Terahertz time-domain spectroscopy is used to study properties of nonpolar amorphous materials. Terahertz absorption spectra and refractive indices were measured in a number of glasses, lubricating oils, and polymers, and the results were correlated with material properties.

Terahertz Time-Domain Spectroscopy for Material Characterization

Terahertz time-domain spectroscopy (THz-TDS) is a powerful technique for material’s characterization and process control. It has been used for contact-free conductivity measurements of metals, semiconductors, 2D materials, and superconductors. Furthermore, THz-TDS has been used to identify chemical components such as amino acids, peptides, pharmaceuticals, and explosives, which makes it particularly valuable for fundamental science, security, and medical applications. This tutorial is intended for a reader completely new to the field of THz-TDS and presents a basic understanding of THz-TDS. Hundreds of articles and many books can be consulted after reading this tutorial. We explore the basic concepts of TDS and discuss the relationship between temporal and frequency domain information. We illustrate how THz radiation can be generated and detected, and we discuss common noise sources and limitations for THz-TDS. This tutorial concludes by discussing some common experimental scenarios and explains how THz-TDS measurements can be used to identify materials, determine complex refractive indices (phase delay and absorption), and extract conductivity.

Tutorial: An introduction to terahertz time domain spectroscopy (THz-TDS)

In this paper, we present an overview of chemical recognition with ultrashort THz pulses. We describe the experimental technique and demonstrate how signals for chemical recognition of substances in sealed containers can be obtained, based on the broadband absorption spectra of the substances. We then discuss chemical recognition in combination with THz imaging and show that certain groups of biological substances may give rise to characteristic recognition signals. Finally, we explore the power of numerical prediction of absorption spectra of molecular crystals and illuminate some of the challenges facing state-of-the-art computational chemistry software.

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Chemical recognition in terahertz time-domain spectroscopy and imaging

We have built a relatively simple, highly efficient, THz emission and detection system centered around a 15 fs Ti:sapphire laser. In the system, 200 mW of laser power is focused to a 120 μm diam spot between two silverpaint electrodes on the surface of a semi-insulating GaAs crystal, kept at a temperature near 300 K, biased with a 50 kHz, ±400 V square wave. Using rapid delay scanning and lock-in detection at 50 kHz, we obtain probe laser quantum-noise limited signals using a standard electro-optic detection scheme with a 1-mm-thick (110) oriented ZnTe crystal or a (110) oriented 0.1-mm-thick GaP crystal. The maximum THz-induced differential signal that we observe is ΔI/I=7×10−3, corresponding to a THz peak amplitude of 95 V/cm. The THz average power was measured to be about 40 μW, to our knowledge, the highest power reported so far generated with Ti:sapphire oscillators as a pump source. The system uses off-the-shelf electronics and requires no microfabrication techniques.

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Design and performance of a THz emission and detection setup based on a semi-insulating GaAs emitter

Climate sensitivity, the long-term warming due to doubled atmospheric CO2 levels, is estimated in the range of 1.5 °C to 4.5 °C. A synthesis of work reveals that whether the value falls at the high or low end, future emissions will have to be strongly limited. Equilibrium climate sensitivity characterizes the Earth's long-term global temperature response to increased atmospheric CO2 concentration. It has reached almost iconic status as the single number that describes how severe climate change will be. The consensus on the 'likely' range for climate sensitivity of 1.5 °C to 4.5 °C today is the same as given by Jule Charney in 1979, but now it is based on quantitative evidence from across the climate system and throughout climate history. The quest to constrain climate sensitivity has revealed important insights into the timescales of the climate system response, natural variability and limitations in observations and climate models, but also concerns about the simple concepts underlying climate sensitivity and radiative forcing, which opens avenues to better understand and constrain the climate response to forcing. Estimates of the transient climate response are better constrained by observed warming and are more relevant for predicting warming over the next decades. Newer metrics relating global warming directly to the total emitted CO2 show that in order to keep warming to within 2 °C, future CO2 emissions have to remain strongly limited, irrespective of climate sensitivity being at the high or low end.

Beyond equilibrium climate sensitivity

Via terahertz (THz) coherent transients and THz time-domain spectroscopy, we have measured the far-wing absorption line profile of the ensembles of collision-broadened ground state rotational lines...

Terahertz Studies of Collision-Broadened Rotational Lines

We use the technique of terahertz time-domain spectroscopy to experimentally and theoretically study the absorption and the dispersion of ammonia vapor. Our measurements show small deviations from the rotationinversion spectrum which originate from transitions in the O2 hot band and from the (NH3)2 dimer. We investigate the molecular tunneling in ammonia and apply our new molecular response theory to analyze the inversion spectrum of this molecule. The theory includes the molecular response of polar molecules to an external electric field over the duration of a collision with the molecular response time as a control parameter, and it unifies the basic collision theories of Lorentz, van Vleck -Weisskopf, and Debye. The calculated absorption and dispersion based on this theory fits the measurements over the full spectral range of the THz spectrum and allows to determine a response time of the order of 100 fs.

THz Time-Domain Spectroscopy on Ammonia

A newly developed optoelectronic source for generating well-collimated beams of subpicosecond pulses of terahertz radiation was used to study coherent propagation effects in a spectrally thick medium of methyl chloride vapor. After the passage of the excitation pulse, we observe the subsequent emission of a coherent terahertz pulse train in the free-induction decay of the molecular vapor. The individual pulses of the train are separated by 38 ps, corresponding to the frequency separation between adjacent rotational lines of the excited manifold of more than 70 transitions. An analysis of the data determines the rotational constants and anharmonicity factors of the two naturally abundant methyl chloride isotopes. From the decay and reshaping of the pulse train the coherence relaxation time T2 is obtained as a function of vapor pressure. Our observations indicate a strong nonmonotonic collision broadening and a van Vleck–Weisskopf line shape of the individual rotational lines.

Terahertz coherent transients from methyl chloride vapor

To evaluate the role of Total Solar Irradiance (TSI) on Northern Hemisphere (NH) surface air temperature trends it is important to have reliable estimates of both quantities. 16 different TSI estimates were compiled from the literature. 1/2 of these estimates are low variability and 1/2 are high variability. 5 largely-independent methods for estimating NH temperature trends were evaluated using: 1) only rural weather stations; 2) all available stations whether urban or rural (the standard approach); 3) only sea surface temperatures; 4) tree-ring temperature proxies; 5) glacier length temperature proxies. The standard estimates using urban as well as rural stations were anomalous as they implied a much greater warming in recent decades than the other estimates. This suggests urbanization bias might still be a problem in current global temperature datasets despite the conclusions of some earlier studies. Still, all 5 estimates confirm it is currently warmer than the late 19th century, i.e., there has been some global warming since 1850. For the 5 estimates of NH temperatures, the contribution from direct solar forcing for all 16 estimates of TSI was evaluated using simple linear least-squares fitting. The role of human activity in recent warming was then calculated by fitting the residuals to the UN IPCC's recommended anthropogenic forcings time series. For all 5 NH temperature series, different TSI estimates implied everything from recent global warming being mostly human-caused to it being mostly natural. It seems previous studies (including the most recent IPCC reports) that had prematurely concluded the former failed to adequately consider all the relevant estimates of TSI and/or to satisfactorily address the uncertainties still associated with NH temperature trend estimates. Several recommendations are provided on how future research could more satisfactorily resolve these issues.

Hermann Harde

Papers

Terahertz Studies of Collision-Broadened Rotational Lines

THz Time-Domain Spectroscopy on Ammonia

Terahertz coherent transients from methyl chloride vapor

How much has the Sun influenced Northern Hemisphere temperature trends? An ongoing debate

Photoacoustic spectrometer based on a DFB-diode laser