THz waveguides : a review Alexandre Dupuis École Polytechnique de Montréal M. Skorobogatiy Canada Research Chair in photonic crystals

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. Introduction What 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 challenges 1.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 HE 11 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)  = cm -1 at 1 THz With Cu inner layer,  ~ cm -1 at 1 THz

Recent waveguides Ferroelectric hollow core all-plastic Bragg fibers Skorobogatiy, Appl. Phys. Lett., 90, , (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  = 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 HE 11 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)