CREOL, College of Optics & Photonics, University of Central Florida

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1 OSE Optical Properties of Nanostructured Materials Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 1 Challenge: excite and detect the near field Thus far: Nanostructured materials can act as optically ~homogeneous media - Knowing the field distribution within medium allowed us to find effective index - Nanostructuring allowed for the structural design of new optical properties - Applications: structured Fresnel lens, engineered modes and dispersion, single-material anti-reflection coatings Today: - Go beyond effective medium approach: local fields inside structured media - Discuss technique to visualize these local fields: near-field microscopy! Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 2 OSE6650 Fall Class 3: Optical near fields 1

2 Local fields near polarizable objects Remember: particle placed in external field polarization and surface charge External potential for static applied field: Example: E z (x,z) for 10nm diameter Si particle (n=3.5) in air 3 in out a 1 E z 3 in 2 out r out 0 Small structure can take long wavelength light, and generate local response field Fields close to polarized objects: near fields. Next: define this more precisely Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 3 Spatial field variations - limits Time varying fields (e.g. oscillatory) lead to spatially varying fields Single frequency in free space sinusoidal variations of E(x,t) Field variation described by wavevector k (spatial frequency) given by k =2 / Questions: limits on spatial frequency in free space? (i.e. without nanostructures) Consider plane wave incident on flat substrate: 2 2 Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 4 OSE6650 Fall Class 3: Optical near fields 2

3 Wavevector magnitude Given the magnitudes of k x and k z : sin cos we have a well-known wavevector conservation law : sin cos with In free space: total length of k-vector components adds up to n /c In three dimensions: with if fields are rapidly varying along x, they are not varying rapidly along y and z Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 5 Limitations of free space optics Example: illumination of flat surface from free space: Smallest feature size ~ /2 / 2 sin with n=1 sin In free space we cannot generate arbitrarily fine field distributions Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 6 OSE6650 Fall Class 3: Optical near fields 3

4 Evanescent field decay Illumination from within high index material: maximum k x = n k 0 For high angle k x > k 0 (above critical angle) no radiation into free space Check: above sample We find an imaginary k z exponential field decay into air: evanescent wave k x > k 0 High spatial frequency components may exist, but do not radiate into free space Used in immersion microscopy, immersion lithography Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 7 Interpretation of evanescent decay Microscopic interpretation of evanescent field: Incident wave generates oscillating dipole moment along surface We all know that oscillating dipoles radiate. But apparently not here? Reason:..... Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 8 OSE6650 Fall Class 3: Optical near fields 4

5 The near-field Far-field illumination - Spatial frequency of EM fields inside homogeneous media has upper limit of nk 0 The near-field - small objects (=inhomogeneous medium!) can be polarized by incident fields E in which have large wavelength - resulting localized surface charges generate E(x,t) contributions with spatial variations that can occur over distances x << - These field distributions cannot be detected in the far field E Mathematical method for finding radiative and non-radiative field contributions: Take spatial Fourier transform, look for k > k Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 9 Fourier description of spatial field distribution For any arbitrary field distribution in the x-y plane E(x,y,z) at a given frequency Can find the Fourier transform Tells us How much of each spatial frequency is present Some of these wavevector contributions have radiation But: wavevectors may exist with invisible in the far field Example: nanoparticle with d E x highly localized large range of wavevectors present, including nonradiative Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 10 OSE6650 Fall Class 3: Optical near fields 5

6 Spatial Fourier transform: check Example: normal field distribution in x,y plane along a surface. Corresponding F.T.? k y 500 nm k x Assume that 0 = 500nm Does this field contain near-field components along z? How can you tell? k z =? Draw all possible (k x, k y ) values for plane =500nm wave in arbitrary (x,y,z) direction Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 11 Spatial Fourier transform: check Example: E x field distribution in x,y plane. What is the corresponding F.T.? k y k x Assuming oscillation at =2 c/500nm. Draw k max in vacuum. Does this field distribution contain near-field components? k z =? If this field were oscillating in free space, would there be radiation? Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 12 OSE6650 Fall Class 3: Optical near fields 6

7 Near fields vs. frequency nonrad rad Low frequency: k max small High frequency: k max large Only E(k x,k y, ) components that lie within light cone can be detected in far field along z nonrad rad Near-field exist and contain important information, but: near-fields are invisible Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 13 Challenge: excite and detect the near field To visualize the near-field: bring field to detector, or bring detector to field - use conversion element that can scatter the near field (couples NF to far field) - general idea: small (<< ) polarizable structure (probe) will polarize E local and generate scattered/reradiated dipole radiation E local Detect scattered field vs. position of probe relative map of local field! - need: movable nanoprobe E Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 14 OSE6650 Fall Class 3: Optical near fields 7

8 Challenge: excite and detect the near field Idea not new: E. H. Synge: A suggested model for extending microscopic resolution into the ultra-microscopic region, Phil. Mag. 6, 356 (1928). Obtain image by 2D scanning: Near-field Scanning Optical Microscopy (NSOM) Experimental realization took several technological advances: - reproducible nanoprobes - accurate probe or sample movement - accurate probe position detection - low noise optical detectors Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 15 Challenge: transmission efficiency through nano-aperture Classically: transmitted power proportional to area a 2 BUT: Far-field Poynting vector S T through nanoscale circular aperture with radius a in thin perfectly conducting plate is Transmitted power integrated over 2 independent of r, with P T (a/ ) 4 P i Aperture based NSOM high resolution results in rapidly reduced throughput Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 16 OSE6650 Fall Class 3: Optical near fields 8

9 Near field detection : Types of tips Aperture style probe Transparent uncoated Non-transparent Aperture style probe - Metal coated (typically Al high plasma frequency: good blocking ability ) - Can be fabricated on optical fiber, or on hollow/transparent AFM-type cantilever - Acts as nano-radiator or nano-collector, aperture ( << ) - Low transmission efficiency (~ for vis. light, nm aperture) Transparent uncoated: Apertureless uncoated: tip apex couples near-fields to guided modes tip apex scatters near-fields to far-field Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 17 Types of probes Tapered bent metal coated fiber probe hand made, fragile, thermally sensitive Collection / illumination path shielded Tapered metal-coated fiber probe hand made, fragile, thermally sensitive Collection / illumination path enclosed AFM style microfabricated probe Robust, not very temperature sensitive Collection path open: risk of background Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 18 OSE6650 Fall Class 3: Optical near fields 9

10 Typical illumination mode NSOM measurement setup Fiber nanostructures Laser source x-y scan for image formation Transparent sample on scanning stage z-scan for tipsample distance control Collection objective to detector Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 19 Near-field microscopy modes illumination mode Light incident through fiber, generates localized point dipole radiator near surface Scan sample or tip, Collect transmitted light vs. position High resolution transmission map Requires transparent sample Places conductive big object near surface: tip may affect near-fields! Question: effect of buried scatterer? Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 20 OSE6650 Fall Class 3: Optical near fields 10

11 Near-field microscopy modes collection mode Light incident from below, generates near-fields on patterned/rough sample surface Scan sample or tip, Near-fields + incident wave excite electron oscillation on tip aperture weak coupling to fiber modes Collect transmitted light vs. position High resolution transmission map Requires transparent sample Entire surface always illuminated: Not ideal with bleaching samples Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 21 Near-field microscopy modes reflection mode Light incident through fiber, small reflection back into tip, and away along sample Scan sample or tip, Collect transmitted light vs. position High res. reflection/scattering map Works with opaque samples Signal collection tricky: - Outside fiber: large part of 4 angle blocked by tip and sample - Collection lens with high NA needed - Back-reflection into fiber: weak, large background signal Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 22 OSE6650 Fall Class 3: Optical near fields 11

12 Near-field microscopy modes Photon scanning tunneling mode In PSTM, a potential barrier separates two media that support propagating modes picture from PSTM Total internal reflection No light emitted Small air gap: Light tunneling possible Small air gap + fiber Light tunneling into guided mode toward detector Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 23 Near-field microscopy modes photon scanning tunneling mode Light incident under total-internal reflection condition. Small field overlap with tip of fiber, weak coupling into guided mode Scan sample or tip, Collect near-field (+ scattered radiating components) vs. position High resolution map of near-fields Transparent high index samples only Precise distance control needed Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 24 OSE6650 Fall Class 3: Optical near fields 12

13 Near-field microscopy modes scattering mode / apertureless NSOM Light illuminates sharp tip (metal, high index e.g. Si), generates field concentrated near tip apex. Interaction with surface region affects total tip scattering Scan sample or tip, Collect distance dependent part of scattered light vs. position High res. reflection/scattering map Works with opaque and transparent Tip throughput not as much a concern Can work at higher resolution But: requires oscillating tip Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 25 Nano-aperture formation, example 1 pulled fiber 1. Starting point: glass fiber 2. Heat and pull: tapering occurs 3. Pull till break: sharp glass tip (also possible via etching) 4. Evaporate metal at an angle + rotate fiber Result: metal coated fiber with tiny aperture at end (diameter ~50nm possible) Advantage: easy to make Disadvantage: not reproducible, slow Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 26 OSE6650 Fall Class 3: Optical near fields 13

14 Alternative method for taper fabrication : etching Starting point: glass fiber Insert fiber in etchant, start retreating at constant speed Top part: etched the least Bottom: etched the most Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 27 Pulled and etched fiber probes from: Note: metal coating is not ideal wetting and grain structure play major role Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 28 OSE6650 Fall Class 3: Optical near fields 14

15 Nano-aperture formation, example 2 microfabricated tip Starting point : Si wafer expose <111> planes anisotropic etch oxidize surface etch back substrate deposit metal open up tip (e.g. FIB) This is followed by release from the handler wafer (possible due to lateral patterning not shown) Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 29 SEM of near-field probes designed for use with Witec AlphaSNOM Illumination of partially Al coated SiO 2 pyramid generates nanoscale light source Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 30 OSE6650 Fall Class 3: Optical near fields 15

16 Sample movement and distance control Sample can be moved using piezo stage placement accuracy ~1 nm Feedback loop required: tip needs to be close, but cannot withstand much force Accurate measurement of tip angle using beam deflection method: A-B = 0 A-B = -1 V x =0 V x =100 Look at laser beam deflection angle (A-B signal): if A-B becomes negative, move sample stage back (or tip up, depending on system) V z,piezo (x,y) represents height map of the sample Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 31 Scanning Near-field Optical Microscopy (NSOM) 1. Illuminate tip aperture (through fiber, or free space as in this example) 2. Scan sample OR tip and collect transmitted photons in far field e.g. using collection objective under sample intensity tip position optical maps with sub-diffraction limit resolution possible, BUT rather slow (typically several minutes per scan) Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 32 OSE6650 Fall Class 3: Optical near fields 16

17 Distance control using fiber probes For straight pulled fibers, beam deflection method is impractical Alternate common method: tuning fork feedback Fiber is mounted on mechanical resonator small (few nm) lateral amplitude can be driven by piezo block At close (few nm) proximity, tip-sample interaction affects resonance frequency amplitude changes Use piezo driving circuit response as feedback parameter For bent pulled fibers, beam deflection can be used No reflected spot, but reflected line Production of bent fiber probes not trivial Some additional losses in bent fiber part (number?) Images from Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 33 Example system at CREOL: the AlphaSNOM See Sample position Tip mounted on objective Control electronics Piezo stage - Microfabricated tips - Beam deflection feedback - Sample scanning, tip fixed Collection optics + detector (remove lid to see..) Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 34 OSE6650 Fall Class 3: Optical near fields 17

18 Resolution comparison: diffraction limited vs. near-field in transmission Test sample: containing aluminum islands with ~60nm diameter, much smaller than the diffraction limit. Measured with Witec AlphaSNOM 1 micron far-field image: through optical microscope: diffraction limited features observed at 532nm Diffraction limit at this ~ 300 nm 1 micron NSOM image: Sub-wavelength details of the sample are visible (dark dots are Al islands) Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 35 Example: phase detection of waveguide modes Tip as pickup element PSTM mode Mapped mode in straight waveguide SIM See Balistreri et al. - Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 36 OSE6650 Fall Class 3: Optical near fields 18

19 Near-field microscopy Summary: - Nanostructures support non-radiative local field components with k > k 0 - A nanostructured probe in the near-field region ( ) can convert part of these components into far-field radiation (scattered light) - Scanning a nanoscale probe near sample surface allows mapping of near-field. This forms the basis of Near-field microscopy - Different configurations: > Transmission mode (illumination and collection mode) > Reflection mode > Photon Scanning Tunneling Microscopy (PSTM) mode Note: - Transmission of aperture probes d 4 and d < 50nm diameter is difficult - Good/excellent resolution at visible frequencies: ~15-50nm - Exact tip shape affects which components of the field are scattered Watch out for tip artifacts! Optical Properties of Nanostructured Materials Fall 2013 Class 3 slide 37 OSE6650 Fall Class 3: Optical near fields 19