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Related Concept Videos

Aliasing01:18

Aliasing

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Accurate signal sampling and reconstruction are crucial in various signal-processing applications. A time-domain signal's spectrum can be revealed using its Fourier transform. When this signal is sampled at a specific frequency, it results in multiple scaled replicas of the original spectrum in the frequency domain. The spacing of these replicas is determined by the sampling frequency.
If the sampling frequency is below the Nyquist rate, these replicas overlap, preventing the original...
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Atomic Absorption Spectroscopy: Interference01:25

Atomic Absorption Spectroscopy: Interference

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Interference leads to systematic error in atomic absorption (AA) measurements by enhancing or diminishing the analytical signal or the background. These interferences can be grouped into three main categories: spectral interference, chemical interference, and physical interference.
Spectral interference occurs when signals from other elements or molecules overlap with the analyte signal, falsely elevating or masking the analyte's absorbance. This interference can be corrected using Zeeman,...
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¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
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NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

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The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton...
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UV–Vis Spectrometers01:14

UV–Vis Spectrometers

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The absorbance of UV and visible (UV–visible) radiations is measured using a UV–visible spectrophotometer. Deuterium lamps, which emit UV radiation, and tungsten lamps, which produce radiation in the visible region, are used as light sources in UV–visible spectrophotometers. A monochromator or prism is used for diffraction grating, i.e., to split the incoming radiation into different wavelengths. A system of slits is used to focus the desired wavelength on the sample cell.
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Bandpass Sampling01:17

Bandpass Sampling

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In signal processing, bandpass sampling is an effective technique for sampling signals that have most of their energy concentrated within a narrow frequency band. This type of signal is known as a bandpass signal. The key principle of bandpass sampling involves sampling the signal at a rate that is greater than twice the signal's bandwidth to prevent aliasing.
A bandpass signal has a spectrum with a lower frequency limit, denoted as ω1, and an upper frequency limit, denoted as ω2....
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Related Experiment Video

Updated: Dec 25, 2025

Simulation, Fabrication and Characterization of THz Metamaterial Absorbers
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Simulation, Fabrication and Characterization of THz Metamaterial Absorbers

Published on: December 27, 2012

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Spurious Absorption Frequency Appearance Due to Frequency Conversion Processes in Pulsed THz TDS Problems.

Vyacheslav A Trofimov1,2, Nan-Nan Wang1, Jing-Hui Qiu1

  • 1Department of Microwave Engineering, Harbin Institute of Technology, Harbin 150001, China.

Sensors (Basel, Switzerland)
|April 2, 2020
PubMed
Summary

Spurious absorption frequencies arise from frequency conversion during terahertz (THz) pulse propagation. This phenomenon, confirmed experimentally, can aid in substance detection and identification.

Keywords:
THz CW radiationbroadband THz pulsedetection and identification of substancefrequency conversionfrequency doublinglinear combination of the absorption frequenciespulsed THz Time-Domain Spectroscopysubstance emission

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Area of Science:

  • Terahertz (THz) spectroscopy
  • Nonlinear optics
  • Physical chemistry

Background:

  • Broadband terahertz (THz) pulse propagation through various media can exhibit unexpected spectral features.
  • Frequency conversion processes, including frequency doubling and sum/difference frequency generation, are known to occur in nonlinear or non-instantaneous media.
  • Understanding these spectral artifacts is crucial for accurate material analysis.

Purpose of the Study:

  • To theoretically and experimentally investigate the appearance of spurious absorption frequencies during broadband THz pulse propagation.
  • To elucidate the mechanisms behind these spurious frequencies, specifically relating to nonlinear and non-instantaneous medium responses.
  • To explore the potential application of this phenomenon for substance detection and identification.

Main Methods:

  • Theoretical analysis of frequency conversion processes in THz pulse propagation.
  • Experimental validation using terahertz continuous wave (THz CW) signals and broadband THz pulses transmitted through various substances.
  • Computer simulations to analyze the correlation between spectral intensities of fundamental and generated frequencies.

Main Results:

  • Spurious absorption frequencies were observed and confirmed to be caused by frequency doubling and sum/difference frequency generation.
  • These effects are linked to the nonlinear or non-instantaneous response of the medium.
  • Computer simulations demonstrated a high correlation between the time-dependent spectral intensities of the fundamental and generated frequencies.

Conclusions:

  • The observed spurious absorption frequencies are a direct consequence of nonlinear frequency conversion during THz pulse propagation.
  • The correlation between spectral intensities provides a signature of the frequency conversion process.
  • This unique feature of frequency conversion holds promise for developing novel methods for substance detection and identification.