Cavity-Enhanced Third Harmonic Generation

We derive the analytical expression of microcavity-enhanced factor for third harmonic generation in terms of detunings, linewidths, and the Purcell factors of the relevant microcavity modes. It is suitable for microcavities with any dimensions and arbitrary geometric shapes.

Improving contrast and sectioning power in confocal imaging by third harmonic generation in SiOx nanocrystallites

We present a new optical microscope in which the light transmitted by a sample-scanned transmission confocal microscope is frequency-tripled by SiOx nanocrystallites in lieu of being transmitted by a confocal pinhole. This imaging technique offers an increased contrast and a high scattered light rejection. It is demonstrated that the contrast close to the Sparrow resolution limit is enhanced and the sectioning power are increased with respect to the linear confocal detection mode. An experimental implementation is presented and compared with the conventional linear confocal mode.

Pulse Slippage of Resonant Third Harmonic Generation in Electron-Hole Plasma

Effect of pulse slippage on resonant third harmonic generation of a short pulse laser in electron-hole plasma in the presence of wiggler magnetic field has been investigated.The group velocity mismatch of the third harmonic pulse and the fundamental pulse is significant in electron hole plasma.As the third harmonic pulse has higher group velocity than that of fundamental pulse,therefore,it moves faster than the fundamental pulse.It gets slipped out of the domain of fundamental pulse and its amplitude saturates.Phase matching condition is satisfied by applying wiggler magnetic field,which provides additional angular momentum to the third harmonic photon to make the process resonant.Enhancement in the efficiency of third harmonic generation of an intense short pulse laser in electron-hole plasma embedded with a magnetic wiggler is seen.

Optical third-harmonic generation of spherical quantum dots under inversely quadratic Hellmann plus inversely quadratic potential

The third-harmonic generation(THG)coefficient for a spherical quantum dot system with inversely quadratic Hellmann plus inversely quadratic potential is investigated theoretically,considering the regulation of quantum size,confinement potential depth and the external environment.The numerical simulation results indicate that the THG coefficient can reach the order of 10~(-12)m~2V~(-2),which strongly relies on the tunable factor,with its resonant peak experiencing a redshift or blueshift.Interestingly,the effect of temperature on the THG coefficient in terms of peak location and size is consistent with the quantum dot radius but contrasts with the hydrostatic pressure.Thus,it is crucial to focus on the influence of internal and external parameters on nonlinear optical effects,and to implement the theory in practical experiments and the manufacture of optoelectronic devices.

Enhanced wide-angle third-harmonic generation in flat-band-engineered quasi-BIC metagratings

Nonlinear metasurfaces and photonic crystals provide a significant way to generate and manipulate nonlinear signals owing to the resonance-and symmetry-based light-matter interactions supported by the artificial structures.However,the nonlinear conversion efficiency is generally limited by the angular dispersion of optical resonances especially in nonparaxial photonics.Here,we propose a metagrating realizing a quasi-bound-state in the continuum in a flat band to dramatically improve the third harmonic generation(THG)efficiency.A superior operating angular range is achieved based on the interlayer and intralayer couplings,which are introduced by breaking the mirror symmetry of the metagrating.We demonstrate the relation of angular dispersion between the nonlinear and linear responses at different incident angles.We also elucidate the mechanism of these offaxis flat-band-based nonlinear conversions through different mode decomposition.Our scheme provides a robust and analytical way for nonparaxial nonlinear generation and paves the way for further applications such as wide-angle nonlinear information transmission and enhanced nonlinear generation under tight focusing.

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.

Generation of femtosecond laser pulses at 263 nm by K_3B_6O_(10)Cl crystal

The third harmonic generation（THG） of a linear cavity Ti：sapphire regenerative amplifier by use of a K3B6O（10）Cl（KBOC） crystal is studied for the first time. Output power up to 5.9 mW is obtained at a central wavelength of 263 nm,corresponding to a conversion efficiency of 4.5% to the second harmonic power. Our results show a tremendous potential for nonlinear frequency conversion into the deep ultraviolet range with the new crystal and the output laser power can be further improved.

Effect of pulse slippage on density transition-based resonant third-harmonic generation of short-pulse laser in plasma

The resonant third-harmonic generation of a tion was investigated. Because of self-focusing self-focusing laser in plasma with a density transi- of the fundamental laser pulse, a transverse intensity gradient was created, which generated a plasma wave at the fundamental wave frequency. Phase matching was satisfied by using a Wiggler magnetic field, which provided additional angular too- mentum to the third-harmonic photon to make the process resonant. An enhancement was observed in the resonant third-harmonic generation of an intense short-pulse laser in plasma embedded with a magnetic Wiggler with a density transition. A plasma density ramp played an important role in the self-focusing, enhancing the third-harmonic generation in plasma. We also examined the ef- fect of the Wiggler magnetic field on the pulse slippage of the third-harmonic pulse in plasma. The pulse slippage was due to the group-velocity mismatch between the fundamental and third-harmonic pulses.

Generation of VUV/XUV coherent radiation in molecular gases

Tunable coherent radiation of wavelength between 92 nm and 122 nm has been produced in molecular gases of N2, CO, H2 and CH4 by resonant and nonresonant third harmonic generation. Factors with respect to the frequency conversion efficiency, including the line strength of the nonlinear susceptibility, the density of the media and the phase-matching, are discussed. By analyzing the characteristics of the four-wave mixing spectra in molecular gases, some physical parameters and the population of the energy levels are obtained. This indicates that nonlinear optical frequency conversion process provides a useful method to study the structure and spectra of molecules.

High-energy,high-repetition-rate ultraviolet pulses from an efficiency-enhanced,frequency-tripled laser

In this study,a high-energy,temporally shaped picosecond ultraviolet(UV)laser running at 100 Hz is demonstrated,with its pulses boosted to 120 mJ by cascaded regenerative and double-pass amplifiers,resulting in a gain of more than 10^(8).With precise manipulation and optimization,the amplified laser pulses were flat-top in the temporal and spatial domains to maintain high filling factors,which significantly improved the conversion efficiency of the subsequent third harmonic generation(THG).Finally,91 mJ,470 ps pulses were obtained at 355 nm,corresponding to a conversion efficiency as high as 76%,which,as far as we are aware of,is the highest THG efficiency for a high-repetition-rate picosecond laser.In addition,the energy stability of the UV laser is better than 1.07%(root mean square),which makes this laser an attractive source for a variety of fields including laser conditioning and micro-fabrication.

Structure and principles of self-assembly of giant“sea urchin”type sulfonatophenyl porphine aggregates

Principles of molecular self-assembly into giant hierarchical structures of hundreds of micrometers in size are studied in aggregates of meso-tetra(4-sulfonatophenyl)porphine(TPPS_(4)).The aggregates form a central tubular core,which is covered with radially protruding filamentous non-branching aggregates.The filaments cluster and orient at varying angles from the core surface and some filaments form bundles.Due to shape resemblance,the structures are termed giant sea urchin(GSU)aggregates.Spectrally resolved fluorescence microscopy reveals J-and H-bands of TPPS_(4)aggregates in both the central core and the filaments.The fluorescence of the core is quenched while filaments exhibit strong fluorescence.Upon drying,the filament fluorescence gets quenched while the core is less affected,showing stronger relative fluorescence.Fluorescencedetected linear dichroism(FDLD)microscopy reveals that absorption dipoles corresponding to J-bands are oriented along the filament axis.The comparison of FDLD with scanning electron microscopy(SEM)reveals the structure of central core comprised of multilayer ribbons,which wind around the core axis forming a tube.Polarimetric second-harmonic generation(SHG)and thirdharmonic generation microscopy exhibits strong signal from the filaments with nonlinear dipoles oriented close to the filament axis,while central core displays very low SHG due to close to centrosymmetric organization.Large chiral nonlinear susceptibility points to helical arrangement of the filaments.The investigation shows that TPPS_(4)molecules form distinct aggregate types,including chiral nanotubes and nanogranular aggregates that associate into the hierarchical GSU structure,prototypical to complex biological structures.The chiral TPPS_(4)aggregates can serve as harmonophores for nonlinear microscopy.