New class light filament identified in Raman radiation of intense ruby laser bea

Chiao, R. Y.

Garmire, E.

Johnson, M. A.

Krinsky, S.

Smith, H. A.

Townes, C. H.

English

NASA Technical Reports Server

A NEW CLASS OF TRAPPED LIGHT FILAMENTS* 
R. Y.  Chiao, M. A. Johnson, S. Krinsky, H. A. Smith, C. H. Townes 
Massachusetts I n s t i t u t e  of Technology, Cambridge, Massachusetts 
and E.  Garmire 
Nat ional  Aeronautics and Space Administration, 
Elec t ronics  Research Center, Cambridge, Massachusetts 
The ex i s t ence  of self-trapped f i laments  of l i g h t  i n  i n t e n s e  
laser b e a q  and t h e i r  inf luence on s t imulated Raman r a d i a t i o n  
have a l r eady  been discussed. ” 29  Furthermore, f i laments  of 
diameters about 50pwhich are self-focussed by t h e  Kerr e f f e c t  
have been i d e n t i f i e d  and most of t h e i r  p r o p e r t i e s  are found t o  
agree with expectat ions.  3’ 4’ However, i t  is  shown h e r e  t h a t  
t h e  s t imu la t ed  Raman r a d i a t i o n  i s  c h a r a c t e r i s t i c a l l y  a s soc ia t ed  
with a second class of f i laments  of much smaller diamter, of 
’ d i f f e r e n t  threshold p rope r t i e s ,  and with c h a r a c t e r i s t i c s  which 
a f f e c t  s u b s t a n t i a l l y  t h e  na tu re  of Stokes and anti-Stokes 
r a d i a t i o n .  
A t y p i c a l  photomicrograph of l i g h t  produced by a Q-switched 
ruby laser j u s t  as i t  emerges from the  end of a 50 c m  ce l l  of 
carbon d i s u l f i d e  i s  shown i n  Figure la.  A beam from a 0.5 mm 
pinhole  such as t h a t  used i n  reference 4 provided t h e  e x c i t i n g  
l i g h t .  
from the  pinhole form the  background l i g h t  i n  t h e  f i g u r e .  
b r i g h t  c e n t r a l  region is  caused by se l f - t rapping  due t o  t h e  K e r r  
e f f e c t ,  as observed previously.  I n  add i t ion ,  superimposed on t h i s  
The Airy d i s k  and A i r y  r i n g  of t h e  Fraunhofer d i f f r a c t i o n  
The 
~~ 
*Work supported i n  p a r t  by the National Aeronautics and 
Space Administration under Research Grant No. NsG-330, a i d  i n  
p a r t  by t h e  A i r  Force Cambridge Research Laborator ies ,  O i f i ce  of 
Aerospace Research, under Contract Af 19(628)-4011. 
c (THRU) 
:: N 6 6  33863  
UCCESSION NUMBER) 
https://ntrs.nasa.gov/search.jsp?R=19660024573 2020-03-24T02:07:39+00:00Z
- 2 -  
concentrated f i lament  are many b r igh t  f i laments  of laser l i g h t  
of much smaller diameter, about 5 microns. W e  s h a l l  r e f e r  t o  
the  former as large-scale  t rapping and t o  the  l a t te r  as small- 
s c a l e  trapping. 
The Stokes l i g h t ,  a t y p i c a l  photomicrograph of which is  
shown i n  Figure l b ,  occurs i n  small f i laments  of about the  same 
s i z e  as the  small-scale l a s e r  f i laments .  By reZlec t ions  from a 
p a r t i a l l y  r e f l e c t i n g  mirror  i n  the  laser beam i n s i d e  a l i qu id  cell,  
these  small-scale f i laments  have been shown t o  persist for many 
cent imeters .  The technique was similar t o  t h a t  used brevious ly  
f o r  large-scale  t r app ingY4  but the  mir ror  cons is ted  of a d i e l ec -  
t r i c a l l y  coated s i x  micron th ick  mylar f i l m  r a t h e r  than a th icker  
microscope s l i d e  i n  order  no t  t o  destroy the  f i laments  by allowing 
d i f f r a c t i o n  i n s i d e  the  g l a s s  s l i d e .  Cha rac t e r i s t i ca l ly ,  t he  
many f i laments  of Stokes r ad ia t ion  which occurred a t  19 c m  i n s i d e  
t h e  c e l l  p e r s i s t e d  t o  the end of t h e  25 c m  c e l l  i n  the  same 
relative pos i t i ons .  
Photomicrographs of the beam as i t  emerged from cells of 
carbon d i s u l f i d e  of varying lengths  show the  following charac te r -  
i s t i c s .  A t  four  centimeters path length  no focussing e f f e c t s  o r  
s t imula ted  Raman rad ia t ion  were observed i n  a d i f f rac t ion- l imi ted  
beam of 200 kwatts power and 500 microns diameter. 
focussing d i s t ance  w a s  approximately s ix  cent imeters .  I n  cells 
of t h i s  length the  concentrat ion of l i g h t  i n t o  large-scale  
t rapping is very much i n  evidence. 
near  se l f - focus  a s  we  were ab le  t o  obta in  and demonstrates a 
c e n t r a l  b r i g h t  f i lament  about f i v e  microns i n  s i z e .  
The s e l f -  
6 
Figure 2a shows t h e  beam as 
Immediately 
- 3 -  
upon the  onset of large-scale  trapping, b r i g h t  f i laments  of 
Stokes r s d i a t i o n  approximately f i v e  microns i n  s i z e  appeared. 
Associated with such f i laments  w a s  a marked decrease i n  the  
i n t e n s i t y  of laser l i g h t  a t  the pos i t ion  of t hese  f i laments .  
This decrease can be l a r g e r  than an order of magnitude and shows 
up as dark spo t s  t he  order of f i v e  microns i n  s i z e  wi th in  the  
large-scale  self-focussed region, as shown i n  Figure 2b. 
path length,  t he  laser beam w a s  found t o  conta in  b r i g h t  f i laments  
r a t h e r  than dark spots ,  which occurred i n  the  same pos i t i ons  as 
t h e  b r i g h t  Stokes f i laments  and w e r e  a l s o  approximately f i v e  
microns i n  s i z e .  Bright f i laments  about t he  same s i z e  w e r e  
observed a t  twenty-five and f i f t y  cent imeters  cel l  length ( c f .  
Figure 1). By one meter cel l  length these f i laments  no longer 
appeared prominently. 
i n  t h e  other  Raman-active l i q u i d s  w e  invest igated:  
bromobenzene and toluene. 
By 10 c m  
S i m i l a r  small-scale f i laments  w e r e  observed 
nitrobenzene, 
While near  t h e  large-scale focussing d i s t ance  the  Stokes 
i n t e n s i t y  increased very rapidly,  i n  succeeding paths  up t o  f i f t y  
cent imeters  i t  increased i n  i n t e n s i t y  by less than two orders  of 
magnitude. Near the  focussing d is tance  large-scale  trapping w a s  
very r a r e l y  observed without i n t ense  Stokes f i laments  and t h e i r  
assoc ia ted  d a r k . l a s e r  spots .  Hence t h e  region of high gain f o r  
t h e  Stokes l i g h t  must be l e s s  than a few m i l l i m e t e r s ,  with a 
gain as l a rge  as about e30 p e r  centimeter.  This rap id  conversion 
weakens the laser i n t e n s i t y  wi th in  the f i r s t  few m i l l i m e t e r s ,  
producing the  observed dark spots .  The majori ty  of anti-Stokes 
- 4 -  
l i g h t  rad ia ted  i n  cones w a s  observed t o  be emit ted i n  t h i s  high- 
p i n _  region. 
The thresholds  f o r  s t imulated Raman emission and small- 
scale t rapping appear t o  be the  same as f o r  large-scale  focussing. 
Near t h e  self-focussing dis tance,  s t imulated Raman f i laments  were 
always observed i n  conjunction with large-scale focussing. Tran- 
s i e n t  self-focussing may occur a t  power l e v e l s  lower than those 
r e s u l t i n g  i n  s teady-state  trapping. 
f i laments  w e r e  formed which remained trapped f o r  long d is tances  
even when the  large-scale focussing w a s  no longer  apparent.  
t he re  w e r e  no small-scale f i laments  a t  power levels below those 
necessary f o r  self-focussing,  a s m a l l  increase  of i nc iden t  power 
above threshold l ed  t o  many f i laments ,  i n  s t r i k i n g  con t r a s t  t o  
t he  a x i a l l y  symmetric large-scale i n t e n s i t y  d i s t r i b u t i o n .  
I n  t h i s  case small-scale 
While 
The t o t a l  energy t ransmit ted through each small-scale f i l a -  
ment w a s  measured photographically a t  t he  end of twenty-five 
cent imeters  of l i q u i d  and found t o  be a few ergs .  Both the  laser 
and Stokes f i laments  appear t o  last  approximately one nanosecond 
r a t h e r  than the f u l l  30 nanoseconds durat ion of t he  laser pulse .  
Thus the  t o t a l  power i n  each f i lament  w a s  few ten ths  t o  one 
ki lowatt ,  which is an order  of magnitude o r  more below t h a t  
required by theory f o r  e i t h e r  Kerr o r  e l e c t r o s t r i c t i v e  e f f e c t s  t o  
produce trapping. The change i n  index of r e f r a c t i o n  required t o  
t r a p  l i g h t  i n  a f i lament  of diameter d is  given by A n  = - n py. 
For the  f i v e  micron fi laments i n  carbon d i s u l f i d e ,  t h i s  gives  an 
order  of magnitude o r  more l a rge r  than can be obtained from known 
n o n l i n e a r i t i e s  due t o  Kerr or  e l e c t r o s t r i c t i v e  e f f e c t s  i n  the  
o p t i c a l  f i e l d s  which must be  present .  
2, 8 
2 nd 
- 5 -  
The c lose  r e l a t i o n  of small-scale t rapping t o  the s t imulated 
Raman e f f e c t  suggests an increase i n  the  r e f r a c t i v e  index due t o  
molecular exc i t a t ion .  This CZIZ zrise from the  increased molecular 
p o l a r i z a b i l i t y  of exc i ted  v i b r a t i o n a l  states. 
numbers are not  known, the  magnitude of t h i s  e f f e c t  can be esti- 
mated from the  rotat ion-vibrat ion constants .  These show t h a t  
molecules t y p i c a l l y  change moment of i n e r t i a  by a good f r a c t i o n  
of one p e r  cen t  due t o  v ib ra t iona l  exc i t a t ion ,  and hence t h e i r  
average dimensions expand by t h i s  amount. Since the  p o l a r i z a b i l i t y  
i s  roughly propor t iona l  t o  the cube of t he  molecular dimension, 
the  molecular expansion due t o  e x c i t a t i o n  would then increase  t h e  
index of r e f r a c t i o n  of the  mater ia l  by an amount comparable with 
one per  cent  i f  a l l  t he  molecules are exci ted.  
about ten  per  cent  of the  molecules w i l l  lead t o  r e f r a c t i v e  
index changes cons is ten t  w i t h  those observed i n  small-scale 
trapping. 
of t he  required degree of exc i t a t ion  appears somewhat marginal 
from the present  experimental numbers, even when one takes  i n t o  
cons idera t ion  t h a t  a s i n g l e  laser photon may be completely converted 
i n t o  v i b r a t i o n a l  energy by successive Stokes emission and subse- 
quent d i r e c t  i n f r a red  absorption, and t h a t  the t o t a l  energy 
absorbed i n  the  fi lament may be g rea t e r  than t h a t  which is 
t ransmit ted.  
While accura te  
Exci ta t ion  of 
Except i n  the  region of very high Stokes gain,  production 
None of t he  o ther  mechanisms w e  have inves t iga ted  appear t o  
give a l a r g e  enough r e f r a c t i v e  index change i n  the  presence of 
t h e  experimental f i e l d  s t rengths .  Higher order t e r m s  i n  t he  
- 6 -  
simple K e r r  o r  e l e c t r o s t r i c t i v e  e f f e c t s  are expected t o  be 
smll and of t he  wrong s ign  t o  produce an increased change of 
index of r e f r a c t i o n  of t he  observed type. 
K e r r  and e l e c t r o s t r i c t i v e  e f f e c t s  could a t  least have the  
co r rec t  s ign ,  bu t  i t  appears much too small t o  be e f f e c t i v e  i n  
these cases. 
such as c r y s t a l l i z a t i o n  induced by molecular alignment and 
pressure of t he  f i e l d ,  could occur. 
po lar iza t ion ,  however, i s  very much less than kT, and we have 
found experimentally t h a t  t he  p rope r t i e s  of these  f i laments  i n  
mixtures are a continuous funct ion of r e l a t i v e  concentration, 
which ind ica t e s  t h a t  no phase t r a n s i t i o n  i s  present .  
It i s  no t  d i f f i c u l t  t o  understand the  s h o r t  t i m e  dura t ion  
A cross-term betweeii 
One might wonder whether some change i n  phase, 
The energy i n  the molecular 
of t he  f i laments .  The v ib ra t iona l  energy given t o  the  molecules 
through the  s t imulated Raman e f f e c t  w i l l  be converted a f t e r  a 
c h a r a c t e r i s t i c  re laxa t ion  t i m e  i n t o  t r a n s l a t i o n a l  energy, causing 
the  f i lament  t o  hea t  up and expand. Expansion w i l l  decrease the  
r e f r a c t i v e  index and destroy the  trapping. 
terist ic t i m e  f o r  v i b r a t i o n a l  r e l axa t ion  t o  t r a n s l a t i o n a l  energy 
is 2 x sec.9 The c h a r a c t e r i s t i c  t i m e  f o r  des t ruc t ion  of a 
f i lament  by expansion i s  the t i m e  required f o r  sound t o  move a 
s i zeab le  por t ion  of t he  fi lament diameter, a l s o  approximately 
one nanosecond. Thus the  durat ion of small-scale t rapping f o r  
only a few nanoseconds is not su rp r i s ing .  Furthermore, i t  is  
clear t h a t  t he  decrease of r e f r a c t i v e  index due t o  material 
expansion would keep rad ia t ion  out  of t h i s  region once the  l i g h t  
f i lament  has  been destroyed. Thus the  appearance of dark s p o t s  
i n  the laser d i s t r i b u t i o n  near t h e  region of high Raman gain 
may p e r s i s t  t he  e n t i r e  durat ion of the  laser pulse  even when 
the  Stokes r ad ia t ion  l a s t s  only one nanosecond. 
I n  CS2 t he  charac- 
- 7 -  
The cause of the  low Raman gain i n  the  lat ter por t ions  of 
long cells has  n o t  been wwrked out i n  d e t a i l .  While s a tu ra t ion  
of t h e  molecular population d i f fe rence  i s  an  a t t r a c t i v e  m,echn.ism, 
t h e  present  experimental numbers do not  appear t o  allow a s u f f i c i e n t  
f r a c t i o n  of t he  molecules t o  be exc i ted .  
of t he  presence of many orders  of Stokes r ad ia t ion  is c e r t a i n l y  
a cont r ibu t ing  f ac to r ,  bu t  does no t  adequately decrease the  
conversion of laser l i g h t  s ince  the  h ighes t  order  Stokes 
r ad ia t ion  i s  absorbed d i r e c t l y  by the  l i q u i d  and provides a rap id  
lo s s .  Dispersion i n  the  r e f r a c t i v e  index w i l l  c ause - fu r the r  ga in  
decrease i f  t he  exc i t i ng  l i g h t  is  broadened i n  frequency. 
Decrease i n  gain because 
The ex is tence  of small-scale t rapping described here  should 
have a considerable  e f f e c t  on t h e  c h a r a c t e r i s t i c s  of s t imulated 
r a d i a t i o n  through t h e  following new considerat ions:  
(a)  The small diameter of the  f i laments  changes the  phase 
ve loc i ty  of l i g h t  waves by wave-guide e f f e c t s  and 
the  angles of anti-Stokes rad ia t ion .  10 
(b) The l a rge  change i n  index of r e f r a c t i o n  
10 phase v e l o c i t i e s  and r ad ia t ion  angles.  
hence modifies 
a l s o  modifies 
(c)  The l a r g e  change i n  index i s  probably accompanied by 
an  appreciable  change i n  acous t ic  v e l o c i t i e s .  
(d) The index change i n  the  f i laments  produces more powerful 
modulation e f f e c t s  and smearing of r a d i a t i o n  frequencies than 
what is  ca l cu la t ed  assuming Kerr e f f e c t s  alone. 11 
(e)  The rate of change of index of r e f r a c t i o n  and hence of 
phase of the  trapped rad ia t ion  assoc ia ted  with rap id  formation 
- 8 -  
and decay of the small-scale filaments can contribute importantly 
to frequency smearing. 10 
( f )  Variations in molecular saturation aiiii fr: i d e x  of 
refraction along the filament can give different relative 
intensities of Stokes radiation in  the backward and forward 
directions, as is sometimes observed. 
. 
REFERENCES 
1. R. Y. Chiao, E. Garmire, and C. H. Tomes, Phys. Rev. Letters 13, 
r ? n c i \  AlY \.LYU+J. 
2. G. Mayer and G. Hauchecorne, Compt. Rend. 261, 4014 (1965). 
3. P. Lallemand and N.  Bloembergen, Phys. Rev. Letters l4, 1010 (1965). 
Y. R. Shen and Y. J. Shaham, Phys. Rev. Letters l4, 1008 (1965). 
4. E. Garmire, R. Y. Chiao, and C. H. Tomes, Phys. Rev. Letters l.6, 
347 (1966). 
5. C. C. Wang, Phys. Rev. Letters l6, 344 (1966). 
6. P. L. Kelley, Phys. Rev. Letters 15, 1005 (1965). 
V. I. Talanov, JETP Letters 2, 138 (1965). 
7. Demonstrated i n  recent  computer ca l cu la t ions  by P. L. Kelley. 
8. F. J. McClung, Fourth In t e rna t iona l  Conference on Quantum 
Elec t ronics ,  Phoenix (April ,  1966). 
G. Mayer, i b id .  
9. K. E. Herzfeld and T. A. L i tov i tz ,  Abosrption and Dispersion 
of Ultrasonic  Waves, Academic Press, New York (1959). 
10. See a l s o  K. Shimoda, Fourth In t e rna t iona l  Conference on 
Quantum Elec t ronics ,  Phoenix (April,  -1966). 
11. N .  Bloembergen and P. Lallemand, Phys, Rev. Letters 16, 81 (1966). 
Figure 1 
(a) 
CS, and exh ib i t i ng  l a r g e  and small-scale trapping.  
t i o n  is  30x. The b r i g h t  cencra i  poztion Is the  large-scale  
trapped beam; the  many small b r i g h t  f i l aments  demonstrate t h e  
small-scale t rapping.  
t he  untrapped beam d i f f r a c t i n g  from the  i n i t i a l  pinhole.  
Image of a ruby laser beam emerging from a 50 c m  ce l l  of 
Magnifica- - 
The broad d i sk  and r i n g  of l i g h t  are 
(b) Stokes r a d i a t i o n  under condi t ions s i m i l a r  t o  ( a ) .  Magnifi- 
ca t ion  SOX. 
Figure 2 
2 Image of ruby laser beam emerging from a 6 c m  ce l l  of CS 
under magnif icat ion a f a c t o r  of 40x. (a)  Very nea r  se l f - focus  
(b) S l i g h t l y  h igher  power level, i.e., se l f - focus  occurred j u s t  
before  t h e  end of t h e  cell. 
Q b 
FIGURE 1 
a 
F I G U R E  2 
b 


A new class of trapped light filaments

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