6/7/20161 THz near-field imaging and micro-spectroscopy -J. Knab, A.J.L. Adam, N. Kumar R. Chakkittakandy, R. N. Schouten, TUD -M. Nagel, RWTH Aachen -Eric Shaner, Sandia National Laboratories -M.A. Seo, D.S. Kim, Seoul National University, Seoul, Korea -A. C. Strikwerda, K. Fan, X. Zhang, R. D. Averitt, Boston University Paul Planken

The problem: small objects are almost invisible Calculated scattering off a perfectly conducting cylinder: Incident field “Shadow” effectsNo “shadow” effects

Near-field optics: Branch of optics that considers configurations that depend on the passage of light to, from, through, or near an element with sub-wavelength features and the coupling of that light to a second element located a subwavelength distance from the first. From: Near-field optics, Theory, Instrumentation and Applications By: M. A. Paesler and P. J. Moyer (Wiley, 1996) The solution: the near-field

Example: sub-wavelength aperture source THz pulse metal sample ~ to detector See also: Mitrofanov et al. APL 77, 3496 (2000) and subsequent papers

How does light go through a sub-wavelength sized aperture?

6/7/20166 Let's measure it! Spatial resolution determined by near-IR probe beam, not THz beam Allows electric near-field vector measurements, (E x, E y, E z ). EO crystal Metal Focused THz beam  THz 5  m spot =800 nm Opt. Express 15, (2007) Opt. Express 16, 7407 (2008) Other methods: A. Bitzer et al. APL 92, (2008); Opt. Express 17, 3826 (2009) A. Doi et al. Opt. Express 18, (2010)

EO-detection: Crystal orientation dependence PBS /4 P1P1 P2P2 PBS /4 P1P1 P2P2 /2 GaP Opt. Lett. 30, 2802 (2005); JOSA B 18, 313 (2001) JOSA B 21, 622 (2004)

PBS /4 P1P1 P2P2 GaP Opt. Lett. 30, 2802 (2005); JOSA B 18, 313 (2001) JOSA B 21, 622 (2004)

6/7/20169 |E y 0.25 THz, Behind (~30  m) a 200 µm square hole on Si 30  m Si With gap:

No gap: Metal+hole deposited on detection crystal

small gap Si z-component Sharper Image...

Opt. Express 17, (2009) Field distribution is not much affected Measurements:  m thick metal  m diameter

What happens when there's a gap between a thick metal and the crystal? 20  m

Effects of gap... GaP Measurement  m thick metal  m diameter 10  m 20  m Opt. Express 17, (2009)

metal, free-standing metal on substrate Near-field distribution is not affected much However......

6/7/ Transmission spectra are different 100  m diameter hole Cut-off frequency

E z behind 100  m hole in 0.5  m Au Transmission spectrum of the hole resembles that of a filled hole

6/7/ Measuring all three components: circular aperture Opt. Express 17, (2009) “Bouwkamp”

Past measurements of near-field of aperture probes E. Betzig and R. J. Chichester, Science 262, 1422 (1993) Molecules used as probes of the field near an aperture probe Near-field region

E z time-evolution.... GaP Opt. Express 16, 7407 (2008)

21 Holes differentiate the incident field... Near-field spectrum is not the same as spectrum of incident field Round holes

Application: Spectroscopy of filled holes

Near-field of filled apertures 150  m APL 97, (2010)

Waveguide filled With D-tartaric acid Empty waveguide ExEx

Polyethylene powder in aperture Filled aperture: stronger electric field

n eff,Si =1.81 n eff,PE =1.28 Transmission spectra More THz light “fits” inside the aperture.....

empty D-tartaric acid filled Pressed pellet, far-field absorption spectrum Waveguide, near-field APL 97, (2010)

In the THz domain, it's an old idea Fritz Keilmann, Int. J. Infrared Milli., 2, 259 (1981). Far-field transmission through metallic, filled waveguide

Filled holes in thin films CsI Opt. Express 21, 1101 (2013)

CsI on 10  m diameter hole E x vs. time Estimated smallest probed sample volume: ~5x m 3 (0.5 pl)

Spectra Opt. Express 21, 1101 (2013)

CsI n(  ) and  Adapted from: Jepsen et al. Opt. Lett. 30, 29 (2005)

CsI size dependence (10  m hole) CsI 10  m Calculations

CsI on 20  m hole Same crystal, different size! 20  m measurements

Advantage of measuring in the near-field “detector” Near-field: z<d Calculations Opt. Express 21, 1101 (2013)

THz magneto-optic near-field sampling Probe beam H

Split-ring resonators

THz magnetic field of double split-ring xx

2D field distribution

Single split-ring

Integrating over crystal length Strong-field region Signal dominated by near-field

6/7/ The electro-optic effect: measuring light with light n(E THz ) (110) oriented EO crystal THz probe pulse Electro-optic effect: THz E-field produces elliptically polarized probe pulse

E x, E y, E z at “zero” distance (Au on Si) Measured the complete electric field (in a plane behind the sample)

6/7/ EO-detection: Crystal orientation dependence PBS /4 P1P1 P2P2 PBS /4 P1P1 P2P2 /2 GaP Opt. Lett. 30, 2802 (2005); JOSA B 18, 313 (2001) JOSA B 21, 622 (2004)

6/7/ PBS /4 P1P1 P2P2 GaP Opt. Lett. 30, 2802 (2005); JOSA B 18, 313 (2001) JOSA B 21, 622 (2004)

6/7/ THz microsocpy Problem: The diffraction limit... -Cannot see smaller than ~ /2 -How do we circumvent the diffraction limit?

6/7/ Achieving sub wavelength resolution... Spatial resolution determined by near-IR probe beam, not THz beam Allows electric near-field vector measurements, (E x, E y, E z ). Example: propagation through apertures EO crystal Metal Focused THz beam THz ~ 500  m 5  m spot =800 nm Opt. Express 15, (2007) Opt. Express 16, 7407 (2008)

6/7/ THz light directly behind a small circular aperture EzEz

6/7/ Holes differentiate the incident field... Incident field 51

6/7/ Holes differentiate the incident field...

6/7/ Holes differentiate the incident field...

6/7/ Holes differentiate the incident field...

6/7/ Holes differentiate the incident field...

6/7/ The electric field behind slits in a metal plate incident plane waves (planes of constant phase) x z y ?

6/7/ Evolution of the field behind metal slits.... Opt. Express 15, (2007) times slowed down!

A THz

y x The boundary conditions for the electric field are very useful in guessing the directions of the electric near-field Perfect metal: -E-field parallel to metal edge = 0 -E-field can only have a component perpendicular to the metal This explains the ocurrence of a (weak) y-component for a square aperture Measured E y

6/7/ Spatial resolution? 200  m 60  m 20  m Spatial resolution ~10  m

6/7/ THz near-field micro-spectroscopy

6/7/ CsI 200 nm Au GaP THz beam Sampling beam ~5  m Solution: Only measure light that interacts with the sample. Sample material: CsI

THz refractive-index and absorption coefficient of CsI Data extracted from: P. U. Jepsen, Opt. Lett. 30, 29 (2005) TO-phonon

6/7/ CsI 200 nm Au GaP THz beam Sampling beam ~5  m d=20  m

Transmission though 20  m hole filled with CsI

Propagation through CsI of thickness z Model is crude but gives physical insight Near-electric field proportional to complex refractive-index (profile) x y a

6/7/ Numerical calculations (CST Microwave Studio):

6/7/ CsI TO phonon

6/7/ To do list: -Improve spatial resolution -Improve bandwidth -Study/improve “unusual” sources -Measure magnetic near-field of stuctures

Frequency (THz) 20  m aperture Measurement

6/7/ Frequency (THz) 20  m CsI filled aperture with and without resonance Calculations

6/7/ Frequency (THz) 40  m CsI filled aperture with and without resonance Calculations

6/7/ Effect of substrate, thin metal layers

6/7/ Opt. Express 17, (2009)

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6/7/ Effect of substrate, thick metal layers

6/7/ calculations Measurements 150  m diameter hole

6/7/ “Free-standing” vs. “In contact” 100  m diameter hole

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6/7/ E 0.25 THz, behind a 200 µm square hole on GaP E 0.25 THz, Behind (~30  m) a 200 µm square hole on Si

6/7/ Near-field of 10  m x 40  m graphite “rod”

6/7/ Magnetic-field “enhancement” THz light from graphite X.-C. Zhang, et al. Appl. Phys. Lett., pp (1993) Opt. Express 17, (2009)

6/7/ Simple picture quasi-static field picture:

6/7/ Frequency analysis polarisation Simulation at 1 THz From thesis of Janne Brok 1 THz0.25 THz Au on Si Si GaP 20  m

6/7/ Evolution of the field behind a THz

6/7/ Can we produce sharper images? Integrate aperture with detector!

6/7/ x y z THz pulse 200 nm Au 300 nm Ge 150 nm SiO 2 GaP Probing pulse

6/7/ |E z (  )| of Au on GaP THz EzEz GaP EO crystal vac Measurable “transmission” for vac  mm mm! vac

6/7/ Movies at fixed frequencies THz0.098 THz 0.54 THz

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6/7/ THz-TDS can measure things that other techniques cannot or not so easily This makes THz-TDS an ideal optical experimentation platform for EM experiments - Maxwell’s equations are scale-invariant! THz (demonstrated) visible (required) Temporal resolution 100 fs0.1 fs Spatial resolution 10  m 10 nm Pulse duration1 ps1 fs Wavelength range  m nm

6/7/ Circular Aperture Array 200 nm Thick Gold on 300  m GaP Detection Xtal 90  m 60  m

6/7/ THz Near-Field Images E(t=t 1 ) 200  m ExEx EyEy EzEz

6/7/ |E z | Phase 0.47 THz 1.0 THz E z inside aperture = NOT observed in single isolated aperture Array contribution?

6/7/ Detail....

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6/7/ Thank you for listening

6/7/ Literature: F.J. Garcia de Abajo Opt. Express 10, 1475 (2002)

6/7/ How do we improve the spatial resolution to < /2 ? THz pulse copper tip GaP crystal probing pulse sample x y ~ probe + GaP crystal = detector Appl. Phys. Lett. 81, 1558 (2002) Semicon. Sci. & Techn. 20, S121 (2005)

6/7/ Past near-field measurements J.A. Veerman et al. J. Microsc. 194, 477 (1999) Molecules used as probes of the field near an aperture probe

6/7/ Past measurements of near-field of aperture probes E. Betzig and R. J. Chichester, Science 262, 1422 (1993) Molecules used as probes of the field near an aperture probe Near-field region

6/7/ Far/near-field measurement e-beam induced plasmon emissionfar-field transmission Degiron et al. Opt. Commun. 239, 61 (2004)

6/7/ Can we also measure the magnetic near-field? I(t) Yes, use the Faraday effect We use TGG (terbium gallium garnet)

6/7/ Difficult to measure.... Signature of magnetic near-field? I(t) Flipping sample 180 degrees should flip B-field TGG

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