October 15, 1994 / Vol. 19, No. 20 / OPTICS LETTERS 1601

Phase conjugation of an optical near field

Sergey I. Bozhevolnyi, Ole Keller, and Igor I. Smolyaninov*

Institute of Physics, University of Aalborg, Pontoppidanstrede 103, DK-9220 Aalborg, Denmark

Received May 31, 1994

Using degenerate four-wave mixing in an Fe:LiNbO3 0.04-wt. % crystal and an external-reflection near-field opticalmicroscope, we have achieved phase conjugation of light emitted by a fiber tip. We observe that the phase-conjugated light at a wavelength of 633 nm can reach a power of -0.1 nW and produce a 180-nm-wide spot imagein the near-field microscope. This is the first direct demonstration, to our knowledge, of the phase conjugationof near-field components of optical fields.

In recent years scanning near-field optical microscopy(SNOM) has attracted a good deal of attention be-cause the technique permits one to obtain subwave-length resolution. In turn, this fact has opened thedoorway to a number of qualitatively new opticaltechnologies based on SNOM, such as local spec-troscopy, biochemical sensing, fluorescence imag-ing, and surface modification.' The development ofSNOM has also stimulated theoretical investigationsof various fundamental aspects of the electromag-netic probe-surface interaction.2

The interaction of atoms with a phase-conjugatingmirror (PCM) and the related problem of phase con-jugation of evanescent optical field components havebeen studied theoretically for the past ten years.3 4However, the physical picture of phase conjugationof evanescent waves is still far from being clear,partly because of the lack of appropriate experimen-tal data. Since one can achieve direct observation ofevanescent waves by means of the SNOM technique,it seems natural to address the problem of phase con-jugation of evanescent waves by near-field methods.The numerous applications of conventional far-fieldphase conjugation, ranging from photolithography tooptical computing,7 indicate that the SNOM-PCMcombination, because of its inherent spatially local-ized nature, should be of significant importance.

In this Letter we present what we believe to bethe first direct demonstration of phase conjugation ofevanescent waves. In our experiment we have usedextemal-reflection SNOM with an uncoated fibertip8 and Fe:LiNbO3 0.04-wt. % crystal as the sample(Fig. 1). The degenerate backward four-wave mix-ing configuration has been chosen to produce phaseconjugation of light emitted by the fiber tip, sincein this case the phase-matching condition is auto-matically satisfied for all components in the angularspectrum of the signal beam. 9 However, there is asignificant difference between the geometry of thepump beams we have used and that of the con-ventional methods.9 Thus, to enhance the phaseconjugation of the evanescent (nonpropagating) com-ponents, one must place the counterpropagatingpump beams as close to the sample surface as possi-ble. Furthermore, the presence of pump beams onthe fiber-tip side of the sample should be avoided so

as to decrease the influence of such beams on the de-tected optical signal. For the above-mentioned rea-sons we decided to use beams that are incident at anoblique angle to the surface from the side of the sam-ple (Fig. 1). In our configuration the pump beamsthus exhibit periodic intensity variations (Wienerfringes) over the beam cross sections because of in-terference between the incident and the totally inter-nally reflected light fields. Since the fiber tip emitsstrongly divergent light, phase conjugation shouldoccur in a few Wiener fringe regions just beneaththe surface, a condition that is quite suitable forour purpose.

The technical details of our experiment are as fol-lows. The main portion (-80%) of the output of a 10-mW He-Ne laser (A = 633 nm) is directed througha lens (focal length -20 cm) onto a side facet ofthe crystal (Fig. 1). The beam is adjusted so that,after total internal reflection from the sample sur-face, it is incident perpendicularly upon the oppositecrystal facet, which is covered by a reflective Al layer(thickness -100 nm). The angle of incidence of thepump beams on the surface is -70°, which corre-sponds to the Wiener fringe width of -400 nm. Thediameters of the pump beams in the interaction re-

RC

Fe: LiNBO3 MFig. 1. Experimental setup for studying near-field phaseconjugation: LS, laser source; BS's, beam splitters; S1,2,shutters; 0, objective; OF, optical fiber; PT, piezoelectrictranslator; RC, reflective coating of a crystal facet (axis cpoints in the optical axis direction); PMT, photomultipliertube; M's, mirrors; L, lens. The polarization of the lightis perpendicular to the figure plane.

0146-9592/94/201601-03$6.00/0 © 1994 Optical Society of America

1602 OPTICS LETTERS / Vol. 19, No. 20 / October 15, 1994

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S 2000

0

w 1000

0.0 50 150 200

time (s)Fig. 2. Time dependencies of the phase-correflected light power measured for tip-surfof -5 nm (curve 1) and -1 ,am (curve 2).

gion are estimated to be -0.2 mm. An(the laser beam is coupled into a singlh(coupling efficiency -10%), which terna tip fabricated by etching the fiber intion of hydrofluoric acid for 55 min.'tive Fe:LiNbO3 crystals are known torecording-erasure time constants (-Opening shutter SI (Fig. 1) during reclosing it afterward permits us to observeter $2 open) the phase-conjugated reflectarately from the signal light reflectecThe position of the fiber tip is controlleccoordinate piezotranslator based on biaddition, a shear-force-based feedbackshown in Fig. 1) is incorporated into oumanner similar to that recently proposfield micropattern generation.' 2 By useforce feedback it is possible to maintai:tip-surface distance (-5 nm) and to imEtopography while recording simultaneofield optical image either in a conventioireflection configuration8 (open shutterphase-conjugated reflection mode of opeclosed and S2 is open).

First, we studied the time characterrecording and erasing processes. Typipendencies of the phase-conjugated refmeasured (after recording a few minute:ter S1 closed are shown in Fig. 2. In tments the fiber tip was kept at the sandifferent for different curves) duringing and readout processes. On the bEdata we have chosen a value of expoh-3 min. A typical power of the phaslight measured immediately after suchwas -10 pW. Considering all losses ofphase-conjugated light on the way frcple surface to the photomultiplier, we (power near the surface to be -0.1 nMbe mentioned that no intensity modullaser light (along with synchronous d(been used, a circumstance that explain.high noise level (as high as -1 pW) inlight signal. This level was apparentlythe expected signal from the evanescentpump waves since we were not able toof the fields' characteristic features (for

exponential dependence of the signal on the tip-surface distance).

In another series of experiments we recorded phaseconjugation approximately near a center of the fieldof view of our microscope and then imaged this fieldwith S1 closed and with the same tip-surface dis-tance as during the exposure. Topographical andnear-field optical images obtained before and af-ter the exposure are shown in Fig. 3. Comparisonof the topographical images indicates that imageswere indeed taken from the same place, whereas the

250 300 optical images clearly demonstrate that the phase-conjugated reflected light is concentrated near the

ijugated (PC) surface in a subwavelength-sized spot. The elonga-ace distances tion of the spot image [Fig. 3(c)] can be accounted

for by the drift of the sample with respect to thepiezotranslator, as seen on the topographical images

)ther part of [compare Figs. 3(b) and 3(d)]. The same experimente-mode fiber was carried out with a tip -surface distance of - 1 ,exmlinates with (without shear-force feedback). Again, the phase-a 40% solu- conjugated light resulted in a distinct spot, but thePhotorefrac- spot size was considerably larger in this case (Fig. 4).have large Averaged over several scans, the distributions of

10-100 s)."1 the detected signal across these spots (Fig. 5) appro-cording and priately yield spot widths of -180 (curve 1) and 5003 (with shut- (curve 2) nm. The difference in the spot sizes can beed light sep-d ordinarily.I by a three-.morphs. Insystem (notr setup in aied for near-

of the shear-ra a constantige a surface a)usly a near-aal external-

s)or in thepration (Sb is

istics of thecal time de-[ected powers) with shut-he measure-ie place (but (b) (d)the record- Fig. 3 (a) (c) Gray-scale near-field optical and (b), (d)

isis of these topographical images of 2 gtm X 2 pm of the sample sur-sure time of face taken (a) (b) before and (c), (d) after exposure Thee-conjugated. maximum depth of the topographical images is 115 nim.an exposurethe detected

im the sam-~stimated itsT. It shouldlation of the

ftection) has

a relativelythe detected

higher than (a) (b)fields of the Fig. 4. Gray-scale optical images of 2 6am )K 2 gm of theobserve any sample surface taken at a tip-surface distance of -1 um

example, the (a) before and (b) after exposure.

October 15, 1994 / Vol. 19, No. 20 / OPTICS LETTERS 1603

800 -

Q

0 500 1000 1500scanning coordinate (nm)

Fig. 5. Averaged distributions (curves 1 and 2) of thephase-conjugated light power across the spot images pre-sented in Figs. 3(c) and 4(b), respectively.

explained by the fact that, for a tip-surface distanceof 1 Am, evanescent waves emitted by the tip cannotreach the sample and therefore do not contributeto the phase-conjugated reflection. At the sametime, a spot width of 180 nm can be obtained onlyif the phase-conjugated light contains an apprecia-ble amount of evanescent-wave components. Actu-ally, taking into account that the external-reflectionSNOM has a resolution limit of -100 nm,'3 '4 onecan presume that the actual spot size of the phase-conjugated light near the surface (in the first case) is-150 nm. Phase conjugation of evanescent wavescan take place only near the surface, implying that itactually occurs only in a few Wiener fringe regions,a condition that explains in turn the relatively weakpower of the phase-conjugated light. The phase-conjugated light power increases with increasingtip-surface distance (Fig. 2) because the active sur-face area is larger for larger distances. Note thatthe imaged light spots [Figs. 3(c) and 4(b)] are notparticulary bright because the imaging time of ourmicroscope (-2 min) is approximately equal to theerasing time (Fig. 2).

In summary, we have demonstrated phase con-jugation of the near-field components of an opticalfield by using a external-reflection SNOM and aspecial configuration of degenerate four-wave mix-ing in a photorefractive crystal. We have observedthat the phase-conjugated light at A 633 nm canreach a power of -0.1 nW and produce a 180-nm-wide spot image in SNOM. One can significantlyimprove the quality of the optical images by usingsynchronous detection of intensity-modulated light

and/or by fixing the photoinduced gratings accompa-nying the phase-conjugation process." The varietyof photorefractive materials having different photo-sensitivities and recording speeds" seems to promiseexciting new applications for the SNOM-PCM com-bination. High-density data storage is one of the ob-vious suggestions. An interesting feature of phaseconjugation, as such, is that the phase-conjugatedlight intensity can be stronger than that of the sig-nal beam.9 This implies that it might be possible toachieve significant enhancement of the optical fieldintensity near the surface of a PCM. This effectcan be useful, for example, in local spectroscopy andfluorescence imaging.

*Permanent address, Institute of Spectroscopy,Russian Academy of Sciences, 142092, Troitsk,Russia.

References

1. E. Betzig and J. K. Trautman, Science 257, 189 (1992);R. Kopelman, W. Tan, and D. Birnbaum, J. Lumin.58, 380 (1994).

2. D. Van Labeke and D. Barchiesi, in Near Field Optics,D. W. Pohl and D. Courjon, eds. (Kluwer, Dordrecht,The Netherlands, 1993), pp. 157-178.

3. G. S. Agarwal, Opt. Commun. 42, 205 (1982).4. R. J. Cook and P. W. Milonni, IEEE J. Quantum

Electron. 24, 1383 (1988).5. E. J. Bochove, J. Opt. Soc. Am. B 9, 266 (1992).6. M. Nieto-Vesperinas and E. Wolf, J. Opt. Soc. Am. A

2, 1429 (1985).7. J. Feinberg and K. R. MacDonald, in Photorefrac-

tive Materials and Their Applications II, P. Gtinterand J.-P. Huignard, eds., Vol. 62 of Springer Topicsin Applied Physics (Springer-Verlag, Berlin, 1989),pp. 151-203.

8. D. Courjon, J.-M. Vigoureux, M. Spajer, K.Sarayeddine, and S. Leblanc, Appl. Opt. 29, 3734(1990).

9. S. G. Odoulov and M. S. Soskin, in Photorefractive Ma-terials and Their Applications II, P. Gunter and J.-P.Huignard, eds., Vol. 62 of Springer Topics in AppliedPhysics (Springer-Verlag, Berlin, 1989), pp. 5-43.

10. S. I. Bozhevolnyi, 0. Keller, and M. Xiao, Appl. Opt.32, 4864 (1993).

11. P. Giinter and J.-P. Huignard, eds., PhotorefractiveMaterials and Their Applications I, Vol. 61 of SpringerTopics in Applied Physics (Springer-Verlag, Berlin,1988), pp. 7-73.

12. A. Shchemelin, M. Rudman, K Lieberman, andA. Lewis, Rev. Sci. Instrum. 64, 3538 (1993).

13. S. Bozhevolnyi, S. Berntsen, and E. Bozhevolnaya, J.Opt. Soc. Am. A 11,609 (1994).

14. S. I. Bozhevolnyi, M. Xiao, and 0. Keller, Appl. Opt.33, 876 (1994).