TRANSVERSE OPTICAL BISTABILITY IN GOLD COLLOID DUE TO SELF-FOCUSING

HAL Id: jpa-00227613

https://hal.archives-ouvertes.fr/jpa-00227613

Submitted on 1 Jan 1988

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

TRANSVERSE OPTICAL BISTABILITY IN GOLD COLLOID DUE TO SELF-FOCUSING

Lingming Wang, Chunfei Li, Zhongxiao Zhang, Yan Jia

To cite this version:

Lingming Wang, Chunfei Li, Zhongxiao Zhang, Yan Jia. TRANSVERSE OPTICAL BISTABILITY

IN GOLD COLLOID DUE TO SELF-FOCUSING. Journal de Physique Colloques, 1988, 49 (C2),

pp.C2-435-C2-438. �10.1051/jphyscol:19882103�. �jpa-00227613�

JOURNAL DE PHYSIQUE

Colloque C 2 , Supplement au n06, Tome 49, juin 1988

TRANSVERSE OPTICAL BISTABILITY IN GOLD COLLOID DUE TO SELF-FOCUSING Lingming WANG, ChunFei LI, Zhongxiao ZHANG* and Yan JIA

Department of Physics, Harbin Institute of Technology, China

" ~ e p a r t m e n t of Chemistry, Heilongjiong University, Harbin, China

Abstract-The demonstration of transverse optical bistabiiity and self-focusing in gold colloid is reported, for the first time, using a continuous Argon Laser.

Recently there has been considerable interested in researching for the art if iciat Kerr medium(l)- The optical nonlinearity in gold cot Loid was first reported in 1985(2), and many related works has been done since that time(3)- Now, we report the observation of self-focusing and

transverse optical bistability by using the CW-Ar Laser in gold colloid- And a theoretical model of self-focusing for metal colloid is proposed.

1-EXPERIMENT AND RESULTS

Our experimental setup, as shown in Fig. 1, consists of a CW-Ar' Laser, a convergent lens with f=8. lmm, optical modulator, aperture, and optical detectors. The maximal output power of the Argon laser is 2w in TEM, model. The modulator converts the continuocls wave into triangular waves

Convergent Len

BS Aperture

\

-

I

laser Modulator

E l t

Refiective Mirror

Fig. 1 The setup of observing transverse optical bistability.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19882103

C2-436 JOURNAL DE PHYSIQUE

The gold colloid exhibiting red colour is formed by dispersing the gold microparticles with a diameter of 35nm in water, which has the density of 1. 12xlOu/cm'

.

The cell containing the medium has length of 10mn, which one side is a reflective mirror with R=83%.

The self-focusing experimental results are shown in Fig.2. When the power of insident beam is smaller than 300mw, the beam passes the medium without any change, so there is no self-focusing, see Fig. 2[a].

When the power of insident beam is larger than 300mw, the self-focusing appears, see Fig- 2 [b]

-

The beam in the medium shows the self-focusing in the center part and diverging in the side part, when the power is between 300mw and 800mw, see Fig. 2 [ c ]

.

Uhen the power is Larger than XOOmw, the self-trapping in the medium occurs, see Fig. 2 [dj ,

Fig. 2 The self-focusing in the medium, [a] P<?OOmw, [b] P=300mw, [cl 300mwsPe800mw, [dl P>800mw.

Taking 8OOnnw as the threshold of self-trapping, based on the formula

we have calculated and obtained the optical nonlinear index n,=3.6X 10-'cnf/w

,

that is similar to the artificial Kerr medium(4)- We have observed the transverse optical bistability due to self-focusing, the threshold is 300mw and the switching time is 0-lms. Fig-3 [a] shows the insiden.t and transmitted light curves, and Fig- 3 [b] shows the optical bistable loop. We think that the thermal absorbtion will affect the medium nonlinearity, but i t is not the main factor of the optical nonlinearity. The reason is that the absorbtion for gold colloid in 514-5nm is larger than that in 488.0nm but n, is the same for two wavelengthes- We . l i o u l d want i on that the gold cot lioid with the density

1 . 1 2 ~ lO"/cm' and the particle size 35nm is most stable-

Fig.3 [a] are the insident and transmitted Light curves [b] is the optical bistable loop

A conductor sphere Levitating in water within an electric field E can behaves as a dipote with polarizability P which is(5)

P= [a'

.

^{<ng-1 >I / [n0+21}

where the

n

is the proportion of the refractive index of the surrounding dielectric medium to that of the conductor sphere, a is the radius of the sphere. A conductor sphere within a field of Gauss beam

E< r, z >= [F/ fc z >I exp 1-rt/vf

- ^f'c

^{z )I}

experiences a force

-L F=<?

.V )s

Taking a time average over a period of light field and ignoring the axial force,we get the radial force

where @ is the radial potential of the dipole- According to N=N,exp [- +/KT]

the distribution of particles can be writted as

No is the homogeneous distribution. The function N<r,z>, for f<z>=0.4, 0.5, 1, 1 - 5 and 3 are plotted in Fig.4. We can see that, for the weak beam, namety larger f<r, z > , the change of the density of the particles

JOURNAL DE PHYSIQUE

with decrease of radius is very smatt and only a Littte tight can pass the medium, for the strong beam, namely smaller f<r,z), the change of the density with decrcase of rasius become very large and most of Light can pass the medium. So the phnomenon of self-trapping is formed.

3 - CONCLUS X ONS

We have observed the self-focusing with power threshold 300mw and self-trapping with power threshold 800mw in gold coLloid. And the optical nonlinear refractive index has been calculated. We have also observed the transverse optical bistabi L i ty due to self-focusing, which switching time is 0. lms.

According to our theoretical interpretation, theself-focusing is caused by strong Laser induced the deep well of particle number-

REFERENCES

N

(I)P. W. Smi th, P. J. Maloney and A. Ashkin Opt-Lett., 7<8), 347<1982>

(Z)D.Ricard, Ph-RoussignoL, andChr-Flytza- nis, Opt-Lett. 10<lO>, 511 <1985)

(3)F.Hache, 11. Ricard, and C-Fly tzanis, J . Opt- Soc.Am.B, 3< 12>, 1647< 19861 (4)A.Ashkin, J-M-Dziedzic, andP.W- Smith, Opt.L.ett.7<6), 276<1982>

(5)Max Born and Emit Wolf, Principles

r

of Optics;, (Pergaman Press, 1975>, 834 Fig. 4 The curve of function N<r, z ) for f<z>=0.4, 0.6, 1, 1-5 and 3