50 years of transition-metal lasers: from ruby to Ti:sapphire

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1 50 years of transition-metal lasers: from ruby to Ti:sapphire ICFO Colloquium Program ICFO The Institute of Photonic Sciences Castelldefels (Barcelona), Spain July 5 th, 2010 Peter Moulton Q-Peak, Inc.

2 Outline Color and light Quick review of transition-metal spectroscopy The ruby laser and its consequences Divalent transition-metal lasers Ti:sapphire background Impact of Ti:sapphire lasers

3 What makes things colored?

4 Electronic transitions giver rise to colors in the visible region of the electromagnetic spectrum Energy -> Electronic transition at red wavelength

5 Absorbed energy by electrons where does it go? Light? Heat Light (fluorescence) Ground state Excited state Energy Exp (-t / τ ), sometimes Time Fluorescence quantum efficiency = Decay rate from light emission / Total decay rate

6 Stimulated emission (thanks to Einstein) Light Stimulated emission Stimulated emission competes with absorption Excited state Gain Loss

7 Thank you, 1917! Physika Zeitschrift, Volume 18 (1917), pp

8 Pick a scheme and win a Nobel prize! 3-level laser 4-level laser Also, for starters, find a system with high fluorescence quantum efficiency and a narrow emission linewidth

9 What makes things colored? Part II Organic Inorganic

10 Chlorophyll green coloring for leaves, from an organic molecule Structure of chlorophyll a Structure of methane

11 Transitions of 3d ions in solids often make inorganic colors Sc [Ar] 3d 1 4s 2 Ti [Ar] 3d 2 4s 2 Number of d electrons Ion(s) V[Ar] 3d 3 4s 2 1 Ti 3+ Cr [Ar] 3d 5 4s 1 Mn [Ar] 3d 5 4s 2 2 Ti 2+, V 3+ Fe [Ar] 3d 6 4s 2 3 Cr 3+, V 2+ H Co [Ar] 3d 7 4s 2 Ni [Ar] 3d 8 4s 2 4 Cr 2+, Mn 3+ Li Be Cu [Ar] 3d 10 4s 1 B C N 5 Fe 3+, Mn 2+ Na Mg Zn [Ar] 3d 10 4s 2 Transition metals Al Si P 6 Fe 2+, Co 3+ 7 Co 2+, Ni 3+ K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As 8 Ni 2+ Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb 9 Cu 2+

12 Outline Color and light Quick review of transition-metal spectroscopy The ruby laser and its consequences Divalent transition-metal lasers Ti:sapphire background Impact of Ti:sapphire lasers

13 d-electron orbitals 5-fold degenerate in free space

14 Energy levels of ions with 3 d-shell electrons

15 Ruby d3 system fluorescence spectra

16 Fluorescence lifetime vs. temperature for d 3 systems What is the quantum efficiency?

17 Outline Color and light Quick review of transition-metal spectroscopy The ruby laser and its consequences Divalent transition-metal lasers Ti:sapphire background Impact of Ti:sapphire lasers

18 Early thoughts on ruby laser from Schawlow Interviewed by Joan Bromberg, 1984 After we finished the paper, I knew that Townes and Cummins and later Abella and Heavens were going to work on trying to make a potassium optical maser at Columbia. And I never want to do what anybody else is doing, because I haven't much confidence in my ability to compete, and I don't like competing. And being at Bell Labs in the transistor era, you felt that if you could do anything in a gas, you could do it better in a solid. And so I started trying to learn about solids. And in fact, in that one paragraph in our paper that mentions that solids have broad bands for absorbing light and sharp lines to emit it, I had just learned that much; I knew that ruby was that way. Now, ruby was a common material around there because a lot of people were working on microwave masers. So you could go down the hall and find somebody who had a drawer full of rubies of various concentrations, and could borrow a few samples which you'd never return. So I just thought well, I'll get my feet wet, I'll try and learn something about this stuff, what's it all about. I had no idea of the theory, or anything at all about it. And I got hold of a copy of Pringsheim's book on Fluorescence and Phosphorescence. Which was one of these wonderful, thoroughly Germanic books that had all the references back to the early 1800s. It was very complete, but it didn't have the answers we wanted. At that time, I asked [lab director Al] Clogston if Icould work on that, and he said "Fine." Then later there was another incident in the fall of 1958 after the fall of 1960, rather, after Maiman had published the pink ruby laser, I was thinking about the dark ruby, and I really knew quite a lot about it, and I knew that those satellite [dark ruby spectrum] lines, or "N" lines, were really very strong, stronger than the [pink ruby s]"r" lines, and I just felt that that dark ruby maser that I had proposed really ought to work. So I asked Clogston if he thought I ought to try it out, and he said, "You owe it to yourself." So, we did, and it worked. Right away. And of course, I should have done it sooner.

19 Ruby quantum efficiency was thought by some to be low (Maiman disagreed)

20 Nature 187, (06 August 1960) First publication on laser Stimulated Optical Radiation in Ruby T. H. MAIMAN Hughes Research Laboratories, A Division of Hughes Aircraft Co., Malibu, California. Schawlow and Townes 1 have proposed a technique for the generation of very monochromatic radiation in the infra-red optical region of the spectrum using an alkali vapour as the active medium. Javan 2 and Sanders 3 have discussed proposals involving electron-excited gaseous systems. In this laboratory an optical pumping technique has been successfully applied to a fluorescent solid resulting in the attainment of negative temperatures and stimulated optical emission at a wave-length of 6943 Å. ; the active material used was ruby (chromium in corundum). 1. Schawlow, A. L., and Townes, C. H., Phys. Rev., 112, 1940 (1958). 2. Javan, A., Phys. Rev. Letters, 3, 87 (1959). 3. Sanders, J. H., Phys. Rev. Letters, 3, 86 (1959). 4. Maiman, T. H., Phys. Rev. Letters, 4, 564 (1960).

21 From digital version of Nature article

22 Pictures of first ruby laser at Hughes

23 Bell Labs gets convinced it s a laser

24 Hughes did more science

25 Sapphire (corundum, Al 2 O 3 ) enabled ruby laser

26 CW ruby lasers with lamp pumping Cryogenic cooling in 1962

27 Laser-pumped ruby laser

28 Ruby laser pumping Sm:CaF2

29 No comment

30 Legacy of early ruby laser development First laser First Q-switched laser First laser-driven nonlinear optics (harmonics, Raman, etc.) First use of cryogenic cooling to improve thermo-optical and spectral characteristics First demonstration of laser pumping of a solid-state laser Argon-ion-pumped ruby laser Ruby-laser-pumped Sm:CaF 2 laser (first 5d-4f laser?)

31 Outline Color and light Quick review of transition-metal spectroscopy The ruby laser and its consequences Divalent transition-metal lasers Ti:sapphire background Impact of Ti:sapphire lasers

32 Tunable lasers organic dyes provided a start

33 Dye lasers, cw and pulsed

34 Isoelectronic traps in Te-doped CdS- try for a tunable laser, but Auger-process won

35 Rediscovery of first broadly tunable lasers, handicapped by cryogenic operation

36 Energy levels of divalent transition metals

37 Divalent Ni in MgF2 : Properties at 77 K pump

38 pump Divalent Co in MgF2 :properties at 77 K

39 Co:MgF2 boule and assorted TM-doped crystals grown at MIT Lincoln Laboratory

40 Photos of cryogenic lasers at MIT/LL ( )

41 Cryogenic operation of Co:MgF 2 laser

42 First room-temperature operation from Co:MgF2

43 Outline Color and light Quick review of transition-metal spectroscopy The ruby laser and its consequences Divalent transition-metal lasers Ti:sapphire background Impact of Ti:sapphire lasers

44 Bill Krupke suggested a possible material for a lamp-pumped fusion-driver laser but no gain

45 Ce:YLF absorption/emission (1979) (with Dan Ehrlich, Rick Osgood)

46 First Ce:YLF laser setup

47 One reviewer was skeptical

48 We did publish, and later made another laser

49 Excited-state absorption (ESA) a pervasive problem Ce 3+

50 Example of complexity in ESA calculations

51 Color-center laser levels inspired search for systems without ESA

52 Energy levels of single d electron in crystal Number of d electrons Ion(s) 1 Ti 3+ 2 Ti 2+, V 3+ 3 Cr 3+, V 2+ 4 Cr 2+, Mn 3+ 5 Fe 3+, Mn 2+ 6 Fe 2+, Co 3+ 7 Co 2+, Ni 3+ 8 Ni 2+ 9 Cu 2+

53 Early work on Ti in sapphire (1962)

54 MIT efforts studied defect diffusion using Ti J. Am Ceramic Soc. 52, 331 (1969)

55 Ti:sapphire absorption/emission (1982) ,000 WAVELENGTH (nm) 0 ABSORPTION COEFFICIENT (arb. units) FLUORESCENCE INTESITY (arb. units) Fluorescence lifetime 3.2 usec

56 Jahn-Teller splitting for upper and lower levels leads to broadened transitions

57 First Ti:sapphire laser operation

58 Ti:sapphire - early photos in

59 MIT couldn t afford (!) to patent Ti:sapphire

60 Parasitic absorption was a party spoiler 7E-20 CROSS SECTION (cm^2) 6E-20 5E-20 4E-20 3E-20 2E-20 1E-20 PI SIGMA ABS. COEFFICIENT (arb. units) ,000 1,200 WAVELENGTH (nm)

61 Work at LL examined Ti 3+ -Ti 4+ as culprit

62 Predictions that were right MIT LL Solid State Research 1982:3

63 Predictions that were (mostly) wrong

64 Technology genealogy Livermore V:MgF2 FUSION DRIVER ESA-crippled Understand ESA Try again COLOR-CENTER LASERS Bell Simple energy levels Crystal engineering? Livermore Cr:LiSAF LASER Aha! Ti:SAPPHIRE LASER Lincoln Not a good fusion driver, but...

65 Outline Color and light Quick review of transition-metal spectroscopy The ruby laser and its consequences Divalent transition-metal lasers Ti:sapphire background Impact of Ti:sapphire lasers

66 200-W average power from lamp-pumped Ti:sapphire

67 My own group s work on Ti:sapphire

68 Laser-pumped, high-energy, ns-pulse Ti:sapphire laser Diode Seed seed Pump #1 Prisms Ti:sapphire crystals Pump #2 GRM ns pulse duration diffraction-limited Developed with NASA Langley, DARPA support, Ti:sapphire output energy (mj) nm 727 nm 911 nm 960 nm Green pump energy (mj)

69 Tuning curve of Titan-CW laser pumped by argon-ion laser 2 7 W pump power Power Output (W) ,000 1,050 1,100 1,150 Wavelength (nm)

70 Rare-earth levels and Ti:sapphire tuning Key tool in development of Er:fiber amplifiers

71 Ti:sapphire gain bandwidth support 5 fs pulses 1 INTENSITY (arb. units) PI 98 THz (4.4 fs) GAIN 0.2 SIGMA WAVELENGTH (nm)

72 Kerr-lens modelocking (KLM) provides a fast switch to enable fs-pulse modelocking

73 Ti:sapphire ultrafast lasers replaced dye lasers in the 90 s

74 Counting optical cycles

75 Significance of femtosecond lasers The Nobel Prize in Chemistry 1999 "for his studies of the transition states of chemical reactions using femtosecond spectroscopy" Ahmed H. Zewail Egypt and USA California Institute of Technology (Caltech) Pasadena, CA, USA b. 1946

76 Time Domain Frequency Domain Time Domain E(t) 2Δφ t Frequency Domain I(f) f rep δ 0 Frequency modes of the fs pulse are offset from f n=0 =0 by δ f 2πδ= Δφ f rep

77 Locking via Self-ReferencingTechnique How can we control the absolute frequencies (and hence the group-phase velocities)? Self-referencing m Δν+δ Fundamental Spectrum n Δν+δ Second Harmonic Spectrum Δν 2(m Δν+δ) Beat frequency at overlap = δ H. Telle et al., Appl. Phys. B 69, Sept 1999 J. Reichert et al., Opt. Comm. 172 pp Dec 1999 D. J. Jones et al, Science 288 p April 2000 Frequency RF Power (10 db/div) Repetition Rate Fundamental- Second Harmonic Beats Frequency (MHz)

78 Hansch and Hall win Nobel Prize for Optical Combs Stockholm December 10, 2005

79 Chirped pulse amplification (CPA) 1985 (G.Mourou & D.Strikland) Courtesy: Wikipedia

80 Under the hood of a high-power Ti:sapphire CPA system

81 Size does matter for high-energy systems

82 Photograph of Ti:sapphire-generated filament for lidar

83 Attosecond pulses, high-harmonic generation

84 CPA pushes to a Zettawatt (courtesy Mourou)

85 Ti:sapphire laser - highlights Broadly tunable ( nm) output used widely for scientific and applied linear and nonlinear spectroscopy of gases and condensed media, atmospheric research Mode-locked output <10 fs has probed ultrafast dynamics of media (Zewail awarded Nobel Prize in Chemistry for work on molecules) Mode-locked systems also can generate new optical frequency standards and allow measurement accuracies of a part in Amplified mode-locked lasers (with CPA) have approached Petawatt (10 15 W) of output (30 J in 30 fs) to study laser-matter interactions at extremely high intensities, generate x-rays Commercial laser sales are on the order of 6000 systems, about $500 million (update, approaching $1B).

86 Thank you!

OPTI510R: Photonics. Khanh Kieu College of Optical Sciences, University of Arizona Meinel building R.626

OPTI510R: Photonics. Khanh Kieu College of Optical Sciences, University of Arizona Meinel building R.626 OPTI510R: Photonics Khanh Kieu College of Optical Sciences, University of Arizona kkieu@optics.arizona.edu Meinel building R.626 Announcements HW #5 due today April 11 th class will be at 2PM instead of