Measurement of propagation constant in waveguides with wideband coherent terahertz spectroscopy

Hadjiloucas, Sillas; Galvão, Roberto Kawakami Harrop; Bowen, James; Martini, Rainer; Brucherseifer, M.; Pellemans, H.P.M.; Bolívar, P. Haring; Kurz, H.; Digby, J.; Parkhurst, G.M.; Chamberlain, J.M.

doi:10.1364/josab.20.000391

Cited by 13 publications

(17 citation statements)

References 34 publications

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“…͓DOI: 10.1063/1.2011773͔ Recently, there has been an increased interest in the search for a good waveguide for the transportation of terahertz radiation. [1][2][3][4][5][6] The latest development in this field is the propagation of terahertz waves along bare metal wires with very little absorption and dispersion. 7,8 Indeed, measurements were shown in which two metal wires were combined to form what could eventually become a medical probe.…”

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confidence: 99%

Effect of a dielectric coating on terahertz surface plasmon polaritons on metal wires

Valk

Planken

2005

Applied Physics Letters

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The authors present measurements and calculations on the effect of thin dielectric coatings on the propagation of terahertz pulses along the surface of metal wires. Our measurements show that propagation over only a few centimeters of wire having a thin dielectric coating, strongly distorts the terahertz pulse, which results in a several tens of picoseconds long chirped signal. We demonstrate that the terahertz pulses propagate along the wire as surface waves, and show how a thin coating of a nondispersive material makes this propagation strongly dispersive, giving rise to the chirped signal observed in the measurements. Our results show the potential of terahertz surface plasmon polaritons on metal wires for the sensitive detection of thin dielectric layers. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2011773͔ Recently, there has been an increased interest in the search for a good waveguide for the transportation of terahertz radiation. [1][2][3][4][5][6] The latest development in this field is the propagation of terahertz waves along bare metal wires with very little absorption and dispersion. 7,8 Indeed, measurements were shown in which two metal wires were combined to form what could eventually become a medical probe. Many metal wires, however, are not bare. Thin dielectric layers can often be found on the surface of metal wires, applied intentionally or unintentionally through oxidation or contamination, and the effects of these layers on the propagation of terahertz pulses along the wire are not wellknown.Here we show measurements and calculations on the propagation of terahertz pulses over copper wires with and without a thin polyurethane coating. Our time-domain measurements of a terahertz pulse propagating along a 4 cm long wire show that a coating of tens of micrometers thickness strongly distorts the terahertz pulse resulting in a chirped terahertz signal that lasts tens of picoseconds. A comparison with calculations based on Maxwell's equations shows that the terahertz pulses propagate along the wire as surface plasmon polaritons, and that the distortion of the terahertz pulse originates from the dispersive propagation of these waves along the coated wire. Remarkably, the propagation is dispersive, although we assume that the coating material itself has a frequency-independent refractive index. Our work indicates that thin coatings can seriously distort terahertz pulses propagating along metal wires. At the same time, however, this offers the possibility of using metal wires as sensitive detectors of thin layers. Figure 1 shows a schematic drawing of our setup. The terahertz pulses from our emitter are focused onto one of two types of metal wires. The first wire is a bare copper wire with a diameter of 1 mm. The second wire is a copper wire with a diameter of 1 mm having a polyurethane coating, specified to be about 34 m thick.9 A sharp copper needle is used to couple the terahertz radiation onto the wire. 8 The terahertz surface plasmon polariton propagates over the wire toward the detecti...

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confidence: 99%

Effect of a dielectric coating on terahertz surface plasmon polaritons on metal wires

Valk

Planken

2005

Applied Physics Letters

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“…9 shows the insertion loss of a unit length waveguide (as calculated both by the ARX and state-space models) after ratioing the spectra obtained with the backshort adjusted to 3 mm with that at 2 mm (denoted as the case), which is, in fact, the frequency response of the identified models. An effective propagation constant for the multimode propagation can, therefore, be calculated [17]. The observed increased insertion loss at higher frequencies is attributed to the variation of the conductivity of gold with frequency.…”

Section: Resultsmentioning

confidence: 99%

“…Let and be the reflection signatures for the shorter (length , where is a length unit) and longer (length ) waveguides, respectively. Ratioing the spectra and yields the effective complex insertion loss function for a unit length waveguide [17] that is given from , where can be regarded as the frequency response of a linear system, which produces an output for an input . Instead of using the ratioing procedure, which is very sensitive to measurement noise, system identification techniques can be employed to obtain the complex insertion loss.…”

Section: A Arx Modelmentioning

confidence: 99%

“…Firstly, can be obtained directly through a parametric approach [8], [18] in which the unit length waveguide is described by an AutoRegressive with eXogenous input (ARX) model of the form (16) where is the response measured at time (sampling interval femtoseconds), are model coefficients, is the number of poles (equivalent to the system order), is the number of zeros, is a pure time delay, and is a residual. can be discretized as a -domain transfer function (17) by making . The roots of (poles of ) are associated with the modes of the waveguide ( modes in total).…”

Section: A Arx Modelmentioning

confidence: 99%

“…The value of can be accurately estimated after observing the delay in the time-domain signatures recorded by the spectrometer from (18) where [17]. The choice of can be made in an empirical way from cross-correlation of the outputs with the residuals.…”

Section: A Arx Modelmentioning

confidence: 99%

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Comparison of Subspace and ARX Models of a Waveguide's Terahertz Transient Response After Optimal Wavelet Filtering

Hadjiloucas

Galvão

Becerra

et al. 2004

IEEE Trans. Microwave Theory Techn.

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Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract-A quasi-optical deembedding technique for characterizing waveguides is demonstrated using wide-band time-resolved terahertz spectroscopy. A transfer function representation is adopted for the description of the signal in the input and output port of the waveguides. The time-domain responses were discretized and the waveguide transfer function was obtained through a parametric approach in the -domain after describing the system with an AutoRegressive with eXogenous input (ARX), as well as with a state-space model. Prior to the identification procedure, filtering was performed in the wavelet domain to minimize both signal distortion, as well as the noise propagating in the ARX and subspace models. The optimal filtering procedure used in the wavelet domain for the recorded time-domain signatures is described in detail. The effect of filtering prior to the identification procedures is elucidated with the aid of pole-zero diagrams. Models derived from measurements of terahertz transients in a precision WR-8 waveguide adjustable short are presented.

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Pattern identification of biomedical images with time series: Contrasting THz pulse imaging with DCE-MRIs

Yin

Hadjiloucas

Zhang

et al. 2016

Artificial Intelligence in Medicine

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We provide a survey of recent advances in biomedical image analysis and classification from emergent imaging modalities such as terahertz (THz) pulse imaging (TPI) and dynamic contrast-enhanced magnetic resonance images (DCE-MRIs) and identification of their underlining commonalities. Methods: Both time and frequency domain signal pre-processing techniques are considered: noise removal, spectral analysis, principal component analysis (PCA) and wavelet transforms. Feature extraction and classification methods based on feature vectors using the above processing techniques are reviewed. A tensorial signal processing de-noising framework suitable for spatiotemporal association between features in MRI is also discussed. Validation: Examples where the proposed methodologies have been successful in classifying TPIs and DCE-MRIs are discussed. Results: Identifying commonalities in the structure of such heterogeneous datasets potentially leads to a unified multi-channel signal processing framework for biomedical image analysis. Conclusion: The proposed complex valued classification methodology enables fusion of entire datasets from a sequence of spatial images taken at different time stamps; this is of interest from the viewpoint of inferring disease proliferation. The approach is also of interest for other emergent multi-channel biomedical imaging modalities and of relevance across the biomedical signal processing community.

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Measurement of propagation constant in waveguides with wideband coherent terahertz spectroscopy

Cited by 13 publications

References 34 publications

Effect of a dielectric coating on terahertz surface plasmon polaritons on metal wires

Effect of a dielectric coating on terahertz surface plasmon polaritons on metal wires

Comparison of Subspace and ARX Models of a Waveguide's Terahertz Transient Response After Optimal Wavelet Filtering

Pattern identification of biomedical images with time series: Contrasting THz pulse imaging with DCE-MRIs

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