In this review, we discuss our recent theoretical work on the nonlinear optical response of graphene and its sister structure in terahertz (THz) and near-infrared frequency regime. Due to Dirac-like linear energy-momentum dispersion, the third-order nonlinear current in graphene is much stronger than that in conventional semiconductors. The nonlinear current grows rapidly with increasing temperature and decreasing frequency. The third-order nonlinear current can be as strong as the linear current under moderate electric field strength of 104 V/cm. In bilayer graphene (BLG) with low energy trigonal warping effect, not only the optical response is strongly nonlinear, the optical nonlinearity is well-preserved at elevated temperature. In the presence of a bandgap (such as semihydrogenated graphene (SHG)), there exists two well separated linear response and nonlinear response peaks. This suggests that SHG can have a unique potential as a two-color nonlinear material in the THz frequency regime where the relative intensity of the two colors can be tuned with the electric field. In a graphene superlattice structure of Kronig-Penney type periodic potential, the Dirac cone is elliptically deformed. We found that not only the optical nonlinearity is preserved in such a system, the total optical response is further enhanced by a factor proportional to the band anisotropy. This suggests that graphene superlattice is another potential candidate in THz device application.
Metamaterials have been developed as a new class of artificial effective media realizing many exotic phenomena and unique properties not normally found in nature. Metamaterials enable functionality through structure design, facilitating applications by addressing the severe material issues in the terahertz frequency range. Consequently, prototype functional terahertz devices have been demonstrated, including filters, antireflection coatings, perfect absorbers, polarization converters, and arbitrary wavefront shaping devices. Further integration of functional materials into metamaterial structures have enabled actively and dynamically switchable and frequency tunable terahertz metamaterials through the application of external stimuli. The enhanced light-matter interactions in active terahertz metamaterials may result in unprecedented control and manipulation of terahertz radiation, forming the foundation of many terahertz applications. In this paper, we review the progress during the past few years in this rapidly growing research field. We particularly focus on the design principles and realization of functionalities using single-layer and few-layer terahertz planar metamaterials, and active terahertz metamaterials through the integration of semiconductors to achieve switchable and frequency-tunable response.
Superconducting terahertz metamaterials have attracted significant interest due to low loss, efficient resonance switching and large-range frequency tunability. The super conductivity in the metamaterials dramatically reduces ohmic loss and absorption to levels suitable for novel devices over a broad range of electromagnetic spectrum. Most metamaterials utilize subwavelength-scale split-ring resonators as unit building blocks, which are proved to support fundamental inductive-capacitive resonance, to achieve unique resonance performance. We presented a review of terahertz superconducting metamaterials and their implementation in multifunctional devices. We began with the recent development of superconducting metamaterials and their potential applications in controlling and manipulating terahertz waves. Then we explored the tuning behaviors of resonance properties in several typical, actively controllable metamaterials through integrating active components. Finally, the ultrafast dynamic nonlinear response to high intensity terahertz field in the superconducting metamaterials was presented.
This paper introduces the terahertz time-domain spectroscopy (THz-TDS) used for the quantitative detection of n-heptane volume ratios in 41 n-heptane and n-octane mixtures with the concentration range of 0-100% at the intervals of 2.5%. Among 41 samples, 33 were used for calibration and the remaining 8 for validation. Models of chemometrics methods, including partial least squares (PLS) and back propagation-artificial neural network (BP-ANN), were built between the THz-TDS and the n-heptane percentage. To evaluate the quality of the built models, we calculated the correlation coefficient (R) and root-mean-square errors (RMSE) of calibration and validation models. R and RMSE of two methods were close to 1 and 0 within acceptable levels, respectively, demonstrating that the combination of THz-TDS and chemometrics methods is a potential and promising tool for further quantitative detection of n-alkanes.
In this paper, we employed terahertz time domain spectroscopy (THz-TDS) to investigate the nitrate () concentration in four types of nitrate solution (sodium nitrate, aluminum nitrate, calcium nitrate and magnesium nitrate). Their absorption coefficient and refractive index were calculated in 0.2–2.5 THz, and a logarithmic relationship was observed between NO-3 concentrations and selected optical parameters regardless of the kinds of nitrate solution. Partial least square (PLS) model was built between THz-TDS and NO-3 concentration. The correlation coefficient of PLS model was calculated. The results make the quantitative analysis of NO-3 concentration possible by THz-TDS and indicate the bright future in practical application.
A terahertz asymmetrically coupled resonator (ACR) consisting of two different split ring resonators (SRRs) was designed. Using finite difference time domain (FDTD), the transmission of ACR and its refractive-index-based sensing performance were simulated and analyzed. Results show that the ACR possesses a sharp coupled transparent peak or high quality factor (Q), its intensity and bandwidth can be easily adjusted by spacing the two SRRs. Furthermore, the resonator exhibits high sensitivity of 75 GHz/RIU and figure of merit (FOM) of 4.4, much higher than the individual SRR sensors. The ACR were fabricated by using laser-induced and chemical non-electrolytic plating with copper on polyimide substrate, the transmission of which measured by terahertz time-domain spectroscopy system is in good agreement with simulations.
In this paper, we experimentally observed generation of the second and the third optical harmonics and the broadband terahertz radiation in the course of 800 nm 120 fs pulse in atmospheric air. The analysis of their polarization properties revealed unity of their nonlinear optical nature. Taking into account only the third-order nonlinear response of the neutral molecules of air, we analytically described the newly generated elliptically polarized 3d harmonic, the linear polarization of terahertz radiation and the stability of terahertz energy yield for the initial circularly polarized ω pump pulse.
The work presented in this paper was performed at the Oriental Institute at the University of Chicago, on objects from their permanent collection: an ancient Egyptian bird mummy and three ancient Sumerian corroded copper-alloy objects. We used a portable, fiber-coupled terahertz (THz) time-domain spectroscopic imaging system, which allowed us to measure specimens in both transmission and reflection geometry, and present time- and frequency-based image modes. The results confirm earlier evidence that THz imaging can provide complementary information to that obtainable from X-ray computed tomography (XRCT) scans of mummies, giving better visualisation of low density regions. In addition, we demonstrated that THz imaging can distinguish mineralized layers in metal artifacts.
When laser intensity varies, the ionization induced by optical field can be described by multiphoton ionization (MPI) in low regime and tunneling ionization (TI) in high regime. An empirical formula was used to fit the ionization happened between these two limitation ionization processes. Based on this, ionization rate and photocurrent induced by two-color femtosecond laser pulses interaction with air-plasma were investigated numerically. It was found that they have different relations with the laser intensity.
The field screening effects in small-size GaAs photoconductive (PC) antenna are investigated via the well-known pump and probe terahertz (THz) generation technique. The peak amplitude of the THz pulses excited by the probe laser pulse as a function of the pump-probe time delay was measured. An equivalent-circuit model was used to simulate the experimental data. Based on the good agreement between the results of simulation and experiment, the time behavior of the radiation and space-charge fields was simulated. The results show that the space-charge screening dominantly determines the device response in the whole time, while the radiation filed screening plays a key role in initial time which strongly affects the peak THz field. The parameter analysis was performed, which may be valuable on the optimum design for the antenna as a THz emitter.
Via constructing a special terahertz time domain spectroscopy (THz-TDS) system in which two femtosecond (fs) laser pulses were used as probe pulses to excite a photoconductive (PC) THz detector, the time behavior of the current from the detector was measured. The corresponding theoretical analysis was performed by a well-known equivalent-circuit model. When the time domain current was transformed to frequency domain, an oscillation effect was observed. The oscillation frequency was decided by the time delay between the two probe pulses. The number of the extrema in the frequency domain current curve was proportion to the pulse interval in 0.1-2 THz. A method to measure the interval of fs laser pulses was proposed. It is important for applications of fs laser pulses or train.