THz waveguides : a reviewAlexandre Dupuis

École Polytechnique de Montréal

M. Skorobogatiy

Canada Research Chair in photonic crystals

http://www.photonics.phys.polymtl.ca/

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

Bridges the gap between the microwave and optical regimes.

= 0.1 THz - 10 THz

= 3000 m - 30 m

Major applications sensing, imaging and spectroscopy.

IntroductionWhat is the THz regime ?

Applications•Imaging of biological tissues (tissue recognition)

Löffler, Opt. Exp., 9, 12 (2001)

Applications•Chemical recognition of gases

Jacobsen, Opt. Lett., 21, 24 (1996)

Time domain spectroscopy

Applications• Tomography

Pearce, Opt. Lett., 30, 13 (2005)

Mittleman, Opt. Lett., 22, 12 (1997)

Applications• Non destructive sensing

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

Combining imaging and spectroscopy for the detection of organic compounds

Applications• Non destructive sensing

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

Applications•Inspecting electrical faults in integrated circuits

Kiwa, Opt. Lett.,

28, 21 (2003)

Technological challenges•Bulky free-space propagation of THz radiation

Goto, Jap. J. Appl. Phys. Lett.,

43, 2B (2003)

Technological challenges1. Virtually no low-loss waveguides

Conventionnal waveguides don’t work in the THz regime

Metals: high loss due to finite conductivity

Dielectrics: high absorption

2. Low dispersion waveguides necessary for spectroscopy

Early waveguides•Coplanar striplines

Frankel, IEEE Transactions on microwave theory and techniques, 39, 6 (1991)

Metal electrodes on a

semiconductor substrate

= ~20 cm-1 at =1 THz

~3

Early waveguides•Plastic ribbon waveguides

Mendis, J. Appl. Phys., 88, 7 (2000)

PE ribbon 150 mm thick

Dispersive single-mode propagation

No cut-off frequency

= ~1 cm-1

Early waveguides•Sapphire fiber

Jamison, Appl. Phys. Lett., 76, 15 (2000)

Single-crystal sapphire fiber

Diameter of 125, 250 and 325 m

= ~1 cm-1

Dispersive propagation, mainly attributed to the waveguide and not the material

Dominance of HE11 mode despite multimode fiber

Early waveguides•Metal tubes

McGowan, Opt. Lett., 24, 20 (1999)

Stainless steel with an inside diameter

of 280 m

= 0.7 cm-1

Very dispersive multimode propagation

Low frequency cut-off at 0.76 THz

Recent waveguides•Parrallel metal plates

Mendis, IEEE Microwave and wireless components letters, 11, 11 (2001)

Two 100 m thick copper plates

separated by a 90 m air gap

= 0.1 cm-1 at 1 THz

Low dispersion

Absorption still high and cross-section too large for medical application

Recent waveguides•Hollow polymer waveguides with inner metallic layers

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

• 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.

Recent waveguides•Hollow polymer waveguides with inner metallic layers

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

8 mm bore hollow waveguide with an inner wall of ferroelectric Polyvinylidene Fluoride (PVDF)

= 0.015 cm-1 at 1 THz

With Cu inner layer, ~ 0.045 cm-1 at 1 THz

Recent waveguides• Ferroelectric hollow core all-plastic Bragg fibers

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

Recent waveguides•Metal wire

Wang, Nature, 432, (2004)

Stainless steel wire with a diameter of 900

m

< 0.03 cm-1

However, coupling efficiency is (very) low

Non polarization maintaning

Recent waveguides•Metal wire

Cao, Opt. Exp., 13, 18 (2005)

Cu wire with a diameter of 450 m should have

= 0.002 cm-1 at 1 THz

Theoretical explanation of Wang’s results:

Azimutely Polarized Surface Plasmon (APSP)

The polarization mismatch with the linearly polarized source leads to a very low coupling efficiency.

Recent waveguides•Metal wire

Cao, Opt. Exp., 13, 18 (2005)

Outside the metal, air 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, m is very large, leaving a field penetration depth of less than 1 m.

Recent waveguides•Metal wire with milled grooves

Cao, Opt. Exp., 13, 18 (2005)

Vain attempt to increase coupling

Recent waveguides• Subwavelength plastic fibre

Sun, Opt. Lett., (Oct. 2005)

200 m diameter PE fiber

~ 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

Recent waveguides• Plastic photonic crystal fibers (PPCF)

Han, Appl. Phys. Lett., 80, (2002)

500 m diameter HDPE tubes

The tubes were 2cm long, stacked in 2D triangular lattice and fused together at 135°C in a conventional furnace.

= 0.5 cm-1 at 1 THz

Material absorption primary loss factor

Relatively low dispersion, mainly due to waveguide dispersion

Recent waveguides• Plastic photonic crystal fibers (PPCF)

Teflon tubes = 0.3 cm-1 at 1 THz

Goto, Jap. J. Appl. Phys. Lett.,

43, 2B (2003)