Resonantly enhanced second-and third-harmonic generation in dielectric nonlinear metasurfaces

Nonlinear dielectric metasurfaces provide a promising approach to control and manipulate frequency conversion optical processes at the nanoscale,thus facilitating both advances in fundamental research and the development of new practical applications in photonics,lasing,and sensing.Here,we employ symmetry-broken metasurfaces made of centrosymmetric amorphous silicon for resonantly enhanced second-and third-order nonlinear optical response.Exploiting the rich physics of optical quasi-bound states in the continuum and guided mode resonances,we comprehensively study through rigorous numerical calculations the relative contribution of surface and bulk effects to second-harmonic generation(SHG)and the bulk contribution to third-harmonic generation(THG) from the meta-atoms.Next,we experimentally achieve optical resonances with high quality factors,which greatly boosts light-matter interaction,resulting in about 550 times SHG enhancement and nearly 5000-fold increase of THG.A good agreement between theoretical predictions and experimental measurements is observed.To gain deeper insights into the physics of the investigated nonlinear optical processes,we further numerically study the relation between nonlinear emission and the structural asymmetry of the metasurface and reveal that the generated harmonic signals arising from linear sharp resonances are highly dependent on the asymmetry of the meta-atoms.Our work suggests a fruitful strategy to enhance the harmonic generation and effectively control different orders of harmonics in all-dielectric metasurfaces,enabling the development of efficient active photonic nanodevices.

In vivo label-free measurement of blood flow velocity symmetry based on dual line scanning third-harmonic generation microscopy excited at the 1700 nm window

Measurement of bloodflow velocity is key to understanding physiology and pathology in vivo.While most measurements are performed at the middle of the blood vessel,little research has been done on characterizing the instantaneous bloodflow velocity distribution.This is mainly due to the lack of measurement technology with high spatial and temporal resolution.Here,we tackle this problem with our recently developed dual-wavelength line-scan third-harmonic generation(THG)imaging technology.Simultaneous acquisition of dual-wavelength THG line-scanning signals enables measurement of bloodflow velocities at two radially symmetric positions in both venules and arterioles in mouse brain in vivo.Our results clearly show that the instantaneous bloodflow velocity is not symmetric under general conditions.

Enhanced picosecond terahertz wave generation based on cascade effects in a terahertz parametric generator

Enhanced terahertz wave generation via a Stokes cascade process has been demonstrated using picosecond pulse pumped terahertz parametric generation at 1 kHz repetition rate.Clear cascade saturation of terahertz output was observed,and the corresponding cascade-Stokes spectra were analyzed.The maximum terahertz wave average power was 22μW under a pump power of 30 W,whereas the maximum power conversion efficiency was 8×10^(-7)under a pump power of 21 W.The THz power fluctuation was measured to be about 1%in 20 min.This THz parametric source with a relatively stable output is suitable for a variety of practical applications.

Polaronic Electron-Phonon Interactions on the Third-Harmonic Generation in a Square Quantum Well

The influence of electron-phonon interactions on third-harmonic generation in a square quantum well is investigated. The first- and third-harmonic generation coefficient is obtained by using compact-density-matrix approach and iterative method, and the numerical results are presented for a GaAs square quantum well. The results show that the third-harmonic generation coefficient is obviously enhanced after considering the influence of electron-phonon interactions.

Deep-skin third-harmonic generation(THG)imaging in vivo excited at the 2200 nm window

The skin is heterogeneous and exerts strong scattering and aberration onto excitation light in multiphoton microscopy(MPM).Shifting to longer excitation wavelengths may help reduce skin scattering and aberration,potentially enabling larger imaging depths.However,previous demonstrations of skin MPM employ excitation wavelengths only up to the 1700 nm window,leaving an open question as to whether longer excitation wavelengths are suitable for deep-skin MPM.Here,in order to explore the longer-wavelength territory,first,we demonstrate characterization of the broadband transmittance of excised mouse skin,revealing a high transmittance window at 2200nm.Then,we demonstrate third-harmonic generation(THG)imaging in mouse skin in vivo excited at this window.With 9mW optical power on the skin surface operating at 1MHz repetition rate,we can get THG signals of 250
m below the skin surface.Comparative THG imaging excited at the 1700nm window shows that as imaging depth increases,THG signals decay even faster than those excited at 2200 nm.Our results thus uncover the 2200 nm window as a new,promising excitation window potential for deep-skin MPM.

A Novel Cascaded PID Controller for Automatic Generation Control Analysis With Renewable Sources

Present day power scenarios demand a high quality uninterrupted power supply and needs environmental issues to be addressed. Both concerns can be dealt with by the introduction of the renewable sources to the existing power system. Thus, automatic generation control(AGC) with diverse renewable sources and a modified-cascaded controller are presented in the paper.Also, a new hybrid scheme of the improved teaching learning based optimization-differential evolution(hITLBO-DE) algorithm is applied for providing optimization of controller parameters. A study of the system with a technique such as TLBO applied to a proportional integral derivative(PID), integral double derivative(IDD) and PIDD is compared to hITLBO-DE tuned cascaded controller with dynamic load change.The suggested methodology has been extensively applied to a 2-area system with a diverse source power system with various operation time non-linearities such as dead-band of, generation rate constraint and reheat thermal units. The multi-area system with reheat thermal plants, hydel plants and a unit of a wind-diesel combination is tested with the cascaded controller scheme with a different controller setting for each area. The variation of the load is taken within 1% to 5% of the connected load and robustness analysis is shown by modifying essential factors simultaneously by± 30%. Finally, the proposed scheme of controller and optimization technique is also tested with a 5-equal area thermal system with non-linearities. The simulation results demonstrate the superiority of the proposed controller and algorithm under a dynamically changing load.

Third-harmonic generation and imaging with resonant Si membrane metasurface

Dielectric metasurfaces play an increasingly important role in enhancing optical nonlinear generations owing to their ability to support strong light-matter interactions based on Mie-type multipolar resonances.Compared to metasurfaces composed of the periodic arrangement of nanoparticles,inverse,so-called,membrane metasurfaces offer unique possibilities for supporting multipolar resonances,while maintaining small unit cell size,large mode volume and high field enhancement for enhancing nonlinear frequency conversion.Here,we theoretically and experimentally investigate the formation of bound states in the continuum(BICs)from silicon dimer-hole membrane metasurfaces.We demonstrate that our BIC-formed resonance features a strong and tailorable electric near-field confinement inside the silicon membrane films.Furthermore,we show that by tuning the gap between the holes,one can open a leaky channel to transform these regular BICs into quasi-BICs,which can be excited directly under normal plane wave incidence.To prove the capabilities of such metasurfaces,we demonstrate the conversion of an infrared image to the visible range,based on the Third-harmonic generation(THG)process with the resonant membrane metasurfaces.Our results suggest a new paradigm for realising efficient nonlinear photonics metadevices and hold promise for extending the applications of nonlinear structuring surfaces to new types of all-optical near-infrared imaging technologies.

Spectral modulation of third-harmonic generation by molecular alignment and preformed plasma

We demonstrate spectral modulation of third-harmonic generation from molecular alignment effects. The third harmonic spectrum is broadened or narrowed under different influences of cross-phase modulations originating from various molecular alignment revivals. Furthermore, the spectrum and spatial distribution of the generated third harmonic pulse change dramatically in the presence of a preformed plasma. Under the influence of a preformed plasma, a narrower third harmonic spectrum is observed, and the conical third-harmonic pulse increases while the axial part decreases. The investigation provides an effective method to modulate the spectral characteristic and spatial distribution of third-harmonic generation from intense femtosecond filament.

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.

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.

Creation of multi-frequency terahertz waves by optimized cascaded difference frequency generation

A new scheme which generates multi-frequency terahertz(THz)waves from planar waveguide by the optimized cascaded difference frequency generation(OCDFG)is proposed.A THz wave with frequencyω_(T1)is generated by the OCDFG with two infrared pump waves,and simultaneously a series of cascaded optical waves with a frequency intervalω_(T1)is generated.The THz wave with a frequency of M-timesω_(T1)is generated by mixing the m-th-order and the(m+M)-th-order cascaded optical wave.The phase mismatch distributions of cascaded difference frequency generation(CDFG)are modulated by changing the thickness of planar waveguide step by step,thereby satisfying the phase-matching condition from first-order to high-order cascaded Stokes process step by step.As a result,the intensity of THz wave can be enhanced and modulated by controlling the cascading order of OCDFG.

Low-Threshold Broadband Spectrum Generation by Amplification of Cascaded Stimulated Raman Scattering in an Ytterbium-Doped Fiber Amplifier

We present an experimental study on low-threshold broadband spectrum generation mainly due to the amplirfication of the cascaded stimulated Raman scattering （SRS） effect in a four-stage fiber master oscillator power amplifier system. The cascaded SRS is achieved by using a long passive fiber pumped by a pulsed fiber laser cen： tered at wavelength 1064 nm. The amplified spontaneous emission during the amplification process is efficiently suppressed by cutting the length of the passive fiber and by using a double-clad ytterbium-doped fiber amplifier. The generated broadband spectrum spans from 960nm to 1700nm with maximum average output 13.6 W and average spectral power density approximately 17. 7 mW/nm.

Nonlinear Cascaded Femtosecond Third Harmonic Generation by Multi-grating Periodically Poled MgO-doped Lithium Niobate

Nonlinear cascaded femtosecond third harmonic generation was experimentally investigated pumped by 100 fs pulses at optical communication band 1550 nm using a multi-grating 5 mol. % MgO-doped periodically poled lithium niobate crystal. The optimized efficiency of 10.8% was achieved with the simultaneous phase-matching of the second harmonic and sum frequency process. And the third harmonic spectrum reached as broad as 8.7 nm because of the choosing of a small group velocity mismatching between the fundamental and second harmonic pulses. Nonlinear cascaded method will provide a reference for the efficient frequency conversion in the high intensity range.

Short-term power generation scheduling rules for cascade hydropower stations based on hybrid algorithm

Power generation dispatching is a large complex system problem with multi-dimensional and nonlinear characteristics. A mathematical model was established based on the principle of reservoir operation. A large quantity of optimal scheduling processes were obtained by calculating the daily runoff process within three typical years, and a large number of simulated daily runoff processes were obtained using the progressive optimality algorithm （POA） in combination with the genetic algorithm （GA）. After analyzing the optimal scheduling processes, the corresponding scheduling rules were determined, and the practical formulas were obtained. These rules can make full use of the rolling runoff forecast and carry out the rolling scheduling. Compared with the optimized results, the maximum relative difference of the annual power generation obtained by the scheduling rules is no more than 1%. The effectiveness and practical applicability of the scheduling rules are demonstrated by a case study. This study provides a new perspective for formulating the rules of power generation dispatching.

A hybrid stochastic fractal search and pattern search technique based cascade PI-PD controller for automatic generation control of multi-source power systems in presence of plug in electric vehicles

A hybrid Stochastic Fractal Search plus Pattern Search （hSFS-PS） based cascade PI-PD controller is suggested in this paper for Automatic Generation Control （AGC） of thermal, hydro and gas power unit based power systems in presence of Plug in Electric Vehicles （PEV）. Firstly, a single area multi-source power system consisting of thermal hydro and gas power plants is considered and parameters of Integral （I） controller is optimized by Stochastic FractaI Search （SFS） algorithm. The superiority of SFS algorithm over some recently proposed approaches such as optimal control, differential evolution and teaching learning based optimization techniques is demonstrated by comparing simulation results for the identical power system. To improve the system performance further, Pattern Search （PS） is subsequently employed. The study is further extended for different controllers like PI, PID, and cascaded PI-PD controller and the superiority of cascade PI-PD controller over conventional controllers is demonstrated. Then, cascade PI- PD controller parameters of AGC searched using the proposed hSFS-PS algorithm in presence of plug in electric vehicles. The study is also extended to an interconnected power system. It is seen from the comparative analysis that hSFS-PS tuned PI-PD controller in single and multi-area with multi sources improves the system frequency stability in complicated situations. Lastly, a three area interconnected system with PEVs with dissimilar cascade PI-PD controller in each area is considered and proposed hSFS- PS algorithm is used to tune the controller parameters in presence of nonlinearities like rate constraint of units, dead zone of governor and communication delay.

Cascading Enhancement of Reflected Optical Third-Harmonic Imaging in Bio-Tissues

A new nonlinear optical third\|harmonic imaging technology in reflected fashion in bio\|tissues by using cascading effect, a process whereby the second\|order effects combine to contribute to a third\|order nonlinear process, has been analyzed. The performance of the reflected optical third harmonic imaging enhanced by cascading effect in bio\|tissues is analyzed with the semi\|classical theory. The microscopic understanding of the enhancement of cascaded optical third\|harmonic imaging in reflected manner in bio\|tissues has been discussed.Some i deas for further enhancement is given.

Athermal third harmonic generation in micro‐ring resonators

Nonlinear high-harmonic generation in micro-resonators is a common technique used to extend the operating range of applications such as self-referencing systems and coherent communications in the visible region.However,the generated high-harmonic emissions are subject to a resonance shift with a change in temperature.We present a comprehensive study of the thermal behavior induced phase mismatch that shows this resonance shift can be compensated by a combination of the linear and nonlinear thermo-optics effects.Using this model,we predict and experimentally demonstrate visible third harmonic modes having temperature dependent wavelength shifts between−2.84 pm/ºC and 2.35 pm/ºC when pumped at the L-band.Besides providing a new way to achieve athermal operation,this also allows one to measure the thermal coefficients and Q-factor of the visible modes.Through steady state analysis,we have also identified the existence of stable athermal third harmonic generation and experimentally demonstrated orthogonally pumped visible third harmonic modes with a temperature dependent wavelength shift of 0.05 pm/ºC over a temperature range of 12ºC.Our findings promise a configurable and active temperature dependent wavelength shift compensation scheme for highly efficient and precise visible emission generation for potential 2f–3f self-referencing in metrology,biological and chemical sensing applications.

Study of narrow-band second harmonic generation from a broad-band fundamental pulse

This paper studies the type-I phase-matched second harmonic generation using 25-fs input laser pulses in a thick BBO crystal. The harmonic signal exhibits a narrow spectrum bandwidth, even though the input pulse has a broad bandwidth. The energy transfer efficiency and modulation of the fundamental spectrum are investigated.

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).

Experimental Study on a Cascade Flapping Wing Hydroelectric Power Generator

A hydroelectric power generator, which is able to extract the water flow energy from the hydroelastic response of an elastically supported rectangular wing, is experimentally investigated. An electric motor is used to excite pitching oscillations of the wing. Both the wing and the electric motor are supported by leaf springs which are designed to work both as a linear guide for the sway oscillations and as elastic elements. The wing mass in sway direction necessary to achieve a hydroelastic response is obtained by utilizing a mechanical snubber mechanism. The appropriate load to generate electricity is provided by magnetic dampers. In the previous paper, the generating power rate and the efficiency were examined through the experiments with a single wing model, and the feasibility of the flapping wing hydroelectric power generator was verified. In this paper, the influence of the neighboring wings is examined by using two experimental apparatuses, with the intention of achieving a practical cascade wing generator. Tests revealed that the cascade moving in-phase with the neighboring wings at narrower intervals has a higher rate of electric power generation.