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Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 1 — Jan. 9, 2006
  • pp: 279–290
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Enhanced coupling of terahertz radiation to cylindrical wire waveguides

Jason A. Deibel, Kanglin Wang, Matthew D. Escarra, and Daniel M. Mittleman  »View Author Affiliations


Optics Express, Vol. 14, Issue 1, pp. 279-290 (2006)
http://dx.doi.org/10.1364/OPEX.14.000279


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Abstract

Wire waveguides have recently been shown to be valuable for transporting pulsed terahertz radiation. This technique relies on the use of a scattering mechanism for input coupling. A radially polarized surface wave is excited when a linearly polarized terahertz pulse is focused on the gap between the wire waveguide and another metal structure. We calculate the input coupling efficiency using a simulation based on the Finite Element Method (FEM) Additional FEM results indicate that enhanced coupling efficiency can be achieved through the use of a radially symmetric photoconductive antenna. Experimental results confirm that such an antenna can generate terahertz radiation which couples to the radial waveguide mode with greatly improved efficiency.

© 2006 Optical Society of America

1. Introduction

There have been significant efforts devoted to the development of terahertz waveguides [9–12

9. G. Gallot, S.P. Jamison, R. McGowan, and D. Grischkowsky, “Terahertz Waveguides,” J. Opt. Soc. Am. B 17, 851–863 (2000). [CrossRef]

]. One principal difficulty has been the lack of materials well suited for guided propagation of radiation at these frequencies. Generally, materials that are transparent at terahertz frequencies are crystalline, which precludes their practical use in waveguide structures. Other materials such as glasses and polymers exhibit unacceptable absorption losses at terahertz frequencies. Consequently, many terahertz waveguide techniques rely on the terahertz pulse propagating through air inside of a confined geometry such as a metal tube. While waveguide structures of this type exhibit low losses, they often suffer from significant pulse reshaping due to group velocity dispersion [9

9. G. Gallot, S.P. Jamison, R. McGowan, and D. Grischkowsky, “Terahertz Waveguides,” J. Opt. Soc. Am. B 17, 851–863 (2000). [CrossRef]

]. A notable exception is the parallel-plate metal waveguide, which has been shown to exhibit dispersionless propagation and very low losses [13

13. R. Mendis and D. Grischkowsky, “Undistorted guided-wave propagation of subpicosecond terahertz pulses,” Opt. Lett. 26, 846–848 (2001). [CrossRef]

, 14

14. R. Mendis and D. Grischkowksy, “THz interconnect with low loss and low group velocity dispersion,” IEEE Microwave & Wireless Comp. Lett. 11, 444–446 (2001). [CrossRef]

].

We have recently described another option, based on the propagation of a surface wave on the exterior surface of a metal cylinder. A simple metal wire can be employed as a terahertz waveguide, exhibiting very low loss and negligible dispersion [15

15. K. Wang and D.M. Mittleman, “Guided propagation of terahertz pulses on metal wires,” J. Opt. Soc. Am. B 22, 2001–2008 (2005). [CrossRef]

]. This guiding structure possesses the appealing feature of structural simplicity, which has for example permitted the first report of a terahertz endoscope [16

16. K. Wang and D.M. Mittleman, “Metal wires for terahertz waveguiding,” Nature 432, 376–379 (2004). [CrossRef] [PubMed]

]. However, compared to the earlier reported case of the parallel plate waveguide, the cylindrical symmetry of the wire waveguide poses a significant challenge. The typical linearly polarized terahertz wave has a very poor spatial overlap with the radially polarized guided mode. As a result, direct end-fire input coupling to a wire waveguide is not feasible using a linearly polarized wave.

In the earlier experiments, a scattering mechanism was employed in order to couple linearly polarized terahertz radiation to the radially polarized surface wave that propagates along the metal wire [15

15. K. Wang and D.M. Mittleman, “Guided propagation of terahertz pulses on metal wires,” J. Opt. Soc. Am. B 22, 2001–2008 (2005). [CrossRef]

, 16

16. K. Wang and D.M. Mittleman, “Metal wires for terahertz waveguiding,” Nature 432, 376–379 (2004). [CrossRef] [PubMed]

]. In this letter, we present Finite Element Method (FEM) simulations of this coupling mechanism and use these results to estimate the coupling efficiency of this scattering mechanism. A photoconductive terahertz antenna with radial symmetry is introduced, and additional FEM simulations show that coupling efficiency is significantly enhanced when the polarization profile of the terahertz source is well matched to the dominant mode of the waveguide [9

9. G. Gallot, S.P. Jamison, R. McGowan, and D. Grischkowsky, “Terahertz Waveguides,” J. Opt. Soc. Am. B 17, 851–863 (2000). [CrossRef]

]. Finally, the radially symmetric photoconductive antenna is experimentally evaluated and compared to the FEM data.

2. Efficiency of dual-wire THz coupling experiment

The experimental results from the coupling configuration described above indicated that less than 1% of the power of the incident terahertz beam is coupled to the wire waveguide by this scattering process. Due to experimental difficulties, it has not been possible to obtain a more accurate estimate of the coupling efficiency. Here, we develop a numerical model of the dual-wire THz coupler, using the finite element method (FEM), that demonstrates the nature of the coupling and that also provides a method to more accurately quantify the coupling efficiency.

The wire waveguide is modeled in three dimensions using a commercially available FEM package. Its outer boundary is defined as a perfect electrical conductor (PEC). It should be noted that a more exact model could consider the surface impedance of the wire’s outer boundary and the Drude conductivity of the metal. This would be necessary in order to correctly model the ohmic loss of the wire waveguide [15

15. K. Wang and D.M. Mittleman, “Guided propagation of terahertz pulses on metal wires,” J. Opt. Soc. Am. B 22, 2001–2008 (2005). [CrossRef]

]. However, it is our intention to model only the coupling efficiency of the experiment and not to consider the waveguide’s inherent loss.

A second wire, the coupler wire, is placed in the vicinity of the wire waveguide, but in a direction perpendicular to it. The coupler wire has the same diameter as the waveguide and is also modeled as a PEC. The distance between the closest outer surfaces of the two wires is 0.5 mm. The linearly polarized terahertz pulse is modeled as a plane wave whose k-vector is incident at a 45 degree angle from the axis of the wire waveguide. The plane wave, 2 cm in diameter, is directed such that its center is incident at the center of the gap between the waveguide and coupler wires. The size of the plane wave is chosen to mirror the size of the loosely focused terahertz beam used in the actual scattering experiment. The linear input polarization is chosen such that its direction lies in the same plane (x-z plane) occupied by the long axis of the wire waveguide, again mirroring the experiment. The simulation domain is bounded by a box of air confining the waveguide and coupler wires and the input plane wave. A low-reflecting boundary condition is selected for the outer walls of the domain. This condition is chosen such that any back reflections of the EM waves in the simulation are minimized.

The 3-D simulation domain is bounded by enclosing the waveguide and the coupler wire in a rectangular box 20 cm by 2.5 cm by 2.5 cm. A cylinder (2 cm diameter, 4 cm length) is placed such that its central axis lies at the center of the gap between the waveguide and coupler wires. Part of the cylinder extends outside of the rectangular box, as shown in Fig. 1. In the simulation, the plane wave is excited on the circular face of the cylinder that lies outside of the box. Prior to solving, the 3-D simulation domain is discretized into approximately 1.8 million tetrahedral mesh elements yielding a computational model consisting of 2.1 million degrees of freedom. The large number of mesh elements is due to the need for at least 3, but generally more, mesh elements per wavelength of the propagating radiation [20

20. J. Jin, The Finite Element Method in Electromagnetics (John Wiley ’ Sons, Inc., New York2002).

]. The experiment is simulated using a time-harmonic solver, so only one input frequency is considered at a time. The model problem is solved using an iterative Generalized Minimal Residual (GMRES) iterative solver with Symmetric Successive Overrelaxation (SSOR) matrix preconditioning [21

21. FEMLAB. 2004, COMSOL AB: Stockholm, Sweden.http://www.comsol.com.

, 22

22. R. Barrett, M. Berry, T.F. Chan, J. Demmel, J. Donato, J. Dongarra, V. Eijkhout, R. Pozo, C. Romine, and H.v.d. Vorst, Templates for the Solution of Linear Systems: Building Blocks for Iterative Methods (SIAM, Philadelphia1994). [CrossRef]

]. For a waveguide 15 cm long, a workstation with dual 64-bit processors and 14 GB of RAM arrives at a solution in less than 20 hours.

Fig. 1 (a) FEM Simulation result of 0.1 THz wave coupling to a wire waveguide using a dual-wire coupling configuration (Ex shown here). The top inset shows a close-up view of the electric field (xz-plane) in the coupling region between the two wires, while the right inset shows the electric field at the end of the wire (xy-plane). Note that the majority of the incident wave scatters into free space, and only 0.4% percent of the incident power is coupled to the guided radial mode. Fig. 1(b) shows the x-component of the electric field along a line 300 microns above the wire and parallel to the wire axis. The inset shows a 0.1 THz sine wave fit to the extracted simulation data. This demonstrates that the simulation is discretized with a mesh density fine enough to resolve the oscillating electric field.

Simulation results for a frequency of 0.1 THz can be seen in Fig. 1. The incident plane wave can be seen scattering at the gap between the wire coupler and waveguide. It can be seen from the surface plot that the majority of the plane wave propagates unimpeded and only a small amount of the incident radiation is coupled to the radial mode of the waveguide. A plot of the x-component of the guided wave’s electric field at the end of the wire is also shown highlighting the radial nature of the propagating mode. At the same location, the y-component of the field is similar to the aforementioned plot, but rotated by 90 degrees. Fig. 1(b) is a plot of the x-component of the electric field 300 microns above the wire and along its z-axis. The large peak at z=0 is part of the incident input wave. It can be seen that both forward and backward propagating modes are excited. Recalling that the wires are modeled as PEC’s, we expect little attenuation once the mode is excited, which is consistent with the results of the calculation. A sine wave with a frequency of 0.1 THz can be easily fit to the oscillating electric field along the waveguide, demonstrating that the model is simulating wave propagation.

These simulations clearly demonstrate the propagation of a surface wave along the cylindrical wire, over a distance of 15 cm. On first glance, this may be a surprising result, since it is well known that a perfect conductor cannot support surface waves [17

17. G. Goubau, “Surface waves and their application to transmission lines,” J. Appl. Phys. 21, 1119–1128 (1950). [CrossRef]

, 23

23. F. Yang, J.R. Sambles, and G.W. Bradberry, “Long-range surface modes supported by thin films,” Phys. Rev. B. 44, 5855–5572 (1991). [CrossRef]

]. In our simulations, the boundary condition chosen for the surface of the waveguide is that of a perfect electric conductor (PEC), which means that the electric field component parallel to the surface (Ez) must vanish at the wire surface. As expected, the analytic Sommerfeld model predicts that, under this condition, the wave is unbounded, with amplitude extending infinitely far from the wire surface. We note, however, that a mode with infinite lateral extent would be impossible to excite using a finite-sized input field. We therefore expect that the simulated field is not single mode, but instead consists of a superposition of many modes, all possessing azimuthal symmetry. Of course, each of these modes has a distinct attenuation spectrum, so extracting accurate attenuation coefficients from these simulations is obviously not possible using the PEC condition. Nevertheless, these simulations do demonstrate the coupling of terahertz radiation into a radially polarized mode of the wire waveguide, and the propagation of this mode over several tens of cm.

It is quite simple to calculate the coupling efficiency at 0.1 THz from the simulation results shown in Fig. 1. The incident linearly polarized wave is defined on the base of a cylinder in our model. The outline of the side of the cylinder can be seen in Fig. 1. The coupled input power is defined by integrating the time-averaged power over the area defined by the circular base of the cylinder, where the input wave is introduced into the model. Likewise, the coupled power is determined by integrating the time-averaged power over a circular area normal to the z-axis at the end of the waveguide (15 cm from the point of excitation). These calculations produce a simulated power coupling efficiency for the dual wire coupling configuration of 0.42%, comparable to the estimate generated from experimental data [15

15. K. Wang and D.M. Mittleman, “Guided propagation of terahertz pulses on metal wires,” J. Opt. Soc. Am. B 22, 2001–2008 (2005). [CrossRef]

]. It should be noted that this number is probably the upper limit on any expected coupling efficiency. The wires in this simulation were modeled as PEC’s, so no loss due to finite conductivity was considered. Finally, while the efficiency is expected to vary with wavelength, wire separation, etc., there is no reason to expect it to increase greatly, since the polarization of the waveguide mode and the incident wave are so poorly mismatched.

3. Enhanced THz wire waveguide coupling

3.1 Photoconductive terahertz antenna with radial symmetry

Fig. 2 (a) Photoconductive antenna with radial symmetry (b) FEM simulation of the power emitted by an “ideal” radial antenna in free space (i.e., no substrate) at 0.5 THz. The antenna sits at the center of the sphere in the xy-plane. The emitted field is zero along the z-axis, as expected for a ‘donut’ mode.

The radially symmetric photoconductive antenna was simulated with the same three-dimensional electromagnetic wave FEM package used to model the dual-wire THz waveguide coupler. The center of the antenna consists of a circular electrode 5 μm in diameter, fed by an electrode 1 μm wide that approaches the center from the negative y-axis direction. The separation between the center electrode and the 10 μm wide outer electrode is 100 μm. At the bottom of the antenna where the outer ring electrode approaches the feed electrode, there is a 7.5 μm gap on each side of the feed electrode. To simplify the model, the boundaries of the electrodes are defined as PEC’s. In order to model an actual experimental configuration, the antenna is placed at the interface between a section of air and a 0.5 mm thick dielectric substrate, with a dielectric constant of 13, roughly equal to that of GaAs. As is the case for standard THz-TDS [27

27. C. Fattinger and D. Grischkowsky, “Terahertz beams,” Appl. Phys. Lett. 54, 490–492 (1989). [CrossRef]

], a substrate-matched aplanatic hyperhemispherical (not collimating) silicon dome (2 mm radius) is placed on the other side of the GaAs substrate in order to couple the beam into free space and collimate it. For comparison, an idealized radial antenna (with no feed electrode and no gap in the outer circular electrode) with a 100 μm radius is also placed on the same GaAs substrate with the same silicon dome configuration.

For the modeling of the antenna configuration, we use a two step approach, in which we model first the DC fields present in the antenna and use those results to simulate the generation of terahertz radiation [21

21. FEMLAB. 2004, COMSOL AB: Stockholm, Sweden.http://www.comsol.com.

]. In the first step, the outer electrode is grounded and a potential is applied to the center electrode. A charge density with a Gaussian distribution and a 1/e width of 40 μm is placed at the center of the antenna. This mimics the charge carriers generated in the GaAs by the optical pump pulse in a typical photoconductive generation scheme. The electrostatic fields are computed using the FEM solvers and then those fields are used as the time-varying input fields for the electromagnetic wave propagation model. In all, these models typically consisted of approximately 90,000 mesh elements and were run in less than 4 hours on Pentium PC with 2 GB of RAM.

The results of the radial antenna simulations are presented in Fig. 3. All four plots show the generated radiation fields at 0.1 THz in the x-y plane immediately after the silicon dome, which is visible in addition to the GaAs substrate. Figures 3(a) and 3(b) show the horizontal and vertical components of the generated electric field from the idealized radial antenna. The generated field from the idealized antenna is perfectly radial. The polarization of the generated field from the actual radial antenna structure (Fig. 3(c) and 3(d)) is not perfectly radially oriented. The lack of radial symmetry in the field generated by the actual antenna is caused by the lack of symmetry in the actual antenna design. Both the break in the outer electrode and the presence of the feed electrode create asymmetry in the lower half of the antenna structure. This results in the y-component of the electric field being stronger in amplitude than the x-component and the strongest parts of the x-component of the generated field not being centered on the antenna.

Fig. 3. FEM simulation results of the radial antenna in a typical THz configuration. (a) & (b) Plots of the x and y component of the electric field for the idealized radial antenna. (c) & (d) Plots of the x and y component of the electric field for the actual radial antenna.

3.2 Simulation of enhanced wire waveguide coupling

Fig. 4. FEM simulation model of radial antenna coupling to a wire waveguide (a) Plot of Ex at the end of a waveguide coupled to an ideal radial antenna. (b) Plot of Ey at the end of a waveguide coupled to an ideal radial antenna. (c) Plot of Ex at the end of a waveguide coupled to an actual radial antenna. (d) Plot of Ey at the end of a waveguide coupled to an actual radial antenna.

FEM simulations at 0.1 THz of the idealized and actual radial antennas coupled to wire waveguides can be seen in Fig. 4. Plots of the field at the end of the wire from the idealized antenna results show that the idealized antenna is capable of exciting the low-order radial mode of the wire waveguide. Likewise, while less power is coupled to the waveguide, the actual antenna design is also shown to be capable of exciting the radial mode. A more quantitative observation can be obtained by performing a coupling efficiency calculation similar to the one performed in Section 2. The power available to be coupled into the waveguide is calculated by integrating the time-averaged power over a boundary in the x-y plane immediately after the silicon dome. This is the same plane where the electric field is plotted in Fig. 3(c) and 3(d). The power coupled into the waveguide is determined in a similar manner. The time-averaged power is integrated over a boundary at the end of the waveguide and normal to it. The areas integrated over in both cases are identical. For the actual radial antenna design, this calculation yields a coupling efficiency of approximately 56%, an improvement of more than 2 orders of magnitude over the 0.4 % coupling efficiency obtained via the scattering mechanism discussed earlier. Simulations with the waveguide removed were also performed. For both the case of the idealized and actual antenna structures, only a very tiny amount of power propagated to the end of the simulation domain.

Fig. 5. Setup for experimental testing of the radially symmetric terahertz emitter antenna. The emitter was pumped with a free-space beam. A 0.9 mm diameter, 27 cm long wire waveguide was end-coupled to the silicon dome of the emitter. THz radiation was detected at the end of the waveguide by a fiber-coupled LT-GaAs detector sensitive to only the horizontal polarization component.

4. Experimental results

Fig. 6. The wire waveguide end-coupled to the silicon dome, which is mounted on the opposite side of the GaAs substrate from the radial antenna.

Fig. 7. Time-domain measurements of the terahertz pulse detected 27 cm away from the radial emitter. Measurements have been performed with the wire waveguide both present (7a) and absent (7b). All other experimental parameters are unchanged. The terahertz pulse exhibits a polarity reversal when measured at opposite locations at the end of the waveguide (upper and lower curves in (a)), a hallmark of the guided mode’s radial polarization.

A qualitative determination of the coupling efficiency of the radial antenna-wire waveguide configuration can be obtained by comparing the two sets of results shown in Fig. 7. At the +3 mm horizontal offset, the peak-to-peak amplitude of the terahertz signal measured at the end of the waveguide is approximately 20 times larger than that for the pulse measured at the same position when the waveguide is not present. While that number does not represent the actual power coupling efficiency since these waveforms were measured at only a single point in space, it provides some insight into the considerable advantages of using the radial antenna-wire waveguide configuration.

4. Conclusion

In conclusion, we have performed finite element method simulations of terahertz experiments in which terahertz beams were scattered at an intersection of two wires. We simulate the same phenomenon observed experimentally, namely that the scattering process excites the low order radial mode of the wire waveguide. Calculations indicate that the coupling efficiency in this simulation, 0.4%, is comparable to earlier experimental estimates. We have shown the importance of spatial overlap between the polarization of the input terahertz beam and the radial mode of the wire waveguide. Simulation results showed that a novel photoconductive antenna with radial symmetry can be used in place of the standard linear dipole antennas to achieve coupling efficiencies greater than 50%. Integrating our antenna design into the standard substrate + silicon dome configuration also uses the vast majority of the generated terahertz radiation. Such antennas have been fabricated and tested. The largely radially polarized beam coupled to a wire waveguide effectively. A radially polarized mode was excited and guided over the length of the 27 cm wire waveguide. The magnitude of the peak terahertz electric field detected at the end of the waveguide was twenty times larger than the radiation detected at the same position with the waveguide removed. This is an important demonstration of the effectiveness of using a radially symmetric photoconductive terahertz antenna end-coupled to a metallic wire waveguide for convenient and efficient guiding of terahertz radiation.

Acknowledgments

This work has been funded in part by the National Science Foundation, the R. A. Welch Foundation, and the Intelligence Community Postdoctoral Fellowship Program.

References and Links

1.

D. Mittleman and e., Sensing with Terahertz Radiation (Springer-Verlag, Heidelberg2002).

2.

P.R. Smith, D.H. Auston, and M.C. Nuss, “Subpicosecond photoconducting dipole antennas,” IEEE J. Quant. Elec. 24, 255–260 (1988). [CrossRef]

3.

M.v. Exter and D. Grischkowsky, “Characterization of an optoelectronic terahertz beam system,” IEEE Trans. Microwave Th. Tech. 38, 1684–1691 (1990). [CrossRef]

4.

K. Kawase, Y. Ogawa, and Y. Watanabe, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Express 11, 2549–2554 (2003). [CrossRef] [PubMed]

5.

D. Crawley, C. Longbottom, V.P. Wallace, B. Cole, D.D. Arnone, and M. Pepper, “Three-dimensional terahertz pulse imaging of dental tissue,” J. Biomed. Opt. 8, 303–307 (2003). [CrossRef] [PubMed]

6.

R.M. Woodward, V.P. Wallace, D.D. Arnone, E.H. Linfield, and M. Pepper, “Terahertz pulsed imaging of skin cancer in the time and frequency domain,” J. Biol. Phys. 29, 257–261 (2003). [CrossRef]

7.

R.H. Jacobsen, D.M. Mittleman, and M.C. Nuss, “Chemical recognition of gases and gas mixtures with terahertz waves,” Opt. Lett. 21, 2011–2013 (1996). [CrossRef] [PubMed]

8.

S. Wang and X.-C. Zhang, “Pulsed terahertz tomography,” J. Phys. D 37, R1–R36 (2004). [CrossRef]

9.

G. Gallot, S.P. Jamison, R. McGowan, and D. Grischkowsky, “Terahertz Waveguides,” J. Opt. Soc. Am. B 17, 851–863 (2000). [CrossRef]

10.

J.A. Harrington, R. George, P. Pedersen, and E. Mueller, “Hollow polycarbonate waveguides with inner Cu coatings for delivery of terahertz radiation,” Opt. Express 12, 5263–5268 (2004). [CrossRef] [PubMed]

11.

H. Han, H. Park, M. Cho, and J. Kim, “Terahertz pulse propagation in a plastic photonic crystal fiber,” Appl. Phys. Lett. 80, 2634–2636 (2002). [CrossRef]

12.

M. Goto, A. Quema, H. Takahashi, S. Ono, and N. Sarukura, “Teflon photonic crystal fiber as terahertz waveguide,” Jpn. J. Appl. Phys. 43, L317–L319 (2004). [CrossRef]

13.

R. Mendis and D. Grischkowsky, “Undistorted guided-wave propagation of subpicosecond terahertz pulses,” Opt. Lett. 26, 846–848 (2001). [CrossRef]

14.

R. Mendis and D. Grischkowksy, “THz interconnect with low loss and low group velocity dispersion,” IEEE Microwave & Wireless Comp. Lett. 11, 444–446 (2001). [CrossRef]

15.

K. Wang and D.M. Mittleman, “Guided propagation of terahertz pulses on metal wires,” J. Opt. Soc. Am. B 22, 2001–2008 (2005). [CrossRef]

16.

K. Wang and D.M. Mittleman, “Metal wires for terahertz waveguiding,” Nature 432, 376–379 (2004). [CrossRef] [PubMed]

17.

G. Goubau, “Surface waves and their application to transmission lines,” J. Appl. Phys. 21, 1119–1128 (1950). [CrossRef]

18.

T.-I. Jeon, J. Zhang, and D. Grischkowsky, “THz Sommerfeld wave propagation on a single metal wire,” Appl. Phys. Lett. 86, (2005). [CrossRef]

19.

Q. Cao and J. Jahns, “Azimuthally polarized surface plasmons as effective terahertz waveguides,” Opt. Express 13, 511–518 (2005). [CrossRef] [PubMed]

20.

J. Jin, The Finite Element Method in Electromagnetics (John Wiley ’ Sons, Inc., New York2002).

21.

FEMLAB. 2004, COMSOL AB: Stockholm, Sweden.http://www.comsol.com.

22.

R. Barrett, M. Berry, T.F. Chan, J. Demmel, J. Donato, J. Dongarra, V. Eijkhout, R. Pozo, C. Romine, and H.v.d. Vorst, Templates for the Solution of Linear Systems: Building Blocks for Iterative Methods (SIAM, Philadelphia1994). [CrossRef]

23.

F. Yang, J.R. Sambles, and G.W. Bradberry, “Long-range surface modes supported by thin films,” Phys. Rev. B. 44, 5855–5572 (1991). [CrossRef]

24.

J.V. Rudd, J.L. Johnson, and D.M. Mittleman, “Cross-polarized angular emission patterns from lenscoupled terahertz antennas,” J. Opt. Soc. Am. B 18, 1524–1533 (2001). [CrossRef]

25.

H. Cao and A. Nahata, “Coupling of terahertz pulses onto a single metal wire waveguide using milled grooves,” Opt. Express 13, 7028–7034 (2005). [CrossRef] [PubMed]

26.

J.A. Deibel, M.D. Escarra, and D.M. Mittleman, “Photoconductive terahertz antenna with radial symmetry,” Elect. Ltrs. 41, 226–228 (2005). [CrossRef]

27.

C. Fattinger and D. Grischkowsky, “Terahertz beams,” Appl. Phys. Lett. 54, 490–492 (1989). [CrossRef]

28.

P. Jepsen and S.R. Keiding, “Radiation patterns from lens-coupled terahertz antennas,” Opt. Lett. 20, 807–809 (1995). [CrossRef] [PubMed]

OCIS Codes
(230.7370) Optical devices : Waveguides
(240.6690) Optics at surfaces : Surface waves
(260.3090) Physical optics : Infrared, far
(320.7160) Ultrafast optics : Ultrafast technology

ToC Category:
Optical Devices

Citation
Jason A. Deibel, Kanglin Wang, Matthew D. Escarra, and Daniel Mittleman, "Enhanced coupling of terahertz radiation to cylindrical wire waveguides," Opt. Express 14, 279-290 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-1-279


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References

  1. D. Mittleman, ed., Sensing with Terahertz Radiation (Springer-Verlag, Heidelberg 2002).
  2. Smith, P. R., D. H. Auston, and M. C. Nuss, "Subpicosecond photoconducting dipole antennas," IEEE J. Quant. Elec. 24, 255-260 (1988). [CrossRef]
  3. Exter, M.V. and D. Grischkowsky, "Characterization of an optoelectronic terahertz beam system," IEEE Trans. Microwave Th. Tech. 38, 1684-1691 (1990). [CrossRef]
  4. Kawase, K., Y. Ogawa, and Y. Watanabe, "Non-destructive terahertz imaging of illicit drugs using spectral fingerprints," Opt. Express 11, 2549-2554 (2003) <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-20-2549">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-20-2549</a>. [CrossRef] [PubMed]
  5. Crawley, D., C. Longbottom, V. P. Wallace, B. Cole, D. D. Arnone, and M. Pepper, "Three-dimensional terahertz pulse imaging of dental tissue," J. Biomed. Opt. 8, 303-307 (2003). [CrossRef] [PubMed]
  6. Woodward, R. M., V. P. Wallace, D. D. Arnone, E. H. Linfield, and M. Pepper, "Terahertz pulsed imaging of skin cancer in the time and frequency domain," J. Biol. Phys. 29, 257-261 (2003). [CrossRef]
  7. Jacobsen, R. H., D. M. Mittleman, and M. C. Nuss, "Chemical recognition of gases and gas mixtures with terahertz waves," Opt. Lett. 21, 2011-2013 (1996). [CrossRef] [PubMed]
  8. Wang, S. and X.-C. Zhang, "Pulsed terahertz tomography," J. Phys. D 37, R1-R36 (2004). [CrossRef]
  9. Gallot, G., S. P. Jamison, R. McGowan, and D. Grischkowsky, "Terahertz Waveguides," J. Opt. Soc. Am. B 17, 851-863 (2000). [CrossRef]
  10. Harrington, J. A., R. George, P. Pedersen, and E. Mueller, "Hollow polycarbonate waveguides with inner Cucoatings for delivery of terahertz radiation," Opt. Express 12, 5263-5268 (2004) <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-21-5263">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-21-5263</a>. [CrossRef] [PubMed]
  11. Han, H., H. Park, M. Cho, and J. Kim, "Terahertz pulse propagation in a plastic photonic crystal fiber," Appl. Phys. Lett. 80, 2634-2636 (2002). [CrossRef]
  12. Goto, M., A. Quema, H. Takahashi, S. Ono, and N. Sarukura, "Teflon photonic crystal fiber as terahertz waveguide," Jpn. J. Appl. Phys. 43, L317-L319 (2004). [CrossRef]
  13. Mendis, R. and D. Grischkowsky, "Undistorted guided-wave propagation of subpicosecond terahertz pulses," Opt. Lett. 26, 846-848 (2001). [CrossRef]
  14. Mendis, R. and D. Grischkowksy, "THz interconnect with low loss and low group velocity dispersion," IEEE Microwave & Wireless Comp. Lett. 11, 444-446 (2001). [CrossRef]
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