Introduction to Laser Physics Laura Corner Cockcroft Institute

Introduction to Laser Physics Laura Corner Cockcroft Institute for Accelerator Science Liverpool University, UK CERN Accelerator School High Gradient Wakefield Accelerators Sesimbra, Portugal, March 2019 1

Outline What is a laser? Basic laser physics Different types of laser systems. High power lasers Suggested reading: ‘Lasers’, A.E. Siegman, University Science Books ‘Principles of Lasers’, O. Svelto, Springer ‘Laser Physics’, S. Hooker & C. Webb, Oxford University Press ‘Optical Electronics in Modern Communications’, A. Yariv, Oxford University Press CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 2

LASER ght Amplification by Stimulated Emission of Radiatio CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 3

What makes lasers special? They produce a directional beam. They have a narrow spectrum (or bandwidth). They are coherent. Slide courtesy Prof. S. Hooker CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 4

Einstein model Einstein identified 3 ways in which atoms exchange energy with a radiation field Absorption Spontaneous emission spontaneous emission stimulated absorption emission Stimulated emission Rate of absorption/stimulated emission dependent on no of photons & number of atoms in lower/upper state. ‣ photon needs to have correct energy Albert Einstein ‣ the number of photons can be amplified Slide courtesy Prof. S. Hooker CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 5

Lasers: the basic idea We want lots of this. . but not much of this Hence we need a population inversion, i.e. more atoms in the upper level than in the lower level. CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 6

Population Inversion Need to pump (add energy to) the gain medium to get an inversion CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 7

3 and 4 level lasers DN1 Rate of change of population population density x pumping rate Absorption & stimulated emission equally likely Can’t get a population inversion in 2 level system. So lasers 3 or 4 level systems. Bestbroadly we cancategorised do is N2 Nas 1 - no population inversion in 2 level laser 3 level - N2 N1 – 1 thermally populated so need raise lots of population from 1 – 3 4 level - N2 0, so any population in 3 is an inversion – less pump, more efficient! Not necessarily monochromatic: Some lasers have broad emission range (although not white light!) CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 8

Making a laser All laser oscillators (as opposed to amplifiers) have 3 parts: Gain medium – gas, solid state, liquid – what provides the lasing transition. Pump – source of energy to create population inversion – usually another light source e.g. flashlamp or another laser, can be electrical discharge or current. Cavity – need to recirculate photons to stimulate emission on lasing transition – often mirrors around gain medium, can be medium itself. Lasing threshold – when gain (no. photons emitted in round trip) exceeds loss (number lost to absorption, through mirrors etc.). Do you want laser light all the time (continuous wave, cw) or pulsed? Pulses can be from femtosceonds - nanoseconds And that’s it! CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 9

Cavity Pump gain medium to upper level A photon decays spontaneously & stimulates more emission The photons bounce back and forth along the cavity – if the number of photons emitted each round trip exceeds losses (mirrors etc.) laser is above threshold One of the mirrors allows a small amount of this light out – laser output! Laser output controlled by gain of medium and longitudinal & transverse modes of cavity CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 10

Types of laser Gas lasers: Usually electrically pumped Wide range of wavelengths Low gain Liquid lasers Solution of complex organic dyes Widely tunable Solid state lasers Widest class of laser systems Lasing ion doped in crystalline host - Nd:YAG, Ti:sapp Ion in glass -Nd:glass Fibre lasers – Er, Yb in glass Semiconductor diode lasers But you don’t get every colour! Dependent on lasing transition CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 11

Gas lasers Probably widest range of wavelengths – uv to infrared. Helium neon – HeNe red (632.8nm) gas laser. Pumped by electrical discharge. uv excimer lasers – 100 – 300nm Photolithography Pumps for chemical reactions Poisonous gases – chlorine, fluorine! CO2 lasers (10.6mm). Industrial & medical applications. Research on increasing peak power at BNL. CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 12

Liquid lasers Less common, usually organic dyes – Rhodamine 6G Can get orange/yellow, hard to get with other gain media. Often carcinogenic materials. CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 13

Solid state lasers Most important class of lasers: many lasers have solid gain media: ‘solid state’ generally means lasing ions doped into crystalline/glass host e.g. Ti:sapp, Nd:YAG, Nd:glass Other solid state lasers referred to separately: Semiconductor diode lasers (often used to pump other lasers) Fibre lasers – usually rare earth ions e.g. Er, Yb, doped into glass. Pumping mechanisms: Broadbandwidth white flashlamps (Nd:YAG) Other lasers (Ti:sapp, fibre lasers) Electricity (semiconductor diodes) CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 14

Titanium sapphire lasers Hugely important laser Bandwidth! – 300nm Shortest pulses. Absorbs at 500nm – pump with green laser. Commercially available petawatt (PW) Ti:sapp lasers Workhorse of science research Ti:sapp crystal CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 15

High power lasers 100s TW & PW lasers - laser plasma wakefield acceleration, laser ion acceleration Usually Ti:sapp (fs), sometimes Nd doped systems How do you make a PW laser without damaging it? Chirped Pulse Amplification (CPA). (Also useful for winning Nobel prizes). But need to know something about ultrashort pulses to know how this works. Fourier synthesis of short pulse requires many frequency components - spectrum! When all components have no phase difference pulse is shortest it can be for a given spectrum. Pulse is Fourier transform limited (FTL). CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 16

Fourier synthesis of a short pulse ‘flat phase’ time CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 17

Chirped pulses Chirp of an optical pulse is the time dependence of its instantaneous frequency Frequency increases with time Positive chirp Both pulses longer in time than if not chirped – flat phase, FTL Frequency decreases with time Negative chirp CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 18

How to chirp a pulse m Gratingcompressor stretcher Grating Red light shorter distance front pulse Red light longer distance closer toof blue Blue light longerdistance distance back ofup pulse Blue light shorter catches CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 19

Chirped Pulse Amplification Short pulse Stretch in time (chirp) Amplify Compress in time CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 20

CPA works! Nobel Prize in Physics 2018 Arthur Ashkin ½ share: optical tweezers Gérard Mourou Donna Strickland ½ share: CPA CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 21

High power lasers – further research CPA hugely successful – operational 10 PW laser! 10,000,000,000,000,000 W So what’s the problem? Is laser research over? Expensive Low repetition rate – 1 shot/hr, /min, /s – still too slow So low *average* power e.g. BELLA laser at LBNL For PP research: Peak power 40J/40fs 1PW Average power 40J/s 40W 32J laser/15kHz 480kW @0.03%: 1.6GW Efficiency BELLA uses 130kW electrical power 0.03% wall plug efficiency C. Schroeder et al. PRST AB 13 101301 (2010) CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 22

Accelerator Laser Research LWFA Compton scattering Ion acceleration Photocathodes FEL seeding Positron generation https://www-bd.fnal.gov/icfabd/WhitePaper final.pdf CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 23

One approach - Fibre lasers Optical waveguides Light guided and amplified in core doped with lasing ions Er 1.5mm, Yb 1mm Efficient - up to 40% wall plug efficiency High average power – 10s – 100s kW Excellent single mode quality Low pulse energy – mJ Long(ish) pulses – 100s fs Core – ion doped, few mm diameter Cladding – few 100 mm diameter CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 24

Combine many lasers Take many efficient, high average, low peak power fibre lasers and add them together HAPPI lasers – High Average and Peak Power Intense lasers (TM Gérard Mourou) CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 25

Combination results Müller et al., Opt. Letts. 41, 3439 (2016) Kienel et al., Opt. Letts. 41, 3343 (2016) Seed split and amplified in 8 large fibre amplifiers 1 MHz, 1.1 mJ, 260fs 8 channels combined spatial & temporal division 12mJ, 56kHz, 260fs 1kW average power 46GW peak power CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 26

Conclusion Lasers hugely important in accelerator physics Conventional: Photocathodes Beam diagnostics – laserwire, electro-optic sampling FEL seeding Heater laser beams Novel: Laser plasma wakefield acceleration Dielectric acceleration Vacuum acceleration Come and do laser research! CERN High Gradient Accelerator School, Sesimbra Portugal, March 2019 27