Third-harmonic Conversion Efficiency Research Articles

Near-Unity All-Optical Modulation of Third-Harmonic Generation with a Fano-Resonant Dielectric Metasurface.

We demonstrate all-optical modulation with a near-unity contrast of nonlinear light generation in a dielectric metasurface. We study third-harmonic generation from silicon Fano-resonant metasurfaces excited by femtosecond pulses at 1480 nm wavelength. We modulate the metasurface resonance by free carrier excitation induced by absorption of an 800 nm pump pulse, leading to up to 93% suppression of third-harmonic generation. Modulation and recovery occur on (sub)picosecond time scales. According to the Drude model, the pump-induced refractive index change blue-shifts the metasurface resonance away from the generation pulse, causing a strong modulation of third-harmonic conversion efficiency. The principle holds great promise for spatiotemporal programmability of nonlinear light generation.

Wavelength and angle-dependent third-harmonic generation in epsilon-near-zero indium tin oxide

Third-harmonic generation (THG) is a typical nonlinear optical phenomenon, which has important applications in the field of photoelectric integration. Here we reported the broadband and incident angle-dependent third-harmonic generation from a 350 nm thick indium tin oxide (ITO) film in near-infrared region, and a significant THG enhancement was observed. Moreover, the maximum third-harmonic conversion efficiency at ENZ wavelength is ∼10 times higher than that in non-ENZ region, and the large nonlinear enhancement effect mainly comes from the electric field enhancement under the ENZ condition. Our results highlight the potential applications of ITO in nonlinear optics and provide a promising platform for the development of efficient nonlinear light sources.

Terahertz Third-Harmonic Generation in Topological Insulators Based on Bismuth and Antimony Chalcogenides

Terahertz third-harmonic generation in topological insulators based on bismuth and antimony chalcogenides has been experimentally investigated. It has been found that the third-harmonic conversion efficiency is inversely proportional to the electron Fermi energy; among the materials under study, antimony telluride has the highest conversion efficiency.

Chiral Third-Harmonic Metasurface for Multiplexed Holograms.

Chiral nonlinear metasurfaces could natively synergize nonlinear wavefront manipulation and circular dichroism, offering enhanced capacity for multifunctional and multiplexed nonlinear metasurfaces. However, it is still quite challenging to simultaneously enable strong chiral response, precise wavefront control, high nonlinear conversion efficiency, and independent functions on spins and chirality. Here, we propose and experimentally demonstrate multiplexed third-harmonic (TH) holograms with four channels based on a chiral Au-ZnO hybrid metasurface. Specifically, the left- and right-handed circularly polarized (LCP and RCP) components of the TH holographic images can be designed independently under the excitation of an LCP (or RCP) fundamental beam. In addition, the TH conversion efficiency is measured to be as large as 10-5, which is 8.6 times stronger than that of a bare ZnO film with the same thickness. Thus, our work provides a promising platform for realizing efficient and multifunctional nonlinear nanodevices.

Polychromatic drivers for inertial fusion energy

Although tremendous achievements have been made toward inertial confinement fusion, laser plasma instabilities (LPIs) remain to be an inevitable problem for current drive schemes. To mitigate these instabilities, significant efforts have been paid to produce high-power broadband ultraviolet lasers. However, no practical scheme has been demonstrated up to now for efficient triple-frequency conversion of broadband laser. Here we propose the design of polychromatic drivers for the generation of multicolor beams mainly based upon the optical parametric amplification, which can significantly enhance the third-harmonic conversion efficiency. Each polychromatic light has four colors of monochromatic beamlets with a full spectrum width of 3%, and the beamlet colors of any two adjacent flanges are different. The suppression effects of such polychromatic lights have been investigated via large scale particle-in-cell simulations, which indicate that more than 35% of the incident energy can be saved from the LPIs compared with monochromatic lasers for the direct-drive scheme, or high-density filled target for the indirect-drive scheme. The proposed polychromatic drivers are based on the matured technologies, and thus may pave the way towards realization of robust and high-efficiency fusion ignition.

Optimal third-harmonic generation in an optical microcavity with χ(2) and χ(3) nonlinearities.

Third-harmonic generation can be realized via both χ(3) and cascaded χ(2) nonlinear processes in a triply-resonant microcavity. It is still unknown how these processes interfere with each other and the optimization of the conversion efficiency still remains as a question. In this work, the interplay between the direct third-harmonic generation and the cascaded process combining of the second-harmonic generation and the sum-frequency generation are investigated. It is found that the interference effect between these two processes can be used to improve the conversion efficiency. By optimizing the cavity resonance and the external coupling conditions, the saturation of the nonlinear conversion is mitigated and the third-harmonic conversion efficiency is increased. A design rule is provided for achieving efficient third-harmonic generation in an optical microcavity, which can be generalized further to the high-order harmonic generations.

Coherent Visible-Light-Generation Enhancement in Silicon-Based Nanoplasmonic Waveguides via Third-Harmonic Conversion.

We report visible third-harmonic conversion at λ=517 nm in subwavelength silicon-based nanoplasmonic waveguides at an unprecedented conversion efficiency of 2.3×10^{-5}. This marks both the highest third-harmonic conversion efficiency in a silicon-based or nanoplasmonic structure and the smallest silicon waveguide structure demonstrated to date. The high conversion efficiency is attributed to tight electric field confinement and strong light-matter coupling arising from surface plasmon modes in the nanoplasmonic waveguide, enabling efficient nonlinear optical mixing over micrometer length scales. The nonresonant geometry of the waveguide enables the entire λ=1550 nm femtosecond pulse spectrum to be converted to its third harmonic, which may be easily extended to the entire visible spectrum. We envisage that third-harmonic generation in silicon-based nanoplasmonic waveguides could provide a platform for integrated, broadband visible light sources and entangled triplet photons on future hybrid electronic-silicon photonic chips.

Enhancement of the Second and Third Harmonic Generations in Nanorods and Nanotubes Based on Metal Electron Nonlinear Responses and Dielectric Nonlinearity

In this paper, we investigated the second harmonic (SH) and the third harmonic (TH) generations in nanorods and nanotubes. Nonlinear free and bound electrons responses of metal combined with the third-order nonlinearity of dielectric core were utilized to study the nonlinear response of nanotubes made of noble metals such as gold. The impacts of the dielectric constant of core medium, thickness of metal shell, and size of nanotube on the SH and TH conversion efficiency are reported. The physical interpretations behind the results are discussed. Finally, we have shown that enhancement of the SH and TH generations are more noticeable and adjustable by varying the core-shell parameters compared to those of nanorods.

Effect of spatial refractive-index nonuniformities existed in a large-scale rapid growth KDP crystal on third-harmonic conversion

Effect of spatial refractive-index nonuniformities existed in a large-scale rapid growth KDP crystal (a type I frequency doubler at the wavelength of 1053nm) on third-harmonic conversion is studied. Spatial refractive-index nonuniformities due to imperfections in the crystal structure which prohibit the attainment of phase matching at all locations in the crystal are measured with the orthogonal polarization interferometry technique. And a third-harmonic conversion experiment is conducted on a large-scale facility to investigate the influence of refractive-index nonuniformities on conversion efficiency and third-harmonic beam nearfield. According to the experimental results, refractive-index nonuniformities in this large-scale rapid growth KDP doubler do not reduce the third-harmonic conversion efficiency, but affect the third-harmonic beam nearfield.

Broadband Third-Harmonic Generation with a Composite KD*P Crystal

We predict that broadband efficient third-harmonic generation (THG) can be achieved with a frequency-doubling crystal and a novel composite KD*P tripler. The composite KD*P tripler is made of two partially deuterated KDP crystals with different deuteration levels by using the thermal bonding technique. The deuteration level of a partially deuterated KDP crystal is used as a degree of freedom to alter the phase-matching (PM) wavelength. Simulations show that the composite KD*P tripler can improve third-harmonic conversion efficiencies over a very wide band of input fundamental frequencies. In terms of robustness, alignment and stability, this THG scheme should be more promising than other broadband THG approaches because the composite KD*P tripler is a monolithic device.

Phase-matched second- and third-harmonic generation in plasmas with density ripple

The generation of second and third harmonics by the interaction of an ultrashort laser pulse with underdense plasma having a density ripple is studied at intensities Iλ2=1016−1019W cm−2μm2 using fully relativistic two-dimensional particle-in-cell simulations with high spectral resolution. A theoretical model is developed for second- and third-harmonic conversion efficiencies. When the laser is plane polarized in the simulation plane even and odd harmonics are excited in the same polarization as the laser polarization. The highest efficiency of generation of a specific harmonic occurs when the ripple wave vector value kq satisfies phase-matching conditions. The efficiency of phase-matched harmonic generation is an order of magnitude higher than the one without phase matching. The efficiency increases rapidly in weak and moderate relativistic regime and tends to saturate in strong relativistic regime. At moderately relativistic intensities and low plasma densities, the simulation and recent experimental results are fairly reproduced by an analytical theory.

Phase-matched third-harmonic UV generation using low-order modes in a glass micro-fiber

We observe phase-matched third-harmonic generation at 355 nm in a low-order mode of a sub-micron diameter glass fiber. The third-harmonic signal exhibits a sharp resonance for a fiber diameter d = 0.49 ± 0.02 μm, in excellent agreement with the value d = 0.506 μm predicted by theory. The third-harmonic conversion efficiency is 2 × 10 −6, and is limited by the pump power (1 kW) and effective fiber length (100 μm).

Phase matching for third-harmonic generation in collisional magnetoplasmas

This paper presents a derivation of the phase-matching conditions for generation of the third-harmonic (3ω1) and combination frequencies (ω1+2ω2), by two fundamental electromagnetic waves of frequencies ω1 and ω2, propagating in the extraordinary mode along a magnetic field in a collisional plasma. Expressions for the corresponding optimum distance of propagation for maximum output and the associated values of the phase functions have been derived. In the case of the third-harmonic generation phase matching occurs when the gyrofrequency of the electrons is twice the wave frequency (for all values of plasma frequency). In the case of the combination frequencies, the phase-matching condition is represented by an expression for plasma frequency ωp in terms of the wave frequencies ω1, ω2 and the gyrofrequency ωc of electrons. Furthermore, it is seen that in the (ωc∕ω1)−(ω2∕ω1) plane, phase matching occurs in a narrow region; for every point in this region, there is a corresponding value of (ωp∕ω1). The dependence of optimum distance of propagation and the corresponding value of the phase function on electron collision frequency νav∕ω1 have been graphically illustrated for a specific case. An estimate of the maximum value of the third-harmonic conversion efficiency has also been made.

Large enhancement of third-harmonic generation induced by coupled gap solitons inχ(3)nonlinear photonic crystals

It is shown that, in a chi((3)) nonlinear photonic crystal, the third-harmonic conversion efficiency can be enhanced by about three orders of magnitude via the formation of self-organized localized states inside the gaps as compared with that in a bulk medium of the same length with a perfect phase-matching condition. These localized states contain both the fundamental and third-harmonic frequencies located in different gaps. Their existence is a result of both the Kerr effect and the large energy transfer between fundamental and third-harmonic waves.

Continuum generation of the third-harmonic pulse generated by an intense femtosecond IR laser pulse in air

The continuum generation by intense femtosecond IR laser pulses focused in air including the effect of third-harmonic generation is investigated. We have used a theoretical model that includes the full spatio-temporal dynamics of both the fundamental and the third-harmonic pulses. Results of our numerical calculations show that a two-color filamentation effect occurs, in which the third-harmonic conversion efficiency remains almost constant over the whole filament length. It is found that this effect is rather independent of the wavelength of the input beam and the focal geometry. During the filamentation process the third-harmonic pulse itself generates a broad continuum, which can even overlap with the continuum of the fundamental pulse for the longer pump wavelengths. In consequence, the continuum generation generated by intense IR laser pulses is further extended into the UV.

Crucial effects of coupling coefficients on quasi-phase-matched harmonic generation in an optical superlattice

Coupling of optical parametric processes in an optical superlattice through quadratic nonlinearity was analyzed theoretically. Solving the coupled equations, we found that efficient quasi-phase-matched third-harmonic (TH) generation depends not only on the magnitude of the coupling coefficients but also on their ratio. Theoretically, all the fundamental energy can be transferred to the TH at a particular ratio. In other cases, there exists an optimum condition that corresponds to a maximum TH conversion efficiency. The result is of practical importance for the design of TH devices.

Singly resonant cavity-enhanced frequency tripling

We use a plane-wave analysis to examine a singly resonant, cavity-enhanced, frequency doubler containing an intracavity sum-frequency interaction that frequency sums the resonant fundamental with the second harmonic to produce the third harmonic of the resonant field. We derive expressions for the steady-state performance of the device. We determine the input coupler intensity reflectivity and the ratio of nonlinear drives between the second-harmonic generation (SHG) and sum-frequency generation (SFG) stages for optimum third-harmonic conversion efficiency. We also examine the optimum SHG interaction length under conditions of limited total interaction length. We find that numerical simulations modeling three spatial dimensions can be closely approximated by appropriately scaled plane-wave results. As an example, we consider frequency tripling of a 350-mW, 1319-nm, cw laser in two consecutive nonlinear gratings in periodically poled lithium niobate and find third-harmonic power-conversion efficiency of 85.3% when first-order quasi-phase-matching (QPM) is used for the SFG process and 51.0% when third-order SFG QPM is used. We also consider frequency tripling of a 20-W, 1064-nm, continuous-wave mode-locked laser in two lithium triborate crystals and find a time-averaged third-harmonic power-conversion efficiency of 56.0% from modeling three spatial dimensions.

Soliton-antisoliton configurations and the linear and nonlinear optical response of degenerate-ground-state conjugated polymers.

The contributions of soliton-antisoliton (SS\ifmmode\bar\else\textasciimacron\fi{}) configurations to the linear and third-order nonlinear optical response are investigated for conjugated polymers having a degenerate ground state. We treat the direct photoproduction of charged solitons as a nonlinear Franck-Condon problem and obtain analytical expressions for the linear optical susceptibility ${\mathrm{\ensuremath{\chi}}}^{(1)}$(\ensuremath{\omega}). With the help of the oscillator strength sum rule, we decompose the \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ oscillator strength into two parts; a contribution where the final states are charged SS\ifmmode\bar\else\textasciimacron\fi{} pairs and a contribution where the final states are free electron-hole pairs (as in the noninteracting rigid lattice). The linear optical coefficients calculated from ${\mathrm{\ensuremath{\chi}}}^{(1)}$(\ensuremath{\omega}) are in general agreement with optical data obtained from trans-polyacetylene. The results imply that approxiamtely 25% of the integrated oscillator strength of trans-polyacetylene arises from the direct photoproduction of solitons.A parallel treatment of the generalized third-order nonlinear optical susceptibility ${\mathrm{\ensuremath{\chi}}}^{(3)}$(${\mathrm{\ensuremath{\omega}}}_{\mathrm{\ensuremath{\sigma}}}$) is presented, demonstrating that, for any third-order process, contributions arising from neutral SS\ifmmode\bar\else\textasciimacron\fi{} pair configurations as intermediate states are one to two orders of magnitude larger than the corresponding rigid-lattice contribution. This mechanism for ${\mathrm{\ensuremath{\chi}}}^{(3)}$ is enabled by nonlinear zero-point motion which provides a finite Franck-Condon overlap between the ground and SS\ifmmode\bar\else\textasciimacron\fi{} excited state lattice wave functions. The large contribution to ${\mathrm{\ensuremath{\chi}}}^{(3)}$ from the SS\ifmmode\bar\else\textasciimacron\fi{} intermediate states results from the large transition dipole moment between the free electron-hole pair excited states of ${\mathit{B}}_{\mathit{u}}$ symmetry and the ${\mathit{A}}_{\mathit{g}}$ symmetric neutral SS\ifmmode\bar\else\textasciimacron\fi{} excited state. This enhanced transition dipole moment is a consequence of the large virtual shifts of oscillator strength associated with the localized SS\ifmmode\bar\else\textasciimacron\fi{} electron-lattice configuration. The third-harmonic conversion efficiency ${\mathrm{\ensuremath{\chi}}}^{(3)}$(3\ensuremath{\omega}) is further enhanced by a condition unique to degenerate-ground-state systems, simultaneous two- and three-photon resonance.

Phase-matched third-harmonic generation of Nd: Glass-laser picosecond pulses in a new cyanine-dye solution

The phase-matched direct tripling of picosecond light pulses of a mode-locked Nd: glass laser in a new cyanine dye PMC is studied. The solvents trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP) are applied. The S 0–S 1 absorption peak of the dye is around λ=480 nm and the absorption cross section at the third-harmonic wavelength of λ3=351.3 nm is only σ3≈1×10−19 cm2. Phase-matching occurred at concentrations of CPM=0.0874 mol/dm3 in HFIP and 0.1088 mol/dm3 in TFE. A third-harmonic energy conversion efficiency of ηE≈0.01 was achieved at a pump-laser peak intensity of I 0L≈2.5×1011 W/cm2 in a 5 mm long sample of PMC in TFE. The conversion efficiency is limited by destruction of phase-matching due to the intensity-dependent nonlinear refractive index of the dye solutions.

Quantum Lattice Fluctuations and the Linear and Nonlinear Optical Susceptibility of Degenerate Ground State Conjugated Polymers

1>.z> We treat the direct photoproduction of charged solitons as a nonlinear Franck-Condon problem and obtain analytical expressions for the linear optical susceptibility, x (w). With the help of the oscillator strength sum rule, we decompose the Jr-Jr* oscillator strength into two parts; a contribution where the final states are SS pairs and a contribution where the final states are free electron-hole pairs (as in the noninteracting rigid-lattice). The linear optical coefficients calculated from x (w0'), is presented, demonstrating that, for any third-order process, contributions arising from neutral SS pair configurations as intermediate states are one to two orders of magnitude larger than the corresponding rigid-lattice contribution. This mechanism for x is enabled by nonlinear zero-point motion which provides a finite Franck-Condon overlap between the ground and SS excited state lattice wavefunctions. The large contribution to x from the SS intermediate states results from the large transition dipole moment between the free electron-hole pair excited states of Bu symmetry and the Ag symmetric neutral SS excited state. This enhanced transition dipole moment is a consequence of the large virtual shifts of oscillator strength associated with the localized SS electron-lattice configuration. The third-harmonic conversion efficiency x (3w) is further enhanced by a condition unique to degenerate ground state systems, simultaneous two and three-photon resonance.