A POLYMERIC FILM EMBEDDED WITH SILVER NAN0 PARTICLES AND A METHOD FOR PREPARING THE SAME FIELD OF THE INVENTION The present invention relates to a polymeric film embedded with silver nano particles and a method for preparing the same, wherein the silver nano-particles grown in situ. More particularly, the said films are useful in applications where antibacterial find utility, such as purification of drinking water. The said films having strong nonlinear absorption, positive nonlinear refraction, and efficient optical limiting in the femtosecond regime. BACKGROUND AND THE PRIOR ART OF THE TNVENTTON .Antibacterial efficacy of silver is known for a long time. Among heavy metals which are antibacterial, silver is the most popular one because of its activity against a wide spectrum of prokaryotes while being relatively harmless to eukaryotes. In recent years, there has been considerable interest in employing nanometric silver particles as antibacterial agents in several , applications. Silver nanoparticles have been used in the colloidal state as well as in the for111 of embeddings in polymers, foams, fibers and textiles. The ultra small dimensio.ns lead to large surface arealvolume ratio and hence enhanced activity; it is also likely to facilitate specific 1 binding to ceU membrane leading to the bactericidal action. In the past yPars several methods have been developed for the bottom-up synthesis of silver nanoparticles through soft chemical protocols. Colloidal silver was directly used in several I j antibacterial qplications reported. 'The US Patent 6894085 discloses that the incorporation of nanosized metal particles into cyclodextrin-containing material leads to a "relative" barrier material having excellent barrier properties. Also, it has been found that the presence of nanosized metal or metal alloy particles in a barrier material such as thermoplastic material, wherein said material comprises cyclodextrin derivatives, may be advantageous in achieving excellent barrier properties. The barrier material of the said invention may provide improved barrier resistance to a variety of permeants andlor impurities. Tie diffusion of volatile substances through the barrier material may be prevented by b adding compatible derivatized cyclodextrin and nanosized metal particles to thc matcrial uscd. Nowever, US6894085 relates to barrier materials, the essential function being thc prevcntion of I passage of permeants including odors. US6894085 uses primarily nanoparticulatc zinc US6894085 does not mention any in situ growth of the nanomaterial inside polymcr films. Another US patent document 7052765 relates to the preparing of master batches and fibres of! polyesters con4aining silver nanoparticles. The said document discloses several reducing agents added externally for forming silver nanoparticles and does not use the polymer as the reducing I agent. Some 1:3f the steps in the preparing procedure disclosed in US7052765 involve temperatures 250 - 3 5 0 ' ~p~re ssures up to 2.5 atm and processing times of 5 - .I2 11. US7052765 does not mention any in situ growth of the nanomaterial inside cast polymer 61n1s1 US7052765 does not address purification of water by antibacterial action. I The inventors nave strong believe that preparing of polymer composites of the nanoparticlcs is I h:ghly desirable since it facilitates: (i) coating of surfaces of varying size and shape, (ii) controlled release of the metal atoms/ions or even restriction of the bactericidal action to contact- Itilling, (iii) rehse and perhaps regeneration, and (iv) monitoring of the bactcricidal agent through I successive action cycles. . Therefore, methodology has been developed for the preparing of silver nano particle e~nbedded poly (vinyl alohol) films, is not only very simple and safe to implement, but also likely to provc highly econoGical. The highlights of the method include the use o l aqueous medium for thc preparing proc.essl deployment of the bio-compatible and bio-degradable polymer itself as the reducing agenf, mild thermal annealing for generating the metal and in situ generation of the I nanoparticles mside the polymer matrix which serves as the stabilizer as well. Among :he several avenues, where anti-bacterial finds utility, purification of drinlcing water is I cone of the <,,st critical and important. The antibacterial agent used should be non-toxic to humans at the concentrations being used, effective against a wide range of pathogens and cheap end easy to fa.3ricate. One of the efficient and convenient ways to deploy the agent in a donlestic I setting would;%e as a coating on stirring devices. The methodology developed for the preparing I 1 -'v' of silver nanoparticle-embedded polymer film is unique and meets all these demands. "l'lie I experiments discussed below establish that the bactericidal agent (Ag-PVA film) can be used efficiently and lrepeatedly in several action cycles, allowing simultaneously, its monitoring using I spectroscopy and microscopy. r ... 1 ae applncants/ have developed an extremely simple and economically viable methodology for the preparing of silver nanoparticle-embedded poly (vinyl alcohol) film. 'The highlights of present method include the use of aqueous medium for the preparing process, deployment of the I bio-compatibl{ and bio-degradable polymer itself as the reducing agent, mild thermal annealing for generatingtthe metal and in situ generation of the nanoparticles inside the polyn~er matrix I which serves a; the stabilizer for the nanoparticles as well. This protocol can be used to fabricate I supported as dell as free-standing films. The metallpolymer ratio and the heating conditions can br tuned to actieve control on the size and shape and therefore the activity of nanoparticles. i. The methodol!,gy applicants have developed for preparing of silver nano particle &bedded polymer film 1s unique and meets all these demands. In addition, the applicants have found that bactericidal a$ent can be used efficiently and repeatedly in several action cycles, allowing I simultaneousl~i,t s monitoring using spectroscopy 'and microscopy. 1 8 OBJECTIVES OF THE INVENTION I r .. 1 he primary o3jective of the present invention to provide a polymeric film embedded with silver I r-ano panticles'and a method for preparing the same. Yet another oAjective of the present invention is to provide a polymeric film having antibacterial capabilities. 1 Yet another objective of the present invention is to provide a silver nano particles are grown in silu inside poiy (vinyl alcohol) film supported by glass substrates. Yet another objective of the present invention is to provide simple & econonlic viable method I for preparing bf silver nano particle embedded PVA film. I I L- SUMMARY OF THE INVENTION The applican!s have developed an extremely simple and economically viable methodology for I the preparing of silver nano particle embedded poly (vinyl alcohol) film. The highlights of present meth?d include the use of aqueous medium for the preparing process, dcploynicnt of thc bio-compatible and bio-degradable polymer itself as the reducing agent, mild thermal annealing for generating the metal and in situ generation of the nano particles insidc thc polymcr matrix which serves as the stabilizer for the nano particles as well. This protocol can bc used to fabricate supported as ! a l l as free-standing films. The metallpolymer ratio and thc hcating conditions can be tuncd to achieve control on the size and shape and therefore the activity of nano particles. Silver nano >articles are formed in situ inside poly (vinyl alcohol) film by a simple and convenient photocol leading to nano particle-embedded films supported on glass substrates. A srotocol is dtveloped for the preparing of freestanding films, PVAIAg-PVAIPVA. Nonlinear optical studiek using femtosecond pulse laser reveal appreciable nonlinear absorption and optical :imiting capability of these films at the. ultra fast time scales. The positive nonlinearity observed is relatively &re andof potential utility in fabricating devices with graded nonlinear refractive indices. The freestanding films facilitate unambiguous estimation of the nonlinear refractive index and su&eptibility which are comparable to those reported earlier fir supported films under femtosecond laser irradiation. The current study establishes a convenient route to the realization of freestandipg metal nano embedded polymer film and demonstrates their optical iimiting capability, serving as a step toward the development of optical power limiters. STATEMENT OF THE INVENTION ! Accordingly, the present invention relates to a polymeric film embedded with silver nano particles whekein the size of silver particles is in the range of 3 to 10 nm. I .%ISOt,h e preynt invention relates to method for preparing a polymeric film embedded with silver nano pbticle, said method comprising the steps ol; . , a. preparing aqueous solution of polymer at a temperature in the range of 60 to 7 0 ' ~ and subsequently c&ling the same up to 30°c,' I b. mixing aqueous solution of step (a) with solution of silver nitrate in a ratio in the range of 0.015 to 0.25, stirring the same for 5 to 15 minutes and obtaining the solution mixture, c. applying the solution mixture on a substrate, annealing the solution misture on the substrate at a temperature in the range of 100 to 1 6 0 ' ~ for time in the range of 1 to 3 hrs, thereby generating a polymeric film embedded with silver nano particle. BRIEF DESCRIPTION OF THE FIGURES Figure 1 Bacterial growth reflected in the increase of optical density (at 600 nin) of thc bacterial suspension (in LB broth) with time; control, samples treated with the films (a) Ag-PVA(1) (b) Ag-PVA(I1) and (c) Ag-PVA(II1) for different periods of times, 10, 30 and 60 min. Figure 2 Bacterial growth reflected in the increase of optical density (at 600 nm) of the bacterial suspension (in LB broth) with time; control, sample treated with PVA film, and samples treated with one Ag-PVA(II1) film in multiple uses are shown. :Figure 3 Absorption spectra of the. same Ag-PVA (111) film before and after multiple uses for 30 rnin each in the bacterial suspension (in LB broth). Inset shows the integrated intensity in each case, normalized to the starting value. Figure 4 AFlM images of the same Ag-PVA (111) film (a> before use and after use .for 30 11iin cach in the bacterial suspension (in LB broth) (b) first (c) second and (d) third time. Magnified images of the same films are shown in (e), (0, (g) and (h) respectively. Figure 5 Photographs of petri plates (after 12 h.incubation) spread with ultrapure water samples containing E. coli. (105 CFU); untreated (C) and treated for 15 min with the same Ag-PVA (Ill) tilm in multiple uses (1 - 20) are shown. Figure 6 Absorption spectra of the Ag-PVA (111) films before and after multiple uses for 15 min each in ultrapure water samples containing 105 CFU of E. coli (the number of uses is indicated on the plot). Figure 7 AFM images of the Ag-PVA (111) film before and after multiple uses for 15 mi11 each in ultrapure water samples containing 105 CFU of E. coli (the number of uses is indicated on the images). mul5ple uses (Fig. 7). The lower concentration of bacteria present in this study and the absence of LB broth and bacteria growth process therein, might be the reasons for the film morphclogy remaining unaffected. This experiment shows that even with fairly large bacterial concentratiohs, Ag-PVA film .is very effective as reusable antibacterial agent for water purification. i I Figure 8 photographs of petri plates spread with ordinary tap water - 15 ml each of untreated sample (C) And samples treated for 5 min with the same Ag-PVA (111) film coated glass rod in I multiple used. upto 2 1 times - and incubated for 12 h. I Figure 9 ~ h c l t o ~ r a o~fh pse tri plates spread with ordinary tap water - untreated samplc (C) and I 200 ml and b 0 ml samples treated for 5 min each with Ag-PVA (Ill) film coated on glass rod - and incubated for 12 h. Figure 10 showing three kinds of AgN03-PVA films, prepared by spin coating. Figure 11 shbwing TEM image of the Ag-PVA films with (a) x=0.029 and (b) x = 0.058. Figure 12 showing colour online (a) Electronic absorption spectra of PVA and Ag-PVA films I coated on g l ~ sO. p en apercture Z-scan traces of (b) glass and PVA on glass and Ag-PVA lilnis on glass with (c) x=0,029 and (d) x=0.058 for different input intensities(I,,) fenitosecond laser I pulses. ~ Figure 13 shlawing colour online Output vs input intensity plots of Ag-PVA films on glass for femtosecond laser pulses. Figure 14 shb~ wing closed aperture Z-scan traces of Ag-PVa films on glass, with (a) x-0.029 and [b) x=0.058,1 for femtosecond laser pulses.with input intensity, 1,,=0.62 T W / C I ~ ~T~he. transmittance is normalized and divided by the corresponding open aperture values. Figure 15 showing colour online photographs of freestanding films of Ag-PVA(PVA/Ag- PVAIPVA), hansparency of the films is demonstrated by placing them on wire framcs abovc a paper on whiLh the corresponding value of x is printed. I Figure 16 &wing colour online (a) Electronic absorption spectra of freestanding PVA and Ag- I PVA films. Open specture Z-scan traces of freestanding (b) PVA films and Ag-I'Va films with 98% alone were chosen since they showed low solubility in water at room temperature. This also implied that the aqueous solutions of PVA had to be prepared in water at 60-70oC, and then cooled to room temperature (-30oC) before mixing with the solution of silver nitrate. Antibacterial action of Ag-PVA was examined on Escherichia coli (NCIM No. 293 1, A'SCC No. 25922); control experiments were conducted in all cases. The effective concentration of silver used in all the experiments is < 6 ppm. Approximately 107 - 108 CFU of E. coli was inoculated in 15 ml Lur-a-Bertani (LB) broth. Ag-PVA film coated on 6 crn2 quartz plate was immersed in this and shaken at 37OC for 10, 30 and 60 min in different runs; the plate was removed immediately after the run. Following this, the bacterial growth was monitored for up to 12 11 by measuring the optical density at 600 nrn. The main results are collected in Fig. 11. Fig. 1 la - c shows that, vihen the films are shaken in the broth for 10 mi'n, the bacterial growth suppres.sion is more effective with Ag-PVA (11) and Ag-PVA (111). However, when shaken for 30 min or more, the effective inhibition of bacterial growth is achieved with Ag-PVA (I) and Ag-PVA (TIT) Ag' s' PVA(II1) pr&duces the maximum bacterial .growth inhibition at 10 h. Based on the strongest ! , effects obserr~ed, the inventors have chosen Ag-PVA(I1T) for further antibacterial experinients discussed below. The main aspect investigated in the following experiments is the reusability of the Ag-PVA tilms; three types of situations were studied. I. Bacteria introduced in LB Brotlz Approximately 107 - 108 CFU of E. coli was inoculated in 15 ml LB broth. Ag- PVA (111) film I coated on a 6 cm2 quartz plate was immersed in this and shaken at 370C Sor 30 min; tlic plate was immediately removed. Following this, the bacterial growth was monitored for up to 12 11 by measuring th':: optical density at 600 nm. The main results collected in Fig. 12 indicatc cfrcctivc inhibition of Sacterial growth for up to 8 h in the first two uses of the film and for up to 4 h in tlie third use of the same film. The changes in the absorption spectra after the reuses arc shown in the I Fig. 13; it is interesting to note that the change in intensity up to third reuse is < 10%. 'l'lie applicants h ~exeam ined the film morphology before and after the reuses, using atomic forcc microscopy (Fig. 14). The films show formation of pits, typically - 30-100 nm in diamctcr Cram I thc second reuse onwards. This could be a reason for the degradation of antibacterial activity. The applicants have carried out control experiments in which LB broth alonc was used with no I bacteria inoc~latedT. he films were found to be unaffected after multiple reuses. 2. ~ a c t b iIan troduced in Ultrapure Water I In the previous experiments, the inventors have used very large concentrations (CFU/niI) of bacteria, which are typically much higher than that encountered in ordinary drinking water. 111 spite of this, !the inventors have observed the bactericidal action of Ag-PVA films. The applicants have now considered slightly lower concentration of bacteria. Approximately 105 CI'U of 12. coli was inoculaizd in 15 ml of ultrapure water (Millipore). Ag- PVA(II1) film coated on a 6 cm2 quartz plate -i?las immersed in this and shaken at 370C for 15 min. The test was repeated using the same f i b 20 times in new samples of bacteria-containing water. Subsequent to each I treatment, thi water sample was plated on LB agar Petri plates and incubated for 12 h at 37oC. The bacterid colonies formed u7ere observed under a microscope. Control sample of water showed > 450 bacterial colonies whereas not a single colony was detected in the experimental I samples from up to the 20th reuse of the Ag-PVA film (Fig.15). Absorptioi~ spectra of tlie l7lm were monitoled throughout the experiment. Selected spectra of the films upto twenty reuse are collected in !he Fig. 16. There is very little variation of the spectrum and its intensity obscrved even after twenty reuses. I 3. Bactiria Introduced in Ordinary Water The AFM im,ages of the film surface show no appreciable change through the Ordinary tap watcr (possibly containing a range of nonpathogenic and pathogenic bacteria) was treated by stirring with Ag-PV4 (111) coated glass rods for 5 min at 370C. With 15 ml fresh samples of watcr, thc test was repeated 2 1 times using the same rod. The effective concentration of silver prcscnt in thc films, in the& experiments is - 0.4 ppm. While the control showcd a bactcrial count oC - 250 CFUlml, the 15 min treated ones showed tremendous reduction; the samplc subjected to thc 20th reuse of the same rod showed 5 30CFUlml (Fig. 18). Similar results were obtaincd wit11 a singlc treatment o f t 200 - 250 ml water (Fig. 19). I I The differed experiments described above demonstrate the antibacterial capability of silver nanoparticle.s in Ag-PVA (111) film. Two general possibilities can be considered: (i) bacteria 1 coming into d.ontact with the silver nanoparticles inside the Ag- PVA film and (ii) silver leaching out from the film into the bacterial medium. The spectroscopy and'microscopy experiments suggest that the Ag-PVA films are strongly affected only when inserted in thc L13 broth containing large initial concentrations of bacteria (107-108 CFU) which grow hrthcr during incubation. rb view of the pit formation observed, leaching of silver from thc film is quitc I possible; pakick aggregation may also occur. The pit formation could facilitate contact of bacteria witk the nanoparticles inside the film. In the case of water samples with lower concentratio4 of bacteria (105 CFU), even though the films are intact, bactericidal action is still observed. be minor decrease in the absorption spectral intensities suggests that the I nanoparticles! are mildly affected. One possibility is that the films swell when submerged in the aqueous medium allowing bacterial contact with the nanoparticles; particle aggregation may result from this. Another possibility is that the film swelling induces controlled leaching of the nanoparticles and consequent bactericidal action. The applican!s report the optical limiting in the nanosecond time scale by silver nano particlcembedded pc/ly (vinyl alcohol) film layered over polystyrene (Ag-PVAIPS) supported on glass. It was demhstrated that films typically a few micrometers thick show characteristics 3 , - comparable to colloidal silver with path lengths of a few millimeters. Nonlinear absorption in the nanosecond regime, of AgIAu-PVA films typically 100 pm thick, has also been reported. 'rhe inventors now investigated the nonlinear absorption/refraction and optical limiting of femtosecond laser pulses by Ag-PVA coated directly on glass and more importantly, as freestanding films with the trilayer structure, PVAIAg-PVAIPVA. The films with optimal concentrations of Ag show efficient responses and are quite stable against the high power laser irradiation. Unlike the case with nanosecond laser pulses, the high . intensities associated with the femtosecond laser pulses can cause the substrates on which tlie films are coated to produce their own nonlinear responses. Open and closed aperture %-scan experiments on the freestanding films allowed unambiguous estimation of the nonlinear absorption cross sections and nonlinear refraction due to the nanoparticles without the complications arising from the contribution of the glass substrate. Ag-PVA shows large nonlinear absorption 2.2 cm/GW and nonlinear refractive index . (n2=1.20) (10-12 esu). Significantly, the n2 dominated by electronic responses has a positivc sign; experiments in the nanosecond domain revealed contrasting behavior, with ~iegative nonlinearity possibly arising due to thermal contributions. The Ag- PVA films show nearly 90% linear transmission coupled with efficient optical limiting for femtosecond pulses; tlie freestanding nature of these composite films is a significant development from the point of view of device applications. ,. . 1 he advantages of the disclosed invention are thus attained in an economical, practical, and facile manner. While preferred aspects and example configurations have been shown and described, it is to be understood that various further modifications and additional conl7gurations will be apparent to those skilled in the art. It is intended that the specific embodiments and configurations herein disclosed are illustrative of the preferred and best modes for practicing tlie invention, and should not be interpreted as limitations on the scope.of the invention. WORKING EXAMPLES 'C A. Film preparing Required weight of silver nitrate (AgN03) dissolved in 1.0 ml water was mixed with 0.4 rnl of a solution of polyvinyl alcohol (Aldrich, average molecular weight =13-23 kDa, % hydrolysis=86) in water (2.4 g PVA in 8 ml water) to prepare different compositions designated using the AgIPVA weight ratio x (for example, 5.5 mg of AgN03 gives x=0.029). The solution mixture was stiired fc.r 5 min at 27-30 "C. Millipore MilliQ purified water was used in all operations. The glass substrate was cleaned by sonication with isopropyl alcohol, methanol, and finally acetone for 10 min cacli. 'l'lie substrate required for fabricating freestanding films of Ag-PVA and san~plesf or transmission electron microscopy (TEM) studies was prepared by spin coating a few drops of a solution of polystyrene, (average molecular weight=280 kDa) in toluene (1 g PS in 8 ml toluene) on glass, using a Laurell Technologies Corporation model WS- 400B-6NPP/LITIZ/8K photoresist spinner, at 1000 rpm for 10 s followed by drying in a hot air oven at 85-90 "C for 15-20 min. Three kinds of AgN03-PITA films were prepared by spin coating at 6000 rpm Sor 10 s: (i) AgN03-PVA solution on glass substrate, (ii) AgN03-PVA solution on PSIglass substrate, and {iii) F'VA, AgN03-PVA, and PVA solutions in succession on PSIglass substrate (schenie 10). The film coated plates were heated in a hot air oven at 130 "C for 60 niin to generate tlie silver nanoparticles in situ inside the PVA matrix. Sample prepared using procedurc (i) is referred to in the text as Ag-PVh film on glass. Samples from procedures (ii) and (iii) werc immersed in toluene taken in a Petri dish to dissolve the PS layer and release free films. 'l'he frcc Ag-PVA film obtained via procedure (ii) was used directly for TEM imaging. Thc film obtaincd via procedur~ (iii) is referred to in the text as freestanding Ag-PVA or PVAIAg-PVAIPVA lil~ii. Thicknesses of the films were measured using an Ambios Technology XP-1 profilomctcr. 'I'hc values estimated for the different films are as follows. Ag-PVA on glass of (0.5 )m; Ag-PVAIPS on glass of (5.5)m; freestanding PVAIAg-PVAIPVA of (1O)m. H. Spectroscopy and microscopy Electronic absorption spectra of the Ag-PVA films coated on glass or freestanding wcre recorded on a Shimadzu model UV-3101 UV-vis spectrometer. Ag-PVA films prepared following P' procedure (ii) described above were placed directly on a 100 mesh copper grid and exaniined . in a TECNAI G2 5EI F12 transmission electron microscope at an accelerating voltage of 120 kV. C. Nonlinear optical studies A Tisapphire laser (800 nm, -110 fs, I kHz) and frequency doubled Nd: YAG (yttrium aluminum garnet) lasers (532 nm, 6 ns, 10 Hz) were used as the excitation sourccs for tlic nonlinear optical studies. Open and closed aperture %-scan measurements were carried out by moving the sample across the foc.us of the laser beam using a computer controlled translation I stage; the scans were repeated several times to ensure reproducibility of the data. Femtosecond '(nanosecond) pulse laser was focused using a lens of 80 mm (60 mm) focal length; the bean1 waist was 2'7.1 pm (13.5 pm) at focus leading to peak intensity in the range of (0.1 5-1.73) TWIcm2 (0.03-0.28 GWIcmZ), i.e., fluences in the range of 0.02-0.19 J lcm2' (0.18-1 7 1 lcm2). , l'he input intensity could be varied using calibrated neutral density filters; closed aperture experiments used an aperture of diameter of 1 or 2 mm after the sample. The transmitted output was col1ecte.j using a calibrated fast photodiode (FND 100) and processed using a data acquisition system consisting of a lock-in amplifier or boxcar averager, adc and computer. The films were st3ble at all the intensities reported in the paper. Optical limiting studies were carried out using f /LO geometry (using 8 cm focal length lens and a beam diameter of 0.2 cm) with the femtosecond pulse laser in the same input fluence/intensity'range as noted above. 111. RESULTS AND DISCUSSION Ag-PVA films spin coated on glass substrate were fabricated with four different compositions (x=0.029, 0.058, 0.087, and 0.1 16). In this invention the inventors present the studies on .films with x=0.02? and 0.058, since they showed superior damage thresholds conipared to the films with higher content of silver; TEM images of the films are shown in Fig. 1 . A uniform distribution of silver nanoparticles is observed with particle sizes ranging froni 5 to 10 nni. The density of p~rticlesin creases with the value of x.18 Figure 2(a) shows the electronic absorption spectra recorded for the different films. The intensity of the plasmon peak increases with increasing si ,ver content. . 8 '.. I Plots of the transmittance versus scan position for the open aperture %-scan studies using the ~ femtosecon; pulse laser are shown in Figs. 12(b)-12(d). Control experiment on glass and plain ~ PVA coated on glass showed weak nonlinear absorption, possibly resulting from the large (I I mm) interaction length of glass involved, since both show nearly identical response. With 1 comparable hnd higher input laser power, Ag-PVA films coated on glass exhibit strong nonlinear absorption. Films with x00.06 show perceptible damage at input intensity exceeding (1.3 Tw/crn2). st;ldies on silver nanoparticles in the form of suspensions and doped in glass matrices indicated saturable absorption; this could be due to longer lifetimes of the surface plasm011 state I in these endronments. The applicants have attempted to fit the Z-scan curves of the Ag-PVA films using ltnodels for nonlinear absorption. However, the fitting was not very satisfactory, possibly bedause of the non-negligible effect of the glass substrate at these high laser powcrs. Results of the optical limiting experiments are collected. in Fig. 3. The films show high lincar I transmittany (87%) at low laser intensity. The plots of output versus input intensity indicatc appreciable I Optical limiting with a threshold (1112) of 1.62 Tw/crn2 and output clamped at 0.70 Tw/cm2 (1112 is defined & the input intensity at which the transmittance reduces to half of the linear I transmhtance). The dynamic range estimated as the ratio of the damage and limiting thresholds is 1 . I . The ability of these polymer films to sustain high laser power is of practical intcrcst. 'flic contribution' from nonlinear refraction was assessed using closed aperture %-scan cxpcrimcnts The kduced transmittance before focus and increased transmittance after focus indicate that Ag-PvA possesses positive nonlinearity for refraction in the femtosecond regi~iic.'f liennal buildup wo$d typically lead to negative non-linearity; the positive nonlinearity of the lilnis with the fedosecond pulses. Traces of the transmittance of the Ag-PVA films coated 011 glass, observed even with 1 kHz repetition rate therefore suggests that the responses are dominated by electronic ekects. As in the case of the open aperture %-scan curves, theoretical fitting of thc closed apertlre data is not dependable; the value of nonlinear refractive index obtaincd is divided by 1 comparable ti2 that of glass alone. I the corresponding values in the open aperture experiment so as to reveal the effcct of nonlinear rekaction alone, are shown in Fig. 14. ? Experiments using nanosecond laser pulses revealed interesting contrast with the nonlinear b' response for femtosecond pulses. Open and closed aperture %-scan experiments with the nanosecond laser pulses were carried out on pure PVA on glass as well as Ag-PVA films on glass with two different c.ompositions. PVA film did not show any detectable nonlinear absorption or refraction with the nanosecond pulses. Ag-PVA films, however, showed appreciable nonlinear absorption and the closed aperture experiment clearly revealed a negative nonlinearity. The latter is a consequence of resonant abs~ptiona t 532 nrn and considerable thermal effects operative with the longer laser pulses in the nanosecond regime. These experiments reaffirm that the strong positive nonlinear response of rlg-PVA films to femtosecond laser pulses arises primarily due to electronic effects. I-Towever, as noted above, the %-scan experiments on the films supported on glass substrates do not permit an accurate evaluation of the nonlinear susceptibility of the Ag- PVA films. 'I'hc reason for this is the following: the glass substrate, in spite of its weaker nonlinear susceptibility compared to Ag-PVA, is a couple of orders of magnitude thicker than the polymer film and hence e:xerts a comparable or even overwhelming impact on the total nonlinear response for the femtosecond pulses. In order to unambiguously demonstrate the nonlinear absorption, quantitatively estiniatc the nonlinear susceptibility, and project the potential advantage from the point of view of device applicarions, the applicants have fabricated freestanding films of Ag-PVA having no glass substrate and carried out the nonlinear optical studies on them. As noted earlier, these I'llms were prepared by successively spin coating PVA, AgN03-PVA, and PVA and generating the Ag nanoparticles by thermal treatment. Photographs of the lop m thick freestanding films are shown in Fig. 5. l'he larger thickness of these films precluded direct TEM imaging. I2lectronic absorption spectra of the freestanding films are shown in Fig. 16(a); the plasmon absorption peaks are quite similar to those observed in the supported films, suggesting that the nanoparticles are similar in size. Data from the open aperture %-scan experiments using femtosecond lascr pulses are provided in Fig.16. Pure PVA film of comparable thickness (10 pm) does not show any nonlinear absorption Fig. 16(b); once again proving that the weak absorption seen in Fig. 2(b) is due to the glass substrate. On the other hand, the freestanding films of Ag-PVA show appreciable nonlinear sbsorption similar to that observed in the glass supported films [Figs. 16( c) and 16(d):(. The data +T were fined t( the equation for the transmittivity TOzO, taking into account the spatial extent ofa Gaussian beah, zo is the Rayleigh range for the beam with intensity Ioo at focus, L is the thickness of the film, i and f3 is a boss nonlinear absorption coefficient. The value of f3 is found to increase linearly with 109 with a small but nonzero intercept, suggesting that it is likely to arise horn an association Gth two and three photon absorptions, leading to reverse saturable absorption. ! At thb highest input fluence of 1.22 TWIcm2, .the values of P are found to be 2.0 and 2.2 cmIGW, respectively, for films with x =0.029 and 0.058. -She plots 04 output versus input intensity (Fig. 7) indicate a limiting threshold (1112) of 1.4 TWIcm2, output clamped at 0.70 TWIcm2, and dynamic range of 1.'2 for the fill11 with ~ 0 . 0 2 9 ; I similar characteristics are observed in the case of x=0.058 as well. It may be noted that the linear transmittance of the freestanding films is higher than that of the glass supported ones; I transparency of the film with x =0.029 is quite significant. The silver nanoparticle- embedded films k e cpile stable for several months under ambient conditions; the stability of the material ' 1 under laser itradiation is indicated by reproducible responses in %-scan experinients repeated several times for up to 10 min, keeping the peak intensities below the damage threshold. Closed sperture %-scan experiments using femtosecond laser pulses .on the freestanding fi111is of Ag- PVA confirm that they exhibit positive nonlinearity (Fig. 18). More importantly, these %-scan I . [races allow ;:he direct estimation of the nonlinear refractive index n2 and the real part of the I rhird order susceptibility x3 of the Ag-PVA films. The data were fitted to the equation where Avo is the phase change. The values of Avo estimated for Ag-PVA filnis with ~ 0 . 0 2 9 and 0.058 are very similar, -0.39 and -0.41, respectively. Based on the preparing proccdure, the active layer jf Ag-PVA in the freestanding PVAIAg-PVAIPVA films is likely to be l p111 thick. Even though:SEM and TEM images of the film cross sections support this, in spite of several attempts we were unable to establish the thickness of the Ag-PVA layer unambiguously with corroborating elemental composition or electron diffraction data. Therefore, the inventors have used the full thickness (10 urn) of the PVA/Ag- PVA/PVA films in the estimation of n2 and X even though the contribution of the PVA layer is negligible. Hence the values we report are likely to be the lower bounds for these materials. It is notable that even then they are comparable to the nonlinear susceptibility for silver nano particles in colloidal films on silicon substrates. The freestanding films which exhibit strong nonlinear response and optical limiting capability are highly advantageous from an application point of view. WE CLAIM 1 . A polymeric film embedded with silver nano particles wherein the size of silver particles is in th? range of 3 to 10 nm. 2. The polymeric film as claimed in claim 1, comprising nonlinear absorption, positive non linear refraction and efficient optical limiting in the femtosecond regime. 3. The polymeric film as claimed in claim 1, wherein the polymer is selected from the group of bio-compatible and bio-degradable polymer. 4. The film as claimed in claim 1, wherein the polymer is poly vinyl alcohol. 5. 'She film as claimed in claim 1, having antibacterial properties for utilization in purification of drinking water. 6. A met!od for preparing a polymeric film embedded with silver nano particle, said method comprising the steps of; d. preparing aqueous solution of polymer at a temperature in the rangc of 60 to 7 0 " ~ and subsequently cooling the same up to 30°c, e. mixing aqueous solution of step (a) with solution of silver nitrate in a ratio in the range of 0.015 to 0.25, stirring the same for 5 to 15 minutes and obtaining the solution mixture, f. applying the solution mixture on a substrate, annealing the solution mixhre on the substrate at a temperature in the range of 100 to 1 6 0 ' ~ for time period in the range of 1 to 3 hrs, thereby generating a polymeric film embedded with silver nano particle. 7. "l'he method as claimed in claim 6, wherein in step (a) the polymer is selected from the group of bio-compatible and bio-degradable polymer. 8. The method as claimed in claim 6, wherein in step (a) the polymer is poly (vinyl alcohol) (PVA). , 9. The method as claimed in claim 6, wherein in step (a) the ,poly (vinyl alcohol) having average molecular weight in the range of 20 to 300 KDa and extent of hydrolysis is in the range of 70 to 99.5 %. I .' , I I 7 10. The method as claimed in claim 6, wherein in step (b) the mixing of aqueous solution of i step (a:) with solution of silver nitrate in a ratio 0.029 or 0.058 and stirring the same for 10 minutes. 11. The mzthod as claimed in claim 6, wherein in step (c) substrate are quartz plate or glass rod. 12. The method as claimed in claim 6, wherein in step (c) applying the solution niisture either Dn quartz plate by spin coating or on glass rod by dip coating, 13. The method as claimed in claim 6, wherein in step (c), annealing results in .y~/u generztion of silver nano particle. 14. The rethod as claimed in claim 6, wherein in the particle size of nano particles is in tllc range of 3 to 10 nrn. 15. A polymeric film embedded with silver nano particles and a mcthod for preparing the same substantially describsd with reference to the forgoing examples and figures.