Guided-wave THz time-domain spectroscopy of highly doped silicon using parallel-plate waveguides

Abstract

A novel spectroscopy technique that uses parallel-plate waveguides for the characterisation of highly conductive materials in the terahertz (THz) frequency regime is presented. This guided-wave technique resolves some of the fundamental problems associated with standard THz time-domain spectroscopy (THz-TDS) as applied to these optically dense materials. The technique is demonstrated by measuring the conductivity of highly phosphorus doped silicon.Introduction: Ever since undistorted subpicosecond terahertz (THz) pulse propagation was demonstrated using parallel-plate metal waveguides [1,2] there has been considerable interest in these waveguiding structures for THz applications. These parallel-plate structures have been used to demonstrate two-dimensional interconnect layers [3], sense nanometre water layers [4], study photonic crystals [5], and for building biosensing systems [6]. This Letter describes another novel use of these structures for the characterisation of highly conductive, optically dense materials resolving some of the fundamental problems associated with standard THz time-domain spectroscopy (THz-TDS) [7,8].THz-TDS is generally carried out in two configurations, one where the THz beam is transmitted through the sample [7], the other where the beam is reflected off of the sample [8]. The transmission method is not effective for highly conductive materials, as the sample thickness has to be reduced to impractical limits to obtain a measurable signal. In this case, the reflection method is preferred, provided it is possible to discriminate the sample signal from the reference signal. If this is not possible, this would also breakdown. Furthermore, obtaining precise sample and reference positioning for the reflection method is also quite challenging [8]. The guided-wave spectroscopy technique presented here overcomes these problems, and would be ideal for materials such as highly doped semiconductors, superconductors, and conducting polymers. To demonstrate the technique, highly phosphorus-doped silicon (Si) with a carrier density >10 18 cm À3 is used as the candidate material.

Global modeling of carrier-field dynamics in semiconductors using EMC–FDTD

“…Since the advent of THz time-domain spectroscopy (THz-TDS) 20 years ago, extensive experimental work has examined the THz characteristics of doped bulk silicon [72][73][74][75][76][77][78][79][80][81][82][83][84]. Pure undoped silicon is almost completely transparent and nondispersive under THz-frequency radiation, more so than quartz, sapphire, or fused silica, making silicon a very attractive THz-optics material [73].…”

“…[79]. While careful implementation of reflecting THz-TDS produces excellent results, the sensitivity of the technique has prompted research into other THz-regime characterization methods [81][82][83][84].…”

A terahertz band-pass resonator based on enhanced reflectivity using spoof surface plasmons

“…We choose a PPWG as a design platform because of its popularity as a prototype guided wave structure in the THz region. With a TEM mode having no cut-off, negligible group velocity dispersion, and low ohmic losses, the PPWG has proven to be a useful platform for many applications involving THz pulses [21][22][23][24][25]. As is well known, an abruptly terminated PPWG exhibits a reflection at the output end, resulting from an impedance mismatch between the waveguide and free space [26,27].…”

Multifaceted terahertz applications of parallel-plate waveguide: TE 1 mode

“…Undistorted pulse propagation was achieved by exciting the waveguide's dominant transverse-electromagnetic (TEM) mode that exhibits virtually no groupvelocity-dispersion (GVD) due to the absence of a low-frequency cutoff. This capability to propagate 'clean' THz pulses within a two-dimensional metallic environment has enabled numerous THz applications including pulse generation [3,4], spectroscopy [5][6][7], sensing [8,9], imaging [10,11], signal processing [12], and even super-focusing [13].Recently, we demonstrated THz pulse propagation by exciting the waveguide's lowest-order transverse-electric (TE 1 ) mode, which was not previously considered to be a viable wave-guiding option owing to the presence of a low-frequency cutoff. This cutoff causes spectral filtering and introduces high GVD that results in undesirable broadening and reshaping of the input THz pulses.…”