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Introductory Research in Optical Physics

by: Mrs. Peter Toy

Introductory Research in Optical Physics PHYS 3340

Marketplace > University of Colorado at Boulder > Physics 2 > PHYS 3340 > Introductory Research in Optical Physics
Mrs. Peter Toy

GPA 3.96

Kyle McElroy

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This 29 page Class Notes was uploaded by Mrs. Peter Toy on Friday October 30, 2015. The Class Notes belongs to PHYS 3340 at University of Colorado at Boulder taught by Kyle McElroy in Fall. Since its upload, it has received 16 views. For similar materials see /class/232093/phys-3340-university-of-colorado-at-boulder in Physics 2 at University of Colorado at Boulder.


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Date Created: 10/30/15
Physics 3340 Spring 2004 Polarized Light and verification of the Fresnel Equations Purpose This experiment will test the electromagnetic theory of transmission and re ection of polarized light from a dielectric surface as expressed in Fresnel s equations The lab begins with an introduction to methods for producing and analyzing linear and circularly polarized light You will go on to measure the angular dependence of re ection and transmission for both p and s polarizations of a NeNe laser beam incident on a lucite surface Intro du ction 1 Description of polarized light Polarized Light The plane of polarization of an electromagnetic wave is taken to be the plane containing the electric eld vector E and the direction of propagation I Coherent linearly polarized light is described by the equation E EOYX cosltZ at This equation represents a wave of amplitude EOYX wavelength A 27rk propagating in the zdirection polarized in the xz plane See Figure 81a below Unpolarized light can be Visualized as a stream of photons whose individual polarizations are randomly oriented Any on photon behaves like an electromagnetic wave packet up to tenthousand wavelengths long whose polarization is unrelated to that of its companions See Figure 81 b below For incoherent linearly polarized light the photon Efields are parallel to a fixed plane In circularly polarized light the vector E rotates about the direction of propagation at the frequency of the light wave ie one revolution per period of the wave A circular polarized wave can be represented by superposing two perpendicular waves of equal amplitude whose phases differ by 90 degrees For example E0 E0 coskz wtfc cosltZ at i The case with the positive phase is righthand circularly polarized light while the negative phase represents the case of lefthand circular polarization See Figure 81 c below Spring 2004 a I r 1 x V 7 W W E w a 39 r a 6quot 41 b I K Egg 81 a Coherent hnearly polanzed hght b Uhpolarrzed hght CR1ghtrhanded mularty polanzed hght Polanzed Lrght 8 2 Z Productxon ofgolanzed hght Tm arn tan anon Almearn hm er uha n hm d component othe Er eld perpendrcularto thrs axrs 15 erther absorbed or re ected h a drtrereht imam n L mun e W t related m that emerghg from thepolarrzer as E2 analyzer E patauzemosd Smce cos2 d7 H m V phase The Ervecior rotates at the equmcy ofthe hght about the drrectror of propagation It may be generated by passrhg lmearly polanzed hght through a quarter a pmw hurdchapterzt hr qumtu y Wmrcular F H r b b E mm nz a r plane tatytah t at degees then the resultmg hghhs elhpucallypolanzed Spnng 2004 11727st f in 5 391 sunrm 13 E 8 2 mm mmvmnx 4155 E5 mm um nm 3 A n ma WWW mm u the Fremel Equauons The muos ofxntmsxues are gwen by IR funded 1 74sm2 cosz R Zl39sxn aa TquotInf sxn gt 5 1r 3mm 3 1 4sm agtcos zgt aim M3 talismwawwoswaea 939 3 anew nsm nsm Note that aLBreWsUer s angle where 3 3 IrZ we have 13 o All the re ected lrgdt polanzed ugm They are used m producmg polanzed laser sources second mponantmethod w produce polanzed ugm Furtha more we nd r 100 for pr rm a mm vsnmvmmx d rmmmm ymmm Fxg E6 8 3 sr and pepolanzzhon geomemes Polanzed my 8 3 Spnng 2004 Apparatus Fe M mum Cumin m wmnr rh yi m Manama upfnm L VRW km aim F gure 8 4 Apparatm w measure Lmnsml ed and re eued Imensmes ofpolanzed hght and the analyzer polamd sheeLP3 The mta medxate posmoa pz 15 emaea empty or 15 occupxed r ave 3 Fa H M can b mu mnya measured thh a photodxode or obse39ved on a screen F1gure84b a an The sample 15 mmwr mi AuaunLuO the Iowa romwr so at can move mdepmdmtly Polanzed Lxght 8 4 Spnng 2004 surface is the horizontal plane in this experiment The angle of incidence on the plane face is varied by rotating the upper table and is measured by viewing the beam from above against the angular graph paper on the table surface The transmitted ray exits along a radius of the curved face so it is not bent by refraction The table can be rotated further enabling the beam to enter radially through the curved face and exit through the plane face With the later configuration you can observe the critical angle 6C and total internal re ection The beam can be observed either by its glow from scattering inside the lucite or with a white card Angles can be measured relative to a piece of circular graph paper xerox copies in the lab which is stuck onto the top of the table Black paper just under the sample makes the beam inside it more visible The narrow laser beam is linearly polarized by passing through the polaroid polarizer P1 The polarization of the beam can be set either vertical to give vector E perpendicular to the plane of incidence or horizontal E parallel to the plane of incidence by rotating Pl about the beam axis The intensity of incident light 10 re ected light I R and transmitted light I T is measured with a photodiode You may need to change the resistor in your present metering circuit to accommodate the different light intensity in this experiment The photodiode is attached to the lower of the pair of rotation tables enabling it to be positioned independently in the beam it is to measure Complete collection of the beam at the photodiode aperture presents a problem Verify that the total of the re ected and transmitted powers DOES EQUAL the incident power throughout the experiment in order to verify that the collection efficiency remains constant Problems 1 Polarization problems What is the fraction of light intensity transmitted by the second of two polaroids whose axis of transmission are 30 degrees apart 3 V 5quot V Suppose that your laser produces light polarized vertically and you need to make the measurements with horizontally polarized light Show that this can be accomplished by placing an additional polaroid at 45 degrees between the laser and the final polaroid What fraction of the laser intensity is incident on the sample Can you think of another choice of polarizers and wave plates that also solves the problem 2 Assuming that the refractive index of lucite is nl50 calculate the critical angle and Brewster s angle for comparison with your data Prepare theoretic graphs of the four Fresnel equations 3 Polarization analysis a Devise a procedure to distinguish unambiguously between unpolarized light and circularly polarized light b Find a convincing explanation for the quotoneway light valvequot See page 86 Polarized Light 85 Spring 2004 Outline of the Experiment N E 4 V39 Polarized Light Set up the laser and rotating table on an optic bench Align the optic axis so that the laser beam passes through the vertical axis of the rotator Determine whether or not the light from your laser is polarized using the polarization analyzer P1 Test Malus39 law quantitatively using the polarizer P1 and analyzer P3 Measure the incident and transmitted beam powers with the photodiode The intensity of a laser beam may be controlled by inserting two polaroids and adjusting their relative orientation to give the required beam power Set the polarizer P1 and analyzer P3 axes to be mutually perpendicular ie crossed Examine the effect of placing between them the following inserts a A third polaroid sheet P2 Observe how the intensity of light now emerging from the analyzer P3 changes as you rotate the inserted sheet P2 b A quarterwave plate Demonstrate that light emerging from the waveplate is circularly polarized when the fast axis is turned 45 degrees relative to the polarization plane of light from the polarizer What happens at other angles c Show that your polarizer and waveplate jointly function as a oneway light valve Try returning the light with a mirror placed beyond P2 d Try various pieces of plastic and sheet and scotch tape For the lucite sample a Using several angles of incidence between 10 and 60 degrees determine the refractive index using Snell s Law n1 sinQ n2 sin 92 b Measure the critical angle and test the relation sin 6C 1 n c Determine the Brewster angle and test the prediction tan 93 n d Measure the intensities of the incident re ected and transmitted beams as a function of angle of incidence for both p and spolarized light Plot the re ection and transmission coefficients versus the angle of incidence on the theoretical graphs you produced for Problem 2 Do you con rm the predictions of electromagnetic theory Verify that the re ected and transmitted powers do sum to the original incident power at each angle as is required by conservation of energy Spring 2004 Physics 3340 Spring 2005 Holography Purpose The goal of this experiment is to learn the basics of holography by making a two beam transmission hologram Introduction A conventional photograph registers a scene as a two dimensional distribution of light intensity recorded on a piece of film at the image plane of a camera lens The three dimensional character of the original scene as perceived through parallax and focal depth is lost in the lm image Parallax refers to the change in perspective that occurs as the viewing angle is changed and focal depth refers to the need to refocus the eyes as portions of the scene at different distances are examined Information is lost in the photograph because the film records only the light intensity but not the phase information that would be necessary to reconstruct the original wave fronts The three dimensional character of the scene could be restored if one could reconstruct the detailed wave fronts emitted from the original scene Amazingly the method of holography first proposed by Gabor in 1948 accomplishes just this A hologram is a direct record on film of the interference fringes formed by superposing a coherent reference beam on the light scattered from an illuminated object The camera lens is eliminated Fully three dimensional images are reconstructed by illuminating the film in its original position by the coherent reference beam alone The fringes on the film behave as a grating that diffracts the incident light from the reference beam giving rise to several images from the various orders of diffraction Scientific applications of holography are widespread along with applications in art and entertainment Especially 39 r are 39 J of J 39 interferometry pattern recognition and storage and image processing There are several methods for producing real time holographic motion pictures which have important applications in the analysis of mechanical vibrations and other small motions References 1 Welford Chapter 7 2 Heavens and Ditchburn Chapter 13 available in the lab 3 Reynolds DeVelis Parrent amp Thompson Chapters 25 26 amp 27 4 G Saxby Practical Holography Holography 61 Spring 2005 Problem Set 6 1 See Fig 61 below Suppose that L1 is a 60x microscope objective and the distance from the focal point PR to the lm plane is 40 cm How long should the shutter be left open for proper exposure of the film See the attached Kodak data sheet for useful information on the lm N See Fig 62 below Suppose the reference beam point source PR has yz coordinates 0 40 cm and a point P0 on the object has coordinates 10cm 15cm Where are the orthoscopic and pseudoscopic images located Which images are real and which virtual Holography Apparatus A typical set up for writing dual beam holographs is shown in Fig 61 The laser beam is divided into two mutually coherent beams by the beam splitter BS The re ected beam is steered by mirror M1 then expanded by lens L1 to create a divergent spherical wave that serves as the reference beam This beam must illuminate the entire area of the film H The beam transmitted by the beam splitter is re ected by mirror M2 and then expanded by microscope lens L2 to form the illumination beam The geometry must be chosen so that the illumination beam fully illuminates the side of the object O that faces the lm Light scattered by O interferes with the reference beam to produce the fringes that are recorded by the film H When the film is developed the fringes appear as a pattern of ne dark lines which may be examined with a microscope Many variations on the geometry are possible The reference and illumination beams must be expanded to a area large enough to illuminate the entire filmobject typically a 60x objective are needed to expand a He Ne beam The distance from the beam splitter to each mirror is in the neighborhood of 16 cm and the distance from PR to the film plane is about 40 cm The object can be placed on either side of the reference beam above or below referring to Fig 61 Not shown in the figure are the shutters that control the film exposure time To reconstruct the image the developed film should be placed in a film holder at location H the same location where it was exposed The illumination beam is then blocked by a beam stop placed between BS and M2 and the original object is removed The fringes in the hologram now serve as a diffraction grating which splits the reference beam into several distinct images One order of diffraction produces a virtual image at the position of the original object This orthoscopic image which possesses the same three dimensional character as the original scene may be viewed by looking through the film towards the object position Another image called the pseudoscopic image can also be seen in most cases The location and magnification of this image depends on the details of the geometry If the pseudoscopic image is real it may be observed by placing a screen in Holography 62 Spring 2005 front of the hologram 85 Ha Hn LLSZY 4 RYERMlt Baum H Fig 51 Dualbeam Transmission Holography Apparatus The olden lower power lasers and to have have longer coherence length because they operate closer to Lhreshold rnlerference pattern aLLhe fllm plane H the opucal paLh dlfference along the two path from the beam splrner the lm plane must be less than the coherence lenth of the laser To be safe you should almfor a path length dlfference in your setup of at most 172 cenumeters The path length dlfference can be adjustgd by moyrn g mnor M Holograms are very yrbrauon sensluveivery small mouons of the apparatus can shrmhe frlnges on ll L b b l Thu solr y mounlecl Funhermore even the acuon of the shutter in the camera back can cause fatal yrbrauons Thus 39 39 39 wr an opaque card that you are holding whrle berng careful not lo much the table Then rmke the exposure manually moyrng the carol away whrle counung exposure then moyrng the card back in e way ln lhr way you can p pr rorau n free as possrble whrle clorng the exposure Take a number of progressryely longer exposures on one fllm smpr Holographic imag forma on and reconstruction ln ths secuon we grye a srmple theory fothe formation ancl reconsmlcuon of holographrc rmages complexity of an extended ObJECL we conslder only a polnt source on the Object at Lhe polm Pun llolography o 3 Spnng 2005 of sphenoal waves embed from the pornllR We flx the yrcooldlnate of PR lo be zero Normally the zrcoordlnatg of P and PR would both be negauve numbers r e ln Lhe leftehand srde of the plane r p ru r u an the plane 20 nd P ls a polnt on the fllmplane When the rmage rs reooneruoled there wlll be a drmaoled a ray leavlng PHaLLhe angle 9Dmeasure from the posluve zraxls PD labels a polnt on the drmaoled Fig 52 Geometry 0 Theon ol Twobeam Hologram Our hrsuaslr rs to flnd the fringe spaorng My on the hologram The superposed eleomo elds from the object and the reference beam have the form E e E emf Egeiml on the fllm plane The fllm records the lntenslly lee averaged 1rxReEz 7a E ZEDERcosMRD eRR whlch contalns Lhe oruoral ooskRO 7RRlnLerference term The wave number k rs equal lo when the path length dlfference ReelR changes by one wavelength A Holography o 4 Spnng 2005 Fig 53 Fig 63 shows tha the pah length from the objea point P0 to the film changes by Asin 39a from one fringe to the next Similarly the pah length from the reference beam source PR to the film changes by AsinS39R from one fringe to the next This the fringe spacing A is fixed by the condition AsinFR e Asin a A inS39R asin 39a Next we will use the fringespacing formulato find the direoion of the diffracted rays 9D when the hologram is illuminated only by the reference beam Fig 64 shows the pah lengths along rays AsinS39D rAsinS39R n Fig 54 Holography 65 Spring 2005 Combining this result with the formula for the fringe spacing yields an equation for the diffraction angle 0 a This is the fundamental result describing two beam transmission holography For n0 we have sin SD sin 0R which says that the zero order diffracted beam is simply the undeflected reference beam For n1 we get the more interesting result sin SD sin 00 This means there will be diffracted rays that appear to come from the original position of the object P0 Thus there is a virtual image at the original object location This is called the orthoscopic image For other values of n the position of the image can only be found by further analysis To keep things reasonably simple we will now restrict ourselves to the paraxial approximation for which BR 60 are all much less than one The angles are then related to the coordinates of the points P PH P0 and PR by 0D YDYH0R y711 ZD 00 yH yO ZR 10 Using these expressions and the small angle approximation in Eqn 1 gives YD YH n1y711nyny0 ZD ZR 10 which may be solved for yD to yield yD n1zi nZi1yH nZiyo ZR ZR 10 The image point with coordinates 3 13quot 21339 is that point on the diffracted ray where yDzD is independent of yH This occurs when n1iDniD10 ZR 10 or at the coordinates 2 Eqn 2 may be used to reproduce the previous results for n0 and n 1 diffraction The case n1 is called the pseudoscopic or conjugate image and Eqn 2 shows that it may be either real or virtual zD may be either positive or negative depending on whether zR is greater than or less than 2 zo Holography 66 Spring 2005 mqu may 59 Holoqnphlc Fllm 50251 gun mu m DAK 532111quot 2 131 I Int 1 nzscx n on s emume This aw mm prnvidcc zxnmurdiniry upud when prntgd with alumnum 533 m7 or krypton 547 um lucrs n m Emu rim xu mitrufint rlin m umm Ana mm mullion nhrcncrxnicn conbin m a high diffrlctian afiiciancy ma rccunstwclxmu of hamu 1 r 1su cyclzc I 1 chzrfu39wecxy gens x m z s recomendzd primlfily f0 nlozrlphi nd maman d u 1 palmum unful in 1 holographic procedurgr with luvpawn am 1 us fill Ann ulna b rm c expound szicignzly vilh halimrcadmxm um um lrgnn 515 mu m1 fraquany daubltd mus 52 ma hm A mm vavelcn ths film speed in u km to b pasted from an mm m ruin slzl nd 1 1n n y 2 petfom nze x o b xpnczcd 3n progrmmxy quotsun 1 Rayleigh itann in m c s m u cmummnc nE all nuvmmud u mulniun 1 Arm cried 9 5 may an a 125139 1 um um sun yam suppDrL A a m gelatin Puma an ht hm ma mom ntihn15210n vrouuiun uud psimm canvaninnt handling m huh You And mm shczt or short srrzv forman ma mucm an m scum data are based an 0mm m mm illumuucxon with n mum mrmq Ylizgr No 29 Detp Red nd pxoczssing My new mum 7 s m 5 minutes m um um cunununus iguana m crnnunamy s h an m mum dizmzn r n s m pmm dxun rum 4r ulL diffu e Ss m c at us damuy cf 1 o T o 5 mo 1 1250 Ismsum 2 a 1 son linesnan Reso vlnz oucr has Vlluas far rgcolvlng pone vzrc dcrzmineu h nonhomguphn whens The should be we r be Dgrlyhic xcxoh39ng pa mr for much no slu r Vidaly cuma mm or rx scs sans rum Khuuld m 150 rm a rocnnstruct holclunms racurde a slumquot canespandmg nxxnalaiy 50 u x gum b Hologmphy Spnng 2005 Exl osmrr L PROCESSING And Uhnn ex 0 a nun lIeNc mm or m a 11m from kryycun lanzn c 39 of s xx p luauroman no u processin x cnntinucd 11 A 6570 135 u 11C Rlnr in numan can of mm mama Slap Huh or 10qu 5 am 53 1 with mum far 10 m 113 quotmm m using KODAK rim or xozmx Fixing Bath r 5 umx uglutian so 5 m o m um um um nudenu marten for mnur Rinse in u mixnon ol IconAk Hyvo clza Lug Axum vnh agitation for A airman Rush 4th uuzcr 39e nguiuan or 3 ninuur Rinse in a solulLon of Inc parLs Huianal and am part um um giuuon m s zinuus n ash um odeute agitauon for s ainqus 05 1 n val flow nu sufflcxan fur one ham n hm cvcry s munus wry m n dus free nmosyhgn Drying mark can b minimum by treating m in La xomy PHCTDFLO Sumner preparnd n amazed on the mm mm rur ushzng The use of won a Solution 11 rnmotc w r r 1 mm a 57nure on xy 3 0 results dry ftlu slowly The rim 1n KODAK Hypa mum L7 can vnums 0 mm mess cquivalen o nunon or Archival k p1ng c v c n ms 54 PH 148l methanal rinse x nqumd 3 rcmavc n 1 g v 0 rutduu39 z dye from h cmulslan The dye ln disanczly bl 1n Ippcsvnr unqu guuzlv n uce mmnuwczian unsung vhcn op latlrg 1 zd emumg blur Hologmphy 68 Spring 2005 RECIPROCITY ADJUSTMENTS Dvez Eh range from i to in Ices this film Ihuus virtually n raciprocity effectsl Data for hotter the If no AVlilnhle For crpulure durations of 10 to 100 leconds xpusuru Ihould bl lpproxu mcly 2 z greatgr than than calculaced Exam trial exposure of l lecond or 155 duznziona MENUS DMGE DECAY Like mail ulna uinh Extraner fin grains 50253 Film xhibizs aigniucan lugn image fading during ch hours ju fallaving txpolure rur xampln an exposure sufficient to Leld a dennity film adi u h of 10 when ch procnnd 11 quot11 result in danslcy of 08 is pro cssing is deferred or on o r I i good pxnctlc in dazemlning an opt mu m xpu ure 1 1 f given 2 v or a hole phi setup to process as soon are cxpo ur as possible prov ded mat the elapsed tin can be mincainad for 11 subnquan operatlun ulzh 2 5mm setup STORAGE Unaxnaaad film hculd be and in a cool place vor or lower in cm original sealed package is noted in a rasriuaxanux nmava on n n before opening a przvznt Condensanian of Anna hcrlc cola 11m Otherulsz npnu ng ferratyp mg or sticking in addition hzmal expansion during zxpasu39re Ulll rgsulz in wearing of halo raphic fringzs Free the film 55 nut requir d 39 x in of to assure lung shalf llfc SAFELIGHT RECOMI NDATI DNS Tonal dlrlmes5 is recommended when handlini this film lndznd its unusually hign red sensitivity will genara 1y require greater care in shialding he Llu from laser radiation extraneous m the aczua halegrxghic exposure than you Ian I accuszcund to prev ding with unteria s prcvluuily available Greenfluarzicinz 1552 5k or gr y subdued 1 b hologra M lubnruary ol photo phi darkrnm bu nuy shnuld be used on y atner pranziaal filmgagging tests indicate no discernibll incxaaaa in minlmml film danalcy aim normal 39prnccssinx Saianzl a Pnocezrapny lam ken EASTHAN KODAK Comm ROCRESTER NEW YORK lasso Hologmphy 69 Spring 2005 Holography 610 Spring 2005 g Senllllvlty 7 0 Anrplltud n miltlnce I 5 Emu saw 19 Continunus AskKlan Komx ounc er nvw Yroccu39 hpnnurr ergcmz7 Densky 0 swam 139 1 SZ IS TVTTY vxvr 00 Cont mum 10 In bwolure upm1 uponquot 8 633 mu Sprctrll mm a an rm danal mwm 1 In up an r macawquot menu y vucnume M mmquot m 1412va mam mudr mm my Speed Manypm ma mu mg 50251 mamas m5 PK Mmm mm H Dualibeam Transmisslcn Biography Appaau Figure 61 HeNe Lam SH h R Laxach S nulL M was 4 a Magma aw B vanc Re from ltam Irrummnrrng H L LH VSKFLHL FA 0h 4 L n 35 Mm 59 ux 7 new 053mg 0 s Asks Fujurlt 5 P chmY Fpurol us 3 wrung Ha ogmn was Bmwsrc k a Balm mmx 1a Hack nu um Kat Jaw rL e uj r Holography Spring 2005 Dimensions v09 1 M New MI 3 H 01 ms h m 72 m m h 0 KM LcurmH Amm WM ammg Holography Spring 21105 Processing of 35mm Kodak SO253 Holographic Film darkness with lid on tank until film has been fixed Hologram keeps longer with step 5 but can be deleted Allow about 45 minutes to complete the processing Examine the hologram by eye The degrees of blackening should be quite weak with about 50 of the incident light being transmitted if you are to obtain a good bright holographic image Holography 613 Spring 2005 Advanced Lab Spring 2007 Xray Photoelectron Spectroscopy XPS Purpose In this lab you will study the photoelectron spectra of silicon and silicon dioxide using a commercial XPS instrument running in ultrahigh vacuum UHV conditions You will make use of the surface sensitivity as well as the chemical selectivity of XPS to estimate the thickness of a very thin SiOz layer on top of a Silicon wafer Introduction The process of photoelectron spectroscopy was first explained by Einstein and garnered him the Nobel prize in physics according to his explanation that a photon transfers all of its energy hv upon being absorbed Photoemission also was the subject of the physics Nobel given to the Swede Kai Siegbahn for his development of the technique of ESCA Electron Spectroscopy for Chemical Analysis which we usually now just call XPS Siegbahn showed how different valence configurations of an atom give rise to slightly different energy levels chemical shifts observed in XPS Very brie y a monochromatic beam of photons Xrays impinges on the sample to be studied exciting an electron in the sample to a higher energy state If the excited energy state is higher than the vacuum level of the sample the Fermi energy plus the work function the electron may be ejected into the vacuum and collected and energy analyzed by the electron spectrometer With knowledge of the final state collected electron energy and the energy of the xray energy conservation then allows us to precisely determine the initial state energy of the electron before it was ejected from the sample using the equation EkinhvBE where Ekin is the kinetic energy of the ejected electron hv is the photon energy BE is the binding energy of the electron in the solid and I is the work function typically a few eV The peaks as a function of energy thus tell us the chemical elements in the sample that the electrons were ejected from The strength or intensity of the peaks are proportional to the number of atoms of any kind from which the electrons are ejected so that the stoichiometry of a mystery sample can be identified as well as the valence of chemical state of the internal elements Finally the surface sensitivity on the order of 550 Angstroms allows a unique view of the chemistry and physics of a materials surface ESCA or XPS has grown into a very widely used and indispensable spectroscopy that is utilized in most major companies universities and national labs and also from special analytical services companies such as Evans Analytical Group httpwww eaolabs com enUSser 39 html or RBD Enterprises httpwwwsurfaceanalysislabcom These companies typically charge upwards of 250hour to study a sample and the instruments themselves have replacement value over 500000 So have fun but also be careful with our instrument Jason Underwood who has been performing his PhD thesis using the XPS instrument will be available to help you with this experiment he is being partially supported to do this He can be reached at underwjmgmailcom or on his cell phone at 7204700564 Outline of the experiment Begin by reading the attached pages from the Handbook of XPS written by the instrument manufacturer Physical Electronics abbreviated Phi and answer the prelab questions You will then clean your Si and SiOz samples for UHV conditions and mount each sample to a transferable sample puck You will load a puck into the fastentry vacuum loadlock which you will pump out to below 10396 torr using a turbo pump You will then transfer the puck and sample into the UHV 10399 torr ion pumped analysis chamber and carefully turn on all electronics You will take both a broadenergy survey scan of each sample and a higher resolution higher statistics multiplexed scan of a few individual regions You will perform some data analysis routines on your spectra including a removal of satellite peaks due to additional weak Xray lines a removal of the background of scattered electrons and finally a calculation of the area under the individual peaks so that you may determine the stoichiometry of each sample For your Silicon sample you should see a Si 2p peak due both to pure silicon and due to silicon which has been bonded to oxygen in SiOz You will use the included fitting routine to separate out the contribution from each of these peaks and determine the relative strength of Si and SiOz in your spectra From this and a knowledge of the photoelectron mean free path in SiOz you will make an estimate of the thickness of the SiOz layer on top of your Si sample Your lab report will include the answers to the included prelab questions as well as plots of your raw data reduced data and your analysis and results The instrument The two key pieces of the instrument are the hemispherical electron energy analyzer which is a fully electrostatic instrument no magnetic fields and the dualanode Mg and Al Xray source as well as the associated controldetection electronics with each unit The following website has some information about the design of a typical XPS instrument including some key concepts such as electron pass energy etc httpwwwcasaXpscomhelpimanualXPSInformationXPSInstrhtm The vacuum system It is important that you understand the details of the vacuum system before you begin to use the chamber The main vacuum chamber is pumped by a large 110 litersecond ion pump which is hanging below the chamber When in operation the ion pump supply Varian VacIon pump control should be almost 6 KV with the ion pump current proportional to the gas pressure in the chamber For our chamber at N 3X10399 torr the ion pump current will be around 250 HA Please note this value in your log book You also should note the pressure in the main vacuum chamber which is read off of the Granville Phillips ion gauge controller in the top position of the right rack You should always keep an eye on the ion gauge controller with the vacuum staying below a few X 10397 torr at all times The ion gauge itself is a nude gauge with a center pin collector and 5 pins around the outer circle The load lock is pumped by a turbo pump which is backed by an oillubed rough pump The load lock is separated from the turbo by a pneumatic angle valve and separated from the main chamber by a manual gate valve There is a coldcathode vacuum gauge MKS instruments which reads the loadlock vacuum red dial as well as the pressure of the foreline black dial using a Pirani gauge The load lock should go to about leO396 torr about 30 minutes before opening it to the main vacuum chamber See the picture below with all the main components labeled A W n The gate valve requires a fairly strong twist to get it fully closed You should be able to feel the stop when it is closed and you don t want to torque it very hard beyond this If you have questions whether it is closed ask Jason Professor Dessau or one of the Advanced Lab instructors The angle valve is electropneumatically operated and the turbo vent valve is electric with a spring supplying force in the opposite direction Both are controlled by plugging or unplugging a 110 V plug into the power strip on the oor behind the instrument The angle valve is normally open NO while the vent valve is normally closed NC This means that while plugged in the normal state the angle valve is open and the vent valve is closed If you wish to vent the transfer arm e g to remove a sample we do this with the angle valve open to the turbo plugged in but of course with the gate valve to the UHV chamber solidly closed Turn off the cold cathode MKS gauge Hit stop on the Leybold orange turbo pump controller and wait about 30 seconds for the pump to partially spin down Then you should unplug the vent valve which will let air into the bottom portions of the turbo At this time you should almost immediately turn OFF the mechanical roughing pump as it will now be pumping on atmosphere which makes more noise and puts more oilmist into the air You can then open the green Nupro valve by the sample entry portion of the loadlock to speed the venting of the system When you wish to pump down again loosely shut the fastentry sample door it can cock sideways and make it harder to seal if you crank it down too hard If you position it properly the vacuum forces will keep it shut and then shut the green Nupro valve shut the turbo vent valve plugging it in and turn on the MKS gauge Then you can turn on the rough pump and then the turbo pump press start Unless you left a valve open the black dial on the MKS gauge Pirani guage should come down very quickly in a matter of a few to 10 seconds You should usually wait about 30 minutes for the transfer arm vacuum to come close to bottoming out below 10396 torr on the red coldcathode gauge dial before you are ready to transfer through the gate valve into the UHV chamber When transferring through the gate valve we like to first verify that the transfer arm vacuum is good convectron gauge better than 10396 torr and then we shut the angle valve unplug it to isolate the turbo from the UHV chamber This is a safety precaution in case the turbo should fail while the gate valve is open and also to keep oil turbo or rough pump oil from backstreaming to the UHV chamber After transferring and closing the gate valve remember to reopen plug in the angle valve X ray and high voltage safety Although this instrument produces Xrays there is no way for them to get out of the chamber unless a port was opened and if one was opened there would be no way to produce the Xrays We have checked that there is zero leakage of radiation from this instrument and so you are not required to use Xray monitoring equipment The ion gauge pins are somewhat exposed and are high voltage so you don t want to touch them when the gauge is energized However the controller can not source much current so it is not terribly dangerous The ion pump power supply and the Xray source supply will put out lethal amounts of voltage and current though you would have to work quite hard to put yourself in either of these circuits Please do not try to do this Surface sensitivity Photoemission is surface sensitive because as it is leaving the sample the photoelectron has a high probability for inelastic scattering causing it to lose energy and show up as a background or secondary electrons as opposed to the primary photoelectrons which give rise to the peaks The inelastic mean free path or attenuation length 7 of the photoelectrons sets the length scale of the surface sensitivity and is typically 1040 Angstroms depending upon the material and the photoelectron kinetic energy The falloff in intensity of the photoelectron signal as a function of depth follows what is called the BeerLambert law varying as e39Z A where z is the depth beneath the surface and 7 is the attenuation length We can therefore ask the question 7 if a photoelectron is detected what is the probability that it emerged from somewhere within a specified distance of the surface We thus sum the probabilities of escape of the photoelectrons d P0 lt d Ie xmdx 1 eW 0 where the proportionality factor 1 comes from requiring that PX be normalized 7 integrating from 0 to in nity should yield unity as the electron must have come from somewhere Setting d7t we see that 63 of the detected electrons have come from within one attenuation length 7 of the surface and 95 come from within 37 Prelab questions 1 Why do we need to clean our pa1ts well before entering them into the UHV chamber 2 What do the labels 32 12 in the Si 2p core level stand for What is the ideal peak area ratio of the two peaks within a Si 2p core level How about inside a 4f level 3 What is a chemical shift in XPS What is a typical orderofmagnitude of this size of effect 4 Why is there an increasing background signal as you go to increasing binding energy 5 What is the Auger process and how could one operationally distinguish an XPS peak from an Auger peak 6 Brie y describe how the electron energy analyzer works Brie y describe how the X ray source works 7 What does it mean when we discuss the analyzer pass energy What is the effect of varying this on the spectrum that you take 8 In a uniform material what percentage of electrons that you detect at normal emission are emitted from within the first two attenuation lengths 9 You wish to study a silver core level but it is covered by a 10 Angstrom thick layer of gold at the surface Assuming a photoemission escape depth of 20 Angstroms how much weaker will the silver core level be than if the gold layer was absent What if we are at glancing emission of 75 degrees 10 What happens if your sample charges slightly Detailed Procedure and explanations a Clean your samples for UHV There should be acetone first and isopropyl or ethyl alcohol to finish for this purpose as well as an ultrasonic cleaner You should use rubber gloves to keep all fingerprints off of your sample and the sample holder Note that latex gloves will not hold up to acetone and so you should use nitryl gloves when using acetone b Mount your sample it is recommended that you start with the thermally grown SiOz to one of the transferable sample holders the round stainless steel discs with two grooves along the sides Ideally you should just be able to gently wedge the sample underneath one of the berylliumcopper springs which will make physical as well as electrical contact to your sample electrical contact is necessary so that the ejected electrons will be replenished If they are not your sample will charge and the kinetic energies you measure will be incorrect c At this point you may wish to begin warming up the voltage supplies which set the voltages for the electron detector This will give them time to stabilize while you are getting things ready Do this only for the top 5 pieces of Leybold brand electronics There is a round push button on the left of each of these units BEFORE doing this you should check that the electron multiplier high voltage dial is off fully CCW so that there will be no high voltage applied You do not want this HV on in case there are any vacuum problems in the next steps this could cause arcing and serious damage to the in vacuum detector assembly d Following the included ow chart and initially reviewing with Jason Professor Dessau or one of your instructors load the sample onto the xyz manipulator in the vacuum chamber Brie y you will begin by first loading it into the transfer arm grabbing the sample puck on the lower groove with the sample fork the xyz manipulator should grab the puck at the top groove You should pump on the transfer arm with the turbo for about 2030 minutes bringing the transfer arm vacuum to about 1XlO396 torr or better Following that you will shut the angle valve to isolate the transfer arm from the UHV chamber open the gate valve checking that the main chamber vacuum does not go past the low 7 scale vacuum Transfer the sample puck to the xyz manipulator grabbing it on the top groove and then remove the aim and shut the gate valve You should then reopen the angle valve to keep the transfer arm pumping As a starting point you should expect the transfer position to be at about Xyz 1251310 and the analysis position to be 1258510 where you are reading the black scale and the units are mm 250 is having the manipulator fully retracted from the chamber center which is the safest location when moving the transfer aim in e You have a sample loaded into the UHV chamber Congratulations You now want to slowly turn on the Xray source after con rming that the vacuum in the UHV chamber is still good 8 scale or better To turn on the Xrays you should first turn on the cooling water using the white pushbutton switch on the Xray controller it is helpful to hold this in for a few seconds You should wait a few seconds to let all air bubbles in the water lines disappear After the water is on check that the large HV high voltage knob is at the minimum fully CCW and the emission regulation Xray source is in standby mode Then push the HV push button on You should now see about 016 KV high voltage 0 mA of emission both on the digital meters and about 3 amps of filament current on the analog meter f You now want to SLOWLY begin turning up the Xray source high voltage using the large knob on the panel You should shoot for about 10 KV taking about 1 minute to get there You will begin to see first one and then 2 mA of current on the emission current digital display This is not actually emission current but is a slight bit of leakage current through the cooling water lines the water is deionized but is still not in nitely resistive When you have reached 10 KV you can start to get a real electron emission current These emitted electrons will strike the Al or Mg target creating the Xrays you need You should check that the knob on the left is set to an emission current setting of only 20 mA Then you should turn the emission regulation knob from standby to operate and you should see that you have 20 mA of emission current on the digital display You now have Xrays Typically we run the system with the Mg anode If you wish to change from the Mg to the Al anode it is imperative that you first turn the Xray source to standby no emission current and then turn the HV knob slowly down to below SKV before switching over Afterwards bring HV back to 10 KV and go from standby to operate to get emission current g You now need to turn up the voltage on the electron multiplier which is an avalanche style device inside the vacuum at the exit of the hemisphere which amplifies a single electron which makes it around the hemisphere into a pulse of about 1 million electrons This pulse of electrons is then further amplified with a preamp and a postamp both outside the vacuum and then shaped into a TTL pulse which is counted by the computer We are doing pulsecounting here as opposed to measuring an electric current Pulse counting is preferable for low signal levels while analog current measurements are preferable at high signal levels as the counting electronics may not be able to keep up Check that the detection mode is XPS and then slowly turn the electron multiplier supply to 38 KV using the potentiometer h You are ready to take data Launch AugerScan 24 which is available by double clicking on the icon on the computer desktop on the small black computer desk There is a data acquisition card in this computer with a DAC digital to analog converter which puts out a single 010V analog signal which goes to the analyzer control to set the hemisphere voltages determining the electron kinetic energy being studied NO other voltages can be set from the computer 7 neither the analyzer pass energy multiplier voltage etc The computer also has a countertimer facility which counts the number of TTL pulses coming from the ratemeter photoelectron counts AugerScan will repeatedly ramp the kinetic energy over a predetermined range for you counting the photoelectron signal and averaging the new sweep with the previous scans It is recommended that you start with a broad energy survey scan of your sample which we usually do using a pass energy of 150 eV this is called transmission energy on the Leybold electron energy analyzer power supply Under the file menu select new and then select survey Then under the acquisition menu select settings which you can use to view the scan parameters that you will use These are typically set at good values and you can just hit acquire to start your measurements or you can save and then hit sta under the acquisition menu Take enough scans until you see that you have good statistics Save your scan you should make your own folder and save them in this You also should do some analysis on this data Under the data menu there is a routine called satellite subtraction that you should run For your writeup you should include both the raw spectra and the satellitesubtracted spectra showing and discussing the differences and the physical reason for the differences Note that this subtraction program needs to know whether you are using the Mg or Al source the satellite energies and intensity ratios are a function of the source material which is listed at the left side of your data window If you wish to change the source in the software you can access this under menu system hardware properties xps You should note that there are a variety of toolsicons in the tool bar above the spectral window which may be helpful for you If you hold your mouse cursor over the icon it will tell you what the tool does There is a magnifying glass toolicon that you can use to look at only a portion of the spectra There also is a MarldTrack tool that you can use to read off the peak energies and count rates as well as a Select Endpoints tool that you can use to define regions that you will use for background subtractions etc You should make sure you understand the origin label and describe in your report of every one of your peaks You should also verify that the energy position is approximately correct within 05 eV or so for each of these peaks i You can now take a more detailed multiplex spectrum by selecting a new file The multiplex setting lets you spend your counting time taking finer higher resolution scans over only the energy regions of interest ie on a few core levels while leaving the intermediate energy space between them unscanned To set up a multiplex range you will create a new file of multiplex type and then under the menu acquisition and settings you will be able to set it up to your liking When setting up the multiplex regions you should make sure that the atomic sensitivity factor is filled in for each of your regions This will enable the software to determine the elemental composition of your material A table of these atomic sensitivity factors for our instrument is included at the back of this writeup Each individual spectrum e g C ls Si 2p of the multiplex data can be viewed from the lower pulldown menu labeled Region The default setting probably says All though if you pull it down you can access the individual data regions It is recommended that you do some data reduction to your data in each individual multiplexed region First you should do a satellite subtraction like you did for the survey spectrum Then you will wish to do a background or baseline subtraction so that your program can integrate the area under your peak or peaks You can do this by selecting an energy window for your peaks using the Select Endpoints tool and dragging between the starting and stopping energies Under the data menu you can then select baseline and then background subtraction It is preferable to use the Integrated Shirley type background as opposed to the simple linear background this gives an sshaped background with the idea that the number of scattered secondary electrons is proportional to the total number integrated over energy of primary electrons at a higher kinetic energy Using the view menu you should be able to toggle between your raw data and your transformed data If you don t like some of your transformations you can hit the revert tool and start transforming your data again You should present both your raw and your transformed data in your lab writeup When you have successfully transformed all of your individual multiplexed spectra you can select Atomic concentrations under the Data menu You can then go to All in the Regions pull down menu and see a plot of the atomic concentrations from your measurement For SiOz it would ideally show you that the concentration of oxygen is exactly twice that of Si though you probably won t find this Can you explain why this may be off Is there possibly another source of oxygen on your surface Can you tell this from your spectra j This is all that you need to measure on the SiOz sample Now that you are done with this you should prepare to remove it from the chamber First turn the Xray emission to standby rotate the Xray HV knob CCW all the way to 0 min and turn off the HV Turn off the 38KV of the multiplier supply Check that the transfer arm vacuum is still good lxlO396 torr or better on the cold cathode gauge and if so close the angle valve 0 separate the turbo from the UHV chamber Remove the SiOz sample into the load lock Close the gate valve open the angle valve and prepare to vent the transfer arm k Follow the same procedure to load the Si sample into the transfer arm chamber and then into the main chamber Take a set of survey and multiplexed scans For this sample you should be able to observe both the contribution from the top SiOz layer and the underlying pure Si which you can distinguish due to the difference in the chemical shifts ofthe Si 2p core levels 1 As before process your data via the satellite subtraction and background baseline removal printing both raw and transformed data and do the atomic concentration analysis To go further you will probably wish to separate out the contribution from the Si and SiOz peaks and so you may find it helpful to use the curve fitting routine that is available under the Data menu of Augerscan 24 To get started note that you can tell it to include a peak to fit using the button on the lower left portion of the top panel that pops up You will probably wish to include two peaks one from Si and one from SiOz and try to find the area ratio between them Putting your mouse over or right clicking on various options may help get you additional information It is recommended that you use the GL lineshape which is a mixture of a Gaussian term usually due to resolution broadening and a Lorentzian term usually due to lifetime effects 7 Fourier transform of a decaying exponential m According to the softwaredatabases from NIST available in the small red binders the attenuation length of a photoelectron of about 1150 eV why this energy through SiOz is about 36 Angstroms Using this info and your fits make an estimate of the thickness of the SiOz layer which is on top of the pure Si 1 Turn off all HV remove your sample and clean up your work space Suggestions for future studies e g an endofsemester project 0 Run the sputter ion gun so that you can clean the top layer off of a sample before or even during a measurement to make sputter depthprofiles 0 Study a sample or set of samples from your own research group or any sample of interest 7 different types of stainless steels coins etc Materials of special interest would be thin films which can take advantage of the surface sensitivity or materials where the valence state is relevant and which you can study using the chemical shifts of XPS 0 Make your own thin film samples using the thinfilm evaporator in the Advanced Lab and study it using XPS


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