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Coupling of terahertz pulses onto a single metal wire waveguide using milled grooves

Hua Cao and Ajay Nahata

Open Access Open Access

Abstract

We demonstrate a novel approach for coupling freely propagating THz pulses onto a metal wire waveguide utilizing grooves fabricated directly into the metal. Using broadband THz pulses incident on the wire, we use metal segments containing zero, one, three, and eight uniformly spaced grooves to launch surface propagating multi-cycle pulses along the wire. We observe a one-to-one correspondence between the groove number and the number of oscillations in the THz waveform radiated from the end of the waveguide. We further demonstrate that this coupled radiation is radially polarized. Although the cross-sectional parameters of the grooves are identical in the present measurements, alteration of the individual grooves in a controlled manner should allow for arbitrarily shaped THz pulses to be launched on the waveguide.

©2005 Optical Society of America

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Figures (5)
Fig. 1.
Fig. 1. Properties of the grooves milled into the cylindrical metal wire (a) The relationship between a large area metal film with inscribed linear grooves and a cylindrical wire with circumferentially milled grooves. Using the former sample, with slightly different groove patterns [21,22], we showed that each groove was able to couple the incident THz radiation to a propagating surface wave. By using multiple grooves, multiple oscillations, delayed in time from one another in accordance with the groove separation, would coherently superpose. If we roll the sheet about the axis perpendicular to the groove length, we will have a solid cylindrical metal conductor with circumferentially milled grooves that should allow for coupling of THz radiation. (b) Photograph of a section of the 1 mm diameter stainless steel wire containing 3 milled grooves.

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Fig. 2.
Fig. 2. Schematic diagram of the experimental setup. The emitter was oriented so that the resulting THz radiation was polarized in the plane of the schematic diagram and parallel to the wire length. The fiber-fed photoconductive detector was offset from the center of the wire by approximately 3 mm in order to measure the radially polarized THz electric field. The wire length, from the excitation region to the end (near detector), was approximately 5 cm.

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Fig. 3.
Fig. 3. (a) Measured time-domain waveform of THz pulse incident on the wire. The waveform was measured by replacing the cylindrical lens and wire with the photoconductive detector (b) Corresponding normalized amplitude spectrum.

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Fig. 4.
Fig. 4. (a) Measured time-domain THz waveforms for THz pulses coupled to a 1 mm diameter stainless steel wire with eight grooves (red trace), three grooves (blue traces), one groove (green traces), and no grooves (black trace). Broadband THz radiation is coupled to the wire via multiple periodically spaced grooves. The rectangular cross-section grooves are 500 μm wide and 100 μm deep with a center-to-center spacing of 1 mm. The waveforms have been offset from the origin for clarity (b) Corresponding normalized amplitude spectra using the same color scheme noted above.

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Fig. 5.
Fig. 5. Measured time-domain THz waveforms for THz pulses coupled to a 1 mm diameter stainless steel wire with 3 grooves for two different detection points. The blue trace is the data shown in Fig. 4(a) with the photoconductive detector located 3 mm to one side of the wire center. The black trace corresponds to the observed waveform taken with the detector placed 3 mm to the other side of the wire center. The inversion of the observed waveform with the change in the detector position demonstrates clearly the radial polarization of the wave.

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