THz waveguides : a review

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THz waveguides a review

• Alexandre Dupuis
• École Polytechnique de Montréal
• M. Skorobogatiy
• Canada Research Chair in photonic crystals
• http//www.photonics.phys.polymtl.ca/

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Outline

• Introduction
• Applications in the THz regime
• Early waveguide attempts
• - Coplanar striplines, plastic ribbons,
• sapphire fibers, metal tubes
• Recent breaktroughs
• - Metal wire, microstructured fiber,
• plastic fiber,
• hollow plastic tubes with inner metal
  layer
• Perspectives

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Introduction
What is the THz regime ?

• Bridges the gap between the microwave and optical
  regimes.
• n 0.1 THz - 10 THz
• 3000 mm - 30 mm
• Major applications sensing, imaging and
  spectroscopy.

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Applications

• Imaging of biological tissues (tissue
  recognition)

Löffler, Opt. Exp., 9, 12 (2001)
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Applications

• Chemical recognition of gases

Time domain spectroscopy
Jacobsen, Opt. Lett., 21, 24 (1996)
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Applications

• Tomography

Pearce, Opt. Lett., 30, 13 (2005)
Mittleman, Opt. Lett., 22, 12 (1997)
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Applications

• Non destructive sensing

Combining imaging and spectroscopy for the
detection of organic compounds
Kawase, Opt. Exp., 11, 20 (2003)
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Applications

• Non destructive sensing

Kawase, Opt. Exp., 11, 20 (2003)
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Applications

• Inspecting electrical faults in integrated
  circuits

Kiwa, Opt. Lett., 28, 21 (2003)
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Technological challenges

• Bulky free-space propagation of THz radiation

Goto, Jap. J. Appl. Phys. Lett., 43, 2B (2003)
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Technological challenges

• Virtually no low-loss waveguides
• Conventionnal waveguides dont work in the THz
  regime
• Metals high loss due to finite conductivity
• Dielectrics high absorption
• 2. Low dispersion waveguides necessary for
  spectroscopy

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Early waveguides

• Coplanar striplines

Metal electrodes on a semiconductor substrate a
20 cm-1 at n1 THz a n3
Frankel, IEEE Transactions on microwave theory
and techniques, 39, 6 (1991)
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Early waveguides

• Plastic ribbon waveguides

PE ribbon 150 mm thick Dispersive single-mode
propagation No cut-off frequency a 1 cm-1
Mendis, J. Appl. Phys., 88, 7 (2000)
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Early waveguides

• Sapphire fiber

Single-crystal sapphire fiber Diameter of 125,
250 and 325 mm a 1 cm-1 Dispersive
propagation, mainly attributed to the waveguide
and not the material Dominance of HE11 mode
despite multimode fiber
Jamison, Appl. Phys. Lett., 76, 15 (2000)
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Early waveguides

• Metal tubes

Stainless steel with an inside diameter of 280 mm
a 0.7 cm-1 Very dispersive multimode
propagation Low frequency cut-off at 0.76 THz
McGowan, Opt. Lett., 24, 20 (1999)
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Recent waveguides

• Parrallel metal plates

Two 100 mm thick copper plates separated by a 90
mm air gap a 0.1 cm-1 at 1 THz Low
dispersion Absorption still high and
cross-section too large for medical application
Mendis, IEEE Microwave and wireless components
letters, 11, 11 (2001)
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Recent waveguides

• Hollow polymer waveguides with inner metallic
  layers

• Using liquid-phase chemistry methods, a metal or
  dielectric layer is deposited inside a silicon or
  polymer hollow waveguide.
• It has been shown in the mid-IR region that
  hollow waveguides suffer a bending loss of 1/R,
  where R is the radius of curvature. It is
  possible to eliminate this effect with photonic
  bandgap structures.
• The losses in Cu hollow waveguides can be
  significantly reduced if a dielectric coating of
  the correct optical thickness is deposited over
  the metallic layer.

Harrington, Opt. Exp., 12, 21 (2004)
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Recent waveguides

• Hollow polymer waveguides with inner metallic
  layers

• 8 mm bore hollow waveguide with an inner wall of
  ferroelectric Polyvinylidene Fluoride (PVDF)
• a 0.015 cm-1 at 1 THz
• With Cu inner layer,
• a 0.045 cm-1 at 1 THz

Hidaka, Optical information, data processing and
storage, and laser communication technologies,
Proc. SPIE, 5135, 11 (2003)
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Recent waveguides

• Ferroelectric hollow core all-plastic Bragg
  fibers

Skorobogatiy, Appl. Phys. Lett., 90, 113514,
(2007)
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Recent waveguides

• Metal wire

Stainless steel wire with a diameter of 900 mm a
lt 0.03 cm-1 However, coupling efficiency is
(very) low Non polarization maintaning
Wang, Nature, 432, (2004)
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Recent waveguides

• Metal wire

Cu wire with a diameter of 450 mm should have a
0.002 cm-1 at 1 THz Theoretical explanation of
Wangs results Azimutely Polarized Surface
Plasmon (APSP) The polarization mismatch with
the linearly polarized source leads to a very low
coupling efficiency.
Cao, Opt. Exp., 13, 18 (2005)
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Recent waveguides

• Metal wire

Outside the metal, kair is very small, so the
field decays very slowly in the radial direction
and extends several 10 times R outside of the
metal. Inside the metal, km is very large,
leaving a field penetration depth of less than 1
mm.
Cao, Opt. Exp., 13, 18 (2005)
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Recent waveguides

• Metal wire with milled grooves

Vain attempt to increase coupling
Cao, Opt. Exp., 13, 18 (2005)
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Recent waveguides

• Subwavelength plastic fibre

200 mm diameter PE fiber a 0.01 cm-1 at 0.3
THz Single-mode HE11 propagation
Fig. Ponyting vector a) 0.3 THz b) 0.5 THz c)
0.7 THz d) 0.9 THz
Sun, Opt. Lett., (Oct. 2005)
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Recent waveguides

• Plastic photonic crystal fibers (PPCF)

500 mm diameter HDPE tubes The tubes were 2cm
long, stacked in 2D triangular lattice and fused
together at 135C in a conventional furnace. a
0.5 cm-1 at 1 THz Material absorption primary
loss factor Relatively low dispersion, mainly due
to waveguide dispersion
Han, Appl. Phys. Lett., 80, (2002)
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Recent waveguides

• Plastic photonic crystal fibers (PPCF)

Teflon tubes a 0.3 cm-1 at
1 THz
Goto, Jap. J. Appl. Phys. Lett., 43, 2B (2003)