Poster CLEO2007 final

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Low Loss, Low Dispersion T-ray Transmission in Microwires S. Afshar Vahid1, S. Atakaramians1&2, B.M. Fischer2, H. Ebendorff-Heidepriem1, T.M. Monro1 and D. Abbott2 1Centre of Expertise in Photonics, School of Chemistry & Physics, The University of Adelaide, SA 5005, Australia 2Centre for Biomedical Engineering and School of Electrical & Electronic Engineering, The University of Adelaide, SA 5005, Australia Introduction Great demand for: Low loss and low dispersion transmission media The state of the art: Metal parallel plates [2,3] a = 0.3 cm-1 almost dispersion-free Bare metal wire [4,5,6] a = 0.03 cm-1 almost dispersion-free Plastic fiber [7] a = 0.01 cm-1 Optical Nanowires Dielectric filament D << l optic~ nm Electromagnetic field Mainly outside waveguide Practical issues fragility, surface roughness, susceptible to environment, bend loss Question: Apply optical nanowire concepts to T-rays? Field Distribution Power Fraction Optical nanowires [8] Fiber Effective Loss Conclusion References K. Sakai, Terahertz optoelectronics. Berlin, Heidelberg: Springer, 2005. S. Coleman and D. Grischkowsky, “A THz transverse electromagnetic mode two-dimensional interconnect layer cooperating quasi-optics,” Applied Physics Letters, vol. 83, no. 18, pp. 3656–3658, 2003. R. Mendis and D. Grischkowsky, “Undistorted guided-wave propagation of subpicosecond terahertz pulses,” Optics Letters, vol. 26, no. 11, pp. 846–848, 2001. K. Wang and D. M. Mittleman, “Metal wires for terahertz wave guiding,” Nature, vol. 432, no. 7015, pp. 376–379, 2004. T.-I. Jeon, J. Zhang, and D. Grischkowsky, “THz Sommerfeld wave propagation on a single metal wire,” Applied Physics Letters, vol. 86, art. no. 161904, 2005. M. Wachter, M. Nagel, and H. Kuz, “Frequency-dependent characterization of THz Sommerfeld wave propagation on single-wires,” Optics Express, vol. 13, no. 26, pp. 10815–10822, 2005. L.-J. Chen, H.-W. Chen, T.-F. Kao, J.-Y. Lu, and C.-K. Sun, “Low-loss subwavelength plastic fiber for terahertz waveguiding,” Optics Letters, vol. 31, no. 3, pp. 308–310, 2006. L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Maxwell, and E. Mazur, “Subwavelength-diameter silica wires for low-loss optical wave guiding,” Nature, vol. 426, no. 6968, pp. 816–819, 2003. R. Mendis and D. Grischkowsky, “THz interconnect with low-loss and low-group velocity dispersion,” IEEE Microwave and Wireless Components Letters, vol. 11, no. 11, pp. 444–446, 2001. D. Mittleman, Sensing with Terahertz Radiation. Berlin Heidelberg: Springer, 2003. S. Atakaramians, S. Afshar Vahid, H. Ebendorff- Heidepriem, B. Fischer, T. Monro, and D. Abbott, “Terahertz waveguides and materials,” in Conference Digest of the 2006 Joint 31st International Conference on Infrared and Millimeter Waves and 14th International Conference on Terahertz Electronics, Shanghai, China, 2006, p. 281. A. W. Snyder aand J. D. Love, Optical Waveguide Theory. London: Chapman and Hall, 1995. K. Okamoto, Fundamentals of Optical Waveguides. San Diego: Academic Press, 2000. S. Atakaramians, S. Afshar Vahid, B. Fischer, H. Ebendorff- Heidepriem, T. Monro, and D. Abbott, “Microwire fibers for lowloss THz transmission,” in Proc. SPIE Smart Structures, Devices, and Systems III, vol. 6414, art. no. 64140I, Adelaide, Australia, 2006. J.A. Buck, Fundamentals of Optical Fibers. Wiley-Interscience, 2004. X.-C. Long and S. R. J. Brueck, “Composition dependence of the photoinduced refractive-index change in lead silicate glasses,” Optics Letters, vol. 24, no. 16, pp. 1136–1138, 1999. N. Sugimoto, H. Kanbara, S. Fujiwara, K. Tanaka, Y. Shimizugawa, and K. Hirao, “Third-order optical nonlinearities and their ultrafast response in Bi2O3–B2O3–SiO2 glasses,” Journal of the Optical Society of America B, vol. 16, no. 11, pp. 1904–1908, 1999. Sub-wavelength T-Ray fibers (Modeling Results) Terahertz (THz) spectroscopic techniques have attracted much interest over the last decade due to their applications in detection of biological and chemical materials [1]. In almost all existing terahertz time domain spectroscopy systems terahertz waves propagate in free space due to the lack of low loss and low dispersion terahertz waveguides. Optical nanowires are filaments of dielectric media whose tailorable sub-wavelength dimensions (order of nm) allow a substantial fraction of the guided light (wavelength of 1-1.5 nm) to propagate outside the structure. The key characteristics of nanowires are as follows: The effective loss in a simple rod fiber is the average of the loss coefficients inside (dielectric) and outside (air) the fiber over the transversal field distributions. The field, although guided through the fiber, has the same or a larger amplitude at the dielectric-air interface, resulting in a larger fraction of guided power to be outside of the fiber [13]. The power fraction shows the ratio of the power carried outside the dielectric rod to the total power [13]. f=0.5 THz (l=600 mm) f=0.5 THz (l=600 mm) D >> lT-ray  a = bulk material loss Microwire regime (D << lT-ray)  All a converge to the similar order loss The concept of low loss in plastic fiber is similar to optical nanowires that have recently attracted a lot of interest [6,7]. f=0.5 THz (l=600 mm) 6. Effective Mode Index The phase velocity of a wave is frequency dependent because the refractive index of the medium and the field configuration in a waveguide are in general functions of frequency. As a result, different components of the spectrum propagate at different group velocities, leading to signal distortion, known as group velocity dispersion [14]. Group Velocity Dispersion The effective mode index is 7. Group Dispersion Where b and b0 are the propagation constant of the fundamental mode and free space, respectively. The group dispersion parameter D(l) is found by differentiating the group delay ̶ the first derivative of propagation constant with respect to frequency ̶ with respect to l. For all curves red and black colours are representing Bismuth and PMMA, respectively. There is a great agreement between total dispersion (solid lines) and waveguide dispersion (doted lines); i.e. the The lower the refractive index the less the overall group dispersion Possibility of having negative, positive and zero dispersion Dispersion can be minimized by choosing the substrate glass and core diameter. Center of Expertise in Photonics is supported by DSTO Acknowledgement We gratefully acknowledge Naoki Sugimoto at Asahi Glass Japan for supplying the bismuth glass samples, and David N. Jamieson for supplying the diamond samples. Surface roughness (developing a mathematical model) Fabrication and testing Appendix The Table indicates the PbO concentrations in F2, SF6, and SF57 supplied by Schott-AG and the BiO1.5 concentration in the bismuth glass supplied by Asahi Glass that has a similar composition to BI-3 glass [17]. Other main components of the glasses are SiO2 and B2O3. The references of concentration measurements are given for each glass. Concentration measurement for SF57 is obtained by direct measurement using Energy Dispersive X-ray (EDX) spectroscopy. Composition of heavy metal oxide glasses Bulk Material THz Measurement Optical parameters of 4 glasses, a polymer and a diamond The absorption coefficients and refractive indices were obtained by comparing the sample pulses with a reference pulse propagating through dry air. The pulse propagating through the set up when the sample is present is called the sample pulse and when there is no sample it is called the reference pulse. Therefore the equation for calculating the optical properties can be written as [9]: Experimental setup Picometrix T-ray 2000TM [10,11] T1 and T2 are the total transmission coefficients at the entrance and exit faces, respectively; C is the coupling coefficient, the same for the entrance and exit faces; b0 is the free space phase constant; a is the power absorption coefficient; n the refractive index of the sample; and l is the sample length. Glasses represent huge bulk loss Sub-wavelength waveguiding? 4. Critical Bend Radius Typical example: D= 200 mm, n=1.5 & a=1 cm-1 [7] A radius at which the bend loss of the fundamental mode is equal to 3 dB per loop. 5. Overall loss = Material + Bend Loss Analytical study of THz transmission in T-ray microwires by solving full vectorial Maxwell’s equations for a simple rod geometry. Based on the measured bulk material absorption we can calculate the following parameters: Overall loss ~ effective loss bend loss can be minimized Rayleigh range: 0.013 to 0.35 cm Fiber length in [7]: 17.5 cm Total loss (effective & bend losses) considering different bend radii compared to the results in [7]. Calculated effective loss compared to the experimental and calculated results in [7]. Bend Loss material dispersions are negligible at the range of frequencies shown here. T-ray Microwires (D << lT-ray ~ mm) provide low loss waveguides for THz transmission Bend loss and surface roughness can be minimized for Microwires  Effective material loss is the dominant loss mechanism for short fiber length Microwires, due to their sizes, are easy to handle Almost zero material dispersion By choosing substrate glass and core diameter, waveguide dispersion can be minimized for a range of frequencies Narrowband low loss and low dispersion fibers Future Work