Quasi-phase Matching Conditions Research Articles

Interfacial epitaxy of multilayer rhombohedral transition-metal dichalcogenide single crystals.

Rhombohedral-stacked transition-metal dichalcogenides (3R-TMDs), which are distinct from their hexagonal counterparts, exhibit higher carrier mobility, sliding ferroelectricity, and coherently enhanced nonlinear optical responses. However, surface epitaxial growth of large multilayer 3R-TMD single crystals is difficult. We report an interfacial epitaxy methodology for their growth of several compositions, including molybdenum disulfide (MoS2), molybdenum diselenide, tungsten disulfide, tungsten diselenide, niobium disulfide, niobium diselenide, and molybdenum sulfoselenide. Feeding of metals and chalcogens continuously to the interface between a single-crystal Ni substrate and grown layers ensured consistent 3R stacking sequence and controlled thickness from a few to 15,000 layers. Comprehensive characterizations confirmed the large-scale uniformity, high crystallinity, and phase purity of these films. The as-grown 3R-MoS2 exhibited room-temperature mobilities up to 155 and 190 square centimeters per volt second for bi- and trilayers, respectively. Optical difference frequency generation with thick 3R-MoS2 showed markedly enhanced nonlinear response under a quasi-phase matching condition (five orders of magnitude greater than monolayers).

Single-pass generation of widely-tunable frequency-domain entangled photon pairs.

We demonstrate a technique that generates frequency-entangled photon pairs with strong polarization correlation by using a single-period nonlinear crystal and single pass configuration. The technique is based on the simultaneous occurrence of two spontaneous parametric down-conversion processes satisfying independent type-II collinear quasi-phase matching conditions in periodically poled stoichiometric lithium tantalate. The generated photon pairs exhibit non-degenerate Hong-Ou-Mandel interference, indicating the presence of quantum entanglement in the frequency domain. This method provides a light source capable of wide-range quantum sensing and quantum imaging or high-dimensional quantum processing.

Quasi-phase-matching of resonance-enhanced high-order harmonics in laser plasmas.

The resonance-enhanced harmonics in laser-induced arsenic and selenium plasmas are studied at the quasi-phase-matching (QPM) conditions. We demonstrate that the enhancement of these harmonics was significantly smaller than the one of the neighboring harmonics. Though the enhancement factors of the harmonics in the vicinity of resonance-enhanced harmonics were in the range of 5× to 18×, the resonance-enhanced harmonics were almost unenhanced at QPM conditions. The most probable reason for such restriction in the enhancement of specific harmonics at the conditions of QPM was a stronger influence of free electrons on the phase-matching conditions of the resonance-enhanced single harmonic compared to the QPM-enhanced group of harmonics.

Near-noiseless and small-footprint phase sensitive optical parametric amplifier using AlGaAs-on-insulator waveguides.

Phase sensitive amplifiers (PSAs) based on optical parametric amplification feature near noiseless amplification, which is of considerable benefit for improving the performance of optical communication systems. Currently, the majority of research on PSAs is carried out on the basis of highly nonlinear fibers or periodically poled lithium niobite waveguides, with the impediments of being susceptible to environmental interference and requiring complex temperature control systems to maintain quasi-phase matching conditions, respectively. Here, a near-noiseless and small-footprint PSA based on dispersion-engineered AlGaAs-on-insulator (AlGaAsOI) waveguides is proposed and demonstrated theoretically. The phase-dependent gain and the phase-to-phase transfer function of the PSA are calculated to analyze its characteristics. Furthermore, we investigate in detail the effects of linear loss, nonlinear coefficient, and pump power on the PSA gain and noise figure (NF) in AlGaAsOI waveguides. The results show that a PSA based on an AlGaAsOI waveguide is feasible with a maximum phase sensitive gain of 33dB, achieving an NF of less than 1dB over a gain bandwidth of 245nm with a gain of >15d B, which completely covers the S+C+L band. This investigation is worthwhile for noiseless PSAs on photonic integrated chips, which are promising for low-noise optical amplification, multifunctional photonic integrated chips, quantum communication, and spectroscopy, and as a reference for low-noise PSAs depending on the third-order nonlinearity, χ (3), of the waveguide material.

Sequential Three-Dimensional Nonlinear Photonic Structures for Efficient and Switchable Nonlinear Beam Shaping

Nonlinear beam shaping can be dynamically controlled by temperature or polarization to form switchable nonlinear structured beams, which constitute an unprecedented system for realizing optical encryption at new frequencies and high-dimensional nonclassical light sources. However, previous works were limited by a trade-off between the conversion efficiency and the modulation dimension of the beam. It is still technologically challenging to achieve efficient and switchable multidimensional nonlinear beam shaping. Here, we demonstrate switchable generation of nonlinear structured beams via three-dimensional (3D) nonlinear photonic crystals (NPCs) fabricated by femtosecond laser writing technique with considerable conversion efficiency. The 3D NPCs contain different sequential 3D arrays of spatially modulated χ(2) nonlinearity designed by computer-generated hologram (CGH) along the optical y-axis (LiNbO3 crystal). The output direction of the nonlinear beams can be changed by the carrier frequencies, while their efficiencies can be increased by the quasi-phase matching condition.

Optical frequency combs in dispersion-controlled doubly resonant second-harmonic generation.

We report on the experimental realization and a systematic study of optical frequency comb generation in doubly resonant intracavity second harmonic generation (SHG). The efficiency of intracavity nonlinear processes usually benefits from the increasing number of resonating fields. Yet, achieving the simultaneous resonance of different fields may be technically complicated, all the more when a phase matching condition must be fulfilled as well. In our cavity we can separately control the resonance condition for the fundamental and its second harmonic, by simultaneously acting on an intracavity dispersive element and on a piezo-mounted cavity mirror, without affecting the quasi-phase matching condition. In addition, by finely adjusting the laser-to-cavity detuning, we are able to observe steady comb emission across the whole resonance profile, revealing the multiplicity of comb structures, and the substantial role of thermal effects on their dynamics. Lastly, we report the results of numerical simulations of comb dynamics, which include photothermal effects, finding a good agreement with the experimental observations. Our system provides a framework for exploring the richness of comb dynamics in doubly resonant SHG systems, assisting the design of chip-scale quadratic comb generators.

Equal-efficiency diffraction of unpolarized wideband light with acousto-optic filters

We present the diffraction of unpolarized light over a broad wavelength range using paratellurite Acousto-Optic Tuneable Filters. The diffraction of unpolarized light usually relies on a specific operating point that is sensitive to the wavelength and angle of incidence, resulting in an operating ability over a narrow bandwidth. We present a detuning strategy to eliminate the angular dependence and obtain equal diffraction efficiency for both the linear polarization states over a broad bandwidth. We explain how to find the optimum operating frequency and present its dependency as a function of the wavelength. With the quasi-phase matching condition presented, the diffraction of unpolarized light over the 400 nm to700 nm band is possible with an efficiency of over 80%.

Spectrally separable photon-pair generation in dispersion engineered thin-film lithium niobate.

Existing nonlinear-optic implementations of pure, unfiltered heralded single-photon sources do not offer the scalability required for densely integrated quantum networks. Additionally, lithium niobate has hitherto been unsuitable for such use due to its material dispersion. We engineer the dispersion and the quasi-phasematching conditions of a waveguide in the rapidly emerging thin-film lithium niobate platform to generate spectrally separable photon pairs in the telecommunications band. Such photon pairs can be used as spectrally pure heralded single-photon sources in quantum networks. We estimate a heralded-state spectral purity of >94% based on joint spectral intensity measurements. Further, a joint spectral phase-sensitive measurement of the unheralded time-integrated second-order correlation function yields a heralded-state purity of (86±5)%.

Multi-resonant forward optical parametric oscillations without external mirrors based on nonlinear photonic crystals of LiTaO3

In this paper, we report experimental evidence of forward optical parametric oscillations (OPOs) with multi frequencies conversion processes in a spectral range of 400 nm–1700 nm using nonlinear photonic crystals of periodically poled LiTaO3 without external mirrors. The highly polished end-facets of the samples act as a Fabry–Perot cavity mirrors making the device simple and compact. Oscillation thresholds were found of 97.6 MW cm−2 and 160 MW cm−2 for the 1D and 2D samples, respectively. Moreover, at pump energies above the threshold, optical oscillations of multiple frequencies due to cascaded interactions established within the samples are obtained. In particular, we point-out the generation of two pairs of signal and idler (OPO1 and OPO2) in the same nonlinear photonic crystal. It is found that the second pair OPO2 appears with different quasi-phase matching conditions induced by a local refractive index variation. Our analysis indicates that this index variation results from a competing effect between quadratic cascading and electro-optic induced refractive index changes.

Quasi-phase-matching-division multiplexing holography in a three-dimensional nonlinear photonic crystal

Nonlinear holography has recently emerged as a novel tool to reconstruct the encoded information at a new wavelength, which has important applications in optical display and optical encryption. However, this scheme still struggles with low conversion efficiency and ineffective multiplexing. In this work, we demonstrate a quasi-phase-matching (QPM) -division multiplexing holography in a three-dimensional (3D) nonlinear photonic crystal (NPC). 3D NPC works as a nonlinear hologram, in which multiple images are distributed into different Ewald spheres in reciprocal space. The reciprocal vectors locating in a given Ewald sphere are capable of fulfilling the complete QPM conditions for the high-efficiency reconstruction of the target image at the second-harmonic (SH) wave. One can easily switch the reconstructed SH images by changing the QPM condition. The multiplexing capacity is scalable with the period number of 3D NPC. Our work provides a promising strategy to achieve highly efficient nonlinear multiplexing holography for high-security and high-density storage of optical information.

Robust Geometries for Second-Harmonic-Generation in Microrings Exhibiting a 4-Bar Symmetry

Microring resonators made of materials with a zinc-blend or diamond lattice allow exploiting their 4-bar symmetry to achieve quasi-phase matching condition for second-order optical nonlinearities. However, fabrication tolerances impose severe limits on the quasi-phase matching condition, which in turn degrades the generation efficiency. Here, we present a method to mitigate these limitations. As an example, we studied the geometry and the pump wavelength conditions to induce the second-harmonic generation in silicon-based microrings with a second-order susceptibility χzxy(2)≠0. We found the best compromises between performances and experimental requirements, and we unveil a strategy to minimize the impacts of fabrication defects. The method can be easily transferred to other material systems.

Femtosecond Laser-Micromachining of Glass Micro-Chip for High Order Harmonic Generation in Gases.

We report on the application of femtosecond laser micromachining to the fabrication of complex glass microdevices, for high-order harmonic generation in gas. The three-dimensional capabilities and extreme flexibility of femtosecond laser micromachining allow us to achieve accurate control of gas density inside the micrometer interaction channel. This device gives a considerable increase in harmonics’ generation efficiency if compared with traditional harmonic generation in gas jets. We propose different chip geometries that allow the control of the gas density and driving field intensity inside the interaction channel to achieve quasi phase-matching conditions in the harmonic generation process. We believe that these glass micro-devices will pave the way to future downscaling of high-order harmonic generation beamlines.

Application of Quasi-Phase Matching Concept for Enhancement of High-Order Harmonics of Ultrashort Laser Pulses in Plasmas

Novel methods of coherent short-wavelength sources generation require thorough analysis for their further amendments and practical implementations. In this work, we report on the quasi-phase matching (QPM) of high-order harmonics generation during the propagation of single- and two-color femtosecond pulses through multi-jet plasmas, which allows the enhancement of groups of harmonics in different ranges of extreme ultraviolet. The role of the number of coherent zones; sizes of plasma jets and the distance between them; plasma formation conditions, and the characteristics of the fundamental radiation on the harmonic efficiency at quasi-phase matching (QPM) conditions are analyzed. We demonstrate the ~40× enhancement factor of the maximally-enhanced harmonic with respect to the one generated at ordinary conditions in the imperforated plasma.

High-order harmonics generation under quasi-phase matched conditions in silver, boron, and silver sulfide plasmas of different configurations

The application of advanced methods for high-order harmonics enhancement allows the formation of conditions for phase matching of the generated harmonics in different ranges of extreme ultraviolet. We demonstrate quasi-phase matching (QPM) of the high-order harmonics generated in the plasmas containing silver microparticles and atoms, and silver sulfide molecules. The groups of harmonics in the range of 30th to 40th orders at QPM conditions prevail over the lower orders of harmonics. In most cases, the lower-order harmonics disappeared from the generated spectra at QPM conditions. The 22× enhancement factor of QPM harmonics and the 3 × 10−5 conversion efficiency for a maximally enhanced harmonic were demonstrated in the case of silver microparticle-contained plasma in the 47–67 eV energy range.

Temperature-dependent quasi phase-matching properties of periodically poled LaBGeO5

We report accurate Sellmeier equations at 22℃ for LaBGeO5, which reproduce well our experimental results for the quasi phase-matching conditions with the ee-e, oe-o, eo-o, and oo-e interactions in the 0.2660 − 1.0642 µm spectral range. In addition, the thermo-optic dispersion formula (dn/dT) of this material is also presented.

Effects of Laser Plasma Formation on Quasi-Phase Matching of High-Order Harmonics from Nanoparticles and Atoms.

The application of nanoparticles (NPs) and quasi-phase matching (QPM) each play an important role in the enhancement of high-order harmonics (HHG) of ultrashort laser pulses. We analyze various regimes of nanoparticle plasma formation for the creation conditions for maximal QPM-induced enhancement of the groups of harmonics in the extreme ultraviolet (XUV). Laser plasmas were formed on the surfaces of NPs- and microparticle (MPs)-contained targets using ablation by nanosecond, picosecond, and femtosecond pulses. Different conditions of laser plasma formation (extended and perforated plasma) and variable concentrations of free electrons in these three cases of laser ablation led to modifications of QPM conditions. We demonstrate novel approaches in the optimization of QPM at the conditions of laser ablation of NPs and MPs by pulses of different durations. The formation of QPM conditions using femtosecond and picosecond heating pulses during HHG in such plasmas allowed the growth of conversion efficiency of the groups of harmonics, with the enhancement factors exceeding 25× in different ranges of XUV, contrary to less efficient QPM in the case of nanosecond pulse-induced ablation.

Quasi-phase matching of harmonics generating in laser plasmas: experiment and theory

We compare the results of experimental and theoretical studies of the quasi-phase matching (QPM) of high-order harmonic generation (HHG) during propagation of two-color femtosecond pulses through the indium multi-jet plasmas. The role of number of coherent zones and size of plasma jets on the HHG efficiency at QPM conditions is analysed. We demonstrate the enhancement of the HHG efficiency due to QPM during laser-plasma interaction.

Effect of LiNbO3 polarity on the structural, optical and acoustic properties of epitaxial ZnO and MgxZn1−xO films

The effect of LiNbO3 substrate polarization on a charge density at the interface with epitaxial ZnO and MgxZn1−xO films was studied in comparison with ZnO films on dielectric sapphire substrates. An excitonic emission and optical properties such as band gap energy, refraction index, absorption and reflectivity of the ZnO/LiNbO3 structures were studied. The effect of interface charges on the performance of surface acoustic waves with frequency of 2.7 GHz was investigated, as well. The stimulated cavity mode emission was observed due to attained quasi-phase matching conditions for thin ZnO films on sapphire at high laser excitation power. The positive polarization of the ferroelectric substrate induced the band bending of the semiconductor layer, resulting in the accumulation of the negative charges at the ferroelectric/semiconductor interface, which induced an increased reflectivity, a generation of oxygen emission lines in PL spectra under laser excitation, and an increased electromechanical coupling of surface acoustic waves. The negative polarization of the substrate increased atomic peening during sputtering deposition, which induced deteriorated quality of texture and in-plane orientation and increased the residual stresses in ZnO and MgxZn1−xO films.

Simultaneous Generation of Two Pairs of Stokes and Terahertz Waves from Coupled Optical Parametric Oscillations with Quasi-Phase-Matching

We present a theoretical investigation on simultaneous generation of two pairs of Stokes and terahertz (THz) waves from coupled optical parametric oscillations (OPOs) with a quasi-phase-matching (QPM) scheme. The two pairs of Stokes and THz waves are generated by stimulated polariton scattering (SPS) from periodically-inverted GaP. By analyzing the QPM conditions of coupled OPOs we find that the two THz waves with any frequency below the transverse optical (TO) mode frequency of GaP can be simultaneously generated with a suitable pump wavelength. We calculate the photon flux densities of the two THz waves by solving the coupled wave equations. The calculation results indicate that the two THz waves can be efficiently generated with high pump intensities, particularly in lower THz frequency band.

Theoretical study on broadband second harmonic generation in periodically and aperiodically poled KTP

In this paper, the dispersion property of KTP is analyzed to calculate the group-velocity matching (GVM) wavelength at a given temperature. Then, first-order quasi-phase-matching (QPM) broadband second harmonic generation (SHG) for two different types of interaction is studied at the calculated wavelength in periodically poled KTP (PPKTP). To broaden the acceptance bandwidth of QPM SHG, a structure of aperiodically poled KTP (APPKTP) is designed using a genetic algorithm with the QPM and GVM conditions satisfied simultaneously. Next, the broadband property of APPKTP is studied. Research shows that, for type-I QPM, the bandwidth at the communication band is ~1.9 nm in a 10 mm long PPKTP crystal with a set temperature of 22 °C, while a wide acceptance bandwidth of 73.9 nm can be obtained at the same waveband for type-II QPM, with the conversion efficiency decreasing to 4.64%. In addition, the maximum available acceptance bandwidths are ~9.3 nm for type-I QPM and 417.1 nm for type-II QPM, using the APPKTP designed above. The tradeoff between bandwidth and conversion efficiency is also discussed.