at MIT Nonlinear Optics Elsa Garmire Thayer School of Engineering Dartmouth College garmiredartmouthedu Townes 19581961 1958 SchawlowTownes paper Infrared and Optical Masers Cold War Technical advice to the military ID: 508889
Download Presentation The PPT/PDF document "Charles Townes" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
Charles Townes at MITNonlinear Optics
Elsa GarmireThayer School of EngineeringDartmouth Collegegarmire@dartmouth.eduSlide2
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”Slide3
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
.” Slide4
Second Commercially Sold LaserSlide5
MIT Laser Laboratory (1961-1966)
Stimulated Raman Scattering in Liquids
Oscilloscope
Ruby Laser
Elsa’s FatherSlide6
Townes and Nonlinear Optics at MIT
Explained important aspects
of Stimulated Raman Scattering (SRS
):
coherent molecular vibrations
2) Introduced
Stimulated Brillouin Scattering
(SBS)
Introduced
Spatial Solitons
(se
lf-trapped optical beams
)
4) Demonstrated
filament-formation
and
instabilities.
Introduced
S
elf-steepening
of
Pulses
(
change in pulse shape from Self-Phase Modulation)
Slide7
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 W
rAnti-Stokes light: Scattered light from vibrating molecules. Scattered light is at frequency higher by
W
r
because molecule
loses vibrational energy
at frequency
W
r
.
Ordinary Anti-Stokes Raman Scattering
Vibration thermally induced
Small fraction of molecules
W
eak anti-Stokes
W
r
Light
beam
Anti-Stokes
Stokes
MoleculeSlide8
Stimulated Raman Scattering
SRS: A coherent laser beam at frequency ω
L causes gain for the Stokes wave at frequency ωL -
Wr. Intense Stokes.
Observed up to n = 3 in frequency:
ω
L
-
n
W
r
.
Anti-Stokes light
:
C
omparable intensity to Stokes.
Observed frequencies
ω
L
+
n
W
r
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
W
r
Laser, ωL
Anti-Stokes
Stokes
Molecule
1
2
ω
LSlide9
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 = 3Terhune 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). Slide10
Anti-Stokes as a Parametric Process
Molecular vibration K, driven by Stokes generation.Second laser photon scatters off K to produce anti-StokesPhase-matching means conical anti-Stokes generation
"Coherently Driven Molecular Vibrations and Light Modulation" (Garmire, Pandarese, Townes) Phys. Rev. Lett. 11, 160 (1963).
K = kL
–
k
S
k
L
=
n
L
w
L
/c
k
S
=
n
S
(
w
L
– W o )/c
k
L
= nL
w L/c
k
a
= na (w L + W
o )/cRequires phase coherence over interacting length: Phase Matching Stokes
Anti-Stokes
LaserLaserSlide11
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:
K
m
=
k
L
-
k
s
.
P
eriodic 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
k
AS
=
k
L
+ K
m
= 2k
L
-
k
s
K
m
Laser
k
L
A.S.
k
AS
Molecule
k
s
StokesSlide12
Experimental Proof: SRS in Calcite
Black = Diffuse Forward Stokes
White = Laser Light
White = anti-Stokes cone
Cone of missing Stokes
d
ue 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 theorySlide13
Anti-Stokes from
Benzene Stimulated Raman ScatteringSlide14
Liquids: Anti-Stokes in Acetone
Successively higher power pump. a) Forward-directed b) Filament-emitted
(Cerenkov)c) Volume and forwardd) All three
Phase-Match
Too Big for Phase-Match
Forward-directed
Filament-emitted
Filaments conserve momentum only along laser beam:
k
L
=
k
AS
cos
Slide15
Explanation: Mis-aligned Cell
Stokes
Anti-Stokes
Cell Facets act as mirrors to increase off-axis Stokes.
Enough to generate Anti-Stokes
Volume-matched.
FILTERSlide16
Misaligned Cell at Higher Power
= volume phase-match
S AS L
L= filament phase-match
L AS
S L LSlide17
Evidence of Filaments
The 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 + VolumeSlide18
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.
C
ircular anti-Stokes proves surface-emission generated from filaments.
Weak signs of elliptical volume emission.Slide19
Single Frequency Mode Excitation
Single frequency generated at each anti-Stokes Raman order.
Imaging Spectrograph
LASER frequencySlide20
Multi-mode excitation: slit inserted in spectrograph: (
self-phase modulation)Slide21
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:
W
s
= 2
w
o
(
v
ac
/
v
ph
) sin
(q/2
)
v
ac
<<
v
ph
q
large
ω
L
ω
L
- Ω
S
k
L
k
S
phonon =
k
L
-
k
SSlide22
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
W
s
=
2
w
o
(
v
ac
/
v
ph
)
Retro-reflected Signal
R. Y. Chiao, E. Garmire, C. H. Townes, Proc. Enrico Fermi Summer School of Physics, 1963Slide23
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 rings
M = OPTICAL MASER
B = BRILLOUIN
Brillouin
frequency offset agrees with theory (~30 GHz)Slide24
SBS1; SBS2
Q-switch
gain
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 Liquids
first demonstration of Phase Conjugation (
unrecognized
) Slide25
Early Observation of SBS
Detector
Detector
Beam
Block
“A” reads 10 X power out. Why?
First realized in 1972:
ZeldovichSlide26
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 beamSlide27
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)
2
c/64n
2
, independent of diameter.
P ~ 10
6
W.
R
. Y. Chiao, E. Garmire and C. H. Townes, Phys. Rev. Lett.
13
, (1964)
Divergence by diffraction
Total internal reflectionSlide28
Slab-Shaped Beam (1D confinement)
Solution is E(y) = Eosech(Gy).
where G =
Solution is stable
1D Spatial SolitonSlide29
2D Confinement (cylindrical beam)
“The Townes profile”
Integration gives the critical power
P =
w
hich equals that given before.
Solution turned out to be unstable
in typical nonlinear media Slide30
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 long
With Field
Without FieldSlide31
Laser
Increasing
Laser
Power
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 FilamentsSlide32
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 paperSelf-trapping paperInstabilities in self-trappingSingle mode needed to see self-focusingPhase ConjugationSlide33
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!Slide34
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
z
o
=
0
z
1
=
z
s
/2
z
2
=
z
s
Transforms into
Optical Shock
Trailing edge
Pulse slows downSlide35
Spectrum of Modulated Gaussian Pulse
Ω
M
=
ω
o
/100
Ω
M
=
ω
o
/500
z
2
= 2z
1
z
2
= 2z
1
2000 cm
-1
Phys. Rev.,
164
, 1967, F
.
Demartini
, C. H. Townes, T. K. Gustafson, P. L. Kelley
Self-phase ModulationSlide36
Townes’ Technical Contributions to Nonlinear Optics
Explained important aspects
of Stimulated Raman Scattering (SRS
):
coherent molecular vibrations
2) Introduced
Stimulated Brillouin Scattering
(SBS)
Introduced
Spatial Solitons
(se
lf-trapped optical beams
)
4) Demonstrated
filament-formation
and
instabilities.
Introduced
S
elf-steepening
of
Pulses
(equation for calculation; self-phase modulation)
Slide37
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 womanSlide38
Garmire and Townes, 2007Slide39
Tony
SiegmanSlide40
END