CA2098420C

CA2098420C - Near field scanning optical microscope

Info

Publication number: CA2098420C
Application number: CA002098420A
Authority: CA
Inventors: Robert Eric Betzig; Jay Kenneth Trautman
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1992-08-04
Filing date: 1993-06-15
Publication date: 1999-07-20
Anticipated expiration: 2013-06-15
Also published as: US5272330A; JP3262909B2; JPH06235864A; EP0583112B1; DE69308421D1; DE69308421T2; HK77597A; CA2098420A1; EP0583112A1

Abstract

An optical system for near field scanning optical microscopy (NSOM) includes a probe (20) that comprises a planar optical waveguide. This waveguide is tapered in at least one dimension, and at least an apical portion of the waveguide is coated with an opaque material. The taper is adapted such that radiation propagating in the taper region is substantially interconverted between a dielectric mode and a metallic mode.

Description

NEAR FIELD SCANNlNG OPTICAL MICROSCOPE
AND APPLICATIONS THEREOF

Field of the Invention This invention relates to ill~ JllJ~nl:i for near field scrnning opdcal S mi~ ,scu~ (NSOM).
Ar~ Background Various NSOM IlliClu~ CS have been previously reported. For example, R. E. Bet_ig et al. have described an NSOM .ni-;lusc~e using a probe that S~ light with reladvely high efficiency, is very reliable3 and gives relatively 10 high image resolution. (See European Patent Applicadon No. 91310415.4, published May 27, 1992.) This probe co,~p~;~es a tapered and at least partially m~trl1i7f;d single-mode opdcal fiber.
One dis~ anlage of probes made from ~;ylinJ~ical bodies such as opdcal fibers is that they are difficult to n~mlf~rtl-re in closely spaced arrays, for 15 example, for parallel reading or wridng operations.
S ~ of the In~. ' The invention is an NSOM opdcal system having a probe that comprises a planar waveguide. Planar waveguides are readily ll~ r~ ,d in closely spaced arrays. Thus, one advantage of the ill~entiv~ opdcal system is that it can be made 20 with muldple, closely spaced probes integrated on a single substrate. Anotheradvantage is tha; a planar ~. a~ ,uidc probe can be made very co" ,pa- ~ly Accordingly, the invendon, in a broad sense, is an optical system that cf~ es a probe having a k-ngit~l-lin~l axis and a distal end, an optical aperture defined in the distal end, means for optdcally coupling a light source to the probe 25 such that at least some el~ u~ n~ti~ radiation emitted by the source, at least at a give ~ ,len~lh, enters or exits the probe through the aperture, and means for iOg the probe relative to an objest. At least a portion of the probe is optically transmissive at least at the given wavelength, and the aperture has at least one~limPn~ion smaller than the given wavelength. The probe co"~ es a portion of a 30 planar opdcal waveguide having a core and a c1P~lfling, there being associated with the waveguide at least one guided ~ lectric mode for radiadon of the given wavelength. The waveguide has a taper region that is r~ batic~lly tapered such lh;~t at least a pordon of the taper region can guide radiadon of at least the given wavelength, and this taper region has a taper length and a taper angle. The taper 35 region t~rmin~t~s in a s~bstrntirlly flat end face oriented in a plane substantially 2~9~20 perpendicular to the lon itn(~inAl axis, and the aperture is defined in or adjacent the end face. At least a portion of the waveguide in the taper region is coated with an opaque material which has a relatively small pelle~d~ion depth for electromAgn~tç
radiation of the given wavelength, defining a metallic waveguide portion capable of S guiding a metallic mode of radiation of the given wavelength. The taper angle and taper length are adapted such that radiation propagating in the taper region is substAn*:llly illt~,~.;o~ ed between tne dielectric and metallic modes.
Brief Dcs~ - of the D. ~
FIG. 1 is ~ech~ ..,AI;r drawing of an optical system of the prior art that is 10 useful for near-field sçAnning optical miclosc~.
FIG. 2 is a schemAtic drawing of an alt~ alivG prior art optical system useful for near-field scanning optical ~~CIuSCOpy.
FIGS. 3 and 4 schf, ,A I ;ç~lly depict alternate e~ubo 1; l~f n~ of the iVG probe as a tapered, planar ~a~e~,uide.
15 Detailed D~ ,tion In one embo limrnt~ the invention includes features of optical systems of tne prior art. Turning to FIG. 1, such an optical system includes a light source 10, a probe 20, displ~rempnt means 30 for ~lieplArin~ the probe relative to an object 40 disposed, eyemrl~rily on a stage 50, adjacent the probe tip 60. The optical system 20 further col~ ;ees means for optically coupling light source 10 to probe 20. In the example illustrated in FIG. 1, the optical coupling is provided by a single-modeoptical fiber 70 PYtPn(1in~ between light source 10 and probe 20. Light source 10 is GA.,.~JIa.ily a laser. Light from source 10 is readily injected into the optical fiber by way, e.g., of a single-mode coupler 80, which includes a .ll.e.u~col~c objective 90 25 and a fiber pQsitioner 100. A mode stripper 110 is also optionally included in order to insure that only the single mode in the core is propagated to the probe, and not other modes in the c1 lr1ing. The displ~sç .~ means 30 may, for eyAmple~ be a pi~7r~e1P!ctric tube adapted for moving the probe vertically as well as in two orthognnA1 lateral ~iimp~leinne Alternatively, the ~~ieplp~e ~çt~ means may be 30 mPçh~nir~1 or L ' ~ ~'~ t~'e means for moving the stage rather than the probe, or some c~"..hi.)-l;ol~ of stage-displAremRnt and probe--lieplArçmPnt means.
One possible use for an optical system as ~çsçribed is for direct writing.
That is, the sarnple surface p~w~ a~G the probe tip may be coated with a photoseneitive layer that is capable of being exposed by light emitted forrn the light 35 source. An GAlXJ~ i pattern is created in the pho~u~ensiLi~, layer by displacing ~he probe relative to the sample, while light from the light source is continuously or .

209~20 e~ y emitted from the probe tip.
A second possible use for an optical system as described is for imaging of the sample in a so-called "ill--minAtinn" mode. According to this application, light from the probe tip is l1A..~ d through the sarnple and colleeted below theS stage (as shown in FIG. 1) by miclusc~c objective 120. (Illustrated is an ilhlmin~ticn-tr~ncmi~iion mode; and illnmin~tion-reflection mode is also readilypracdced.) The coll~cted light is directed into detector 130, which is exemplarily a photomultiplier tube. For visual positioning of the probe, it is alio desirable to include a beamsplitter 140, which directs a portion of the collected light into an 10 G~p CCG 150. Si~nifi~ntly, when the sample is scanned by a raster-like displ~eem~nt of the probe, the signals from detector 130 can be recol~sl.u.;lGd to produce an image of the sample portion that has been scanned. Such scAnning methods are employed in near-field sc~nning optical miclosc~ (NSOM), in which the probe tip is brought to within a very small distance of the sample surface, 15 typically less than a wavelength of the light emitted by the light source. NSOM
provides very high optical res- lntion by also employing an aperture in the probe tip that is very small, also typically less than one wavelength.
Yet a third possible use for an optical system as descrihed is shown in FIG. 2. In the ~rrAngem.ont of FIG. 2, the probe tip serves as a collector of light 20 rather than as an emitter of light. Such an arrAn~PmPnt is useful, e.g., for NSOM
imaging in a so-called "coll~ction" mode. (~ tr~tpd is a coll~c~ion-reflection mode. A co1lPction-tlA~-~ ..;cs on mode is also readily practiced.) Light from light source 10 is directed via tilted mirror 160 and tilted annular mirror 170 to annular objective lens 180. Lens 180 focuses the light onto the sample surface. Light 25 reflected or emitted from the surface is collected by the probe tip and directed via fiber 70 and objecliv~ 120 to detector 130.
The detector (or, more generally, the tr~n~dnc~r) 130 converts the detected light to el~ctricAl signals. These signals are readily used to create a two-~im~n~ n~l image on a video display device such as a calhode-ray tube. For such 30 purpose, a scan g~ne.dl." is used to control the ~ .plc-re .~e.-l of the probe relative to the object and to provide a .~rc.~nce signal for con~llu~ g the displayed image.The eltec~r~ signals gen(,lall;d by llni~dll~. 130 are typically analog signals.These are optionally coll~ ,d to digital signals befôre they are displayed. In such a case, a digital memory is optionally provided for storing the di~ti7ed signals, and a 35 digital ~lu~c~or is optionally provided to process the ~igi~i7~d signals (for, e.g., image Pnh~ e-,.f nt) beforç they are displayed.

:
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2 ~ 2 0 A tapered optical fiber as described, e.g., in European Patent Application No. 91310415.4, cited above, is only one example of a broader class of optical waveguides that can be used as probes in acco.dallce with the invention. The general cllalacLe ;~lir of such probes is that they have a portion distal the probe tip which S supports at least one prop~g~ting, dielectric mode of electrom~gnPti~ radiation, and a portion proximal the probe tip at least part of which supports ~,vane5celll or propagating metallic modes. Such a probe has a core region and at least one cladding region. The ~liP1pctric mode or modes are cl~ t ~ ;ncd by the boundary con~liti-)m at the in~ - ri~re between the core and the C1P~ ;ng By contrast, the 10 metallic mode or modes are defined by the bolmdal~/ conl1ition~ at least at one metal layer (or, more generally, an opaque layer having a pellcll~tion depth much smaller than the ~a~,length of the guided radiation) which is situated adjacent the C1~ 1;n~, distal the core region.
At least a portion of the probe is tapered at an angle ,B, and the tapered 15 portion includes both ~iiPlectri~ and metallic ~. a~ ,uide regions. At least partly as a result of the taper, an elecuo.n~gn-Ptir wave propagating in the probe is ~ sro~ ed between ~liP1ectri~ and metallic modes as it passes between the l~s~.;~ , regions.
The angle ,B is ''~ b~t~ in the sense that the dielectric region of the waveguide guides light of the low-order ~1iPlpctrir modes to the metallic region with high20 efficiency. If, by contrast, the taper were too sharp (i.e., ~ too large), a substantial fraction of the r~ tion injected into these modes in the dielectric region would be lost to .~ ;lion and scatter.
It is preferable for the dielectric region to support relatively few, e.g., less than 10, propagating modes, and more preferably, only one such mode. It is an 25 advantageous property of dielectric waveguides that ~1though the injected radiation can be çffi~iently coupled into the cl~1ding, which may present a reladvely large cross sectdon to the radiation source, the injected radiation can be limited to only one or a few prop~g~ting modes that are bound to the core. By contrast, more than 10, and even as many as one million or more, pl~!pa~ ing modes will typically be 30 excited when radiation is injected into a hollow metal waveguide, or a metal waveguide filled with a single, dielectric m~tPri~1, which has a diarneter of, e.g., 100 llm. When such a waveguide is tapered, a substantial fraction of the propagadng radiatdon is likely to couple into radiadve modes which are ~~ ,lGd, absorbed, or scattered out of the waveguide. Consequently, this fraction fails to propagate to 35 the metallic portion, where the surviving light is confined to subwavelength ~limPn~ic.n$.

2098~20 Each propagating dielectric mode has a typical electric field profile. The amplitude of this profile, at the cladding outer surface, is initially in~ignific~nt relative to the amplitude at the core, even if a metal layer or coating is absent. This condition assures that dielectric, and not rn~tAllir" modes are present. In the absence 5 of a metal coating or layer, the amplitude at the cladding outer surface increases within the taper region as the width or L~ull~,te of the waveguide del;lbases. As a con~equrnre there is some part of the taper region where the boundary conditionsimposed by the metal layer or coating become comparable in hll~ol L~lce to the dielectric boundary con-1itic-ni This con~1ition defines a llal~ iOII region. At one 10 end of the transition region, the mode is preclc".,i,-A~-lly dielectric in character, and at the other end it is pl~ o~ Anlly metllic in cl~a~,t~. Energy will be err,ciell~ly coupled between, e.g., the lowest prc~ag~ting dielectric mode and the least ev ~.~t sce -l metallic mode if there is a large overlap integral between these modes.
It is illlpolL~It for the waveguide to be hybrid in ch~ua ;t~,l, with a 15 dielectric portion as well as a metallic portion. The .1i~1Pctrir portion makes it possible to effiri~ntly couple radiation from a co~ ional light source into a few well-confined ~liP1ectrir modes in the probe, and/or to couple radiation from a few such modes into a con~cl-lional light detector. The fli~1ectric portion also provides substantially lossless tr~ncmis~ion of radiation through the tapered region during, 20 e.g., ;llnmin~ion-mode operation. Near the tip, by contrast, the waveguide should be metallic in ~ -, because a metallic waveguide provides much better confinPm~nt of the electrom~gnetic field. Whereas the Ol~lilllUIII CO.~fil~ e.~ in a purely ~liP!~ctrir waveguide (as defined by the full width at haLf~ ;."~-.. of the con~o"ell~ of the electromagnetic field) is about ~ (i.e.,one-half the guided 25 wavelength), a metallic waveguide can provide coufineu-ç--l as good as 1~ or even better. Because light is efficiel-~ly coupled into the ~iva~ ;uide and efficiçntly l-~ulsr~ d between the dielectric and metallic portions, relatively large signal-to-noise ratios can be achieved. This, in turn, makes it feasible to use relatively small a~ u,~,s, thereby achieving relatively high image resoblfi- n. Mo,eo~,~,., coupling 30 into a few well-confin~d modes tends to suppress ladi~ losses through pinholes in the opaque layer or coadng, which can otherwise result in an unde;,ilably high bac~uund radiation level. This enhances the reliability of prùcesses for ",A,.,.ra, ~ ;.,g probes.

2~98~0 One example of a waveguide structure useful in this regard, alternative to an optical fiber waveguide, is a thin-film waveguide disposed on a planar substrate. We believe that useful embof1im~nt~ of the inventive probe can be based on this as well as other alternative waveguide structures, provided that such 5 ~I uclul~,s include the general chalacl~istics described above.
Probes based on optical fibers, thin-film wavegu;des, or other waveguiding ~1, uCLul~,S will typically ~f.~ . in an end flat, with an optical aperture defined in the end flat. However, in some cases it may be desirable to admit or emit light through an aperture formed at or near the edge of the terminal end flat.
10 Such an aperture is readily formed adjacent to the end flat by, e.g., overcoating the end flat with opaque maten~l, and then removing opaque material down to an a~loyl;dte plane. Such a plane will be inclined relative to the ~. a~uide lo~ ..1;n~l axis, and will intersect the end flat at or near an edge thereof.
A thin-film waveguide disposed on a planar s.lb~llate has several 15 advantageous properties. For ~yamrl~, a probe based on this kind of waveguide can be mass produced in large groups on single substrates such as silicon wafers using, e.g. lithographic yl~ces~ g~ resulting in reduced unit cost. Moreover, probes of this kind can be ... ,..~r~ ;d in large arrays for use, e.g., in appli~atit~n~ which require parallel reading or writing operations.
Such a waveguide would c~?mrri~e vitreous or non-vitreous mat~n~l, and would include a core layer, a cladding layer unde.l~h~g the core layer, and,optionally, a cladding layer overlying the core layer. (In some c.llb~;.~ t~, a portion of the upper surface of the core layer may be ~,rr~ hr~,ly in direct contact with air or the ambient ,a, 1 l l ~ h. - ~; or vacuum, which, by virtue of the refractive 25 index dirr~ ,nce across the con~acling surface, would behave like an upper cladding.) An example of a so-called "channel waveguide" 400, formed, e.g., on a planar surface of a silicon ~ub~llate 410, is illustrated in FIG. 3. Well-known methods of material .1~ pG~;I;On, such as ch~mi~al vapor ~el~o~i~io~ or sputter de~G~;l;on, are used to form layers 420 - 460. The lateral extent of one or more30 layers is eYP.mrlanly defined by lithographic pAI t~ and etching. The deposited layers include lower opaque layer 420 and upper opaque layer 460, which are co. nl~osecl of a material having a relatively short penetration depth for the cle~ ...agn~tir radiation that is to be guided. (By a "short" penetration depth is meant a depth which is much smaller than the relevant wavelength ~, exemplarilv~
35 1~ or less.) The depoiit~,d layers further include lower cladding layer 430, op~io .
., 2 ~ 2 0 upper cladding layer 450, and core layer 440. Exemplary coll,posilions for the respective layers are: for opaque layers, ~l,....i~,.... (desirable for relatively high opacity) or chlull~ulll (desirable for relatively high melting point); and for the core and cladding layers, silica-based glass.
S In a planar waveguide of the kind described herein, the TEol mûde will generally be the sûle propagating metallic mode if the width W of the waveguide is less than 2~ ~ where ~ is the guided wavelength and n is the refractive index of the c~ ling (By the width W is meant the thir,knPss of the co..~h;..e-l core and cladding layers.) This mode will be the only mode of practical signifi~nre in the metallic 10 portion of the waveguide. This mode will be strongly linearly polarized, with the electric field vector oriented perpçnr1irulAr to the metal coating. The cle.,llu ..~enPtic field co..~po~ of this mode are described, e.g., in J.B. Marion and M.A. Heald, ~ l Electroma~nPtir R~ rAtion, 2d. Ed., ~r~lPmir Press, New York (1980) p.l91.
For a sllffi~iently small core in the dielectric region, there will generally be two propagating modes, namely the TEo and the TMo modes. The clecl~ gnetic field CO~'l)O~f-~l'; of these modes are ~escribe(l~ e.g., in A.W. Snyder and J.D. Love, Optical Waveguide Theûrv, ~hApm~n and Hall, London (1983), p.242.
At least a portion of core layer 440 is a/liAbAtir~lly tapered at angle ,B.
The taper is eYPmrl~ily formed by varying, ~cor~ling to 1O~ 1A1 position, the CA~JO~ i time of the substrate to the source of ~lepo~ g core rnaterial and the source of cle,~ ;n~ cladding m~teriAl (It is preferable to ll~'~;lllS~;n an ap~ ;"lAt- 1y constant ratio of core thi~n~ss to cladding ill;r~ ~f ~$ in the taper 25 region.) The waveguide r~ t~,s in an end flat 480, which may be defined by the edge of the ~ub~,alG during the deposition process or, alte~llulively, by cutdng and poliS~ ing the substrate after fi~position The opdcal aperture is defined in the end face by the spacing between the opaque layers. As long as the lla,~s~ e length L of the aperture is greater than 30 2~ ~ defined above, the terminal pûrtion of the waveguide will support at least one propagadng metallic mode, regardless of the width W of the waveguide. As a conse~lu~,nce, the ~.d~uide can ~ itionsllly be tapered in the llalls~ G direction~
as long as the relation L~ 2 is ~Ati~fie~ This ~lrlition~l taper, depicted in FIG ~, is exemplarily fo~ned by litho~ap~lically pal~ and etching the waveguide.

2098~20 As noted, the metallic waveguide portion will typically have a single propagating mode that is linearly polarized. This p~l~ri7~ti~n effect is very useful for magneto-optical im:~ging, in which image contrast is produced as a result of the rotation of the plane of pol~ri7~tion of light in an ~ylu~lia~i medium by magnetic S fields.
Appropriate optical elements are rcadily provided for coupling electrom~gn-.tir radiation into or out of probe 400 from or to, e.g., an optical fiber.
However, in plc~ e.llbQ li~ , an optical fiber is not used. Instead, a diode laser is coupled to rear face 500 either directly, or through a planar waveguide. It is 10 expected that mode coupling from probe 400 to a diode laser will be more efficient than mode coupling to an optical fiber because like the probe, a diode laser has a linear ge~m~ , whe~eas an optical fiber has a circular geometry.
A probe such as probe 400 will be especi~lly useful for applications which require ",i~ k~n of colllponenls. Those applir~tion~ might include, for 15 example, col,c ~". . electronics which read data or audio or video i,~rO",.~lion from a dense optical or magneto-optical storage mr(lillm

Claims

1. An optical system, comprising:
a probe, at least a portion of which is optically transmissive at least at a given wavelength, the probe having a longitudinal axis and a distal end;
an optical aperture defined in the distal end, the aperture having at least one dimension smaller than the given wavelength;
means for optically coupling a light source to the probe such that at least someelectromagnetic radiation emitted by the source, at least at the given wavelength, enters or exits the probe through the aperture; and means for displacing the probe relative to an object, wherein:
a) the probe comprises a portion of an optical waveguide having a core and a cladding, there being associated with the waveguide at least one guided dielectric mode for radiation of the given wavelength;
b) the waveguide has a taper region that is adiabatically tapered, at least a portion of the taper region being capable of guiding radiation of at least the given wavelength, the taper region having a taper length and a taper angle;
c) the taper region terminates in a substantially flat end face oriented in a plane substantially perpendicular to the longitudinal axis, the aperture being defined in the end face;
d) at least a portion of the waveguide in the taper region is coated with an opaque material which has a relatively small penetration depth for electromagnetic radiation of the given wavelength, defining a metallic waveguide portion capable of guiding a metallic mode of radiation of the given wavelength;
e) the taper angle and taper length are adapted such that radiation propagating in the taper region is substantially interconverted between the dielectric and metallic modes; and f) the waveguide is a planar waveguide.

2. The optical system of claim 1, further comprising:
a source of electromagnetic radiation, to be referred to as a "light source", the light source coupled to the probe by said optical coupling means;
a scan generator for driving the displacement means such that the probe is displaced in a raster pattern adjacent a portion of a surface of a sample;
transducive means for detecting at least a portion of radiation from the light source which enters or exits the probe, and for generating an electrical signal in response to the detected radiation;

far-field microscopic means for viewing at least a portion of the probe and at least a portion of the object; and video display means, in signal-receiving relationship to the transducive means, for displaying a two-dimensional image which relates to the detected radiation at least at some displacements of the probe relative to the sample, the displacements being part of the raster pattern.

3. The optical system of claim 2, wherein the transducive means are adapted to generate analog electrical signals, and the system further comprises means for converting the analog electrical signals to digital signals and transmitting the digital signal to the video display means.

4. The optical system of claim 3, further comprising storage means for digitally recording at least a portion of the digital signals.

5. The optical system of claim 4, further comprising means for digitally processing the digital signals, the digital processing means being in receiving relationship with the analog-to-digital conversion means and in transmissive relationship to the video display means.

6. The optical system of claim 1, further comprising:
a) a source of electromagnetic radiation capable of stimulating at least a portion of said object to emit fluorescent light, said source optically coupled to the probe such that radiation is transmitted from the source, through the probe, to said object; and b) means for detecting at least a portion of fluorescent light emitted by said object.

7. An optical system, comprising:
a probe, at least a portion of which is optically transmissive at least at a given wavelength, the probe having a longitudinal axis and a distal end;
an optical aperture defined in the distal end, the aperture having at least one dimension smaller than the given wavelength;
a source of electromagnetic radiation capable of stimulating an object to emit fluorescent light at least at the given wavelength;

means for supporting the object relative to the radiation source and the probe such that at least a portion of the radiation emitted by the source impinges on the object and at least a portion of the fluorescent light emitted by the object enters the probe through the optical aperture; and means for displacing the probe relative to the object, wherein:
a) the probe comprises a portion of an optical waveguide having a core and a cladding, there being associated with the waveguide at least one guided dielectric mode for radiation of the given wavelength;
b) the waveguide has a taper region that is adiabatically tapered, at least a portion of the taper region being capable of guiding light of at least the given wavelength, the taper region having a taper length and a taper angle;
c) the taper region terminates in a substantially flat end face oriented in a plane substantially perpendicular to the longitudinal axis, the aperture being defined in the end face;
d) at least a portion of the waveguide in the taper region is coated with an opaque material which has a relatively small penetration depth for electromagnetic radiation of the given wavelength, defining a metallic waveguide portion capable of guiding a metallic mode of radiation of the given wavelength;
e) the taper angle and taper length are adapted such that radiation propagating in the taper region is substantially interconverted between the dielectric and metallic modes; and f) the waveguide is a planar waveguide.

8. The optical system of claim 1 or claim 7, wherein the waveguide is formed on a principal surface of a substrate and comprises a core layer, at least one cladding layer of refractive index n adjacent the core layer, and at least one layer, adjacent the cladding layer, that is opaque to radiation of the given wavelength.

9. The optical system of claim 8, wherein:
a) the aperture has a length L in the direction parallel to the end face and parallel to the substrate surface;
b) L is greater than .lambda./2n, where .lambda. is the given wavelength;
c) the aperture has a width W in the direction parallel to the end face and perpendicular to the substrate surface; and d) W is .lambda./10 or less.