High photoelectric conversion efficiency and fast relaxation time of FA_(0.4)MA_(0.6)PbI_(3) applied in ultrafast modulation of terahertz waves

Active control of terahertz(THz)waves is attracting tremendous attentions in terahertz communications and active photonic devices.Perovskite,due to its excellent photoelectric conversion performance and simple manufacturing process,has emerged as a promising candidate for optoelectronic applications.However,the exploration of perovskites in optically controlled THz modulators is still limited.In this work,the photoelectric properties and carrier dynamics of FA_(0.4)MA_(0.6)PbI_(3)perovskite films were investigated by optical pumped terahertz probe(OPTP)system.The ultrafast carrier dynamics reveal that FA_(0.4)MA_(0.6)PbI_(3)thin film exhibits rapid switching and relaxation time within picosecond level,suggesting that FA_(0.4)MA_(0.6)PbI_(3)is an ideal candidate for active THz devices with ultrafast response.Furthermore,as a proof of concept,a FA_(0.4)MA_(0.6)PbI_(3)-based metadevice with integrating plasma-induced transparency(PIT)effect was fabricated to achieve ultrafast modulation of THz wave.The experimental results demonstrated that the switching time of FA_(0.4)MA_(0.6)PbI_(3)-based THz modulator is near to 3.5 ps,and the threshold of optical pump is as low as 12.7μJ cm^(-2).The simulation results attribute the mechanism of ultrafast THz modulation to photo-induced free carriers in the FA_(0.4)MA_(0.6)PbI_(3)layer,which progressively shorten the capacitive gap of PIT resonator.This study not only illuminates the potential of FA_(0.4)MA_(0.6)PbI_(3)in THz modulation,but also contributes to the field of ultrafast photonic devices.

Dynamic phase assembled terahertz metalens for reversible conversion between linear polarization and arbitrary circular polarization

If a metalens integrates the circular polarization(CP)conversion function,the focusing lens together with circular-polariz-ing lens(CPL)in traditional cameras may be replaced by a metalens.However,in terahertz(THz)band,the reported metalenses still do not obtain the perfect and strict single-handed CP,because they were constructed via Pancharatnam-Berry phase so that CP conversion contained both left-handed CP(LCP)and right-handed CP(RCP)components.In this paper,a silicon based THz metalens is constructed using dynamic phase to obtain single-handed CP conversion.Also,we can rotate the whole metalens at a certain angle to control the conversion of multi-polarization states,which can simply manipulate the focusing for incident linear polarization(LP)THz wave in three polarization conversion states,in-cluding LP without conversion,LCP and RCP.Moreover,the polarization conversion behavior is reversible,that is,the THz metalens can convert not only the LP into arbitrary single-handed CP,but also the LCP and RCP into two perpen-dicular LP,respectively.The metalens is expected to be used in advanced THz camera,as a great candidate for tradi-tional CPL and focusing lens group,and also shows potential application in polarization imaging with discriminating LCP and RCP.

Energy scaling and extended tunability of a ring cavity terahertz parametric oscillator based on KTiOPO_(4) crystal

A wide terahertz tuning range from 0.96 THz to 7.01 THz has been demonstrated based on ring-cavity THz wave parametric oscillator with a KTiOPO_(4)(KTP)crystal.The tuning range was observed intermittently from 0.96 THz to 1.87 THz,from 3.04 THz to 3.33 THz,from 4.17 THz to 4.48 THz,from 4.78 THz to 4.97 THz,from 5.125 THz to 5.168 THz,from5.44 THz to 5.97 THz,and from 6.74 THz to 7.01 THz.The dual-Stokes wavelengths resonance phenomena were observed in some certain tuning angle ranges.Through the theoretical analysis of the dispersion curve of the KTP crystal,the intermittent THz wave tuning range and dual-wavelength Stokes waves operation during angle tuning process were explained.The theoretical analysis was in good agreement with the experiment results.The maximum THz output voltage detected by Golay cell was 1.7 V at 5.7 THz under the pump energy of 210 mJ,corresponding to the THz wave output energy of5.47μJ and conversion efficiency of 2.6×10^(-5).

Terahertz metasurface zone plates with arbitrary polarizations to a fixed polarization conversion

Metasurfaces that can realize the polarization manipulation of electromagnetic waves on the sub-wavelength scale have become an emerging research field.Here,a novel strategy of combining the metasurface and Fresnel zone plate to form a metasurface zone plate is proposed to realize the conversion from nearly arbitrary polarizations to a fixed polarization.Specifically,when one polarized wave is incident on adjacent ring zones constructed by different types of meta-atoms,the transmitted waves generated by odd-numbered and even-numbered ring zones converge at the same focus and superimpose to generate a fixed polarized wave.As function demonstrations,we have designed two types of metasurface zone plates:one is a focused linear polarizer,and the other can convert nearly arbitrary polarized waves into focused circularly polarized waves.The simulated and measured results are consistent with theoretical expectations,suggesting that the proposed concept is flexible and feasible.Our work provides an alternative platform for polarization manipulation and may vigorously promote the development of polarization photonic devices.

High-sensitive terahertz detection by parametric up-conversion using nanosecond pulsed laser

A high-sensitive terahertz detector operating at room temperature was demonstrated based on parametric upconversion.A nanosecond 1064-nm Nd:YAG laser was used to pump the parametric up-conversion detector and the upconversion from terahertz wave to NIR laser was realized in a lithium niobate crystal.The minimum detectable terahertz energy of 9 p J was realized with the detection dynamic range of 54 d B,which was three orders of magnitude higher than that of commercial Golay cell.The detectable terahertz frequency range of the detection system was 0.90 Thz–1.83 THz.Besides,the effects of pump energy and effective gain length on the detection sensitivity were studied in experiment.The results showed that higher pump energy and longer effective gain length are helpful for improving the detection sensitivity of parametric up-conversion detector.

Dynamically tunable terahertz passband filter based on metamaterials integrated with a graphene middle layer

The dynamic tunability of a terahertz（THz） passband filter was realized by changing the Fermi energy（EF） of graphene based on the sandwiched structure of metal-graphene-metal metamaterials（MGMs）. By using plane wave simulation, we demonstrated that the central frequency（ f0） of the proposed filter can shift from 5.04 THz to 5.71 THz; this shift is accompanied by a 3 dB bandwidth（ f） decrease from 1.82 THz to 0.01 THz as the EFincreases from 0 to 0.75 eV.Additionally, in order to select a suitable control equation for the proposed filter, the curves of f and f0 under different graphene EFwere fitted using five different mathematical models. The fitting results demonstrate that the Dose Resp model offers accurate predictions of the change in the 3 dB bandwidth, and the Quartic model can successfully describe the variation in the center frequency of the proposed filter. Moreover, the electric field and current density analyses show that the dynamic tuning property of the proposed filter is mainly caused by the competition of two coupling effects at different graphene EF, i.e., graphene-polyimide coupling and graphene-metal coupling. This study shows that the proposed structures are promising for realizing dynamically tunable filters in innovative THz communication systems.

Terahertz two-pixel imaging based on complementary compressive sensing

A compact terahertz（THz） imaging system based on complementary compressive sensing has been proposed using two single-pixel detectors. By using a mechanical spatial light modulator, sampling in the transmission and reflection orientations was achieved simultaneously, which allows imaging with negative mask values. The improvement of THz image quality and anti-noise performance has been verified experimentally compared with the traditional reconstructed image, and is in good agreement with the numerical simulation. The demonstrated imaging system, with the advantages of high imaging quality and strong anti-noise property, opens up possibilities for new applications in the THz region.

Single-mode antiresonant terahertz fiber based on mode coupling between core and cladding

Based on the index-induced mode coupling between the higher-order mode in core and the fundamental mode in cladding tubes,the single-mode operation can be realized in any antiresonant fibers(ARFs)when satisfying that the area ratio of cladding tube and core is about 0.46:1,and this area ratio also should be modified according to the shape and the number of cladding tubes.In the ARF with nodal core boundary,the mode in core also can couple with the mode in the wall of core boundary,which can further enhance the suppression of high-order mode.Accordingly,an ARF with conjoint semi-elliptical cladding tubes realizes a loss of higher-order mode larger than 30 dB/m;simultaneously,a loss of fundamental mode loss less than 0.4 dB/m.

THz wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves

We propose a novel scheme for THz wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves(HSWs).The repeated frequency conversions are accomplished by oscillations of Stoke waves in resonant cavity(RC)where low-order Stokes waves(LSWs)are converted to high-order Stokes waves again and again.The continuous frequency conversions are accomplished by optimized cascaded difference frequency generation(OCDFG)where the poling periods of the optical crystal are aperiodic leading to the frequency conversions from low-order Stokes waves to high-order Stokes waves uninterruptedly and unidirectionally.Combined with the repeated and continuous frequency conversions,the optical-to-THz energy conversion efficiency(OTECE)exceeds 26%at 300 K and 43%at 100 K with pump intensities of 300 MW/cm^(2).

High sensitivity dual core photonic crystal fiber sensor for simultaneous detection of two samples

The optical control ability of photonic crystal fiber(PCF)is a distinctive property suitable for improving sensing and plasma performance.This article proposes a dual-core D-channel PCF sensor that can detect two samples simultaneously,which effectively solves the problems of coating difficulty and low wavelength sensitivity.The PCF has four layers of air holes,which dramatically reduces the optical fiber loss and is more conducive to the application of sensors in actual production.In addition,by introducing dual cores on the upper and lower sides of the central air hole,reducing the spacing between the core and the gold nanolayer,a stronger evanescent field can be generated in the cladding air hole.The optical fiber sensor can detect the refractive index of two samples simultaneously with a maximum sensitivity of 21300 nm/RIU.To the best of our knowledge,the sensitivity achieved in this work is the highest sensitivity with the dual sample synchronous detection sensors.The detection range of the refraction index is 1.35-1.41,and the resolution of the sensor is 4.695×10^(-6).Overall,the sensor will be suitable for medical detection,organic chemical sensing,analyte detection,and other fields.

High-efficiency terahertz wave generation with multiple frequencies by optimized cascaded difference frequency generation

High-efficiency terahertz(THz) wave generation with multiple frequencies by optimized cascaded difference frequency generation(OCDFG) is investigated at 100 K using a nonlinear crystal consisting of a periodically poled lithium niobate(PPLN) part and an aperiodically poled lithium niobate(APPLN) part.Two infrared pump waves with a frequency difference ω_(T1) generate THz waves and a series of cascaded optical waves in the PPLN part by cascaded difference frequency generation(CDFG).The generated cascaded optical waves with frequency interval ω_(T1) then further interact in the APPLN part by OCDFG,yielding the following two advantages.First,OCDFG in the APPLN part is efficiently stimulated by inputting multi-order cascaded optical waves rather than the only two intense infrared pump waves,yielding unprecedented energy conversion efficiencies in excess of 37% at 1 THz at 100 K.Second,THz waves with M timesω_(T1) are generated by mixing the mth-order and the(m+M)th-order cascaded optical waves by designing poling period distributions of the APPLN part.

Theoretical research on terahertz wave generation from planar waveguide by optimized cascaded difference frequency generation

A novel scheme for high-efficiency terahertz(THz)wave generation based on optimized cascaded difference frequency generation(OCDFG)with planar waveguide is presented.The phase mismatches of each-order cascaded difference frequency generation(CDFG)are modulated by changing the thickness of the waveguide,resulting in a decrement of phase mismatches in cascaded Stokes processes and an increment of phase mismatches in cascaded anti-Stokes processes simultaneously.The modulated phase mismatches enhance the cascaded Stokes processes and suppress the cascaded anti-Stokes processes simultaneously,yielding energy conversion efficiencies over 25%from optical wave to THz wave at 100 K.

Spider Structure of Photoelectron Momentum Distributions of Ionized Electrons from Hydrogen Atoms for Extraction of Carrier Envelope Phase of Few-Cycle Pulses

The spider structure in the photoelectron momentum distributions(PMDs)of ionized electrons from the hydrogen atom is simulated by solving the time-dependent Schrodinger equation(TDSE).We find that the spider structure exhibits sensitive dependence on carrier envelope phase(CEP)of the few-cycle pulses.To elucidate the striking CEP dependence of the spider structure,we select three physical parameters IL,IR,and IR/IL to quantitatively characterize the variations of the spider structure induced by altering the CEPs.IL is the sum of the left half panel of the transverse cut curves(i.e.,the sum of all the negative momenta along the laser polarization direction),IR is the sum of the right half panel of the transverse cut curves(i.e.,the sum of all the positive momenta along the laser polarization direction),and IR/IL is the ratio between the two sums.These parameters are shown to have monotonic relation with the CEP value,which is exploited to extract the CEPs.We anticipate that our method will be useful for obtaining CEPs encoded in the spider structure of PMDs.

Spectroscopy Studies on Stream Sediments in the Terahertz Range

In order to study stream sediments in the terahertz range, we have measured six reference stream sediment samples by terahertz time-domain spectroscopy (THz-TDS). We obtained the absorption coefficients and refractive indexes. By analyzing the spectra, we got different drops in amplitude and delays in time. The absorption and refractive properties of samples changed with its components and types. In addition, we also found there was a nearly linear relationship between the absorption coefficient and the frequency. We calculated the slope value (K) of each sample by linear fitting, and find the K was corresponding to the contents of the samples. The results showed THz-TDS was an effective method to the analysis of stream sediments.

The Hybrid Light Source for Projection Display

A scheme of hybrid light source using lasers and LEDs for projection display is presented;the si-mulation results show that projectors with the proposed scheme can achieve high brightness of over 1000 lm, good uniformity of illumination and chromaticity. It also provides an improved me-thod for light source with longer life, wider color garment and higher adaptability to environment which can replace UHP in traditional projectors.

Statistical Transform of Signal Field with ASE Noise through a Fiber Amplifier

While the signal field + ASE noise pass through a span of transmission fiber, a dispersion compensation grating and a fiber amplifier(with the generation of ASE noise), the nonlinear Fokker-Plank equations, describing the probability transforms of the field, are established and solved. Based on these statistical theories, the probability distributions of the signal + ASE noise field through 50km NZDSF, a dispersion compensation grating and a fiber amplifier link, are obtained. The dispersion and nonlinear effects in transmission fiber induce frequency offsets in the probability distribution of field and they cannot be dissipated by dispersion compensation. The generation of ASE noise in the amplifier will accelerate this frequency offset.

A continuous-wave Nd:YVO_(4)-KGW intracavity Raman laser with over 34% diode-to-Stokes optical efficiency

We demonstrate a continuous-wave(CW)Nd:YVO_(4)-potassium gadolinium tungstate(KGW)intracavity Raman laser with a diode-to-Stokes optical efficiency of 34.2%.By optimizing the cavity arrangement and reducing the cavity losses,8.47 W Stokes output at 1177 nm was obtained under an incident 878.6 nm diode pump power of 24.8 W.The influence of cavity losses on the power and efficiency of the CW Raman laser,as well as the potential for further optimization,was investigated based on the numerical model.The observation of thermally-induced output rollover was well explained by the calculation of the thermal lensing and cavity stability,indicating that the end-face curvature played an important role when the end-face of the crystal was highly reflective coated to make the cavity.A 10.9 W Stokes output under 40.9 W incident pump was also demonstrated with a cavity arrangement less sensitive to the end-face curvature,which is the highest output power of CW intracavity Raman lasers reported.

Full-Stokes metasurface polarimetry requiring only a single measurement

Polarization is crucial in various fields such as imaging,sensing,and substance detection.A compact,fast,and accurate polarization detection device is vital for these applications.Herein,we demonstrate a multifocus metalens for terahertz polarization detection that requires only a single measurement to obtain complete polarization parameters and reconstruct the polarization state of the incident field.The individual subarrays of this metalens convert each of the six polarized components into the same polarization,which in turn links the Stokes parameters to these six foci.The incident linear polarizations and elliptical polarizations are characterized by Stokes parameters and polarization ellipses.Simulations and experimental results show that the scheme can accurately detect the incident polarization with a single measurement.The proposed metasurface polarimetry may find applications in the fields of real-time terahertz detection and integrated optics.

Hybrid metasurface using graphene/graphitic carbon nitride heterojunctions for ultrasensitive terahertz biosensors with tunable energy band structure

Integrating novel materials is critical for the ultrasensitive,multi-dimensional detection of biomolecules in the terahertz(THz)range.Few studies on THz biosensors have used semiconductive active layers with tunable energy band structures.In this study,we demonstrate three THz biosensors for detecting casein molecules based on the hybridization of the metasurface with graphitic carbon nitride,graphene,and heterojunction.We achieved lowconcentration detection of casein molecules with a 3.54 ng/m L limit and multi-dimensional sensing by observing three degrees of variations(frequency shift,transmission difference,and phase difference).The favorable effect of casein on the conductivity of the semiconductive active layer can be used to explain the internal sensing mechanism.The incorporation of protein molecules changes the carrier concentration on the surface of the semiconductor active layer via the electrostatic doping effect as the concentration of positively charged casein grows,which alters the energy band structure and the conductivity of the active layer.The measured results indicate that any casein concentration can be distinguished directly by observing variations in resonance frequency,transmission value,and phase difference.With the heterojunction,the biosensor showed the highest response to the protein among the three biosensors.The Silvaco Atlas package was used to simulate the three samples'energy band structure and carrier transport to demonstrate the benefits of the heterojunction for the sensor.The simulation results validated our proposed theoretical mechanism model.Our proposed biosensors could provide a novel approach for THz metasurface-based ultrasensitive biosensing technologies.

Binary encoding-inspired generation of vector vortex beams

Vortex and vector terahertz(THz)beams with inhomogeneous polarization states have become a considerable hot topic in the last decade due to their important roles in multiplexed wireless data transmission.However,the technical instruments to form and evaluate such beams are rather limited,as they are mainly collected in the visible range.The emergence of coded metasurfaces has reinvigorated the design of functional devices with multiple degrees of freedom.Here,we demonstrate a versatile design inspired by a binary encoding approach for generating off-axis vectorial THz beams on all-silicon metasurfaces.Experimentally,simultaneous manipulation of the encoding method within the orthogonal circularly polarized(CP)channel provides strong support for developing versatility in vector THz beams,including the generation of arbitrary polarization profiles and the directional emission of multiplexed vector vortex beams(VVBs)with specified polarization states.The introduction of the latitudinal polarization control factor guarantees that continuously rotating or independently designed vector polarization profiles can be obtained on a pre-defined focal plane.Subsequently,a universal parametric theoretical model is developed to analyze the evolutionary trend of off-axis VVB in detail.Notably,the proposed monolayer vector metasurface bypasses the requirement of multiple cascaded optical elements,further inducing the advancement of ultra-thin vector THz sources,and also facilitating the generation of vector fields with tailored polarization distributions in 3D space.