined the term “laser” with help from hisstudents. On May 16, 1960, Theodore H. Maiman operated the first functioninglaser i.e., a pulse mode operation of solid- state flash lamp -pumped
L.A.S.E.R Light Amplification by Stimulated Emission of Radiation
BASIC LASER Light Sources Gain medium Mirrors I I0 I1 I3 Laser medium I2 R = 100% R < 100% R. Trebino
GAIN MEDIUM Einstein Coefficients E2 AN2 = rate of Spontaneous emission E1 E2 BN2I = rate of Stimulated emission E1E = hν E2 BN1I = rate of Stimulated absorption E1
TO ACHIEVE LASING: Stimulated emission must occur at a maximum (Gain > Loss) Loss: Stimulated Absorption Scattering, Reflections Energy level structure must allow for Population Inversion E2 E1
OBTAINING POPULATION INVERSION 2-level system 3-level system 4-level system 3 3 Fast decay Fast decay 2 2 2 N2 I sat Pump Laser Pump Laser Laser Transition Transition Transition Transition 1 N1 1 1 Fast decay 0d N d N d N 2 BI N AN AN BIN BI N AN AN BIN BI N AN dt dt dt N 1 I / I sat I / I sat N N N N N 1 I / I sat 1 I / I sat 1 I / I sat Population Inversion is obtained for ΔN < 0 (ΔN = N1 – N2)
LASER SYSTEM Active Medium 3 Active medium can be of following types Liquid Fast decay Solid 2 gases Pumping Source Pump Laser Optical pumping Transition Transition Chemical pumping Nuclear pumping Discharge technique 1 Laser pumping Fast decay Electron beam pimping 0 Resonators Transverse Mode Longitudinal mode
Tunnable Lasers wavlength of operation can be altered in controlled manner.Dye lasers use complex organic dyesGas lasers are pumped by current.Solid-state lasers have lasing material distributed in a solid matrix(such The Nd:YAG laser emits infrared light at 1.064 nm.Semiconductor lasers, sometimes called diode lasers, are p-njunctions. Current is the pump source. Applications: laser printers orCD players.Excimer lasers (from the terms excited and dimers) use reactivegases, such as chlorine and fluorine, mixed with inert gases such asargon, krypton, or xenon. Excimers lase in the UV.Free electron Lasers is a laser that shares the same opicalproperties as conventional lasers such as emitting a beam ofcoherent EMR radiations which can reach high power R. Trebino
SPECTROSCOPY Study of interaction of light with matter all atoms and molecules absorb and emit light at certain wavelengths so we can identify and read their properties In essence, every element has a unique atomic "fingerprint" that takes the form of a set of wavelengths, or a spectrum.
LASER SPECTROSCOPY INSTRUMENTATION LASER as Source of Light Gratings and Monochromators Interferometers Michelsons Interferometers Fourier Transform Spctrometer Dtectors Thermal Detectors Flourescent detectors etc. Recorder
LASER-INDUCED BREAKDOWNSPECTROSCOPY (LIBS) advanc-ing significantly over the last decade. It can analyze solids, liquids and gases and can return results rapidly, with very little damage to the sample. It can do its work from a distance, unlike some analytical tools that require samples being brought to a lab.
WORKING OF LIBS The laser, of course, Generally, LIBS systems use a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at fundamental wavelength of 1,064 nanometers(but many different lasers have been used. The laser doesnt blast the sample with a nonstop beam) The laser light passes through a lens, which focuses the energy onto the sample. "laser spark” produced. Excitation Relaxation The spectrometer contains a prism and a camera to photograph the spectra for further study.
Fig: LIBS Spectra for identification of different elements in sample
LASER ABLATION INDUCTIVELY COUPLEDPLASMA OPTICAL EMISSION SPECTROSCOPY(LA-ICP-OES) The "P" in ICP stands for plasma, an ionized gas consisting of positive ions and free electrons. The Plasma torch consists of three concentric tubes of silica surrounded by a metal coil. A nozzle at the end of the torch acts as an exit for the plasma. Now the instrument is ready to analyze a sample. In the laser-based version of ICP-OES, a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser is used to cut, or ablate, a few microscopic particles from the samples surface. The ablated particles are then carried to the pl-asma torch, where they become excited and emit light.
LASER-INDUCED FLUORESCENCE (LIF) Laser-induced fluorescence (LIF) is a spectroscopic method used for studying structure of molecules, detection of selective species and flow visualization and measurements. Experimental Method The species to be examined is excited with a laser. The wavelength is often selected to be the one at which the species has its largest cross section . The excited species will after some time, usually in the order of few nanoseconds to microseconds, de-excite and emit light at a wavelength longer than the excitation wavelength. This fluorescent light is typically recorded with a photomultiplier tube (PMT).
RAMAN SPECTROSCOPY C.V. Raman ,Indian scientist discovered Raman spectroscopy Raman spectroscopy is a spectroscopic technique used to study vibrational , rotational, and other low-frequency modes in a system Principle: It relies on inelastic scattering , or Raman scattering, of monochromatic light, usually from a laser in the visible , near infrared , or near ultraviolet range. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. This happens because the laser light interacts with phonons. The shift in energy gives information about the phonon modes in the system and ultimately about the molecules present in the sample.
Experimental Procedure: The beam from an argon-ion laser is directed by a system of mirrors to a lens, which focuses monochromatic light onto the sample. Most of the light bouncing off the sample scatters at the same wavelength as the incoming light, but some of the light does scatter at different wavelengths and goes to detector This happens because the laser light interacts with phonons. we use photomultiplier ,CCD detectors etc. and determine vibrations kinds and finally sample molecule.
APPLICATIONS OF LASER SPECTROSCOPY Medical field Analytical Chemistry Industrial Applications Environmental Applications
LASER SPECTROSCOPY IN MEDICINE ANDBIOLOGY Medical diagnostics by breath trace gas analysis Real-time monitoring of exhaled gases (therapeutic monitoring, toxicology, occupational health) Tissue analysis Mapping of drug delivery Insect studies Plant physiology
IDENTIFICATION OF BACTERIALCONTAMINATION OF PLATELETS (LIF) Blood transfusion carries a risk of infection (hepatitis, HIV…) or consequent sepsis every platelet concentrate should be checked before use (after donation and shortly before transfusion» Fluorescent stain attaches to the DNA ofbacteria (platelets don’t contain DNA!)» Frequency doubled Nd-laser (532 nm) toexcite LIF» Scattered light also measured» Certain thresholds for both signals
REAL-TIME MONITORINGReal-time monitoring OF HEMODIALYSISof hemodialysis» Hemodialysis is usedin treatment of renalfailure» Urea, creatinine, etc.removed» Treatment 3 times aweek, 2-12 hours» Over million patientsworldwide, growing fast
LIF SPECTROSCOPY OF TISSUESThere are cellular or subcellular differencesbetween normal and tumorous tissues» LIF can be used to visualize tissue characteristicsand detection of anomalies» Fluorescing compounds or autofluorescence» Non-invasive procedure, no photosensitization orphotodestruction
RESPIRATION OF INSECTS Respiration of insects - real-time, on-line measurement of CO2 - very small quantities sensitive detection method, small volume of sample line and cell photoacoustic spectroscopy - mid-IR should be used if possible (CO2 at 4.234 μm) - OPO (between 3.9 and 4.8 μm) continuous-wave, single mode operation - detection limit 0.7 ppb. - sporadic release of CO2 observed
LASER SPECTROSCOPY OF BREATH ISLIMITED TO SMALL MOLECULES single vibration-rotationlines are measured -the lines have a certainlinewidth (Voigt profile) -the bigger the molecules,the more congested the spectrum becomes(lines start to overlapeach other) -typical laser wavelength 1.5 to 10 μm -sensitivity ppt – ppm -normal pressure cannotusually be used (typical p = 0.05 – 0.2 atm)
IN ANALYTICAL CHEMISTRY Laser Spectroscopy in Analytical Chemistry Chemical Reactions Detection of Atoms Study of Transition States Separation of isotopes (In Nuclear Reactors) Study of Bond Energies and Angles Type of Material
LIF SPECTROSCOPY OF INTERNAL COMBUSTIONENGINES LIF spectroscopy of internal combustion engines Goals: to improve combustion efficiency to reduce emission of pollutants how well air and fuel are mixed chemical intermediates rate constants of key reactions l = air/fuel ratio ArF ,KrF lasers Molecules: NO, CO, CO2, hydrocarbons…
CONCLUDING THOUGHTS The key to managing today’s rapidly evolving technology it to constantly analyze how each advance affects us as individuals and as a society as a whole. “ “Our Advancing Technology , if separated from the human factor, I take to be part of the advance in the evolving quality of existence, something that gives added meaning and higher dimension to the human venture…” - Roger Sperry Neuroscientist and Nobel Laureate
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