Quasi-phase-matching Research Articles

Ultrabroadband nonlinear Raman–Nath diffraction against femtosecond pulse laser

Nonlinear Raman–Nath diffraction (NRND) offers an effective way to realize multiple noncollinear parametric processes based on the partially satisfied transverse phase-matching conditions in quadratic nonlinear media. Here, the realization of ultrabroadband NRND (UB-NRND) driven by a high-peak-power ultrashort femtosecond pump laser in two types of nonlinear crystals is reported: periodically poled lithium niobate (PPLN) and chirped PPLN (CPPLN). Multi-order ultrabroadband Raman–Nath second-harmonic (SH) signal outputs along fixed diffraction angles are simultaneously observed. This distinguished transversely phase-matched supercontinuum phenomenon is attributed to the synergic action of natural broad bandwidth of an ultrashort femtosecond pump laser and the third-order nonlinear effect induced spectral broadening, in combination with the principal ultrabroadband noncollinear second-harmonic generation processes. The NRND process with multiple quasi-phase matching (QPM) interactions from CPPLN leads to the SH output covering a wide range of wavelengths between 389 and 997 nm and exhibiting an energy conversion efficiency several orders of magnitude higher than previous studies. This UB-NRND scheme would bring better techniques and tools for applications ranging from ultrashort pulse characterization and nondestructive identification of domain structures to accurate parameter monitoring of second- and third-order nonlinear susceptibilities within solid-state nonlinear microstructured materials.

Tunable short-wave near-infrared continuous wave source based on a 532 nm pumped singly resonant optical parametric idler oscillator

We report a continuous wave (CW) singly resonant optical parametric idler oscillator (i-SRO) capable of providing high power, high power conversion efficiency, widely tunable radiation in the short-wave near-infrared (NIR) range. The i-SRO is pumped by a fiber-laser-based laser at 532 nm with a multi-grating, MgO doped periodically poled stoichiometric lithium tantalate (MgO:sPPLT) as quasi-phase matching (QPM) crystal. By utilizing eight grating periods, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, and 8.8 µm of the QPM crystal, with grating tuning and temperature tuning, the i-SRO can be continuously tuned across 951–793 nm and the corresponding idler tunable over 1212–1620 nm. A maximum signal power of 4.9 W for a pump power of 9.1 W and a relevant pump to signal conversion efficiency of ∼53.8% are achieved at the signal of 821 nm. At a pump power of 5.4 W, the output power stability of the signal at 837 nm is better than 2.1% rms, within one hour in Gaussian spatial beam profile. The SRO performance in tuning rate has also been investigated.

Tailored Multi‐Color Dispersive Wave Formation in Quasi‐Phase‐Matched Exposed Core Fibers (Adv. Sci. 8/2022)

Dispersive Waves The optical properties of optical fibers with exposed cores can be straightforwardly tuned by the deposition of high refractive index nano-films (green) inducing tailored resonances. In article number 2103864 by Markus A. Schmidt and co-workers, a precise dispersion design and tailored supercontinuum generation is enabled through periodically structured nano-films. Multicolor, user-defined femtosecond frequency conversion to higher order dispersive waves by quasi-phase matching is demonstrated.

Fourth harmonic generation of Laguerre Gaussian beam in BBO crystal by total internal reflection-quasi phase matching technique

This work deals with analytical exploration of a highly multimodal nonlinear guided wave approach based Fourth harmonic frequency conversion scheme of Laguerre Gaussian beam through Total Internal Reflection Quasi phase matching technique. The analysis has been carried out in a rectangular slab geometry of anisotropic crystal made of Beta beryllium borate. Compared to the traditional Gaussian beam-based frequency conversion scheme, the proposed scheme achieves nearly 7 times better efficiency of 4.8% in a 2.6 mm long crystal corresponding to an application wavelength of 266 nm. The optical losses like absorption, GH-shift, reflection, and effect due to nonlinear law of reflection have also been incorporated in the analysis. The impact of various parametric changes like variations in slab length and thickness has also been studied to give a better perspective of the whole analysis.

Optically Induced Nonlinear Cubic Crystal System for 3D Quasi‐Phase Matching

Quasi‐phase matching (QPM) is a technique in nonlinear optics for achieving efficient energy exchange among optical waves at different frequencies, by spatially modulating the quadratic nonlinearity (χ (2)) of the medium. To realize the full potential of QPM, 3D spatial modulation of χ (2) is required. This has become experimentally feasible recently thanks to the invention of femtosecond laser‐based nonlinearity engineering in ferroelectric crystals. Herein, the first experimental demonstration of QPM second harmonic generation (SHG) in a nonlinear cubic crystal system is presented, in which χ (2) modulations form simple cubic, body‐centered cubic, face‐centered cubic, and diamond cubic lattices, respectively. The experimental results indicate that these nonlinear cubic structures share the same primary reciprocal lattice vectors (RLVs), but possess different Fourier coefficients (in conventional cells), leading to SHG with similar angular resonances but various intensity distributions in the far field. This work contributes to a comprehensive understanding of nonlinear optical processes in 3D periodic media, and thus sheds light on the development of high‐performance QPM devices.

Second and cascaded harmonic generation of pulsed laser in a lithium niobate on insulator ridge waveguide.

Nonlinear crystalline ridge waveguides, e.g., lithium niobate-on-insulator ridge waveguides, feature high index contrast and strong optical confinement, thus dramatically enhancing nonlinear interaction and facilitating various nonlinear effects. Here, we experimentally demonstrate efficient second-harmonic generation (SHG) and cascaded fourth-harmonic generation (FHG) in a periodically poled lithium niobate (PPLN) ridge waveguide pumped with pulsed laser at the quasi-phase matching (QPM) wavelength, as well as simultaneous SHG and cascaded third-harmonic generation (THG) waves when pumped at the non-QPM wavelength. Furthermore, the ridge waveguide achieves an efficient single-pass SHG conversion efficiency of picosecond pulsed laser at ∼62%. These results may be beneficial for on-chip nonlinear frequency conversion.

Structure design and numerical simulation of chirped periodically polarized lithium niobate crystal for broadband mid-infrared laser generation

Mid-infrared band 3–5 <inline-formula><tex-math id="M1">\begin{document}${\text{μm}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20220016_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20220016_M1.png"/></alternatives></inline-formula> laser light source has important applications in many fields such as medical treatment, basic science, communication, and industry. Owing to the limitation to available efficient gain media in the mid-infrared band, the traditional methods of generating and amplifying lasers , such as regenerative amplification, are no longer applicable. In order to produce broadband and high-energy mid-infrared laser, in this work we combine quasi-phase matching technology and chirped periodically polarized lithium niobate (CPPLN) crystal for theoretical analysis and numerical design. The second-order nonlinear difference-frequency generation (DFG) process is used to implement the generation of mid-infrared laser via CPPLN. In the differential frequency process, the pump light used is 800 nm in wavelength and the wavelength range of signal light is 0.95–1.6 <inline-formula><tex-math id="M2">\begin{document}${\text{μm}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20220016_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20220016_M2.png"/></alternatives></inline-formula>. By calculating the dispersion curve of CPPLN crystal, the phase mismatch of difference frequency generation processes with different light signals is obtained. Under the condition of quasi-phase matching, the CPPLN with deliberately poling structures is designed and used to provide phase mismatch compensation in a broad bandwidth. The designed structure can meet the generation of mid infrared laser in a 1.6–5<inline-formula><tex-math id="M3">\begin{document}$ {\text{μm}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20220016_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20220016_M3.png"/></alternatives></inline-formula> band according to the numerical simulations. The conversion efficiencies of mid-infrared laser with different wavelengths at different positions in the crystal are obtained by using nonlinear coupled wave equations and fourth-order Runge-Kutta method. The results show that the mid-infrared laser in a wavelength range of 1.6–5 <inline-formula><tex-math id="M4">\begin{document}$ {\text{μm}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20220016_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20220016_M4.png"/></alternatives></inline-formula> can be produced efficiently in a single CPPLN crystal, with an average conversion efficiency of about 15%. The theoretical analysis and numerical simulation for the designed CPPLN crystal can provide good schematic reference and theoretical support for further experimental exploration on generation of mid-infrared laser.

Quasi-Phase-Matched Four-Wave Mixing Enabled by Grating-Assisted Coupling in a Hybrid Silicon Waveguide

Phase matching must be performed to realize four-wave mixing (FWM) in a silicon waveguide, which is challenging when a signal wavelength is highly deviated from an idler wavelength. To solve this problem, quasi-phase-matching (QPM) based on grating-assisted directional coupling (GADC) in a hybrid structure is theoretically investigated. The proposed system consists of a silicon strip with periodic width modulation as the grating and a silicon nitride strip that is vertically aligned to the silicon strip. GADC occurs between the TE_S mode (mainly confined in the silicon strip) and TE_N mode (mainly confined in the silicon nitride strip) at an idler wavelength. This phenomenon compensates for the phase mismatch occurring in FWM among the TE_S modes at the pump, signal, and idler wavelengths. Results of analysis of the hybrid structure show that the TE_S and TE_N modes at an idler wavelength of 2.1177 μm are efficiently generated with the TE_S modes at a pump wavelength of 1.58 μm and signal wavelength of 1.2601 μm. Moreover, the signal TE_S mode can be efficiently implemented with the pump TE_S mode and idler TE_N mode. Owing to the GADC characteristics, the conversion bandwidth of the signal is 0.8 nm; however, the signal wavelength can be thermally tuned, with a temperature change of 50 °C corresponding to a signal wavelength change of 3.6 nm. The hybrid structure with the GADC-based QPM can be used to generate and detect mid-infrared light with well-developed O-band and L-band devices.

Intense Two-Octave Ultraviolet-Visible-Infrared Supercontinuum Laser via High-Efficiency One-Octave Second-Harmonic Generation.

Intense ultrabroadband laser source of high pulse energy has attracted more and more attention in physics, chemistry, biology, material science, and other disciplines. We report design and realization of a chirped periodically poled lithium niobate nonlinear crystal that supports ultrabroadband second-harmonic generation covering 350-850 nm by implementing simultaneously up to 12 orders of quasiphase matching against ultrabroadband pump laser covering 700-1700 nm with an average high conversion efficiency of about 25.8%. We obtain a flat supercontinuum spectrum with a 10 dB bandwidth covering more than one octave (about 375-1200 nm) and 20 dB bandwidth covering more than two octaves (about 350-1500 nm) in the ultraviolet-visible-infrared regime and having intense energy as 0.17 mJ per pulse through synergic action of second-order and third-order nonlinearity under pump of 0.48 mJ per pulse Ti:sapphire femtosecond laser. This scheme would provide a promising method for the construction of supercontinuum laser source with extremely broad bandwidth, large pulse energy, and high peak power for a variety of basic science and high technology applications.

Efficient and tunable blue light generation using lithium niobate nonlinear photonics

Thin-film lithium niobate (LN) has recently emerged as a playground for chip-scale nonlinear optics and leads to efficient frequency conversions from near-infrared to near-visible bands. For many nonlinear and quantum photonics applications, it is desirable to operate deep into the visible band within LN's transparency window. However, the strong material dispersion at short wavelengths makes phase-matching difficult, necessitating sub-micrometer scale control of domain structures for efficient phase-matching. Here, we report the operation of thin film LN in the blue wavelength and high fidelity poling of the thin-film LN waveguide to this regime. As a result, quasi-phase matching is realized between IR (871 nm) and blue (435.5 nm) wavelengths in a straight waveguide and prompts strong blue light generation with a conversion efficiency (1040% ± 140%/W). This blue second harmonic generator exhibits stable temperature tunability, which is important for applications that require precise frequency alignment, such as atomic clocks.

Cascaded chirped quasi-phase-matching grating for broad spectrum generation via a complementary reciprocal lattice vector

Broad spectra are of interest for a variety of applications, including pulse compression and ultrafast optical signal processing. We report a scheme of a cascaded chirped quasi-phase-matching (QPM) grating with a complementary reciprocal lattice vector bandwidth. The large reciprocal lattice vector bandwidth provided by first- and second-order QPM can simultaneously achieve second and third harmonic generations. Combined the effective nonlinear coefficient model with the transfer function method, it reveals that spectrum width increased with increasing fundamental wave bandwidth, reciprocal lattice vector bandwidth, chirp rate, and crystal length. The two-section cascaded chirped QPM grating can offer a reciprocal lattice vector bandwidth as large as 0.3 to 1.26 μm − 1, which corresponds to a broadband spectrum covering 450 to 820 nm upon illumination with a tunable femtosecond laser. The proposed grating could extend supercontinuum generation toward potential spectroscopic applications.

Controlling the transverse correlation in quasi-phase matching parametric down-conversion

In this work we study the transverse spatial correlation of the pair of photons generated via the process of spontaneous parametric frequency down-conversion, in periodically poled non-linear crystals illuminated by a pulsed laser beam. It is well known that the two-photon state generated in quasi-phase matching (QPM) configurations depends explicitly on the characteristics of the pump beam, on the crystal modulated non-linearity, and on the detection geometry. This has allowed the development of several techniques for controlling the biphoton spectral and spatial properties. Here we discuss another technique for implementing the spatial entanglement modification in QPM gratings. We show, theoretically and experimentally, that in nearly collinear geometries, the spatial shape modulation of the pump beam allows for the control of the biphoton transverse spatial correlation.

Theoretical solution of second-harmonic generation in periodically poled lithium niobate and chirped periodically poled lithium niobate thin film via quasi-phase-matching

We present an effective nonlinear coupled mode coefficient model to evaluate and analyze the second harmonic generation (SHG) process under the quasi-phase-matching (QPM) scheme in periodically poled lithium niobate (PPLN) thin film. The theoretical model derives directly from the nonlinear coupled mode equations and simplifies the QPM process in PPLN thin film. The simulated results agree excellently with the rigorous numerical calculations in a variety of situations. We also propose using the chirped PPLN (CPPLN) thin film to achieve the SHG of ultrashort laser pulse and develop a broadband nonlinear coupled mode theory to evaluate this complex nonlinear coupling interaction process. All the frequency components in fundamental wave (FW) and second harmonic wave (SHW) have been considered in the theory, and the spectral evolution of FW and SHW can be obtained by solving a series of nonlinear coupled mode equations. CPPLN thin film supports broadband up-conversion in telecommunications bands and can generate broadband entangled photons in infrared region, which shows great potential in optical communication and quantum optics. The developed theories provide an efficient tool to design and optimize the PPLN and CPPLN thin film for various applications based on nonlinear frequency conversion and help to achieve high-efficiency on-chip SHG of the ultrashort laser pulse.

Fabrication and optical characterization of GaN quasi-phase matching crystal by double polarity selective area growth in metal organic vapor phase epitaxy

The fabrication of ultra-violet (UV) second-harmonic generation (SHG) (UV-SHG) devices requires GaN quasi-phase matching (GaN-QPM) crystals with periodically arranged polar GaN. For fabricating GaN-QPM crystals, the double polarity selective area growth (DP-SAG) using carbon mask technique is employed. However, the growth of narrow (2–4 [Formula: see text]m) pitch pattern GaN-QPM crystals, which is necessary for UV-SHG devices, has not been reported using this technique. Herein, we report the successful fabrication of 4 [Formula: see text]m pitch pattern GaN-QPM. We fabricated a thick GaN-QPM crystal at the optimized V/III ratio. Through optical characterization, we observed SHG generation from the GaN-QPM crystal fabricated using DP-SAG.

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.

Non-Invasive Visualization of Ferroelectric Domain Structures on the Non-Polar y-Surface of KTiOPO4 via Raman Imaging

Potassium titanyl phosphate (KTP) is a nonlinear optical material with applications in high-power frequency conversion or quasi-phase matching in submicron period domain grids. A prerequisite for these applications is a precise control and understanding of the poling mechanisms to enable the fabrication of high-grade domain grids. In contrast to the widely used material lithium niobate, the domain growth in KTP is less studied, because many standard methods, such as selective etching or polarization microscopy, provides less insight or are not applicable on non-polar surfaces, respectively. In this work, we present results of confocal Raman-spectroscopy of the ferroelectric domain structure in KTP. This analytical method allows for the visualization of domain grids of the non-polar KTP y-face and therefore more insight into the domain-growth and -structure in KTP, which can be used for improved domain fabrication.

Efficient parametric amplification via simultaneous second harmonic generation.

We introduce a concept for efficient optical parametric amplification (OPA) based on simultaneously phase-matched idler second harmonic generation (SHG), which together exhibits the dynamical behavior of parametric amplification but with damped conversion-back-conversion cycles. This enables amplification efficiency exceeding that of conventional OPA by several-fold for femtosecond and picosecond signal pulses with bell-shaped intensity profiles by allowing a near-uniform spatiotemporal depletion of the pump wave. We develop a Duffing oscillator model that unifies the description of conventional OPA and amplification accompanied by idler photon displacement by either linear absorption or SHG. A spatiotemporal analysis of devices based on birefringent or superlattice quasi-phase matching in common bulk media predicts energy conversion up to 55%.

Cavity-enhanced frequency doubling with a third-order quasi-phase-matched PPKTP crystal

Blue lasers are very useful for applications in fundamental sciences and advanced technologies. Although the frequency doubling with a first-order quasi-phase-matched (QPM) crystal is an effective approach to obtain blue and UV lasers, the crystal should be poled with a short period, which is technically difficult to fabricate compared with a long period. Using a third-order QPM can be an alternative way to generate blue light, in which the poling period is 3 times larger than that in first-order QPM. In this work, we report on the generation of a 402.5 nm laser by using cavity-enhanced second-harmonic generation and a periodically poled KTP (PPKTP) crystal with a poling period of 10.02 µm. About 36.49 mW output power and 15.3% conversion efficiency are achieved in our experiment.

Quasi-phase-matched laser wakefield acceleration of electrons in an axially density-modulated plasma channel

Quasi-phase matching in corrugated plasma channels has been proposed as a way to overcome the dephasing limitation in laser wakefield accelerators. In this study, the phase-lock dynamics of a relatively long electron bunch injected in an axially-modulated plasma waveguide is investigated by performing particle simulations. The main objective here is to obtain a better understanding of how the transverse and longitudinal components of the wakefield as well as the initial properties of the beam affect its evolution and qualities. The results indicate that the modulation of the electron beam generates trains of electron microbunches. It is shown that increasing the initial energy of the electron beam leads to a reduction in its final energy spread and produces a more collimated electron bunch. For larger bunch diameters, the final emittance of the electron beam increases due to the stronger experienced transverse forces and the larger diameter itself. Increasing the laser power improves the maximum energy gain of the electron beam. However, the stronger generated focusing and defocusing fields degrade the collimation of the bunch.

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.