The hydrogen bond (H-bond) in organic-water molecules is essential in nature. Combining with the charge - transfer analyses, we investigated the penetrating molecular-orbitals in glycine-water clusters, which give evidences of the covalent-like characteristics of H-bonds in this system. Besides, the infrared spectral features provide a rare opportunity to discover the exceedingly-evident redshifts of symmetric stretching modes (Symst) in water on forming H-bond, in contrast to the slightly-redshifted asymmetric stretching modes (Asyst) in water. To explain these intriguing behaviors, we further analyzed the nuclear vibrating patterns, which clearly reveal that H-bond retains two unexpected effects on nuclear motions in water: (i) Intensifying donor Symst, and (ii) Inhibiting donor Asyst. Furthermore, we also quantified the impact of anharmonic quantum fluctuations on each hydrogen bond. For the stretching modes involved in H-bonds, red shifts up to more than one hundred wave numbers are observed under anharmonic vibration, explicitly indicating the increased ‘covalency’ of H-bonds. These finds shed light on the essential understanding of H-bonding comprehensively, and should provide incentives for future experimental studies.
Metamaterials with subwavelength structural features show unique electromagnetic responses that are unattainable with natural materials. Recently, the research on these artificial materials has been pushed forward to the terahertz (THz) region because of potential applications in biological fingerprinting, security imaging, and high frequency magnetic and electric resonant devices. Furthermore, active control of their properties could further facilitate and open up new applications in terms of modulation and switching. In our work, we will first present our studies of dipole arrays at terahertz frequencies. Then in experimental and theoretical studies of terahertz subwavelength L-shaped structure, we proposed an unusual-mode current resonance responsible for low-frequency characteristic dip in transmission spectra. Comparing spectral properties of our designed simplified structures with that of split-ring resonators, we attribute this unusual mode to the resonance coupling and splitting under the broken symmetry of the structure. Finally, we use optical pump–terahertz probe method to investigate the spectral and dynamic behaviour of optical modulation in the split-ring resonators. We have observed the blue-shift and band broadening in the spectral changes of transmission under optical excitation at different delay times. The calculated surface currents using finite difference time domain simulation are presented to characterize these resonances, and the blue-shift can be explained by the changed refractive index and conductivity in the photoexcited semiconductor substrate.
Ultrafast carrier dynamics in Schottky barriers is an extremely active area of research in recent years. The observation of the generation of terahertz pulses from metal/semiconductor interfaces provides a technique to characterize electronic properties of these materials. However, a detailed analysis of these phenomena has not been performed satisfactorily. In this work, the measurements of optically generated terahertz emission from Au/GaAs interfaces are investigated in detail. We observe that, under high laser power excitation, terahertz signals from bare GaAs wafers and Au/GaAs samples exhibit an opposite polarity. The polarity-flip behaviors in the terahertz beams are also observed in the temperature-dependent measurements and the femtosecond pump-generation studies of the Au/GaAs interfaces. These effects can be fully explained in terms of the dynamics of carrier transfer in the Au/GaAs Schottky barriers, which involves the internal photoelectric emission and the electron tunneling effect, and picosecond time constants are found for these processes.
Metamaterials with subwavelength structural features show unique electromagnetic responses that are unattainable with natural materials. Modulation property is regarded as one of the most important features of metamaterials. At present, the development of such terahertz switches and modulators are relatively slow. So the research of the terahertz metamaterial is very meaningful. The light-control modulation, for example, could control the resonance characteristics of split ring resonators (SRRs) by changing dielectric property of the light layer. Due to the complicated effect in photo-excited layer, we could simplify the research to firstly study the influence of substrate’s refractive index on the resonant behaviors, providing the way to further the investigation of complex problems. In addition, the shape and size of metal microstructure can produce important effect on electromagnetic response. Therefore, based on the finite-difference time-domain method, we have also simulated several SRR structures with different geometry. We find the calculated terahertz transmission spectra exhibit remarkable change, showing that the resonant dips have a red-shift phenomenon and the bandwidth gets narrow with the increased refractive index as well as the structure size. Compared with the geometry effect, the red-shift is more sensitive to the change in refractive index. This work could help us to choose the suitable substrate materials for sample fabrication to realize the specific features.
The spatiotemporal and spectral characteristics of ultrawide-band terahertz pulses after passing through a Fresnel lens
are studied by using scalar diffraction theory. The simulation shows that the transmitted terahertz waveforms compress
with increasing propagation distance, and the multi-frequency focusing phenomenon at different focal points is
observed. Additionally, the distribution of terahertz fields in planes perpendicular to the axis is also discussed, and it is
found that the diffraction not only induces focusing on axis but also inhibit focusing at off-axis positions. Therefore,
the Fresnel lens may be a useful alternative approach to be a terahertz filter. Moreover, the terahertz pulses travelling as
basic mode of Gaussian beam is discussed in detail.
Terahertz time-domain spectroscopy (THz-TDS) is used to investigate the absorption and dispersion of polycrystalline
α - and γ -glycine in the spectral region 0.5-3.0 THz. The spectra exhibit distinct features in these two crystalline
phases. The observed far-infrared responses are attributed to intermolecular vibrational modes mediated by hydrogen
bonds. We also measure the Raman spectra of the polycrystalline and dissolved glycine in the frequency range 28-3900
cm-1. The results show that all the vibrational modes below 200 cm-1 are nonlocalized but are of a collective (phonon-like)
nature. Furthermore, the temperature dependence of the Raman spectra of α -glycine agrees with the anharmonicity
mechanism of the vibrational potentials.
The method of spectral dynamics analysis (SDA-method) is used for obtaining the2D THz signature of drugs. This
signature is used for the detection and identification of drugs with similar Fourier spectra by transmitted THz signal. We
discuss the efficiency of SDA method for the identification problem of pure methamphetamine (MA),
methylenedioxyamphetamine (MDA), 3, 4-methylenedioxymethamphetamine (MDMA) and Ketamine.
Because of its unique nature, terahertz radiation has wide application prospects in physics, chemistry,
biomedicine, communications, radar and security checks, etc. Liquid crystal, which is a kind of
macromolecule soft material with special properties in physical and chemical, has been widely used in
planar displays. Recently, much attention has been paid to non-display studies of liquid crystals,
covering the fields of biology, chemistry, physics, material and engineering. In this paper, the
transmission spectra of several nematic liquid crystals such as 5CB, TEB300, RDP-92975 are
measured by THz time-domain spectroscopy technique and free-space electro-optic sampling method.
The absorption coefficients of liquid crystals in the THz range are calculated. Furthermore, the optical
parameters are compared and analyzed, expecting to fill the spectrum gap of liquid crystals in the THz
range and provide the experimental and theoretical foundation for the application of liquid crystals.
Engine oil, most of which is extracted from petroleum, consist of complex mixtures of hydrocarbons of molecular
weights in the range of 250-1000. Variable amounts of different additives are put into them to inhibit oxidation,
improve the viscosity index, decrease the fluidity point and avoid foaming or settling of solid particles among others.
Terahertz (THz) spectroscopy contains rich physical, chemical, and structural information of the materials. Most
low-frequency vibrational and rotational spectra of many petrochemicals lie in this frequency range. In recent years,
much attention has been paid to the THz spectroscopic studies of petroleum products. In this paper, the optical properties
and spectroscopy of selected kinds of engine oil consisting of shell HELIX 10W-40, Mobilube GX 80W-90, GEELY
ENGINE OIL SG 10W-30, SMA engine oil SG 5W-30, SMA engine oil SG 10W-30, SMA engine oil SG 75W-90 have
been studied by the terahertz time-domain spectroscopy (THz-TDS) in the spectral range of 0.6-2.5 THz. Engine oil with
different viscosities in the terahertz spectrum has certain regularity. In the THz-TDS, with the increase of viscosity, time
delay is greater and with the increase of viscosity, refractive indexes also grow and their rank is extremely regular. The
specific kinds of engine oil can be identified according to their different spectral features in the THz range. The
THz-TDS technology has potentially significant impact on the engine oil analysis.
Huygens-Fresnel principle and half-wave method are usually used to explain the diffraction effect as the
light passing through a hole. We present a new simple method to enhance the terahertz signal intensity by
using a small hole. We found that in the terahertz time-domain spectroscopy, the terahertz signal
enhances by 13.87% as a hole with certain size is put in a certain position of the terahertz beam path.
This effect can be fully explained by using Huygens Fresnel principle. Therefore, in the condition of
terahertz emitter and detector are unchanged, by using this method to enhance the terahertz signals is of
The carrier dynamics and terahertz photoconductivity in the n-type silicon (n-Si) as well as in the p-type Silicon (p-
Si) have been investigated by using femtosecond pump-terahertz probe technique. The measurements show that the
relative change of terahertz transmission of p-Si at low pump power is slightly lower than that of n-Si, due to the lower
carrier density induced by the recombination of original holes in the p-type material and the photogenerated electrons. At
high pump power, the bigger change of terahertz transmission of p-Si originates from the greater mobility of the carriers
compared to n-Si. The transient photoconductivities are calculated and fit well with the Drude-Smith model, showing
that the mobility of the photogenerated carriers decreases with the increasing pump power. The obtained results indicate
that femtosecond pump-terahertz probe technique is a promising method to investigate the carrier dynamics of
The ultrafast carrier dynamics and surface photoconductivity of unbiased semi-insulating GaAs have been investigated in
detail by using terahertz pump-emission technique. Through theoretical modeling based on Hertz vector potential, it is
found that transient photoconductivity plays a very important role in the temporal waveform of terahertz radiation pulse.
Anomalous enhancement in both terahertz radiation and transient photoconductivity is observed subsequent to the
excitation of pump pulse, and our modeling gives successful analyses for the dynamics of photogenerated carriers in the
GaAs. We attribute these phenomena to carrier capture in the EL2 centers. Moreover, the pump power- and temperaturedependent
measurements are also performed to verify this model.
Femtosecond pump-terahertz probe studies of carrier dynamics in semi-insulating GaAs have been investigated in detail
for various pump powers and at electric fields up to 15 kV/cm. The pump-induced attenuation of THz transmission
reduces obviously at high field, and the carrier relaxation time is also found to be correlated with photo-injected carrier
density and electric field. These effects can be fully explained in terms of the carrier intervalley scattering and the
surface states filling in GaAs, which may influence the carrier recombination process. Moreover, the carrier screening
effect at high pump powers is also discussed.