Scanning near-field optical microscopy (SNOM) and its application in mineralogy

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(1)

Scanning near-field optical microscopy (SNOM)

and its application in mineralogy

Autor(en):

Gutmannsbauer, Winfried / Huser, Thomas / Lacoste, Thilo

Objekttyp:

Article

Zeitschrift:

Schweizerische mineralogische und petrographische Mitteilungen

= Bulletin suisse de minéralogie et pétrographie

Band (Jahr): 75 (1995)

Heft 2

Persistenter Link: http://dx.doi.org/10.5169/seals-57155

PDF erstellt am:

21.06.2016

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SCHWEIZMINERALPETROGRMITT75,259-264, 1995

Scanning near-field

optical microscopy

(SNOM)

and

its application

in

mineralogy

byWinfriedGutmannsbauer12, Thomas Huser1,ThiloLacoste1,HarryHeinzelmann1 and Hans-Toachim Guntherodt1

Abstract

Scanningnearfield optical microscopy (SNOM)isamemberof the family ofscanningprobe microscopes

It

com

binesthe high three dimensional resolution ofascanningforce microscope with the contrastmechanismsofan op

tical microscopeAnoptical resolution beyond the diffractionlimitcan be achieved We showthe first apphcation of this technique in the field of mineralogy,and wepoint out its future potential

KeywordsScanningnear field opticalmicroscopy, scanningforce microscopy, optical microscopy, diffraction limit, resolution,shearforceimage, spectroscopy

Introduction

Optical microscopyisstill themostpopulärmi¬

croscopytechnique today

It

isfast, mexpensive, providesawealthof Informationand allowstoin¬

vestigate samples atambient conditions There

aremany contrastmechanisms,depending on

whether polarization, amplitude orphaseofthe

lightis detected Also,itsapplicapabilityranges

from imaging of livingspecimensto spectro-scopic analysis

of

mineralogicalsamples Toex¬

tract more Informationofagivenarea,thereis a

growingneed

for

higher spatial resolution,lead¬

ing automatically intothenanometncregime Al¬

thoughoptical interferometrygivesaverticalres¬

olution down tothenanometerscale, thelateral resolutionislimited by diffraction Thiswasde¬

rived by Abbe inhisworkondiffractionand mi¬ croscopicimaging (Abbe,1873)The theoretical diffraction

limit

of resolution,alsoknownasthe

Raleigh criterion(1), isessentiallygivenbythe

wavelengthAoftheemployed radiationandthe

numeric aperturen sm@

A

x *

0 61 A

n sin

q

(i)

where n is therefractive indexof the medium

betweenthe objectandthe microscope,and

0

is the acceptance angle

of

thedetectionsystemAs

aconsequence, thelateral resolutioncanonlybe

improvedby applyingahigh numerical aperture (NA nsmö) or bydecreasingthe wavelength,

suchasinelectron or X-ray microscopy

Success-ful effortshavebeen madein this direction by

Mansfield

and

Kino

(1990) andby van

der

Oord

et al (1992) To get evenhigher resolution

onehasto gotoaregimewherewavelengthisof

no relevance Studiesin this direction resulted in another relativelynew anddifferentclassofmi¬ croscopesthathavealateral resolution m thena¬

nometerränge the scanningprobemicroscopes

(SPM), of which the most prominent memberis the scanningtunnehng microscope (STM)

(Bin-nig

etal,1986)

All

ofthesedevelopmentsare

very interesting but onlyaminontyhasproven theirusefulnessto mineralogical or geological problems Nevertheless, dueto the high lateral resolutionofthese techniques,new insights mto mineralogicalquestions have beengained(Ho¬

chella

et

al,

1989,

Hochella

et

al,

1990,

Hartman

et

al,

1990,

Weisenhorn

et

al,

1990,

Johnssonet

al,

1991,

Rachlin

et

al,

1992,

Hill-1

PhysikalischesInstitut, UniversitätBasel,Khngelbergstr82,CH4056 Basel,Switzerland 1

Corresponding authoremailgutmannsbaue@ubaclu umbas ch,Phone+416126737 30,Fax+41 61 267 37 84

(3)

260 W.GUTMANNSBAUER, TH. HUSER,T.LACOSTE, H.HEINZELMANN ANDH.-J.GÜNTHERODT

ner

et

al,

1992;OhnesorgeandBinnig,1993;

Scandella

et

al,

1993;Stippet al., 1994; Hen¬

derson

et al., 1994;

Nagy

andBlum,1994).

It

is

thereforeclearthatmuchcouldbelearnedby

combiningtheinteractionmechanismof optical

microscopywiththeresolutionofthe scanning

probemethods.Thescanningnear-field optical

microscope(SNOM)combinestheproperties of

ascanningforcemicroscope(SFM)operatedin

thenon-contactmode and anopticalmicroscope.

Theresultisasimultaneousacquisition of optical

andtopographic information withalmosttheres¬

olutionofaSFM.

HOW TO OVERCOME THEDIFFRACTION LIMIT?

Thebasicideatoovercome thediffractionlimit inmicroscopy wasalreadydiscussedby Synge

(1928). To understand Synge'sproposalwe have

to introduce two notions thatarenotapplicable

to conventionalopticsbut to SNOM:thefar-field

andthe near-field.Withrespecttoalightsource,

the observer stays in the far-field when the

distance betweenhim andthe lightsource is

more than the wavelength X ofthe emitted light (d

»

X).Thenear-field, consequently, de-scribesthe rängewithinthewavelength ofthe

light (d

«

X).Whentheobserver staysinthe

near-fieldtheRaleigh

criterion(l)

is nomoreap¬

plicableandtheresolutionisonly determinedby

thesizeof the lightsource and its Separationfrom thesurface.

A

resolution beyond the diffraction

limit

isthereforepossible. Synge'sproposalwas based on thefollowing knowledge(seeFig. 1):

A

tiny apertureisilluminated fromthebackand actsasasubwavelength sizedlightsourcewhich

isscannedin opticalnear-field distancetoasam¬

plesurface, i.e., atadistance d

«

X.No

diffrac-light source

d

«

X

T

sample

Fig.1 Genericscanningnear-field opticalmicroscope

(SNOM)setup: asubwavelengthlightsource is

raster-scanned acrossthesample atsmalldistance.

tion limitedlens is neededforthe image forma¬

tionataresolution functionallydependent on

onlytheprobesizeand theprobe-to-sampleSep¬

aration,eachof whichcan be made muchsmaller than thewavelengthof light. But more than40 years passeduntilSynge's ideas weretakenup and realizedby AshandNicholls, whoachieved

alateral resolution

of

X/60inthemicrowave wavelengthregime (X 3cm) (Ashand Ni¬

cholls,

1972).

The realization ofthe scanningnear-field optical microscope (SNOM),sometimes also

termed NSOM,wasonlypossibleafterthe inven-tionoftheSTM, withitsability toactuate probes

inacontrolled mannerat distancesof nanome-tersoversurfaces

of

interest.Indeed,theSNOM borrowedmany partsfrom STMand especially SFM.Shortly after theappearanceoftheSTM,

Pohl

et al. (1984)first demonstrated near-field microscopy intheoptical wavelengthregimewith

aspatialresolutionofX/20 (25nm). Despite the attractive possibilities, SNOM

wasnearlyforgottenbecauseofthebigrush on theotherscanningprobe methods suchasSTM

andSFM.But nowmany scientists realizethat certainsample-specificinformation, especially chemicalidentificationand spectroscopy is near¬

lyimpossibleto obtain withSFM,or just under very restrictivecircumstances, suchasfriction forcemicroscopy(FFM)(Frommerand

Meyer,

1991;

Meyer,

1992).SNOM providesmostof the contrastmechanismsknownfromthe conven¬

tionalmicroscopy, suchas:absorption,polariza¬

tion,fluorescence, andreflectivity.

In

somecases

theresultingcontrast issimilar to thatobserved

in the far-field (conventional microscopy),

whereasinothers,near-fieldspecific eflfects are observed.

Experimental

Figure2showsaschematicdiagram ofaSNOM

as

it

was usedin thisstudy(Topometrix Aurora). Laser light

(Ar

488nm)iscoupled intoa

near-field probe formed fromanaluminium

(Al)-coat-ed,taperedoptical fiber(seeinsetinFig. 2).The apertureat the apexofthe-fiber, usuallyaround

50 nmindiameter, is theonlyplacethatisnot

coatedwithAI. Thetapered endisproduced by

pullingthefiber while heating withaC02 laser.

This probeisheldin near-fielddistanceof the

sampleby a force feedback similar to

non-contact SFM.Thefiberisattachedtoapiezo tube,thatoscillatesthefiberend near itsreso¬

nancefrequency.Approaching the probe tothe sample surface causes aninteraction thatresults

(4)

SCANNINGNEAR-FIELDOPTICAL MICROSCOPY (SNOM) 261

laser

(488 nm)

^

fiber

scan

piezo

fiber "probe"

sample

lens

photomultiplier

fiber

Al-coating

near-field

far-field

Fig. 2 SchematicofaSNOM in transmission. Insetshowstheendof the opticalsensorwithanaperture of about

50nm indiameter.

phase shift.This dampingisafunction ofthedis¬

tance andisusedto keep the probe-to-sample

distance constant (shearforce feedback).

After

transmission throughtheapertureand the sample, thelightiscollectedwithanobjective

lensoftheconventionalsystem.

It

isalso possible

to collect light thatwasreflectedfromthe sample surface

(Bielefeldt

et al., 1994).Thismodeof detectioncan bevery useful

for

opaque samples suchasores, and

for

local

reflection-spectros-copy.

In

both cases the collected light is detectedbyaphotomultiplier. The sampleis

scannedundertheprobe with the aid of

piezo-electrictransducers, andtheSNOM(transmis¬ sionor reflection)and shearforcesignals arecol¬

lectedforeverypoint (forareviewand an intro¬

ductionintotheotherSNOMtypessee

Heinzel-mannand

Pohl,

1994).Asaconsequence, non-contact (shearforce)andopticalimages areal¬

ways collected simultaneously. Because the

oscil-lationoftheprobeisparallel to thesurfacein SNOM,insteadof the perpendicular motion in non-contactSFM,this modeiscalledshearforce

mode.

Results and discussion FIRSTMINERALOGICALAPPLICATIONS

Figure3ashowsaphotomicrograph ofa

well-pol-ishedthinsection (25um)

of

asymplectiticinter¬

growth of amphibole, diopsideandplagioclaseat therim ofanidiomorphic garnet crystal. The

symplectite appearsonlyasablackarea(marked withawhite arrow)attheborder

of

the relatively idiomorphicgarnet.Identificationof thephases

formingthesymplectitewasperformed byelec¬

tron probe microanalysis. Figure3bshowsacor¬ responding shearforceimageofasymplectite fromthe samethinsection.Theshearforceim¬

agegivesinformation onthetopography of the

investigated area.

It

showsmainlyScratches,re¬

sulting from thepolishingprocess(arrows1,2).

Figure3c displaysthe transmission modeSNOM

imagewhichwassimultaneously recorded with

the shearforceimagein figure3b.Onecansee

thatthisimaging mode yields completely differ¬

entinformation. Forexample,the grainbound¬ aryofgarnet isclearly visible (arrow1), and so is thesymplectiticintergrowthat the edgesof the

garnet.This picturereveals,that the symplectite

isamixtureof differentphases(arrow2).

It

ap¬

pearsthatthe symplectitegrowsinto the garnet, forming pear-like structures orbulges(arrow3),

whichareindicativeof grainboundary migration.

It

isnot thateasyto attribute the clear contrasts toaspecific processin this near-field opticalim¬ age,butitisprobablyamixtureof absorption

and changes

of

the polarization.

Figure4servesto illustrate that SNOMisalso suitable to imageinclusionsin minerals.

It

em-phasizes thepower

of

gaining information that

(5)

262

W.

GUTMANNSBAUER.

TH. HUSER.

T.

LACOSTE.

H.

HEINZELMANN AND

H.-J.

GÜNTHERODT

J

1

%

A-y

fe

200

y

m

%^

5

|jm

'

i

4 M

1

-if

^m.

- *

's

**

JF-®y

¦

i-"-?¦¦;

'F\

4*

R

Fig. 3

a)

Photomicrograph

of

a

garnet

with

a

symplectite

at its

border (arrow).

b. c)

Simultaneously acquired

to¬

pography (b)

and

SNOM

images (c)

of

a

selected area.

Note

the

different contrast

in the scanning near-field opti¬

cal microscope image.

Details

are

explained

in

the text.

was

not

accessible

until

now.

Figure

4a

shows

the

shear

force image

of

a

thin

section

of

a

rock

con¬

taining

quartz

and

feldspar. The

grainboundaries

are

clearly visible

due

to

the

different

relief

of

quartz

and feldspar developed

during

the

polish¬

ing

process

(arrow

1

and

bright line).

The quartz

crystal

is

harder

and consequently higher than

the feldspar. These structures

can be

seen

in

the

shear

force (Fig. 4a)

as

well

as

in the

transmission

mode

SNOM

image (Fig. 4b).

But

in transmis¬

sion

mode.

however.

there

are some

additional

(6)

SCANNING

NEAR-FIELD

OPTICAL MICROSCOPY

(SNOM)

263

im

\

WM

*

^

4

um

i

*;

0

Fig.

4

Topographic

(a) and

SNOM (b)

images

of

a

thin

section

containing quartz

and feldspar. The

solid

inclusion marked

on the

left

is

shown

in

a

separate

scan

in

(c).

crystal

is

not totally uniform.

There

are

lighter

and

darker

regions arranged in

parallel

bands.

This

shows

local

variations

of

the

sample

absorp-tivity.

that could

be

the

result

from

former

tec¬

tonic

stress

(deformation-bands).

Second,

there

are

three

black

spots

(circled),

that

show

a

differ¬

ent

absorptivity

and

probably

represent mineral

inclusions. Figure

4c

shows

a

smaller

scan

of

the

upper

left

cycled spot. which

has

a

diameter

of

about

200

nm.

With

conventional optical microscopy. it

would

bc

very hard

just

to

see

these

and

other

small structures,

such

as

solid and

fluid

inclu¬

sions.

symplectites.

myrmecites,

exsolution-lamellas and

polysynthetic

twins.

An

optical

identification of

these

minerals

and

structures

would

be

impossible.

With SNOM, it

is

possible

to

additionally perform optical

spectroscopy

with

a

spatial

resolution in

the ränge

of

nanomelers.

The

only prerequisite

is

a

tuneable

laser.

that

is

a

laser that

can

continuously emit different

wave¬

lengths.

Other

types

of

spectroscopy that

are

at-tainable,

and

where

different

groups

are

working

on. are

fluorescence,

cathodoluminescence.

Ra¬

man

and

infrared

spectroscopy.

If

one

sueeeeds

to

dye

the

different

phases

with

a

dye

that

is

excit-able by

the wavelength

of

the

laser

used, an easy

identification

with

high

resolution

is

possible.

The

Al-coating

of

the

fiber

can bc used

to very

lo¬

cally

emit

electrons

at

the

apex

of

the

probe,

causing

cathodoluminescence effects

which

can

be

subsequently detected

with SNOM.

In very

similar

ways

the

fiber

can be used

to perform

Ra¬

man and

infrared

spectroscopy

(Paesler

et al.,

1990:

Tsai

etal..

1994).

1

um

Conclusions

SNOM

is

a

promising

technique

for

the

mineral¬

ogical and geological field.

Il

combines the

high

spatial

resolution

of

a

non-contact SFM

with

the

well

established contrast mechanisms

of optical

microscopy.

In

the near

future, it

will

be

useful

in

order

to

perform

chemical analysis and

point

spectroscopy

in

the nanometer ränge.

It

is

also

possible to gain

very

small

scale

optical

informa¬

tion

on rock

formation

and

possible

former

tec¬

tonic activities (mineral deformation. mineral

re¬

crystallization

at

the

rims

due

to

tectonic

stress)

(7)

264 W GUTMANNSBAUER, TH HUSER, T LACOSTE,H HEINZELMANN ANDH-JGUNTHERODT

Like in conventionalmicroscopy, one isnot restricted to transparentsamples opaque miner¬

alsalso can beinvestigated withthe same appara¬ tus

A

majoradvantageof SNOMisthat

it

does

not require newsamplepreparationtechniques

SNOMcan becarried outon the samethinsec¬

tions and Single crystals usedfor microprobe

analysis.Attentionhastobe paidparticularlyto

avery goodpohshof thesurfaceofthe sampleso

thatthetip of the SNOMsensor canfollowthe surfaceprofile

Acknowledgements

Wethank R Giere,CMeyre,C deCapitaniand J

Partzsch,Mineralogical Petrographical Institute of

Basel,forahvely collaboration Further thanksare

extended to RLuthiandE Meyer forfruitfuldiscus¬ sionsThisworkwassupportedby the SwissNational

Science Foundation and the Kommission zur Forderung der wissenschaftlichenForschungin der

Schweiz

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