The carrier frequency of the modern telecommunication system had been raised to hundreds of GHz and aimed at the THz regime. Besides, THz waves have been intensively applied in many fields, e.g., spectroscopy, imaging, and communications. However, there is a rarity of available techniques for modulating few-cycle THz waves on picoseconds timescale. Here we report a simple/reliable system without spatial light modulators (SLMs) for generating circularly polarized THz dual pulses with variable helicity, frequency, and interval. These degrees of freedom allow us to arbitrarily control the THz double pulses of interests, which have potential applications in imaging, spectroscopy, and next-generation communications.
KEYWORDS: Perovskite, Ultraviolet radiation, Solar cells, Titanium, Carrier dynamics, Absorption, Oxygen, Heterojunctions, Diffusion, Electron transport
In this study, the microscopic carrier dynamics that govern the UV stability of perovskite solar cells was investigated using pump-probe spectroscopy. In conventional perovskite solar cells, the UV-active oxygen vacancy in compact TiO2 prohibits current generation after UV degradation. On the other hand, the dominant vacancy type in 2D Ti1-xO2 atomic sheet transporting layer (ASTL) is a titanium vacancy, not UV-sensitive. Consequently, the carrier recombination are suppressed and further extends UV stability in perovskite solar cells with a 2D Ti1-xO2 ASTL. The dynamics of electron diffusion, electron injection, and hot hole transfer processes are found to be less sensitive to the UV irradiation. The ultrafast time-resolved data shown here clearly represent a close correlation between the carrier dynamics and UV aging of perovskite, thus providing insight into the origin of UV-induced degradation in perovskite solar cells.
To improve the transmittance of THz component and overcome the difficulties of fragile structure as well as ensuring precise alignment of existing methods, a new method involving the mature 3DIC through-silicon via (TSV) technology has been proposed to make anti-reflection layer with suitable effective refractive index based on the robustness of Si wafer. Cu wire-grid polarizers were also fabricated on wafer. The THz polarizers were completed after wafer bonding with Cu sealing ring and In/Sn guard ring. Not only the new method is easier for production with better performance, but also the silicon substrate has several advantages. The novel method has proven that THz optical component could be constructed with a nearly 100% transmittance, or widened the transmittance spectrum range from 0.5 to 2 THz when transmittances is sacrificed to 70% instead of a near 100%. Furthermore, a robust structure could also be expected with broadband transmission and excellent extinction ratio. It is properly optimized for mass production because the fabrication method could be easily done and does not required high cost.
Due to the difficulties faced in fabricating robust Terahertz (THz) optical components with low Fresnel reflection loss, the need to increase the efficiency of THz system with reduced cost is still considered as one of the most essential tasks. In this report, a new low cost THz polarizer with robust structure is proposed and demonstrated. This new THz wire grid polarizer was based on an anti-reflection (AR) layer fabricated with low temperature metal bonding and deep reactive ion etching (DRIE). After patterning Cu wire gratings and the corresponding In/Sn solder ring on the individual silicon wafers, the inner gratings were sealed by wafer-level Cu to In/Sn guard ring bonding, providing the protection against humidity oxidation and corrosion. With the low eutectic melting point of In/Sn solder, wafers could be bonded face to face below 150°C. Two anti-reflection layers on both outward surfaces were fabricated by DRIE. With the mixing of empty holes and silicon, the effective refractive index was designed to be the square root of the silicon refractive index. The central frequency of the anti-reflection layers was designed between 0.5THz to 2THz with an approximate bandwidth of 0.5THz. The samples were measured with a commercial free-standing wire grid polarizer by a THz time domain spectroscopy (THz-TDS) from 0.2THz to 2.2THz. The power transmittance is close to 100% at central frequency. Extinction ratio of the polarizer is between 20dB to 40dB depending on the frequency. The advantages of this new polarizer include high transmittance, robust structure and low cost with no precision optical alignment required.
C. Luo, H.-J. Chen, H. J. Wang, S. A. Ku, K. H. Wu, T. M. Uen, J. Y. Juang, J.-Y. Lin, B. L. Young, T. Kobayashi, R. Sankar, F. C. Chou, H. Berger, G. D. Gu
Ultrafast dynamics of carriers and phonons in topological insulators CuxBi2Se3-y (x=0, 0.1, 0.125, y=0, 1) was studied using femtosecond optical pump-probe spectroscopy. One damped oscillation was clearly observed in the transient reflectivity changes (ΔR/R), which is assigned to the coherent optical phonon (A1g1). According to the red shift of A1g1 phonon frequency, the Cu atoms in CuxBi2Se3 crystals may predominantly intercalated between pair of the quintuple layers. Moreover, the carrier dynamics in the Dirac-cone surface state is significantly different from that in bulk state, which was investigated using optical pump mid-infrared (mid-IR) probe spectroscopy. The rising time and decay time of the negative component in ΔR/R, which is assigned to carrier relaxation in Dirac cone, is 1.62 ps and 20.5 ps, respectively.
Fast calculation of trapping force provides a more direct way for optimizing designs of optical systems which generate
optical traps. In this study, a graphic processing unit (GPU), NVIDIA GTX 275, is used to boost the speed of trapping
efficiency calculation under ray optics approximation. The codes of trapping efficiency calculation are implemented in
C++. The computing power of GPU is utilized through compute unified architecture device (CUDA) toolkit 4.0. The
computing speed is compared with that of central processing unit (CPU), Intel Core 2 Quad Q9550. Over 100x speedup
is achieved when single-precision floating-point numbers were used in the calculation.
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