Scattering-type near-field microscopy for nanoscale optical imaging Rainer Hillenbrand Nano-Photonics Group Max-Planck-Institut für Biochemie 82152 Martinsried, Germany
Infrared light enables label-free contrast IR is highly sensitive to - molecular vibrations chemical composition - crystal lattice vibrations structural properties - plasmons in doped semiconductors electron properties -.. - ω 0 BUT: spatial resolution > λ/2 10 µm +
Optical antennas concentrate light to nanoscale dimensions Strong optical near-fields at a metal tip with apex diameter << λ (numerical calculation) air gold tip hν scanning probe tip d Optical field confinement on the scale of the tip apex d << λ sample Novotny et al. PRL 79, 646 (1997) Anderson et al., Materials Today, May, 50 (2005) Near-field optical excitation of the sample yields Elastic scattering (vis, IR, THz, ) Inelastic scattering (Raman) SHG, SFG,
AFM based near-field microscope illumination elastically scattered light tip nanofocus due to tip-enhanced near-fields laser focus diffraction limited sample
Near-field optical amplitude and phase contrast microscopy is possible IR laser beam Ein reference mirror oscillating AFM tip wavelength λ 9-11 µm Ω detector Esca nω objective scattered light sample heterodyne detection: homodyne detection: scan nω local optical amplitude and phase R. Hillenbrand, F. Keilmann, Phys. Rev. Lett. 80 (2000) T. Taubner, R. Hillenbrand, F. Keilmann, J. Microsc., 210,311 (2002) pseudoheterodyne detection: N. Ocelic, A. Huber, R. Hillenbrand, Appl. Phys. Lett. 89, 101124 (2006)
What we can do with optical nanoscopy I. Map nanometer IR contrast for material sciences II. Identify single (bio)particles with IR antennas III. Study superlensing IV. Characterize optical antennas
1 µm Near-field nanoscopy enables nanoscale infrared material and doping contrast Topography IR amplitude E 2 IR Phase ϕ 2 LDMOS layout W SiO 2 Si
Near-field nanoscopy enables nanoscale infrared material and doping contrast Topography IR amplitude E 2 IR Phase ϕ 2 1 µm W LDMOS layout SiO 2 Si Topography IR amplitude E 3 IR phase ϕ 3 SiO 2 doped Si 500 nm Si 930 cm -1 SEM 500 nm Ti (20nm) TiN SiN (10 nm SiO 2 (55 nm doped Si Si simultaneous infrared material and free carrier contrast 10 nm structures visible (= λ/1000) 30 nm optical resolution
A dipole model explains the local IR material contrast Normalized amplitude of scattered light [a.u.] 6 4 2 0 Al Im(ε) = 1 ω = 1000 cm -1 Au Polariton resonance at ε -2: Surface Phonon-Polaritons (e.g. in SiC) Surface Plasmon-Polaritons (e.g. in doped Si) Cu ε -2 SiO 2 PS Si k in k sca Dipole model p p sample, ε(ω) Metals Si (undoped) Dielectrics -1000-500 -10 Real part of the sample dielectric constant, Re(ε) 0 10
The dipole model predicts a sharp phonon resonance for SiC resonant mirror dipole by phonon excitation in SiC plane ε - 2 nonresonant Pt-sphere Near-field spectrum S = intensity! Dielectric function of SiC-sample ε = ε + iε TO LO
The experiment confirms the prediction of a polariton resonance ε -2 IR s-snom amplitude c-sic Au Frequency (cm -1 )
We can map crystal damage induced by ion beam implantation ε -2 60 kev Be 2+ IR s-snom amplitude c-sic a-sic Frequency (cm -1 ) 2 µm a-sic c-sic Topography Infrared amplitude at 925 cm -1
We are mapping nanoscale strain fields in collaboration with T. Köck (MPI Plasmaphysik) SEM Nanoindent in SiC Topography s-snom Amplitude tensile strain 0 2 µm 956 cm -1 compressive strain
What we can do with optical nanoscopy I. Map nanometer IR contrast for material sciences II. Identify single bioparticles with IR antennas III. Study supersensing IV. Characterize optical antennas
Single nanoparticle vibrational imaging is enabled by field enhancement Label-free high-resolution optical imaging of nanoparticles hν metal tip Probing - tip based chemicaly specific methods IR Raman C sca ~ d 6 /λ 4 C sca ~ 10-28 cm 2 (1 nm particle @ mid-ir) 10-30 cm 2 (typical molecule) extremely small scattering cross-sections! Solution field enhancement!
How others enhance the fields (Polariton) resonant antennas Application for microscopy Optical antenna at the apex of an AFM tip glass SEM 200 nm optical field calculation enhanced field in the antenna feed gap. Mühlschlegel et al, Science 308, 1607 (2005) J.N. Farahani, et. al., Phys. Rev. Lett. 95, 017402 (2005)
How we enhance the fields difficult common metal, SiC simple efficient object in the gap IR Cvitkovic et al., Phys. Rev. Lett. 97, 060801 (2006 Raman Pettinger et al., Phys. Rev. Lett. 92, 096101, (2004 Neacsu et al., Phys. Rev. B 73, 193406 (2006)
Optical field distribution in tip-substrate gap INFRARED excitation 927 cm -1 (λ=10.79 µm) weak dielectric substrate perfect metal mirror Field enhancement phonon polariton resonant substrate Field enhanceme 40 20 0 Pt glass -20 0 20 x (nm) Pt Au -20 0 20 x (nm) SiC Pt -20 0 20 x (nm) 500 400 300 200 100 substrate yields increasing field enhancement calculations: J. Aizpurua ( Donostia International Physics Center, Spain)
Tip-substrate coupling enhances optical particle contrast hν d = 17 nm ε (ω) substrate Au particle on norm. amplitude signal 10 1 0.1-100 0 100 200 x (nm) SiC @ 927 cm -1 ε -2 Au @ 927 cm -1 ε -5000 SiC @ 1080 cm -1 ε 3 (polariton resonant substrate) (perfect metal substrate) (weak dielectric substrate) highly reflective substrates signals increase absolute optical particle contrast increases
IR spectroscopy can be done at a single nanoparticle level Topography Near-field infrared ampliutde spectra PMMA infrared spectrum PMMA bead 200nm Protein infrared spectrum (Amide band) Si TMV virus 500 nm 1600 1650 1700 1750 1800 frequency [cm -1 ] IR spectroscopic identification of single nanoparticles at <λ/100 spatial resolution e.g. label-free mapping of proteins in membranes
What we can do with optical nanoscopy I. Map nanometer IR contrast for material sciences II. Identify single bioparticles with IR antennas III. Study superlensing IV. Characterize optical antennas
A superlens captures evanescent waves n 1 = 1 n 2 = -1 Ray picture superlens d/2 d d/2 object plane image plane Enhancement of evanescent waves near-field amplitude high spatial frequencies are restored subwavelenth-scale resolved imaging
Near-field superlenses are already demonstrated in the visible Enhancement of evanescent waves due to excitation of surface polaritons in thin slabs ε slab = -ε environ. slab thickness d slab << λ Visible/UV frequencies: plasmonic superlenses - surface plasmon excitation on thin silver films - topographical readout via photoresist - gratings with λ/3 period imaged ng et al., Science 308, 534 (2005)
We first demonstrated an IR near-field superlens Image plane Objects (holes) topography IR amplitude SEM image (mirrored) 2 µm 500 nm = λ/20! 800 nm λ= 9.26 µm no superlensing λ= 10.85 µm superlensing 1200 nm hole diam.
What we can do with optical nanoscopy I. Map nanometer IR contrast for material sciences II. Identify single bioparticles with IR antennas III. Study superlensing IV. Characterize optical antennas
Small particles exhibit optical resonances I = E + E in particle 2 hν + + + - - - E = ε ε m 2 E ε + 2ε m in resonance at ε = 2ε m E / 30 20 10 E in 10 wavelength λ (µm) 1 SiC Phonon polariton resonance 0.5 ε m =1 Plasmon polariton resonance Ag Au 0 1000 frequency (cm -1 ) 10000
Nanoscale optical field patterns can be mapped by weak dielectric tips sca ( E E ) E + i p 90 nm diameter gold dis 20 nm heigh resonance @ 624 nm imaged @ 633nm theory Ei+Ep experiment optical amplitude optical phase 160 nm
Summary Scattering-type near-field optical microscopy (s-snom) is a powerful analytical tool for nanoscale material characterization and quality control studying fundamental phenomena and problems in many nanosciences Examples shown in this talk: nanoscale material and doping contrast in semiconductor devices mapping of crystal structure, defects and nanoscale strain fields nanoparticle imaging and identification verification of superlensing optical eigenfield mapping
Thank you to my collaborators Nano-Photonics Group N. Ocelic A. Huber T. Taubner A. Cvitkovic U. Männl O. Keller Molecular Structural Biology M. Brehm F. Keilmann R. Guckenberger O. Medalia M. Beck W. Baumeister Infineon Munich (failure analysis) J. Wittborn MPI Plamaphysics (strain mapping) T. Köck D.I.P.C., San Sebastian, Spain (Theory, opt. antennas) J. Aizpurua J. García de Abajo University of Austin, USA (Superlens) G. Shvets D. Korobkin Y. Urzhumov Supported by BMBF within the Young Scientist Competition in Nanotechnology 2002. Grant number 03N8705
Near-field probes can beat the diffraction limit hω hω hω > λ/2 50 nm < 10 nm Far-Field Optics diffraction-limited Scanning Near-Field Optical Microscopy (SNOM) aperture-limited Scattering-type SNOM (s-snom)