Applications of LASERs Jeremy Allam Optoelectronic Devices and Materials Research Group Tel +44 (0) Fax +44 (0) University of Surrey School of Physics and Chemistry Guildford, Surrey GU2 7XH, UK

Applications of lasers 1. General lasers Interferometry Holography coherent monochromatic dynamics of physical, chemical, biological processes spectroscopy, pulse shaping high energy processes, wavelength conversion short pulses (<5fs) broadband gain(>300nm) high peak powers (>TW) 3. ‘Ultrafast’ lasers material processing medical applications nuclear fusion 2. High power lasers high CW power high pulsed powers

Applications of lasers 1. General lasers Interferometry Holography coherent monochromatic

Longitudinal Coherence of Laser Light phase noise or drift (spontaneous emission, temperature drift, microphonics, etc) leads to finite spectral width phasor at t=0 phasor at t=t 1 leads to finite coherence time  coh. (or length l coh. )  coh. (or l coh. )

Measuring Longitudinal Coherence use interferometer e.g. Michelson interferometer  (path length) = 2L 1 -2L 2 << coherence length l coh. M1M1 M2M2 L2L2 L1L1 BS detector M1M1 M2M2 L1L1 BS detector optical fibre for long coherence lengths, use optical fibre delay 2L 1 -2L 2 ~ l coh.

LINEAR TRANSLATION: interferometric translation stage FLATNESS/UNIFORMITY: e.g. Twyman-Green interferometer LINEAR VELOCITY OF LIGHT: famous Michelson-Morley experiment c is independent of motion of reference frame DETECTING GRAVITATIONAL WAVES: minute movement of end mirrors ROTATION (e.g. of earth): Sagnac interferometer as an optical gyroscope: Applications of interferometers Measurement of length: Measurement of optical properties: REFRACTIVE INDEX: Rayleigh refractometer LIGHT SCATTERING: heterodyne spectrometry ULTRAFAST DYNAMICS: pump-probe / coherent spectroscopy {see Smith and King ch. 11} Numerous other applications... For N loops of area A and rotation rate  phase difference is:

Holography {see Smith and King ch. 19} eye reconstructed image reconstruction beam diffracted reference beam hologram LASER Hologram (photographic plate) reference beam beam expander BS object illuminating beam photographic plate object illuminating beam eye 2D representation of image (no depth) photograph Photography - record electric field intensity of light scattered by object Holography - record electric field intensity and phase RECORDING READING / RECONSTRUCTING

Applications of lasers material processing medical applications nuclear fusion 2. High power lasers high CW power high pulsed powers

 a high-speed, low-cost method of cutting beryllium materials  No dust problem (Be dust is poisonous)  autogenous welding is possible  Achieved using a 400-W pulsed Nd-YAG laser and a 1000-W CW CO 2 laser  Narrow cut width yields less Be waste for disposal  No machining damage  Laser cutting is easily and precisely controlled by computer Laser fabrication of Be components

1kW Nd:YAG cutting metal sheet

Photograph of the laser delivery handpiece with a hollow fiber for sensing temperature. The surgeon is repairing a 1 cm-long arteriotomy. Laser Tissue Welding Laser tissue welding uses laser energy to activate photothermal bonds and/or photochemical bonds. Lasers are used because they provide the ability to accurately control the volume of tissue that is exposed to the activating energy.

Nuclear Fusion: National Ignition Facility

ultrashort pulses (5fs) broadband gain ( nm) high power (TW) THz pulse generation pulse shaping coherent control parametric conversion Why femtosecond lasers? timing physical processes time-of-flight resolution generate: UV X-rays, relativistic electrons (Titanium-sapphire properties)

What is “ultrashort”? 1 minute 10 fs light pulse Age of universe Time (seconds) Computer clock cycle Camera flash Age of pyramids One month Human existence Milli (m)10 -3 Micro (µ)10 -6 Nano (n)10 -9 Pico (p) Femto (f) Atto (a) Kilo (k)10 +3 Mega (M)10 +6 Giga (G)10 +9 Tera (T) Peta (P) Very short pulses!Very high powers!

Mode-locked Ultrafast Lasers '65'70'75'80'85'90' Shortest Pulse Duration (femtoseconds) Year Active mode locking Passive mode locking Colliding pulse mode locking Extra-cavity pulse compression Intra-cavity pulse compression Current record: 4.0 fsec Baltuska, et al Ultrafast Ti:sapphire laser A 4.5-fs pulse… Reports of attosec pulses, too!

–6 10 –9 10 –12 10 –15 Speed (seconds) Year Electronics Ultrafast Optics vs. Electronics Optics No one expects electronics to ever catch up.

Ultrafast Laser Spectroscopy: Why? Most events that occur in atoms and molecules occur on fs and ps time scales. The length scales are very small, so very little time is required for the relevant motion. Fluorescence occurs on a ns time scale, but competing non-radiative processes only speed things up because relaxation rates add: 1/  ex = 1/  fl + 1/  nr Biologically important processes utilize excitation energy for purposes other than fluorescence and hence must be very fast. Collisions in room-temperature liquids occur on a few-fs time scale, so nearly all processes in liquids are ultrafast. Semiconductor processes of technological interest are necessarily ultrafast or we wouldn’t be interested.

Ultrafast Spectroscopy of Photosynthesis Arizona State University The initial events in photosynthesis occur on a ps time scale.

The 1999 Nobel Prize in Chemistry went to Professor Ahmed Zewail of Cal Tech for ultrafast spectroscopy. Zewail used ultrafast-laser techniques to study how atoms in a molecule move during chemical reactions.

Selective photochemistry A chemists dream: control of chemical reaction pathway by selective optical excitation of chemical bond The difficulty with using CW light or long pulses is intramolecular vibrational redistribution: excite one bond, and a few fs later, the whole molecule is vibrating and the weakest bond breaks. Gustav Gerber

Coherent control with shaped fs pulses SOLUTION: (1) Use fs pulse to break bond before IVR occurs (2) shape the pulse to optimise the desired yield Termed “coherent control” of chemical reactions

Pulse shaping in time and frequency domains Intensity and phase of an optical pulse may be specified in either the time or frequency domain: Similarly, modulation can be performed in time or frequency domain: difficult - modulators too slow! easy !

The Fourier-Synthesis Pulse-shaper Amplitude mask Transmission = T(x) = T( ) Phase mask Phase delay =  (x) =  ( ) Fourier Transform Plane f f ff f f grating

Laser ablation with CW and long pulse (ns) : High average power Dominant process: thermal material heated and vaporised expansion and expulsion of target material Possible problems  crater formation  heat affected zone (HAZ)  surface contamination (dross)  shock wave damage to underlying material  limiting precision / resolution  collateral damage  absorption within illuminated region  poor vertical control Micromachining with CW lasers

Extreme conditions* at focus of ultrashort pulse: 1µJ pulse focussed to (1 µm) 3 gives: T~1MK p~10Mbar *Eric Mazur, Harvard University Femtosecond pulses in micromachining Ultrashort high peak intensity (ps or fs) pulses: High peak power, low mean power  Dominant process: creation of plasma  direct and rapid generation by multi-photon ionisation  incident energy absorbed in plasma  negligible cratering, HAZ, shock-wave damage or dross  strong NL effects only at focus -> sub-surface machining

Femtosecond vs. picosecond laser ablation ablation with fs pulses appears to be more deterministic due to (?) statistics of photoionisation (by light field or by multi-photon absorption) and subsequent avalanche ionisation

Applications of femtosecond micromachining high-precision ablation encoding information on micron scale engineering dielectrics for e.g. optical waveguides surgery...

pig myocardium drilled by an USPL showing a smooth-sided hole free of thermal damage to surrounding tissue. pig myocardium drilled by excimer laser, illustrating extensive thermal damage surrounding the hole. small, high precision cuts without kerf no thermal or mechanical damage to surrounding areas i.e. no burning or coagulation sub-surface surgery Surgery with femtosecond laser pulses - 1

thermal damage and cracking to tooth enamel caused by 1-ns laser ablation. smooth hole with no thermal damage after drilling with a USPL. Surgery with femtosecond laser pulses - 2

Femtosecond interstroma Femtosecond LASIK Femtosecond laser surgery of cornea - 1

Femtosecond laser surgery of cornea Lenticle removal using Femtosecond LASIK

(Biomedical) imaging using ultrashort laser pulses Problems with conventional microscopy –transparent objects require staining (toxic, fading) –3D imaging by sectioning –internal structures (e.g. retina) not always accessible –opaque objects cannot be viewed in transmission –low contrast due to background transmission Ultrashort pulse imaging methods address some of these problems : –Multi-photon imaging –ballistic photon imaging –optical coherence tomography –T-rays

Nonlinear microscopy for 3D imaging Third harmonic generation Three photon fluorescence Two photon fluorescence tt tt Linear processes do not favour the focus signal~intensity x area~z -2 x z 2 ~constant Nonlinear (‘multi-photon’) processes favour the focus signal~(intensity) 2 x area~z -4 x z 2 ~ z -2 (2-photon) signal~(intensity) 3 x area~z -6 x z 2 ~ z -4 (3-photon) femtosecond pulse detection of nonlinear signal region of NL interaction filter z

Two-Photon Fluorescence* Imaging Pollen grain (Clivia Miniata) 1.5 µm axial resolution 200 mW in 16 beamlets 46 sections separated by 0.5 µm in the axial dimension. 2 seconds/image ~14 µm Conventional image (using fluorescence) *requires fluorescent dye

Imaging by Third Harmonic Generation (THG) THG occurs at focus of intense ultrashort pulse Uniform material: THG light from either side of focus interferes destructively Discontinous material: allows some constructive interference and THG emission. THG imaging depends on  (3) THG is sensitive to interfaces 125 µm Demonstration using an optical fiber in index-matching fluid (~100 fs pulses at 1.2 µm, 1 kHz repetition rate.) Barad et al, Appl. Phys. Lett. 70, 922 (1997)

Sectional THG images of spiral algae formation Squier et al, Optics Express 3, p. 315 (1998)

More Real-Time THG Images Artificial blood vessel (two cover slips) with real red blood cells flowing in it. Scanning scheme used a Lissajou pattern.

scattering medium ‘ballistic’ photons (early arrival): small lateral scattering, low intensity ‘snake’ photons diffusive photons (late arrival): large lateral scattering, high intensity) Time-resolved imaging for opaque media Scattering is a major problem in e.g. mammography The problem is weak signals: mean free path for photons = L s ~ 0.5 mm for breast tissue sample length = L=25mm fraction of ballistic photons is exp(–L / L s ) = exp(–50) = 10 –22 but … for a pulsed laser with 1 Watt average power, there are only photons per second...

Optical Coherence Ranging and Tomography This work has been pioneered by Jim Fujimoto and coworkers of MIT. Huang, et al., Science, 254 (1991) cross-sectional micron-scale imaging real-time, in-situ, in-vivo optical fibre coupling for internal organs commercial device available for ophthalmologists

OCT can see otherwise invisible micro-tears in the retina Photographs can’t see the tears

Inside a blood vessel (in vitro) IVUS OCT Brezinski, et al., Am. J. Cardiology 77 (1996) The OCT images have significantly higher resolution than intravascular ultrasound (IVUS).

THz imaging for biomedical applications fills “THz gap” between microwave and optical frequencies mixed time / frequency domain spectroscopy chemical fingerprints at THz frequencies (e.g. rotational transitions) strong sensitivity to water content … coherent method (like OCT) imaging on 100 micron scale many variation of imaging method: intensity time-of-flight absorption at key frequencies (f 1 ) relative absorption (f 1 /f 2 )

THz imaging of biomedical samples Centre of Medical Imaging Research University of Leeds TeraVision project (EU-IST)

Principles: System: Surrey Femtosecond high-power broadband source

High rep rate near-infrared system (Spectra) high rep rate (80MHz) for good signal-to-noise workhorse system for communications wavelengths <200fs pulses over range nm

Ti-sapphire oscillator and regenerative amplifier high pulse energies for THz beam generation, material processing, and upconversion of weak luminesence dual parametric amplifiers for non- degenerate pump-probe, and difference frequency generator for mid-infrared wavelength range 550nm to >10  m ultrashort pulse version: < 60fs pulses Dual colour / mid-infrared system (Coherent)

Broadband sources for spectroscopy UVvisibleNIRMIRFIRmmWRF THz FEL Ultrafast electronics OPA Ti-S laser Ti-S SHG Ti-S THG DFMSFM HG-OPA

Laboratory Layout