A THz transverse electromagnetic mode two-dimensional interconnect layer incorporating quasi-optics
S. Coleman, D. Grischkowsky
2003
Applied Physics Letters
We report the demonstration of a planar THz interconnect layer capable of transmitting subpicosecond pulses in the transverse electromagnetic ͑TEM͒ mode over arbitrarily long paths with low absorption and no observable group velocity dispersion. Quasioptical elements are incorporated within the interconnect layer forming a configurable THz bandwidth TEM-mode planar interconnect with negligible group velocity dispersion and low loss. For a 146 mm guided path length, including four reflections,
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... e pulses are broadened by the frequency dependent absorption of the interconnect layer from 0.28 to 0.32 ps, and attenuated by the factor 0.2. Terahertz ͑THz͒ waveguides have recently been demonstrated to be an alternative to high-speed coplanar transmission lines. 1-4 For frequencies up to 3.5 THz, the attenuation coefficient of metal waveguides has been shown to be less than 1/10 that of lithographically defined transmission lines on dielectric substrates. Within the passband of the waveguide, the measured power coupling into the waveguide was typically 40% of the incoming THz power. For the single conductor circular and rectangular metal waveguides the observed excessive THz pulse broadening was caused by the extreme group velocity dispersion ͑GVD͒ near the cutoff frequency. Such pulse broadening does not occur for the transverse electromagnetic ͑TEM͒ mode of a parallel-plate metal waveguide, since it has no cutoff frequency. The group and phase velocities of the TEM mode are determined solely by the dielectric. Recent experiments have shown efficient quasioptic coupling of freely propagating subpicosecond ͑subps͒ pulses of THz radiation into parallel-plate metal waveguides and the subsequent low-loss, single TEM mode propagation exhibiting negligible GVD. 3,4 Consequently, a THz interconnect, 5 capable of propagating subps pulses with minimal loss and no distortion, has been realized. However, all of these demonstrations involved quasioptical input and output coupling of freely propagating THz beams into and out of the THz waveguides. The remaining challenge is to efficiently connect the THz TEM waveguide with integrated circuitry. To address this problem we have begun to study a macroscopic planar interconnect approach using two-dimensional ͑2D͒ quasioptics. The interconnect consists of two relatively large ͑many centimeters͒ metal plates separated from each other by approximately 100 m. Within this 100 m thick, free-space interconnect layer, planar quasioptical components are placed to guide, collimate, or focus the propagating THz TEM waves. This 2D planar quasioptics approach has the potential to realize a spatially localized point-to-point interconnect with the low-loss, broad bandwidth, and negligible GVD of the single-mode THz TEM planar metal waveguides. This 2D interconnect is related to previous works by Mink, in which he proposed and demonstrated the hybrid dielectric slab-beam waveguide ͑HDSBW͒. 6,7 The HDSBW uses two distinct waveguiding principles to guide electromagnetic waves in a dielectric slab. In the direction normal to the slab the guided waves are described by the modes of the slab waveguide, while in the lateral direction the beams freely propagate and can be guided with quasi-optical components. In other work, spatiotemporal electro-optic imaging of propagating subps THz pulses, electro-optically generated by ͑fs͒ optical pulses in 0.5 mm thick nonlinear LiNbO 3 crystals, was performed. 8, 9 For this case dielectric waveguides, diffractive, interferometric, and focusing elements were demonstrated for propagation lengths of several millimeters in LiNbO 3 . Here we report an experimental demonstration of the validity of the 2D interconnect approach, by directing a well characterized beam of subps THz pulses into such a 2D interconnect layer and guiding the beam through the layer with four simple 2D mirrors. The THz pulses in the output beam from the 2D layer showed minimal broadening due to GVD, and their total attenuation coefficient was less than twice the attenuation coefficient of the corresponding planar copper waveguide. The minimal broadening of the output pulses was caused by their frequency dependent attenuation. This successful demonstration of 2D reflective quasioptics implies that more complex reflective optics should allow for confocal, integrated guided wave interconnect structures to be realized. In particular, with the incorporation of focusing optics, efficient point-to-point THz pulse communication within this interconnect plane should be possible. The experimental setup of Fig. 1 consists of a photoconductively switched transmitter and receiver in the standard THz time domain spectroscopy configuration. 1, 4, 5 In this arrangement a frequency dependent beam waist is present in the central confocal position. Here, a lens-interconnect-lens system is placed at this central position. The lenses are highresistivity silicon plano-cylindrical lenses of length 15 mm in the x-direction, 10 mm in the y-direction, with a 5 mm radius of curvature and a thickness of 6.56 mm. At the focus of a͒
doi:10.1063/1.1624474
fatcat:rg2n3xr7mzcklpxn7u5j2fqyle