Charles Townes at MITNonlinear Optics

Elsa GarmireThayer School of Engineering

Dartmouth Collegegarmire@dartmouth.edu

Townes: 1958-1961• 1958: Schawlow-Townes paper “Infrared and Optical

Masers”• Cold War: Technical advice to the military

– Chaired Committee to create interest in mm waves.– Chaired Committee to continue support in infrared.

• 1959-61: Vice President and Director of Research for the Institute for Defense Analysis in Washington

“I felt that there just were not enough good scientists in Washington, and we had a pressing problem with the Russian missiles and other things coming on, and it was just a part of my duty”

1961-1967: Townes at MIT• Responsibilities: Provost Research: Nonlinear Optics• “We were in the early stages of non-linear optics. I was

working on non-linear optics, and various new effects that were being found there.

• “I had also invited Ali Javan, who had been at Bell Telephone Laboratories, to come to MIT as a professor, and the physics department accepted that.

• “So it was quite a group working, and I could come and go and do little parts of it when I had time, and that kept me busy, and I did some moderately important work in non-linear optics at that time.”

Second Commercially Sold LaserFlash Lamp

Ruby Rod

Capacitor

Power Supply

Laser BeamOut

100% reflectivecoating

Partial reflectionon rod

MIT Laser Laboratory (1961-1966)Stimulated Raman Scattering in Liquids

OscilloscopeRuby Laser

Elsa’s Father

Townes and Nonlinear Optics at MIT

1) Explained important aspects of Stimulated Raman Scattering (SRS): coherent molecular vibrations

2) Introduced Stimulated Brillouin Scattering (SBS)

3) Introduced Spatial Solitons

(self-trapped optical beams)

4) Demonstrated filament-formation and instabilities.

5) Introduced Self-steepening of Pulses

(change in pulse shape from Self-Phase Modulation)

1) Raman Scattering • Raman Scattering: Inelastic scattering from molecules with

natural resonance frequencies Wr.

• Stokes light: Scattered light is at frequency lower by Wr because molecule begins vibrating at frequency Wr

• Anti-Stokes light: Scattered light from vibrating molecules. Scattered light is at frequency higher by Wr because molecule loses vibrational energy at frequency Wr.

• Ordinary Anti-Stokes Raman Scattering– Vibration thermally induced– Small fraction of molecules– Weak anti-Stokes

Wr

Light beam

Anti-Stokes

StokesMolecule

Stimulated Raman Scattering• SRS: A coherent laser beam at frequency ωL causes gain for

the Stokes wave at frequency ωL - Wr. Intense Stokes.

• Observed up to n = 3 in frequency: ωL - nWr. • Anti-Stokes light: Comparable intensity to Stokes.• Observed frequencies ωL + nWr with n up to 2. • Anti-Stokes emitted in cones, observed as rings on film. Why? Anti-Stokes from Ruby Laser in Benzene Q-switched 10 ns pulses

Wr

Laser, ωL

Anti-Stokes

StokesMolecule1

2

ωL

Townes’ Inspiration for Coherent Molecular Oscillations

• 3rd Quantum Electronics Conference, Paris; 1963 – Lincoln Laboratories theoretical paper on optical phonons.

• Experiments:– Hughes Research Laboratories: Stokes n = 3– Terhune and Stoicheff: Intense anti-Stokes emission

• Stoicheff visited MIT, so we had early access to his data.

• Townes realized that coherent laser light could drive coherent optical phonons (molecular oscillations).

K = kL – kS

Stokes Anti-Stokes

Laser Laser

Phonon

Anti-Stokes as a Parametric Process

• Molecular vibration K, driven by Stokes generation.• Second laser photon scatters off K to produce anti-Stokes• Phase-matching means conical anti-Stokes generation

"Coherently Driven Molecular Vibrations and Light Modulation" (Garmire, Pandarese, Townes) Phys. Rev. Lett. 11, 160 (1963).

K = kL – kS

kL = nLw L/c

kS = nS (w L – W o )/c

kL = nLw L/c

ka = na (w L + W o )/c

Requires phase coherence over interacting length: Phase Matching

StokesAnti-Stokes

Laser Laser

Coherent Molecular Oscillations• Laser light photons become intense Stokes forward-directed

photons at frequency ω - Wr. • Missing photon energy creates molecular oscillation. • Coherent light transfers its phase coherence to molecular

vibrations: Km = kL - ks.• Periodic vibrations can subsequently be transferred back to

the light wave as coherent anti-Stokes emission• Classic resonant parametric process.• Stokes process begins the vibration• Stokes photon used up

in creating anti-Stokes• kAS = kL + Km = 2kL - ks

Km

Laser

kL

A.S.

kAS

Molecule

ks

Stokes

Experimental Proof: SRS in Calcite

Black = Diffuse Forward Stokes

White = Laser Light

White = anti-Stokes cone Cone of missing Stokesdue to generation of anti-Stokes

“Angular Dependence of Maser-Stimulated Raman Radiation in Calcite,” R. Chiao and B. P. Stoicheff, Phys. Rev. Lett. 12, #11, 290 (1964).

Cone angles agree with theory

Anti-Stokes from Benzene Stimulated Raman Scattering

Liquids: Anti-Stokes in Acetone

Successively higher power pump.

a) Forward-directed b) Filament-emitted (Cerenkov)c) Volume and forward

d) All three

Phase-Match Too Big for Phase-Match

Forward-directed Filament-emitted

Filaments conserve momentum only along laser beam: kL = kAScos

Explanation: Mis-aligned Cell Stokes Anti-StokesCell Facets

act as mirrors to increase off-axis Stokes.Enough to generate Anti-StokesVolume-matched.

FILTER

Misaligned Cell at Higher Power

f = volume phase-match

S AS

L L

y = filament phase-match

L AS

S L L

Evidence of FilamentsThe first evidence of self-trapping of laser beamsAnti-Stokes spatial distribution (no camera lens)

(a) Acetone and (b) Cyclohexane

(a) Two side-by-side Filaments (b) Many filaments + Volume

Cylindrical Lens: More Proof of A.S. Generation from Filaments

Calcite: Cylindrical lens with vertical axis forms volume phase-matched anti-Stokes ellipses.

Benzene: Same Geometry.Circular anti-Stokes proves surface-emission generated from filaments.

Weak signs of elliptical volume emission.

Single Frequency Mode Excitation

Single frequency generated at each anti-Stokes Raman order.

Imaging Spectrograph

LASER frequency

Multi-mode excitation: slit inserted in spectrograph: (self-phase modulation)

2) Brillouin Scattering• Inelastic scattering of light beam from acoustic phonons• Analogous to Raman scattering, but molecular vibration

replaced by acoustic wave with frequency near 30 GHz.• Acoustic wave and scattered light wave are emitted in

specific directions, obeying phase-match. • Brillouin frequency shift depends on angle: Ws = 2wo(vac /vph) sin( /2q ) vac << vph q

large

ωL

ωL - ΩS

kL

kS

phonon = kL - kS

Stimulated Brillouin Scattering• “Stimulated Brillouin scattering of an intense optical maser

beam involves coherent amplification of a hypersonic lattice vibration and a scattered light wave”

• “Analogous to Raman maser action, but with molecular vibration replaced by an acoustic wave with frequency near 30 GHz.”

• “Both the acoustic and scattered light waves are emitted in specific directions.”

• The largest SBS signal is retro-reflected with frequency shift

Ws = 2wo(vac /vph)

Retro-reflected Signal

R. Y. Chiao, E. Garmire, C. H. Townes, Proc. Enrico Fermi Summer School of Physics, 1963

ωL

ωL - ΩS

Stimulated Brillouin Scattering

“Stimulated Brillouin Scattering and generation of intense hypersonic waves” R . Y. Chiao, C. H. Townes, and B. P. Stoicheff, Phys. Rev. Lett. 12, 592 (1964).

SBS was detected in quartz and sapphire.

Fabry-Perot ringsM = OPTICAL MASERB = BRILLOUIN

Brillouin frequency offset agrees with theory (~30 GHz)

SBS1; SBS2

Q-switchgain mirror SBS

Laser

Fabry-Perot Interferogram

"Stimulated Brillouin Scattering in Liquids" (Garmire, Townes) Appl. Phys. Lett. 5, 84 (1964).

Note: drawing did not include phase-conjugation

Multiple orders by ruby amplification

Stimulated Brillouin Scattering in Liquidsfirst demonstration of Phase Conjugation (unrecognized)

Early Observation of SBS

Detector

Detector

Beam Block

“A” reads 10 X power out. Why?

First realized in 1972: Zeldovich

3) Townes’ Inspiration for “Spatial Solitons”Michael Hercher’s photographs of damage in glass block: University of Rochester, New York

Focal spot size = 0.04 cm

Direction of laser beam

Self-Trapping of Optical Beams“An electro-magnetic beam can produce its own dielectric waveguide and propagate without spreading. This may occur in materials whose dielectric constant increases with field intensity, but which are homogeneous in the absence of the electromagnetic wave.”“A crude description can be obtained by considering diffraction of a circular optical beam of uniform intensity across diameter D in material for which the index of refraction may be quadratic in field.”

Divergence angle = 1.22 l/nD set equal to critical angle for TIR.

Threshold power P = (1.22 )l 2c/64n2, independent of diameter.

P ~ 106 W.

R. Y. Chiao, E. Garmire and C. H. Townes, Phys. Rev. Lett. 13, (1964)

Divergence by diffraction

Total internal reflection

Slab-Shaped Beam (1D confinement)

Solution is E(y) = Eosech(Gy).

where = G

Solution is stable

1D Spatial Soliton

2D Confinement (cylindrical beam)

“The Townes profile”

Integration gives the critical powerP = which equals that given before.

Solution turned out to be unstablein typical nonlinear media

Spatial Soliton exists in Photorefractive Materials with Electric Field

Experimental demonstration of optical spatial soliton propagating through 5 mm long nonlinear photorefractive crystal. Top: side-view of the soliton beam from scattered light; bottom: normal diffraction of the same beam when the nonlinearity is 'turned off'

Bismuth titanate crystal 5 mm longWith Field

Without Field

Laser

IncreasingLaserPower

No Pinhole

“Dynamics and Characteristics of the Self-Trapping of Intense Light Beams,” E. Garmire, R. Y. Chiao, and C. H. Townes, Phys. Rev. Lett. 16, (1966)

With Pinhole

Formation of Self-trapping Filaments

Townes and Technical Errors

Divided Loyalties (MIT administration, NASA, Research, Nobel Prize)• Creative (and busy) people have to be willing to be wrong. • Be as sure as you can be.• It’s acceptable to make errors when a field is new.

– Initial Laser paper– Self-trapping paper– Instabilities in self-trapping– Single mode needed to see self-focusing– Phase Conjugation

4) "A New Class of Trapped Light Filaments"

R. Y. Chiao, M. A. Johnson, S. Krinsky, H. A. Smith C. H. Townes, E. Garmire, IEEE J. Quantum Electr. QE-2, 467 (1966).

Simultaneous presence of SRS and SBS. Lots still to explain!

5) Self-Steepening of Light Pulses• Change in temporal shape of light pulses due to propagation in

medium with intensity-dependent refractive index• Phase varies with time: Broadens frequency spectrum • Equation for pulse energy:• (Self-phase modulation)

Phys. Rev., 164, 1967, F. Demartini, C. H. Townes, T. K. Gustafson, P. L. Kelley

Gaussian input pulse in nonlinear medium

zo = 0z1 = zs/2 z2 = zs

Transforms into Optical Shock

Trailing edge

Pulse slows down

Spectrum of Modulated Gaussian Pulse

ΩM = ωo/100 ΩM = ωo/500

z2 = 2z1 z2 = 2z1

2000 cm-1

Phys. Rev., 164, 1967, F. Demartini, C. H. Townes, T. K. Gustafson, P. L. Kelley

Self-phase Modulation

Townes’ Technical Contributions to Nonlinear Optics

1) Explained important aspects of Stimulated Raman Scattering (SRS): coherent molecular vibrations

2) Introduced Stimulated Brillouin Scattering (SBS)

3) Introduced Spatial Solitons

(self-trapped optical beams)

4) Demonstrated filament-formation and instabilities.

5) Introduced Self-steepening of Pulses

(equation for calculation; self-phase modulation)

Elsa’s Personal CommentsTownes Relaxing at his Farm

PhD Students: Elsa Garmire, Ray Chiao (and Paul Fleury)Also Javan’s group; visitors: Boris Stoicheff, Francesco deMartiniAlso Paul Kelley from Lincoln Labs; also undergraduates

• Finding an Advisor• Ray Chiao• Beer in the MIT pub• Paul Fleury• Religion• Pregnancy• Post-doc at NASA• Advising Style On being a woman

Garmire and Townes, 2007

Tony Siegman

END