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

ICFO Colloquium ProgramICFO – The Institute of Photonic Sciences

Castelldefels (Barcelona), Spain July 5th, 2010Peter MoultonQ-Peak, Inc.

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

What makes things colored?

Electronic transitions giver rise to “colors” in the visible region of the electromagnetic spectrum

Electronic transition at red wavelength

Ener

gy ->

Absorbed energy by electrons– where does it go?

Ground state Excited state

Light Heat?

Light(fluorescence)

TimeEn

ergy Exp (-t / τ ), sometimes

Fluorescence quantum efficiency = Decay rate from light emission / Total decay rate

Stimulated emission (thanks to Einstein)

Excited state

Light

Stimulated emission

Stimulated emission competes with absorption

Loss

Gain

Thank you, 1917!

Physika Zeitschrift, Volume 18 (1917), pp 121-128

Pick a scheme and win a Nobel prize!

3-level laser

4-level laser

Also, for starters, find a system with high fluorescence quantum efficiencyand a narrow emission linewidth

What makes things colored? Part II

Organic

Inorganic

Chlorophyll – green coloring for leaves,from an organic molecule

Structure of chlorophyll a

Structure of methane

Transitions of 3d ions in solidsoften make inorganic colors

Number ofd electrons

Ion(s)

1 Ti3+

2 Ti2+, V3+

3 Cr3+, V2+

4 Cr2+, Mn3+

5 Fe3+, Mn2+

6 Fe2+, Co3+

7 Co2+, Ni3+

8 Ni2+

9 Cu2+

H

Li

Na

K Ca Sc

Rb Sr

Be

Mg

Ti V

Y

Cr Mn Fe Cu Zn Ga Ge

Zr Nb Mo

As

B C N

Al Si P

Tc Ru Cd In Sn SbRh Pd Ag

NiCo

Transition metals

H

Li

Na

K Ca Sc

Rb Sr

Be

Mg

Ti V

Y

Cr Mn Fe Cu Zn Ga Ge

Zr Nb Mo

As

B C N

Al Si P

Tc Ru Cd In Sn SbRh Pd Ag

NiCo

Transition metals

Sc [Ar] 3d14s2

Ti [Ar] 3d24s2

V [Ar] 3d34s2

Cr [Ar] 3d54s1

Mn [Ar] 3d54s2

Fe [Ar] 3d64s2

Co [Ar] 3d74s2

Ni [Ar] 3d84s2

Cu [Ar] 3d104s1

Zn [Ar] 3d104s2

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

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

Energy levels of ions with 3 d-shell electrons

d3 system fluorescence spectra

Ruby

Fluorescence lifetime vs. temperature for d3 systems

What is the quantum efficiency?

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

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 Icouldwork 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.

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

First publication on laser

Stimulated Optical Radiation in RubyT. H. MAIMAN

Hughes Research Laboratories, A Division of Hughes Aircraft Co., Malibu, California.

Schawlow and Townes1 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. Javan2 and Sanders3 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).

Nature 187, 493 - 494 (06 August 1960)

From digital version of Nature article

Pictures of first ruby laser at Hughes

Bell Labs gets convinced it’s a laser

Hughes did more science

Sapphire (corundum, Al2O3) enabled ruby laser

CW ruby lasers with lamp pumping

Cryogenic cooling in 1962

Laser-pumped ruby laser

Ruby laser pumping Sm:CaF2

No comment

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:CaF2 laser (first 5d-4f laser?)

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

Tunable lasers – organic dyes provided a start

Dye lasers, cw and pulsed

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

Rediscovery of first broadly tunable lasers,handicapped by cryogenic operation

Energy levels of divalent transition metals

Divalent Ni in MgF2 : Properties at 77 K

pump

Divalent Co in MgF2 :properties at 77 K

pump

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

Photos of cryogenic lasers at MIT/LL (1978-1985)

Cryogenic operation of Co:MgF2 laser

First room-temperature operation from Co:MgF2

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

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

Ce:YLF absorption/emission (1979)

(with Dan Ehrlich, Rick Osgood)

First Ce:YLF laser setup

One reviewer was skeptical

We did publish, and later made another laser

Excited-state absorption (ESA) a pervasive problem

Ce3+

Example of complexity in ESA calculations

Color-center laser levels inspired search for systems without ESA

Energy levels of single d electron in crystal

Number ofd electrons

Ion(s)

1 Ti3+

2 Ti2+, V3+

3 Cr3+, V2+

4 Cr2+, Mn3+

5 Fe3+, Mn2+

6 Fe2+, Co3+

7 Co2+, Ni3+

8 Ni2+

9 Cu2+

Early work on Ti in sapphire (1962)

MIT efforts studied defect diffusion using Ti

J. Am Ceramic Soc. 52, 331 (1969)

Ti:sapphire absorption/emission (1982)

400 500 600 700 800 900 1,0000

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

WAVELENGTH (nm)

ABSO

RPT

ION

CO

EFFI

CIE

NT

(arb

. uni

ts)

FLU

OR

ESC

ENC

E IN

TESI

TY (a

rb. u

nits

)

Fluorescencelifetime3.2 usec

Jahn-Teller splitting for upper and lower levelsleads to broadened transitions

First Ti:sapphire laser operation

Ti:sapphire - early photos in 1982-3

MIT couldn’t afford (!) to patent Ti:sapphire

Parasitic absorption was a party spoiler

400 600 800 1,000 1,2000

1E-20

2E-20

3E-20

4E-20

5E-20

6E-20

7E-20

WAVELENGTH (nm)

CR

OSS

SEC

TIO

N (c

m^2

)

AB

S. C

OEF

FIC

IEN

T (a

rb. u

nits

)

PI

SIGMA

Work at LL examined Ti3+-Ti4+ as culprit

Predictions that were right

MIT LL Solid State Research 1982:3

Predictions that were (mostly) wrong

Technology genealogy

V:MgF2FUSION DRIVER

COLOR-CENTERLASERS

Ti:SAPPHIRELASER

Cr:LiSAFLASER

ESA-crippled

Simple energy levels

Aha!

Understand ESA

Try again

Not a good fusion driver, but...

Crystal engineering?

Livermore

Bell

Lincoln

Livermore

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

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

My own group’s work on Ti:sapphire

Laser-pumped, high-energy, ns-pulse Ti:sapphire laser

Developed with NASA Langley, DARPA support, 1986-1992

0

50

100

150

200

250

300

350

400

450

500

0 200 400 600 800 1000 1200 1400

Green pump energy (mJ)

Ti:s

apph

ire

outp

ut e

nerg

y (m

J)

790 nm727 nm911 nm960 nm

10-20-ns pulse duration

diffraction-limited

Pump #1

Pump #2GRM

Ti:sapphirecrystals

Prisms

Diodeseed

Pump #1

GRM

Ti:sapphirecrystals

Prisms

DiodeseedSeed

Tuning curve of Titan-CW laser pumped by argon-ion laser

700 750 800 850 900 950 1,000 1,050 1,100 1,1500

0.5

1

1.5

2

Wavelength (nm)

Pow

er O

utpu

t (W

)

7 W pump power

Rare-earth levels and Ti:sapphire tuning

Key tool in development of Er:fiber amplifiers

Ti:sapphire gain bandwidth support 5 fs pulses

600 700 800 900 10000

0.2

0.4

0.6

0.8

1

WAVELENGTH (nm)

INTE

NSI

TY (a

rb. u

nits

)

GAINPI

SIGMA

98 THz (4.4 fs)

Kerr-lens modelocking (KLM) providesa fast switch to enable fs-pulse modelocking

Ti:sapphire ultrafast lasersreplaced dye lasers in the 90’s

Counting optical cycles

Significance of femtosecond lasers

"for his studies of the transition states of

chemical reactions using femtosecond spectroscopy"

The Nobel Prize in Chemistry 1999

Ahmed H. ZewailEgypt and USA

California Institute of Technology (Caltech)

Pasadena, CA, USA

b. 1946

Time Domain ↔ Frequency Domain

2πδ= Δφ frep

I(f)

f

δ

0

frepI(f)

f

δ

0

frep

•Frequency modes of the fs pulse are offset from fn=0=0 by δ

Frequency Domain

TimeDomain

2Δφ

t

E(t)

• How can we control the absolute frequencies (and hence the group-phase velocities)? Self-referencing

Frequency

FundamentalSpectrum

SecondHarmonic Spectrum

m Δν+δn Δν+δ

2(m Δν+δ)Δν

460 480 500 520 540

Fundamental-Second Harmonic

Beats

Repetition Rate

RF

Pow

er (1

0 dB

/div

)Frequency (MHz)

D. J. Jones et al, Science 288 p 635 28 April 2000

J. Reichert et al., Opt. Comm. 172 pp 59–68 15 Dec 1999

H. Telle et al., Appl. Phys. B 69, 327–332 8 Sept 1999

Locking via Self-ReferencingTechnique

Beat frequency at overlap = δ

StockholmDecember 10, 2005

Hansch and Hall win Nobel Prize for Optical Combs

Chirped pulse amplification (CPA)

Courtesy: Wikipedia1985 (G.Mourou & D.Strikland)

Under the hood of ahigh-power Ti:sapphire CPA system

Size does matter for high-energy systems

Photograph of Ti:sapphire-generated filament for lidar

Attosecond pulses, high-harmonic generation

CPA pushes to a Zettawatt (courtesy Mourou)

Ti:sapphire laser - highlights

• Broadly tunable (650-1100 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 (Zewailawarded Nobel Prize in Chemistry for work on molecules)

• Mode-locked systems also can generate new optical frequency standardsand allow measurement accuracies of a part in 1018

• Amplified mode-locked lasers (with CPA) have approached Petawatt (1015

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).

Thank you!