Enhancing the stability of a continuous-wave terahertz system by photocurrent normalization

Roggenbuck, A.; Langenbach, M.; Thirunavukkuarasu, Komalavalli; Schmitz, Holger; Deninger, A.; Mayorga, Iván Cámara; Güsten, R.; Grüninger, M.

doi:10.1364/josab.30.001397

Cited by 10 publications

(3 citation statements)

References 12 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…1, for details we refer to Refs. [9][10][11][12]. Continuous-wave terahertz radiation with frequency ν = |ν 2 − ν 1 | is generated and coherently detected by illuminating two photomixers, transmitter and receiver, with the optical beat of two tunable near-infrared lasers with frequencies ν 1 and ν 2 .…”

Section: Methodsmentioning

confidence: 99%

Group Delay in THz Spectroscopy with Ultra-Wideband Log-Spiral Antennae

Langenbach

Roggenbuck

Mayorga

et al. 2014

J Infrared Milli Terahz Waves

View full text Add to dashboard Cite

We report on the group delay observed in continuous-wave terahertz spectroscopy based on photomixing with phase-sensitive homodyne detection. We discuss the different contributions of the experimental setup to the phase difference ϕ(ν) between transmitter arm and receiver arm. A simple model based on three contributions yields a quantitative description of the overall behavior of ϕ(ν). Firstly, the optical path-length difference gives rise to a term linear in frequency ν. Secondly, the ultra-wideband log-spiral antennae effectively radiate and receive in a frequencydependent active region, which in the most simple model is an annular area with a circumference equal to the wavelength. The corresponding term changes by roughly 6π between 100 GHz and 1 THz. The third contribution stems from the photomixer impedance. In contrast, the derivative ∂ ϕ/∂ν is dominated by the contribution of periodic modulations of ϕ(ν) caused by standing waves, e.g., in the photomixers' Si lenses. Furthermore, we discuss the Fourier-transformed spectra, which are equivalent to the waveform in a time-domain experiment. In the time domain, the group delay introduced by the log-spiral antennae gives rise to strongly chirped signals, in which low frequencies are delayed. Correcting for the contributions of antennae and photomixers yields sharp peaks or "pulses" and thus facilitates a time-domain-like analysis of our continuous-wave data.

show abstract

Section: Methodsmentioning

confidence: 99%

Group Delay in THz Spectroscopy with Ultra-Wideband Log-Spiral Antennae

Langenbach

Roggenbuck

Mayorga

et al. 2014

J Infrared Milli Terahz Waves

View full text Add to dashboard Cite

show abstract

“…Details of the setup were described in Refs. [31][32][33], and a quantitative description of the overall phase behavior in our setup was reported in Ref. [34], focusing on the photomixers and the log-spiral antennae.…”

Section: Setupmentioning

confidence: 99%

Terahertz measurements on subwavelength-size samples down to the tunneling limit

Maluski,

Mayorga,

Hemberger

et al. 2022

Preprint

View full text Add to dashboard Cite

For terahertz spectroscopy on single crystals, the wavelength λ often is comparable to the size of the studied samples, emphasizing diffraction effects. Using a continuous-wave terahertz spectrometer in transmission geometry, we address the effect of the sample size on the achievable accuracy of the optical properties, focusing in particular on the phase data. We employ α-lactose monohydrate as a paradigmatic example and compare data that were measured using apertures with diameters D in the range from 10 mm to 0.2 mm. For small D, strong diffraction typically invalidates a quantitative analysis of the transmitted amplitude at low frequencies. The phase data, however, can be evaluated to lower frequency and show a more systematic dependence on D. For a quantitative analysis, we employ a waveguide picture for the description of small apertures with a cylindrical bore. For D as small as 0.2 mm, corresponding to 1/D = 50 cm −1 , a circular waveguide does not support propagating waves below its cut-off frequency 1/λc = ωc/2πc ≈ 29 cm −1 . Experimentally, we confirm this cut-off for cylindrical apertures with a thickness dap = 1 mm. Close to ωc, the measured phase velocity is an order of magnitude larger than c, the speed of light in vacuum. The cut-off is washed out if a sample is mounted on a thin aperture with a conical bore. In this case, the phase data of α-lactose monohydrate for D = 0.2 mm can quantitatively be described down to about 10 cm −1 if the waveguide-like properties of the aperture are taken into account in the analysis.

show abstract

“…All these quantities are sensitive to temperature drift. Negative impact of this phenomenon to the measured spectra can be effectively reduced by normalizing the signal amplitude to the direct photocurrents in both the transmitter and receiver photomixers [9]. In addition, the standing waves originating from the multiple reflections between the photomixers [5] as well as inside and between other optical components (lenses, connectors [10] and optical amplifier [11]) produce coherent noise [12] in the THz signal and might conceal actual features of the investigated samples.…”

Section: Introductionmentioning

confidence: 99%

Standing Wave Pattern Suppression in the Terahertz Frequency Domain Spectroscopy by Means of Windowed Fourier Filtering

Shuba,

Seliuta,

Abramova

2024

J Infrared Milli Terahz Waves

View full text Add to dashboard Cite

Terahertz frequency domain spectroscopy provides a superior resolution in the broad range from 50 GHz to 5 THz. However, temporal frequency drift and the standing wave pattern in the spectrometer, which often act simultaneously, produce intensive coherent noise in the transmittance spectra of the measured samples. The paper presents a spectrum processing method allowing to reduce the coherent noise and thus significantly enhance the accuracy of spectral data analysis. The idea of the method is to remove the small-period (0.25 GHz) oscillations in the standing wave pattern of the measured signal by applying the windowed Fourier filtering. The large-period (4 GHz) oscillations in the measured spectra are broadened and then used to compensate the frequency drift of the two consecutive measurements of the signal with the sample and without it. The proposed approach is tested on the measured spectra of metamaterial and silicon wafer. Its advantage over the classical method based on the averaging of adjacent data points is confirmed. Our work benefits to optimization of the frequency-domain terahertz systems and paves the way for fast and accurate analysis of spectral data.

show abstract

Enhancing the stability of a continuous-wave terahertz system by photocurrent normalization

Cited by 10 publications

References 12 publications

Group Delay in THz Spectroscopy with Ultra-Wideband Log-Spiral Antennae

Group Delay in THz Spectroscopy with Ultra-Wideband Log-Spiral Antennae

Terahertz measurements on subwavelength-size samples down to the tunneling limit

Standing Wave Pattern Suppression in the Terahertz Frequency Domain Spectroscopy by Means of Windowed Fourier Filtering

Contact Info

Product

Resources

About