基于氧化钼薄膜的光控太赫兹传输特性研究

本文采用热蒸发法制备了沉积在硅片上的200nm 氧化钼(MoO₃)的太赫兹调制薄膜。 在室温条件下,通过傅里叶光谱仪和太赫兹时域光谱系统(THz-TDS)技术,研究了MoO₃薄膜在不同激励光功率下的太赫兹传输特性。提出了薄膜重要光学参数的提取模型,利用透 射式太赫兹时域光谱技术测量了薄膜的时域信号,分别计算了太赫兹波在薄膜中的透过率及 调制效率及薄膜的复介电常数变化。结果表明,在980 nm激光器条件 下,随着激光器功率 的提高,MoO₃薄膜的太赫兹调制深度逐渐增加。在激光功率为266 mW时,在0.26 THz处 透过率达到最低为61%,调制效率(Modulation factor)达到最高为10%。通过分析MoO₃薄膜 的复介电常数及载流子密度变化,得出了激发生成的载流子浓度的提高导致介电常数的改变 , 增强了薄膜的导电性,从而减低了太赫兹波在薄膜中的透过率的结论。为MoO₃薄膜应用在 太赫兹波段调制领域提供了实验数据。     

High Mobility 3D Dirac Semimetal (Cd3As2) for Ultrafast Photoactive Terahertz Photonics

The Dirac semimetal cadmium arsenide (Cd₃As₂), a 3D electronic analog of graphene, has sparked renewed research interests for its novel topological phases and excellent optoelectronic properties. The gapless nature of its 3D electronic band facilitates strong optical nonlinearity and supports Dirac plasmons that are of particular interest to realize high-performance electronic and photonic devices at terahertz (1 THz = 4.1 meV) frequencies, where the performance of most dynamic materials are limited by the tradeoff between power-efficiency and switching speed. Here, all-optical, low-power, ultrafast broadband modulation of terahertz waves using an ultrathin film (100 nm, λ/3000) of Cd₃As₂ are experimentally demonstrated through active tailoring of the photoconductivity. The measurements reveal the photosensitive metallic behavior of Cd₃As₂ with high terahertz electron mobility of 7200 cm² (Vs)⁻¹. In addition, optical fluence dependent ultrafast charge carrier relaxation (15.5 ps), terahertz mobility, and long momentum scattering time (157 fs) comparable to superconductors that invoke kinetic inductance at terahertz frequencies are demonstrated. These remarkable properties of 3D Dirac topological semimetal envision a new class of power-efficient, high speed, compact, tunable electronic, and photonic devices.     

Si-defect concentrations in heavily Si-DOPED GaAs: annealing-Induced changes-II

Isothermal annealing produces changes in the free carrier density, defect-induced localized vibrational mode (LVM) infrared absorption, microstructure as measured by transmission electron microscopy (TEM), and critical resolve shear stress of heavily Si-doped GaAs. The changes have been measured and correlated for three different Si concentrations for several annealing temperatures. The measurements reveal temperature dependent annealing-induced changes in several specific defect concentrations. The observations indicate the following behavior for two ingots with [Si] ≳ 2 × 10¹⁹ cm⁻¹³: (1) when the anneal temperature, T_A = 400°C, the concentration of Si_ga donors, as determined from LVM spectra decreases probably due to the generation of V_Ga defects followed by the formation of Si_Ga-V_Ga pairs. This change is responsible for observed decreases in carrier density and the large increase in yield stress. The yield stress shows a dependence of the form σ-σ_o ∝ [Si_Ga-V_Ga]^1/4. (2) When T_A = 500°C, the LVM spectra indicate that all of the observed Si defect concentrations change. The decrease in [Si_Ga] alone cannot explain the decrease in carrier density, and a previous suggestion that a new acceptor is required is confirmed. Both the LVM measurements and the shear stress indicate that only a small fraction of the [Si_Ga] reduction is by the formation of Si_Ga-V_Ga pairs. (3) When T_A = 700°C, a new acceptor is still required and the other experimental observations at T_A = 500°C are also still seen here. There is a large decrease in [Si_Ga] and [Si_As] observed for short anneal times which coincides with the formation of Si-rich extrinsic loops and the loop area/vol increases with [Si]. (4) When T_A > 700°C, all of the changes become smaller as T_A increases. For lower [Si] ~ 1.5 × 10¹⁸ cm⁻³, no significant annealing-induced changes are observed for any of the T_A given above.     

Misfit strain induced tweed-twin transformation on composition modulation Zn1−xMgxSxSe1−y layers and the quality control of the ZnSe buffer/GaAs interface

[100] composition modulation as well as [101] and $$1$$ tweed strain contrast were observed in 0.72 μm thick Zn_1?xMg_xS Se_1?y epitaxial films grown on ZnSe buffer layers. The lattice distortion induced tweed strain contrast disappears in relaxed Zn_1?xMg_xS Se_1?y layers of thicknesses above ～ 0.8—1 μm even though the [100] composition modulation remains. Instead, the formation of microtwins takes place to relieve the strain in the distorted lattice of the quaternary films. The Zn_1?xMg_xS_ySe_1?y layers were obtained by growing a ZnSe buffer layer on Asstabilized GaAs substrates with Zn treatment of the substrate prior to the growth of the film. The samples with film thickness of ～0.72 μm were of very high quality with a defect density of less than 5 x lO⁴/cm². Some samples showed rough ZnSe/ GaAs interfaces and a high density of Frank partial dislocations originating at the ZnSe/GaAs interface. The interface roughness is believed to result from an As-rich GaAs surface after the oxide desorption.     

Characteristics of Gadolinium Oxide Nanoparticles as Contrast Agents for Terahertz Imaging

For the application of gadolinium oxide (Gd₂O₃) nanoparticles as terahertz contrast agents, their optical properties in a solvent were studied using terahertz time-domain spectroscopy. The power absorption and refractive index of the samples were measured with various concentrations of nanoparticles. The power absorption was extremely large, as much as three orders of magnitude higher than that of water, so that a few ppms of Gd₂O₃ nanoparticles were distinguished in terms of their power absorption capacity. The results show that the interaction between the terahertz electromagnetic waves and the Gd₂O₃ nanoparticles is strong enough to allow their exploitation as contrast agents for terahertz medical imaging.     

Real-Time Determination of Absolute Frequency in Continuous-Wave Terahertz Radiation with a Photocarrier Terahertz Frequency Comb Induced by an Unstabilized Femtosecond Laser

A practical method for the absolute frequency measurement of continuous-wave terahertz (CW-THz) radiation uses a photocarrier terahertz frequency comb (PC-THz comb) because of its ability to realize real-time, precise measurement without the need for cryogenic cooling. However, the requirement for precise stabilization of the repetition frequency (f _rep) and/or use of dual femtosecond lasers hinders its practical use. In this article, based on the fact that an equal interval between PC-THz comb modes is always maintained regardless of the fluctuation in f _rep, the PC-THz comb induced by an unstabilized laser was used to determine the absolute frequency f _THz of CW-THz radiation. Using an f _rep-free-running PC-THz comb, the f _THz of the frequency-fixed or frequency-fluctuated active frequency multiplier chain CW-THz source was determined at a measurement rate of 10 Hz with a relative accuracy of 8.2?×?10^?13 and a relative precision of 8.8?×?10^?12 to a rubidium frequency standard. Furthermore, f _THz was correctly determined even when fluctuating over a range of 20 GHz. The proposed method enables the use of any commercial femtosecond laser for the absolute frequency measurement of CW-THz radiation.     

Amplification of terahertz radiation in a plasmon <Emphasis Type="Italic">n–i–p–i</Emphasis> graphene structure with charge-carrier injection

The absorption/amplification spectrum of terahertz radiation in inhomogeneous graphene (n–i–p–i structure) with a periodic dual metal grating is theoretically investigated. It is shown that the amplification of terahertz radiation sharply increases at the plasmon-resonance frequency, when losses due to electron scattering and emission are balanced by the plasmon gain (related to the stimulated radiative interband recombination of electron–hole pairs in the inverted region of graphene).     

Shallow-donor lasers in uniaxially stressed silicon

The effects of the terahertz-stimulated emission of Group-V donors (phosphorus, antimony, arsenic, bismuth) in uniaxially stressed silicon, excited by CO₂ laser radiation are experimentally studied. It is shown that uniaxial compressive stress of the crystal along the [100] direction increases the gain and efficiency of stimulated radiation, significantly decreasing the threshold pump intensity. The donor frequencies are measured and active transitions are identified in stressed silicon. The dependence of the residual population of active donor states on the uniaxial compressive stress along the [100] direction is theoretically estimated.     

Edge effects in propagation of terahertz radiation in subwavelength periodic structures

Improving detection sensitivity of biological molecules with low absorption characteristics in the terahertz gap still remains an important issue in terahertz vibrational resonance spectroscopy. One possible way to increase coupling of incident terahertz radiation to molecules is to exploit local enhancement of electromagnetic field in periodic slot arrays. In this work, we show that periodic arrays of rectangular slots with subwavelength widths provide for local electromagnetic field enhancements due to edge effects in our low frequency range of interest, 10–25 cm⁻¹. Periodic structures of Au doped Si and InSb were studied. The half power enhancement width is ∼500 nm or less around the slot, edges in all cases, thereby possibly bringing terahertz sensing to the nanoscale. InSb is confirmed to offer the highest results with local power enhancements on the order of 1100 at frequency 14 cm⁻¹. InSb and Si have large skin depths in our frequency range of interest and so the analysis of their structures was done through the Fourier expansion method of field diffracted from gratings. Surface impedance boundary conditions were employed to model the Au structure. The applications possibly include development of novel biosensors, and monitoring biophysical processes such as DNA denaturation. The text was submitted by the authors in English.     

Study of Optimal Cavity Parameter in Optically Pumped D2O Gas Terahertz Laser

Heavy water gas (D₂O gas) which owns special structure property, can generate terahertz radiation by optically pumping technology, and its 385 μm wavelength radiation can be widely used. In this research, on the base of semi-classical density matrix theory, we set up a three-level energy system as its theoretical model, a TEA-CO₂ laser 9R (22) output line (λ?=?9.26 μm) acted as pumping source, D₂O gas molecules were operating medium, the expressions of pumping absorption coefficient G _p and Terahertz signal gain coefficient G _s were deduced. It was shown that the gain of Terahertz signal was related with the energy-level parameters of operating molecules and some operating parameters of the Terahertz laser cavity, mainly including cavity length. By means of iteration method, the output power density of Terahertz pulse signal was calculated numerically. Changing the parameter of cavity length and keeping others steady, the relationship curve between the output power intensity (Is) of Terahertz pulse laser and the operating cavity length (L) was obtained. The curve showed that the power intensity (Is) increased with cavity length (L) in a certain range, but decreased when the length (L) exceeded some value because of the absorption effect, and there was an optimal cavity length for the highest output power. We used a grating tuned TEA-CO₂ laser as pumping power and a sample tube of variable length in 70–160 cm as terahertz laser operating cavity to experiment. The results of theoretical calculation and experiment matched with each other, and it is helpful for miniaturizing terahertz laser volume to make it practical.     

Industrial prospects of Pb1 − x Sn x Te:In with x > 0.3 solid solutions for photodetectors with extended sensitivity spectral range

Photoelectric properties of Pb_{1 ? x} Sn _x Te:In films with composition x > 0.3 in the temperature range from 4.2 to 80 K have been investigated. High sample sensitivity to black-body radiation has been discovered at the temperature of helium, and as the temperature of the radiation source decreases the sensitivity increases, which can be connected with the optical-frequency transition in the short-wavelength infrared and terahertz spectral range. The detectivity value D* = 8.2 × 10¹⁶ cm · Hz^1/2/W corresponding to the NEP = 3.1 × 10^?18 W/Hz^1/2, has been obtained at detector temperature 4.2 K and T _BBR = 15 K.     

The cyclotron resonance in Heterostructures with the InSb/AlInSb quantum wells

The absorption of two-dimensional electrons in InSb-based quantum wells in the quantized magnetic fields in the terahertz spectral region are studied. A p-Ge-based cyclotron laser was used as the radiation source. The effective mass of carriers at the Fermi level equal to 0.0219m ₀ (m ₀ is the mass of a free electron) is determined from the cyclotron resonance spectra. It is shown that the electron spectrum is described by the Kane model in a wide range of magnetic fields. An anomalously pronounced splitting of the cyclotron resonance line not associated with the nonparabolicity of the conduction band of InAs is observed in low magnetic fields, which can be attributed to the effect of the spin-orbit interaction.     

偏振选择可调谐双带太赫兹吸收器

提出并研究了一种偏振选择可调谐双带太赫兹吸收器。吸收器由顶层方形劈裂石墨烯环、中间SiO₂介质层以及底层金反射层组成。基于时域有限差分法的仿真结果显示,该吸收器在不同偏振光入射下均可以实现双带高效率吸收。x偏振光时在7.86和12.63THz处的吸收率分别为97.9%和91.2%;y偏振光时在6.30和10.52THz处的吸收率分别为94.1%和93.2%。通过改变石墨烯费米能级,可以对两个偏振的双带吸收峰波长进行调谐。此外,研究了介质层厚度和石墨烯劈裂环的物理参数对共振吸收峰的影响。因为在两个偏振状态下都能产生双带高吸收,所以此吸收器在太赫兹偏振成像、太赫兹传感、选择性光谱检测和偏振复用等领域有重要的潜在应用价值。     

Temperature-Induced Self-Decomposition Doping of Fe3GeTe2 to Achieve Ultra-High Tc of 496 K for Multispectral Compatible Strong Electromagnetic Wave Absorption

Fe₃GeTe₂ provides an ideal system for the study of itinerant ferromagnetism. However, the Curie temperature (Tc) below room temperature limits its further in-depth research and wide applications. Here, the Tc of 224 K for Fe₃GeTe₂ is greatly increased to 496 K for Fe₃GeTe₂/FeTe₂/Fe₃Ge by temperature-induced self-decomposition doping, contributing to improve the electromagnetic response for enhancing the absorption strength and bandwidth and boosting application potentials at higher temperatures. Then, to achieve the multispectral compatible strong absorption, room temperature ferromagnetic dielectric graphene@Fe₃GeTe₂/FeTe₂/Fe₃Ge absorber with 3D porous network structure is prepared by synergistic self-assembly. It has high Tc of 443 K with strong electromagnetic synergistic loss, achieving reflection loss of −61 dB at 12.6 GHz. Moreover, its absorption loss in the terahertz band (0.1–1.6 THz) is 76 dB, and the average loss is greater than 50.4 dB. Furthermore, the absorber is very potential as stealth skin to efficiently reduce satellite RCS with a multi-angle and multi-band manner in the gigahertz and terahertz bands, thus achieving the purpose of stealth. Compared with the reported conventional absorbers, the absorber has a multispectral compatible strong absorption performance covering gigahertz and terahertz bands, thus very promising for electromagnetic protection of electromagnetic communication and space vehicles.     

N,7,7‐Tricyanoquinomethanimine (TCQMI) Based Organic Magnetic Materials

The synthesis and characterization of a new family of magnetic materials based on the electron accepting cyanocarbon N,7,7‐tricyanoquinomethanimine, TCQMI, its radical anion [TCQMI], and its σ‐dimer, σ‐[TCQMI]₂^2?, are reported. [Fe^IIICp*₂][TCQMI] (where Cp* is pentamethylcyclopentadienide) forms parallel chains of alternating [TCQMI]^?? and [FeCp*₂]^?+ and magnetically orders at 3.4 K as a weak ferromagnet. M[TCQMI]₂?zCH₂Cl₂ (M = V, Fe) are amorphous solids with [TCQMI]^?? coordinated to metal centers through the nitrile groups. The Fe compound magnetically orders as a weak ferromagnet at ≈4 K, whereas the V compound shows no evidence of magnetic ordering. {[Mn^IIITPP]⁺}₂[TCQMI]₂^2? (TPP = tetraphenylporphyrin) results from the reaction of TCQMI with Mn^IITPP(py) due to the formation of the [TCQMI]₂^2? σ‐dimer in situ, and is a weak ferromagnet below 3.7 K. The lack of magnetic ordering in V[TCQMI]₂?zCH₂Cl₂ is not currently understood, and is in strong contrast to V[TCNE]₂?zCH₂Cl₂, which magnetically orders above room temperature.     

Half Cycle Terahertz Pulse Generation by Prism-Coupled Cherenkov Phase-Matching Method

Nonlinear optical terahertz wave generation is a promising method for realizing a practical source with wide frequency range and high peak power. Unfortunately, many nonlinear crystals have a strong absorption in the terahertz frequency region. This limits efficient and widely tunable terahertz wave generation. The Cherenkov phase-matching method is one of the most promising techniques for overcoming these problems. We propose a prism-coupled Cherenkov phase-matching method, in which a prism with a suitable refractive index at terahertz frequencies is coupled to a nonlinear crystal. We demonstrate prism-coupled Cherenkov phase-matching terahertz generation using the DAST and LiNbO₃ crystals. With a DAST crystal, we obtain a spectral flat tunability up to 10 THz by difference frequency generation. With a LiNbO₃ crystal, we observe a spectral flat broadband terahertz pulse generation up to 5 THz pumped by a femto second fiber laser. The obtained temporal waveform is an ideal half cycle pulse suitable for reflection terahertz tomography.     

Enhanced Electrochromism with Rapid Growth Layer‐by‐Layer Assembly of Polyelectrolyte Complexes

In this work, a facile method to deposit fast growing electrochromic multilayer films with enhanced electrochemical properties using layer‐by‐layer (LbL) self‐assembly of complex polyelectrolyte is demonstrated. Two linear polymers, poly(acrylic acid) (PAA) and polyethylenimine (PEI), are used to formulate stable complexes under specific pH to prepare polyaniline (PANI)/PAA‐PEI multilayer films via LbL deposition. By introducing polymeric complexes as building blocks, [PANI/PAA‐PEI]_n films grow much faster compared with [PANI/PAA]_n films, which are deposited under the same condition. Unlike the compact [PANI/PAA]_n films, [PANI/PAA‐PEI]_n films exhibit porous structure that is beneficial to the electrochemical process and leads to improved electrochromic properties. An enhanced optical modulation of 30% is achieved with [PANI/PAA‐PEI]₃₀ films at 630 nm compared with the lower optical modulation of 11% measured from [PANI/PAA]₃₀ films. The switching time of [PANI/PAA‐PEI]₃₀ films is only half of that of [PANI/PAA]₃₀ films, which indicates a faster redox process. Utilizing polyelectrolyte complexes as building blocks is a promising approach to prepare fast growing LbL films for high performance electrochemical device applications.     

Growth and visible photoluminescence of highly oriented (1 0 0) zinc oxide film synthesized on silicon by plasma immersion ion implantation

High-quality (1 0 0) ZnO films with smooth surface topography have been synthesized on Si substrate by plasma immersion ion implantation. The materials exhibit compressive stress because of room temperature growth. After annealing at different temperatures, various visible photoluminescence bands are observed. The optical phenomenon as well as the transition mechanism which may involve defects such as [Zn_I], [V_Zn], and [O_i⁻] induced by the high substrate bias are discussed in this paper.     

A thermally tunable terahertz bandpass filter with insulator-metal phase transition of VO2 thin film

A terahertz bandpass filter with the sandwich structure consisting of thermally tunable vanadium dioxide （VO2） thin film, silica substrate and subwavelength rectangular Cu hole arrays is designed and theoretically analyzed. The results show that the transmittance of the filter can be actively tuned by controlling the temperature of VO2, the narrow band terahertz （THz） waves with the transmittance from 85.2% to 10.5% can be well selected at the frequency of 1.25 THz when the temperature changes from 50 ℃ to 80 ℃, and the maximum modulation depth of this terahertz bandpass fil- ter can achieve 74.7%.     

Modulation instability of transversely limited electromagnetic waves of terahertz range in strontium titanate paraelectric

Inthe framework of developed approximate phenomenological model we have performed the investigation of modulation instability of transversely limited electromagnetic waves of terahertz range in paraelectric crystals SrTiO₃ at the temperatures of about 77 K. Cubic nonlinearity and frequency dispersion correspond to the existence of modulation instability of long input pulses. The results of numerical investigations of the modulation instability are presented. Transversal boundedness of the input pulses can stabilize the modulation instability. Modulation instability threshold is reduced in the presence of reflections from the crystal boundaries. In the case of development of the modulation instability it is possible to generate a sequence of short terahertz pulses at the output of the crystal. The focusing of input pulses reduces the threshold of modulation instability.