THz Absorption Spectra Research Articles

Terahertz probe of photoexcited carrier dynamics in the Dirac semimetal Cd3As2

The relaxation dynamics of photoexcited quasiparticles of three-dimensional (3D) Dirac semimetals are vital towards their application in high performance electronic and optoelectronic devices. In this work, the relaxation dynamics of photoexcited carriers of 3D Dirac semimetal Cd3As2 are investigated by transient terahertz spectroscopy. The visible pump-THz probe spectroscopy measurement shows clear biexponential decays with two characteristic time constants. According to the pump-power and temperature dependence, these two characteristic time constants are attributed to the electron phonon coupling (1-4 ps) and anharmonic decay of hot coupled phonons to electronic uncoupled phonons (2-9 ps), respectively. An anomalous electron-optical phonon coupling reduction and a bottleneck slowing of hot optical phonons relaxation are observed with higher excitation intensities similar to that in graphene. On the other hand, the electron-optical phonon coupling can be enhanced due to the phonon frequency broadening and softening at elevated lattice temperature. Furthermore, the transient THz spectrum response is strongly modified by the phonon assisted intraband absorption of hot carriers from a pure electronic Drude model, which is evidenced by a characteristic THz absorption dip in the transient THz absorption spectrum. This absorption dip is pinned by the discrete optical phonon energy that assists the intraband transition enabled by photoexcitation of hot carriers.

Minority and Majority Charge Carrier Mobility in Cu2ZnSnSe4 revealed by Terahertz Spectroscopy

The mobilities of electrons and holes determine the applicability of any semiconductor, but their individual measurement remains a major challenge. Here, we show that time-resolved terahertz spectroscopy (TRTS) can distinguish the mobilities of minority and majority charge carriers independently of the doping-type and without electrical contacts. To this end, we combine the well-established determination of the sum of electron and hole mobilities from photo-induced THz absorption spectra with mobility-dependent ambipolar modeling of TRTS transients. The method is demonstrated on a polycrystalline Cu2ZnSnSe4 thin film and reveals a minority (electron) mobility of 128 cm2/V-s and a majority (hole) carrier mobility of 7 cm2/V-s in the vertical transport direction relevant for light emitting, photovoltaic and solar water splitting devices. Additionally, the TRTS analysis yields an effective bulk carrier lifetime of 4.4 ns, a surface recombination velocity of 6 * 104 cm/s and a doping concentration of ca. 1016 cm−3, thus offering the potential for contactless screen novel optoelectronic materials.

THz absorption spectrum employed for characterize the mixed Gallium selenide-Tin disulfide saturable absorber and its application for passively Q-switched solid state laser

Due to the ultra-short recovery time and the wider absorption range, graphene-like 2D material have been widely employed as saturable absorbers (SAs) in passively Q-switched and mode-locked lasers. Due to the low photon energy, noninvasive and high resolution, THz wave was widely used to characterize samples. In this paper, the nonlinear saturable absorption property and THz absorption spectrum of Gallium selenide/Tin disulfide (GaSe/SnS2) is reported. Passively Q-switched Nd:GdTaO4 laser with GaSe/SnS2 SA is demonstrated. The maximum output power of 551 mW under the pump power of 5 W. The minimum pulse width is 365 ns at the pulse repetition rate of 89 kHz. Our results indicate that GaSe/SnS2 nanosheets could be used in Q-switched lasers. The GaSe/SnS2 SA with good nonlinear absorption properties and easy fabricating process will promote the ultrafast photonics applications in solid state lasers.

Terahertz spectroscopic analysis of crystal orientation in polymers

Terahertz time-domain spectroscopy (THz-TDS) is attracting keen attention as a new spectroscopic tool for characterizing various materials. In this research, the possibility of analyzing the crystal orientation in a crystalline polymer by THz-TDS is investigated by measuring angle-resolved THz absorption spectra for sheets of poly(ethylene terephthalate), poly(ethylene naphthalate), and poly(phenylene sulfide). The resultant angle dependence of the absorption intensity of each polymer is similar to that of the crystal orientation examined using pole figures of X-ray diffraction. More specifically, THz-TDS can indicate the alignment of molecules in polymers.

THz absorption spectra of glucose and its polymers

THz absorption spectra of glucose and its polymers

Terahertz time-domain spectroscopy of l-histidine hydrochloride monohydrate

The low-frequency absorption spectrum of l-histidine hydrochloride monohydrate (LHHM) over 0.5–4.0 THz was measured by terahertz time-domain spectroscopy (THz-TDS). The comparison between the THz absorption spectra of LHHM and l-histidine (LHis) was made. The two distinctive THz spectra of LHHM and LHis indicate that THz spectroscopy is sensitive to different specimens. The low-temperature THz spectral features of LHHM presented the blue-shift and splitting as the temperature decreased. Solid-state density functional theory (DFT) calculation of the vibrational modes of LHHM was performed for better understanding the THz characteristic spectrum, and the validity of the calculation modes was confirmed by comparing with the experimental observations. Powder X-ray diffraction (PXRD) was also carried out to check the structures of LHHM and LHis.

The effect of the flexibility of hydrogen bonding network on low-frequency motions of amino acids. Evidence from Terahertz spectroscopy and DFT calculations

Low-frequency modes of L-Asp and L-Asn were studied in the range from 0.1 to 3.0THz using time-domain Terahertz spectroscopy and density functional theory calculation. The results show that PBE-D2 shows more success than BLYP-D2 in prediction of THz absorption spectra. To compare their low-frequency modes, we adopted “vibrational character ID strips” proposed by Schmuttenmaer and coworkers [Journal of Physical Chemistry B, 117, 10444(2013)]. We found that the most intense THz absorption peaks of two compounds both involve severe distortion of their hydrogen bonding networks. Due to less rigid hydrogen bonding network in L-Asp, the side chain (carboxyl group) of L-Asp exhibits larger motions than that (carboxamide group) of L-Asn in low-frequency modes.

Wavelength Selection Method Based on Differential Evolution for Precise Quantitative Analysis Using Terahertz Time-Domain Spectroscopy.

Quantitative analysis of component mixtures is an important application of terahertz time-domain spectroscopy (THz-TDS) and has attracted broad interest in recent research. Although the accuracy of quantitative analysis using THz-TDS is affected by a host of factors, wavelength selection from the sample's THz absorption spectrum is the most crucial component. The raw spectrum consists of signals from the sample and scattering and other random disturbances that can critically influence the quantitative accuracy. For precise quantitative analysis using THz-TDS, the signal from the sample needs to be retained while the scattering and other noise sources are eliminated. In this paper, a novel wavelength selection method based on differential evolution (DE) is investigated. By performing quantitative experiments on a series of binary amino acid mixtures using THz-TDS, we demonstrate the efficacy of the DE-based wavelength selection method, which yields an error rate below 5%.

Probing the global potential energy minimum of (CH2O)2: THz absorption spectrum of (CH2O)2 in solid neon and para-hydrogen.

The true global potential energy minimum configuration of the formaldehyde dimer (CH2O)2, including the presence of a single or a double weak intermolecular CH⋯O hydrogen bond motif, has been a long-standing subject among both experimentalists and theoreticians as two different energy minima conformations of Cs and C2h symmetry have almost identical energies. The present work demonstrates how the class of large-amplitude hydrogen bond vibrational motion probed in the THz region provides excellent direct spectroscopic observables for these weak intermolecular CH⋯O hydrogen bond motifs. The combination of concentration dependency measurements, observed isotopic spectral shifts associated with H/D substitutions and dedicated annealing procedures, enables the unambiguous assignment of three large-amplitude infrared active hydrogen bond vibrational modes for the non-planar Cs configuration of (CH2O)2 embedded in cryogenic neon and enriched para-hydrogen matrices. A (semi)-empirical value for the change of vibrational zero-point energy of 5.5 ± 0.3 kJ mol-1 is proposed for the dimerization process. These THz spectroscopic observations are complemented by CCSD(T)-F12/aug-cc-pV5Z (electronic energies) and MP2/aug-cc-pVQZ (force fields) electronic structure calculations yielding a (semi)-empirical value of 13.7 ± 0.3 kJ mol-1 for the dissociation energy D0 of this global potential energy minimum.

Terahertz Absorption by Cellulose: Application to Ancient Paper Artifacts

Artifacts made of cellulose, such as ancient documents, pose a significant experimental challenge in the THz transmission spectra interpretation due to their small optical thickness. In this Letter we describe a method to recover the complex refractive index of cellulose fibers from the THz transmission data obtained on single freely standing paper sheets in the 0.2–3.5 THz range. By using our technique, we were able to eliminate Fabry-Perot effects and recover the absorption coefficient of the cellulose fibers. The obtained THz absorption spectra are explained in terms of absorption peaks of the cellulose crystalline phase superimposed to a background contribution due to a disordered hydrogen bonds network. The comparison between the experimental spectra with THz vibrational properties simulated by density functional theory calculations confirms this interpretation. In addition, evident changes in THz absorption spectra are produced by natural and artificial aging on paper samples and the final stage of aging is characterized by a spectral profile with only two peaks at about 2.1 THz and 3.1 THz. These results could be used to provide a quantitative assessment of the state of preservation of cellulose artifacts. This dataset contains the data for figures 1 - 4 in the paper.

Competition between weak OH⋯π and CH⋯O hydrogen bonds: THz spectroscopy of the C2H2-H2O and C2H4-H2O complexes.

THz absorption spectra have been recorded for the weakly bound molecular complexes of H2O with C2H4 and C2H2 embedded in cryogenic neon matrices at 2.8 K. The observation and assignment of a large-amplitude acceptor OH librational mode of the C2H2-H2O complex at 145.5 cm-1 confirms an intermolecular CH⋯O hydrogen-bonded configuration of C2v symmetry with the H2O subunit acting as the hydrogen bond acceptor. The observation and assignment of two large-amplitude donor OH librational modes of the C2H4-H2O complex at 255.0 and 187.5 cm-1, respectively, confirms an intermolecular OH⋯π hydrogen-bonded configuration with the H2O subunit acting as the hydrogen bond donor to the π-cloud of C2H4. A (semi)-empirical value for the change of vibrational zero-point energy of 4.0-4.1 kJ mol-1 is proposed and the combination with quantum chemical calculations at the CCSD(T)-F12b/aug-cc-pVQZ level provides a reliable estimate of 7.1 ± 0.3 kJ mol-1 for the dissociation energy D0 of the C2H4-H2O complex. In addition, tentative assignments for the two strongly infrared active OH librational modes of the ternary C2H4-HOH-C2H4 complex having H2O as a doubly OH⋯π hydrogen bond donor are proposed at 213.6 and 222.3 cm-1. The present findings demonstrate that the relative stability of the weak hydrogen bond motifs is not entirely rooted in differences of electronic energy but also to a large extent by differences in the vibrational zero-point energy contributions arising from the class of large-amplitude intermolecular modes.

Wavelength selection of amino acid THz absorption spectra for quantitative analysis by a self-adaptive genetic algorithm and comparison with mwPLS

For researchers interested in the quantitative analysis of mixture components by terahertz time-domain spectroscopy, the wavelength selection of the full spectrum obtained by the experiments is of great importance. The full spectrum is not only composed of the mixture components' characteristic features, but also many types of noises, such as the scattering and the instrumental disturbances. Because of this, the accuracy of the quantitative analysis may be very unsatisfactory if the full spectrum is used. Therefore, the full spectrum must be deeply selected, which means retaining the useful information points and dropping the noises. In this paper, a self-adaptive genetic algorithm (sa-GA) was proposed for point-by-point wavelength selection of mixture THz absorption spectra, with the crossover and mutation probabilities dynamically adjusted, which is helpful for improving the method's performance. The sa-GA was compared with the moving window partial least squares (mwPLS) method, which has been proved to be an effective method for wavelength selection in THz spectroscopy. The quantitative analysis experiments of a series of amino acid mixtures proved that the sa-GA was superior to mwPLS, with lower error (1.94±3.79%) and better convergence.

Terahertz spectra of l-phenylalanine and its monohydrate

The low-frequency vibrational property of l-phenylalanine (l-Phe) and l-phenylalanine monohydrate (l-Phe·H2O) has been investigated by terahertz time-domain spectroscopy (THz-TDS) at room and low temperature ranging from 0.5 to 4.5THz. Distinctive THz absorption spectra of the two compounds were observed. Density functional theory (DFT) calculations based on the crystal structures have been performed to simulate the vibrational modes of l-Phe and l-Phe·H2O and the results agree well with the experimental observations. The study indicates that the characterized features of l-Phe mainly originate from the collective vibration of molecules. And the characterized features of l-Phe·H2O mainly come from hydrogen bond interactions between l-Phe and water molecules. l-Phe and l-Phe·H2O were also verified by differential scanning calorimetry and thermogravimetry (DSC-TG) and powder X-ray diffraction (PXRD) examinations.

Research on the Terahertz Absorption Spectra of Histidine Enantiomer (L) and its Racemic Compound (DL).

Terahertz time-domain spectroscopy (THz-TDS) is used to investigate the absorption spectra of polycrystalline L- and DL-histidine in the frequency range of 10-100 cm-1. The spectra exhibit distinct differences in peak frequencies between the enantiomer (L-histidine) and racemic compound (DL-histidine). The observed spectral differences are attributed to the intermolecular interactions. With the density function theory (DFT) method, the frequencies of vibrational modes of L-histidine and DL-histidine in the THz range are calculated and well assigned according to the measured spectra. The origin of the observed vibrational modes is found to be non-localized and of a collective (phonon-like) nature, which points to the lattice and skeleton vibrations mediated by the hydrogen bond. Furthermore, we propose and demonstrate a method for determining the composition ratio of histidine mixtures based on the THz absorption spectra.

Theoretical and Experimental Investigation of Terahertz Absorption Spectra for Water Clusters (H2O) n (n = 4, 5, 6) in the Atmosphere

The atmosphere transmission spectrum is measured from 0.2 to 1.6 THz with a continuous wave spectrometer. Three ring-formed water clusters of (H 2 O) n (n = 4, 5, 6) are proposed and their THz absorption spectra are calculated with the density functional theory. The calculated absorption lines of (H 2 O)6 agree well with the measured spectrum, which indicates that the resonant absorptions in the atmosphere are caused by the ring-formed water clusters in the studied frequency range. The discrepancy between the calculation and experiment results on the absorptions around 1.2 THz should be ascribed to the deformation of the cluster.

Characteristics of Low-Frequency Molecular Phonon Modes Studied by THz Spectroscopy and Solid-State ab Initio Theory: Polymorphs I and III of Diflunisal.

THz absorption spectra of two polymorphs of diflunisal, form I and form III, exhibit distinct features due to the influence of packing conformations on the frequency distributions and IR activities of gamma point phonon modes within the 100 cm(-1) region. In order to understand the origins of these THz modes, we perform a detailed mode analysis. The result shows that although the spectral features are different, these low-frequency phonon modes of the two molecular polymorphs have similar vibrational characteristics in terms of harmonic couplings of intermolecular and intramolecular vibrations.

Wavelength Selection for Quantitative Analysis in Terahertz Spectroscopy Using a Genetic Algorithm

In the quantitative analysis of the components of mixtures based on terahertz time-domain spectroscopy (THz-TDS), wavelength selection of the spectra holds great importance for quantitative accuracy. The raw spectrum obtained in the experiments contains both high signal-to-noise ratio (SNR) and low SNR information, along with many types of noise, such as the scattering effect and random disturbances. Usually, the frequency band used for analysis is determined over multiple attempts to obtain the most satisfactory results. Currently, there is no common method for selecting the appropriate frequencies. In this paper, a wavelength selection method for the THz absorption spectra based on a genetic algorithm is presented, with the main focus on quantitative analysis by THz-TDS. Using this method, most of the useful data with high SNR were retained. The comparison between quantitative analysis errors obtained by the raw spectra and by the wavelength-selected spectra showed that this method could significantly decrease quantitative errors. After wavelength selection, the quantitative analysis errors of 12 mixture samples with widely differing concentrations are predominantly below 6% with a standard deviation of 0.0344.

بررسي طيف جذب بلورهاي فوتوني شامل تک لايه هاي MoS2 در ناحيه تراهرتز (Investigation of THz Absorption Spectrum of Photonic Crystals Including MoS2 Monolayers)

بلور دو بعدی مولبیدیم دی سولفاتPerisian Abstract: ، MoS_2، در بازه فرکانسی THzبه علت کاربردهای اپتوالکترونیکی بسیار مورد توجه قرار گرفته است و لذا کاربری آنها در ادوات فوتونیکی می تواند سودمند باشد. در این مقاله، برای استفاده همزمان از ویژگی های بلور فوتونی و تک لایه ی MoS_2، ساختارهای متناوب بلور فوتونی با مواد Si، SiO_2 و MoS_2 بررسی شده است که ضریب شکست MoS_2 در ناحیه ی THz از رابطه درود بدست می آید. طیف عبور و جذب این ساختارها در بازه ی فرکانسی THz از روش ماتریس انتقال برای هر دو قطبش TE و TM رسم شده و نشان داده شده است که طیف جذب وابسته به ضخامت کل MoS_2 در ساختار است و به چیدمان قرارگیری MoS_2 در بلور فوتونی وابستگی کمی دارد. English Abstract: The two dimensional Molybdenum disulfide, MoS2, has attracted intense attention for its application in the THz optoelectronics and thus can be used in photonics devices. In this paper, to simultaneous application of photonics crystals and MoS2, periodic photonic crystal structures including Si, SiO2 and MoS2 are demonstrated. The THz refractive index of MoS2 is taken from Drude model. Transmission and absorption spectra in the THz range for either of TE and TM modes are calculated by transfer matrix method (TMM). It is found that the absorption spectrum is dependent upon MoS2 thickness and weakly depends upon the MoS2 position in the structure.

Terahertz Absorption Spectra Simulation of Glutamine Based on Quantum-Chemical Calculation

With simulation of absorption spectra in THz region based on quantum-chemical calculation, the THz absorption features of target materials can be assigned with theoretical normal vibration modes. This is necessary for deeply understanding the origin of THz absorption spectra. The reliabilities of simulation results mainly depend on the initial structures and theoretical methods used throughout the calculation. In our study, we utilized THz-TDS to obtain the THz absorption spectrum of solid-state L-glutamine. Then three quantum-chemical calculation schemes with different initial structures commonly used in previous studies were proposed to study the inter-molecular interactions' contribution to the THz absorption of glutamine, containing monomer structure, dimer structure and crystal unit cell structure. After structure optimization and vibration modes' calculation based on density functional theory, the calculation results were converted to absorption spectra by Lorentzian line shape function for visual comparison with experimental spectra. The result of dimmer structure is better than monomer structure in number of absorption features while worse than crystal unit cell structure in position of absorption peaks. With the most reliable simulation result from crystal unit cell calculation, we successfully assigned all three experimental absorption peaks of glutamine ranged from 0.3 to 2.6 THz with overall vibration modes. Our study reveals that the crystal unit cell should be used as initial structure during theoretical simulation of solid-state samples' THz absorption spectrum which comprehensively considers not only the intra-molecular interactions but also inter-molecular interactions.

Study on identification methods in the detection of transgenic material based on terahertz time domain spectroscopy

Terahertz time-domain spectroscopy system was used to measure the characteristic absorption spectra of 8 transgenic cotton seeds in the frequency range from 0.3 to 1.5 THz. Comparing the similarities and differences of THz absorption spectrum of eight kinds of transgenic cotton seeds, which is based on spectral analysis, and using principal component analysis to perform qualitative analysis, the relevant data of spectral analysis was obtained. Three principal component factors were extracted in this paper, which the cumulative percentage variance reached 71.537 %. Therefore, different categories of transgenic cotton seeds were clearly distinguished according to the principal component analysis score chart. The experimental results indicated that nondestructive testing of transgenic cotton seeds could be achieved by using Terahertz time-domain spectroscopy system technology, which could be widely applied in the fields of agricultural seed selection, agricultural security and so on.