1. 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

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

3. 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

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

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

6. 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, 11781 (2007) Opt. Express 16, 7407 (2008) Other methods: A. Bitzer et al. APL 92, 231101 (2008); Opt. Express 17, 3826 (2009) A. Doi et al. Opt. Express 18, 18419 (2010)

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)

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

9. 6/7/20169 |E y | @ 0.25 THz, Behind (~30  m) a 200 µm square hole on Si 30  m Si With gap:

10. No gap: Metal+hole deposited on detection crystal

11. small gap Si z-component Sharper Image...

12. Opt. Express 17, 17412 (2009) Field distribution is not much affected Measurements: - 0.5  m thick metal - 100  m diameter

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

14. Effects of gap... GaP Measurement - 200  m thick metal - 150  m diameter 10  m 20  m Opt. Express 17, 17412 (2009)

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

16. 6/7/201616 Transmission spectra are different 100  m diameter hole Cut-off frequency

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

18. 6/7/201618 Measuring all three components: circular aperture Opt. Express 17, 15072 (2009) “Bouwkamp”

19. 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

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

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

22. Application: Spectroscopy of filled holes

23. Near-field of filled apertures 150  m APL 97, 031115 (2010)

24. Waveguide filled With D-tartaric acid Empty waveguide ExEx

25. Polyethylene powder in aperture Filled aperture: stronger electric field

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

27. empty D-tartaric acid filled Pressed pellet, far-field absorption spectrum Waveguide, near-field APL 97, 031115 (2010)

28. 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

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

30. CsI on 10  m diameter hole E x vs. time Estimated smallest probed sample volume: ~5x10 -16 m 3 (0.5 pl)

31. Spectra Opt. Express 21, 1101 (2013)

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

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

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

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

36. THz magneto-optic near-field sampling Probe beam H

37. Split-ring resonators

38. THz magnetic field of double split-ring xx

39. 2D field distribution

40. Single split-ring

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

44. 6/7/201644 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

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

46. 6/7/201646 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)

47. 6/7/201647 PBS /4 P1P1 P2P2 GaP Opt. Lett. 30, 2802 (2005); JOSA B 18, 313 (2001) JOSA B 21, 622 (2004)

48. 6/7/201648 THz microsocpy Problem: The diffraction limit... -Cannot see smaller than ~ /2 -How do we circumvent the diffraction limit?

49. 6/7/201649 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, 11781 (2007) Opt. Express 16, 7407 (2008)

50. 6/7/201650 THz light directly behind a small circular aperture EzEz

51. 6/7/201651 Holes differentiate the incident field... Incident field 51

52. 6/7/201652 Holes differentiate the incident field...

53. 6/7/201653 Holes differentiate the incident field...

54. 6/7/201654 Holes differentiate the incident field...

55. 6/7/201655 Holes differentiate the incident field...

56. 6/7/201656 The electric field behind slits in a metal plate incident plane waves (planes of constant phase) x z y ?

57. 6/7/201657 Evolution of the field behind metal slits.... Opt. Express 15, 11781 (2007) 10 13 times slowed down!

58. A THz - - - - - - - + + + + +

59. 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

60. 6/7/201660 Spatial resolution? 200  m 60  m 20  m Spatial resolution ~10  m

61. 6/7/201661 THz near-field micro-spectroscopy

62. 6/7/2016 62 CsI 200 nm Au GaP THz beam Sampling beam ~5  m Solution: Only measure light that interacts with the sample. Sample material: CsI

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

64. 6/7/201664 CsI 200 nm Au GaP THz beam Sampling beam ~5  m d=20  m

65. Transmission though 20  m hole filled with CsI

66. 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

67. 6/7/201667 Numerical calculations (CST Microwave Studio):

68. 6/7/201668 CsI TO phonon

69. 6/7/201669 To do list: -Improve spatial resolution -Improve bandwidth -Study/improve “unusual” sources -Measure magnetic near-field of stuctures

70. 0.5 1.0 1.5 2.0 2.5 3.0 Frequency (THz) 20  m aperture Measurement

71. 6/7/201671 0.0 0.5 1.0 1.5 2.0 2.5 Frequency (THz) 20  m CsI filled aperture with and without resonance Calculations

72. 6/7/201672 0.0 0.5 1.0 1.5 2.0 2.5 Frequency (THz) 40  m CsI filled aperture with and without resonance Calculations

73. 6/7/201673 Effect of substrate, thin metal layers

74. 6/7/201674 Opt. Express 17, 17412 (2009)

75. 6/7/201675

76. 6/7/201676 Effect of substrate, thick metal layers

77. 6/7/201677 calculations Measurements 150  m diameter hole

78. 6/7/201678 “Free-standing” vs. “In contact” 100  m diameter hole

79. 6/7/201679

80. 6/7/201680 E y @ 0.25 THz, behind a 200 µm square hole on GaP E y @ 0.25 THz, Behind (~30  m) a 200 µm square hole on Si

81. 6/7/201681 Near-field of 10  m x 40  m graphite “rod”

82. 6/7/201682 Magnetic-field “enhancement” THz light from graphite X.-C. Zhang, et al. Appl. Phys. Lett., pp. 2477 (1993) Opt. Express 17, 16092 (2009)

83. 6/7/201683 Simple picture.... + - quasi-static field picture: + + + - - -

84. 6/7/201684 Frequency analysis polarisation Simulation at 1 THz From thesis of Janne Brok 1 THz0.25 THz Au on Si Si GaP 20  m

85. 6/7/201685 Evolution of the field behind a hole @0.5 THz

86. 6/7/201686 Can we produce sharper images? Integrate aperture with detector!

87. 6/7/201687 x y z THz pulse 200 nm Au 300 nm Ge 150 nm SiO 2 GaP Probing pulse

88. 6/7/201688 |E z (  )| of Au on GaP THz EzEz GaP EO crystal vac Measurable “transmission” for vac  mm mm! vac

89. 6/7/201689 Movies at fixed frequencies.... 0.2 THz0.098 THz 0.54 THz

90. 6/7/201690

91. 6/7/201691 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 250-3000  m 250-3000 nm

92. 6/7/201692 Circular Aperture Array 200 nm Thick Gold on 300  m GaP Detection Xtal 90  m 60  m

93. 6/7/201693 THz Near-Field Images E(t=t 1 ) 200  m ExEx EyEy EzEz

94. 6/7/201694 |E z | Phase 0.47 THz 1.0 THz E z inside aperture = NOT observed in single isolated aperture Array contribution?

95. 6/7/201695 Detail....

96. 6/7/201696

97. 6/7/201697 Thank you for listening

98. 6/7/201698 Literature: F.J. Garcia de Abajo Opt. Express 10, 1475 (2002)

99. 6/7/201699 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)

100. 6/7/2016100 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

101. 6/7/2016101 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

102. 6/7/2016102 Far/near-field measurement e-beam induced plasmon emissionfar-field transmission Degiron et al. Opt. Commun. 239, 61 (2004)

103. 6/7/2016103 Can we also measure the magnetic near-field? I(t) Yes, use the Faraday effect We use TGG (terbium gallium garnet)

104. 6/7/2016104 Difficult to measure.... Signature of magnetic near-field? I(t) Flipping sample 180 degrees should flip B-field TGG

105. 6/7/2016105 xx

106. 6/7/2016106

107. 6/7/2016107