Periodically Poled Research Articles

Free-space quasi-phase matching.

We report a new, to the best of our knowledge, approach to phase matching of nonlinear materials based on the free-space multipass cells. This technique is applicable to noncentrosymmetric nonlinear crystals, including crystals that cannot be birefringent phase-matched or quasi-phase matched by periodic poling. Notably, by using this approach, the crystalline quartz is quasi-phase matched with the demonstrated increase of the second harmonic generation efficiency by a factor of 40. The method can be extended toward UV and THz ranges. This promises to revolutionize experimental nonlinear optics and all applications by increasing the number of available crystals for quasi-phase matching by at least one order of magnitude and brings fresh motivation for developing novel nonlinear materials.

Periodic poling of thin-film lithium tantalate by applying a high-voltage electric field

Periodically poled lithium tantalate on insulator (PPLTOI) was successfully fabricated by applying a high-voltage electric field. The shape of the electrode, which determines the electric field distribution, as well as the poling time, and the strength of the electric field, are investigated in detail for the fabrication of periodically poled LTOI. By optimizing the poling parameters, the duty cycle of the inverted domain can be flexibly adjusted as well as be controlled to the optimal value of 50%. Moreover, the fabricated domain structure is uniform, and the standard deviation is less than 4.8%. The study presented in this work will pave the way for applications of LTOI in nonlinear integrated photonics.

Adapted poling to break the nonlinear efficiency limit in nanophotonic lithium niobate waveguides.

Nonlinear frequency mixing is a method to extend the wavelength range of optical sources with applications in quantum information and photonic signal processing. Lithium niobate with periodic poling is the most widely used material for frequency mixing due to its strong second-order nonlinear coefficient. The recent development using nanophotonic lithium niobate waveguides promises to improve nonlinear efficiencies by orders of magnitude thanks to subwavelength optical confinement. However, the intrinsic nanoscale inhomogeneity of nanophotonic lithium niobate waveguides limits the coherent interaction length, leading to low nonlinear efficiencies. Here we show improved second-order nonlinear efficiency in nanophotonic lithium niobate waveguides that breaks the limit imposed by nanoscale inhomogeneity. This is realized by developing the adapted poling approach to eliminate the impact of nanoscale inhomogeneity. We realize an overall second-harmonic efficiency of 104% W-1 (without cavity enhancement), approaching the theoretical performance for nanophotonic lithium niobate waveguides. The ideal square dependence of the nonlinear efficiency on the waveguide length is recovered. Phase-matching bandwidths and temperature tuneability are improved through dispersion engineering. We finally demonstrate a conversion ratio from pump to second-harmonic power greater than 80% in a single-pass configuration with pump power as low as 20 mW. Our work therefore breaks the trade-off between the conversion ratio and pump power, offering a potential solution for highly efficient and scalable nonlinear-optical sources, amplifiers and converters.

Quasi-phase matching using frozen waves without periodic poling.

In this article, we show that quasi-phase matching can be accomplished by manipulating one of the pump beams without any periodic poling. We analyze a simple case wherein one of the interacting beams has a periodic pattern, and the others are assumed to be planewaves. We present comparisons of the efficiency of some nonlinear processes with quasi-phase matching achieved through our method and the conventional method. We demonstrate that some patterns of the pump beam can be more efficient than conventional periodic poling.

Facilitated reversible domain switching at multiphase boundary in periodically orthogonal poled KNN‐based ceramics: Strain versus non‐180o domain

AbstractAlthough many approaches have been implemented to tailor the strain in potassium sodium niobate (KNN)‐based ferroelectrics, they still suffer from poor strain compared to shape memory alloys and giant magnetostrictive materials. Herein, a strategy of periodic orthogonal poling is implemented in KNN‐based ceramics with multiphase boundaries, and the correlation between the amount of non‐180° domain and strain is established, revealing that the mechanisms of interfacial stresses facilitate reversible domain switching during the periodic orthogonal poling. Owing to the self‐generated interfacial stresses between the adjacent regions with different poling directions, an enhanced strain benefits from non‐180° domain switching, which is reversible during periodic orthogonal poling. The enhancement in strain decreases from O‐T to R‐O‐T to the R‐T phase boundary, which corresponds to the different quantity of the non‐180° domain, indicating that a large amount of non‐180° domains can further boost high strain under periodic orthogonal poling. Notably, a slight frequency‐dependent strain was observed across the frequency range of 1‒50 Hz. Therefore, an ideal strain can be further induced by enhancing the amount of reversible non‐180° domain switching in the multiphase boundary during periodic orthogonal poling, which can serve as a guide for the design of high‐performance KNN materials.

Integrated Backward Second-Harmonic Generation through Optically Induced Quasi-Phase-Matching.

Quasi-phase-matching for efficient backward second-harmonic generation requires sub-μm poling periods, a nontrivial fabrication feat. For the first time, we report integrated first-order quasiphase-matched backward second-harmonic generation enabled by seeded all-optical poling. The self-organized grating inscription circumvents all fabrication challenges. We compare backward and forward processes and explain how grating period influences the conversion efficiency. These results showcase unique properties of the coherent photogalvanic effect and how it can bring new nonlinear functionalities to integrated photonics.

A parametric poles assignment algorithm for high-order linear discrete periodic systems

The periodic state feedback pole assignment problem of high-order periodic discrete systems is investigated, and the pole assignment problem for such systems is transformed into a class of problems for resolving periodic Sylvester matrix equations with constraints. Using the technique of cyclic lifting, such equations can be transformed into high-order time-invariant Sylvester matrix equations and solved using a parametric method. Lastly, a numerical example is provided to demonstrate the validity of the procedure.

End effects of Halbach field magnet‐type linear motor

AbstractA linear motor that uses a finite length Halbach array as its magnetic field causes a thrust force ripple owing to end effects. In this paper, the theoretical and numerical analysis methods of the Halbach array are investigated. The results of the electromagnetic field analysis are compared to confirm the validity of the theoretical and numerical solutions. Moreover, we focus on the magnetic pole period and coil arrangement of the finite length Halbach field type linear motor to eliminate the influence of the end effects, and report the relationship with the end effects.

Mid-infrared Spectrally Pure Single-Photon States Generation from 22 Nonlinear Optical Crystals

We theoretically investigate the preparation of pure-state single-photon source from 14 birefringent crystals (CMTC, THI, LiIO3, AAS, HGS, CGA, TAS, AGS, AGSe, GaSe, LIS, LISe, LGS, and LGSe) and 8 periodic poling crystals (LT, LN, KTP, KN, BaTiO3, MgBaF4, PMN-0.38PT, and OP-ZnSe) in a wavelength range from 1224 nm to 11650 nm. The three kinds of group-velocity-matching (GVM) conditions, the phase-matching conditions, the spectral purity, and the Hong-Ou-Mandel interference are calculated for each crystal. This study may provide high-quality single-photon sources for quantum sensing, quantum imaging, and quantum communication applications at the mid-infrared wavelength range.

Tunable and efficient ultraviolet generation with periodically poled lithium niobate.

On-chip ultraviolet (UV) sources are of great interest for building compact and scalable atomic clocks, quantum computers, and spectrometers. However, few material platforms are suitable for integrated UV light generation and manipulation. Of these materials, thin-film lithium niobate offers unique advantages such as sub-micron modal confinement, strong nonlinearity, and quasi-phase matching. Despite these characteristics, its utilization in the UV has remained elusive because of the substantial sensitivity of standard quasi-phase matching to fabrication imperfections, the photorefractive effect, and relatively large losses in this range. Here, we present efficient (197 ± 5%/W/cm2) second harmonic generation of UV-A light in a periodically poled lithium niobate nanophotonic waveguide. We achieve on-chip UV powers of ∼30 µW and linear wavelength tunability using temperature. These results are enabled with large cross section waveguides, which leads to first-order UV quasi-phase-matching with relatively long poling periods (>1.5 µm). By varying the poling period, we have achieved the shortest reported wavelength (355 nm) generated through frequency doubling in thin-film lithium niobate. Our results open up new avenues for UV on-chip sources and chip-scale photonics through compact frequency-doubling of common near-IR laser diodes.

A Pulsed Lidar System With Ultimate Quantum Range Accuracy

A light detection and ranging (lidar) system using a multi-output quantum pulse gate (mQPG) for quantum uncertainty limited range resolution is presented. Due to the inherent phase-sensitivity of the process, the proposed mQPG enhanced lidar system is able to beat the theoretical Rayleigh range resolution limit and the single detection Craméer-Rao bound (CRB) of classic pulsed lidar. To achieve this behavior, gating pulses interact with the backscattered lidar pulse while propagating through the optically non linear waveguide within the mQPG. The intensity of the discrete spectral channels of the mQPG contains finally the target distance informations. The mQPG was fabricated in a periodically poled (PP) titanium-indiffused lithium niobate waveguide and the entire system was built up in a lab environment. Measurement up to 1500 μm were performed around an arbitrary set reference point, and show excellent agreement with the theory up to 600 μm for a pulse length of 1.56ps, and up to 1100 μm for a pulse length of 2.12ps.

Fanout periodic poling of BaMgF4 crystals

The angle-dependent poling properties of 0.5-mm-thick BaMgF4 (BMF) crystals are investigated by means of calligraphically poled, radially oriented domains. We present a fanout periodic poling pattern for quasi-phase matching (QPM) applications, which covers an angular spectrum of about 14°. Within this range there is no angle-dependent degradation of the domain quality. This is the first realization of non-parallel domains for QPM in BMF-crystals.

Carrier-resolved real-field theory of multi-octave frequency combs

Optical frequency combs are pillars of precision spectroscopy, and their microresonator realization serves applications where miniaturization and large tooth separation are important. Microresonator combs cover an enormous range of time scales varying from the femtosecond periods of optical oscillations to milliseconds corresponding to the kilohertz linewidth of the comb teeth. Here, we develop and implement the carrier-resolved real-field model for multi-octave frequency combs, which allows for nearly ab initio capture of all the time scales involved. As an example, we consider a microresonator that has a mix of second- and third-order nonlinearities and uses periodic poling. By applying the real-field approach, we demonstrate how to surpass traditional limitations and model the spectral broadening and soliton mode-locking across three optical octaves.

Custom Nonlinearity Profile for Integrated Quantum Light Sources

Heralded single-photon sources are a fundamental building block for optical quantum technologies. These sources need to be unfiltered and integrated to have good scalability and avoid unnecessary losses. To attain this goal, it is necessary to control the effective nonlinearity seen by the fields as they mix and propagate in a waveguide source. In this paper, we introduce a method to design nonlinear waveguides with arbitrarily shaped effective nonlinearity profiles. The method takes advantage of the fact that the second-order nonlinear response is a tensor quantity and thus the local effective nonlinearity of a material depends on the propagation direction of the fields participating in the interaction. Thus, by locally changing the propagation direction of the fields we can modulate the wave-mixing process. Our method allows for the waveguide fabrication process to be significantly simplified. The material structure of the waveguide is made by a single crystal, no longer needing oriented patterning or periodic poling. We use our method to design waveguides with a nonlinearity profile that is Gaussian in the propagation length, allowing the generation of perfectly pure heralded single photons.

Efficient first-order quasi-phase-matched backward second-harmonic generation.

We demonstrate first-order quasi-phase-matched backward second-harmonic generation (BSHG) with an efficiency of 18.7%. This represents an increase by two orders of magnitude from earlier experiments employing higher-order quasi-phase-matching. The efficient BSHG is demonstrated in bulk periodically poled Rb:KTiOPO4 with a poling period of 317 nm. Using these structures, the frequency doubling in the backward direction is achieved for the fundamental wavelength of 2309 nm. Here we report on the experimental investigation of the BSHG properties such as spectral bandwidth, temperature tuning, and temperature bandwidth by employing broadband and narrowband fundamental wavelength sources. The BSHG properties are compared with those of co-propagating second harmonic generation to reveal the BSHG potential for novel applications that were proposed theoretically but have not been realized in practice so far.

Non-destructive periodic poling quality evaluation of MgO:PPLN and Rb:PPKTP crystals based on crystal translation and parametric light generation

The work discloses a flexible, non-destructive, aberration-free, and automated periodic poling quality evaluation technique of the periodically poled crystals in the whole volume of the crystal with very high control and resolution. The method is based on a crystal translation in the directions perpendicular to the pump laser beam and the simultaneous recording of the parametric signal conversion efficiency in a simple single-pass parametric light generator setup. This technique allowed us to investigate and quantitatively describe the ferroelectric domain structure homogeneity of the periodically poled lithium niobate and potassium titanyl phosphate crystals with single, multi, and fan-out grating designs and offers a practicable overall efficiency assessment of the quasi-phase-matched device.

Forward growth and formation of 1D domain arrays by focused ion beam in Y-cut MgOLN

The ion beam-induced domain growth in Y-cut MgO-doped lithium niobate (MgOLN) covered by the resist layer was studied. A comparative study of domain structure in the samples with and without surface dielectric layer is crucial for periodical poling. Differences in the domain shape with changes in the irradiation dose, energy, and current of the ion beam were revealed. The maximum domain depth was 400 nm at 100 pC. A detailed analysis of interaction for different distance between irradiated dots in row revealed formation of 1 D arrays with variating length of the wedge-like domains. These findings allow optimizing the poling process.

Structural Modal Parameter Identification Method Based on the Delayed Transfer Rate Function under Periodic Excitations

The dynamic response transfer rate function (TRF) is increasingly used in the field of structural modal parameter identification because it does not depend on the white noise assumption of the excitation. In this paper, the interference of periodic excitation on structural modal parameter identification using TRF is analyzed theoretically for a class of civil engineering structures with obvious periodic components in excitation, and then an identification method of structural real modal parameters is proposed. First, a delayed TRF is constructed, and the pseudo-frequency response function is further obtained to identify the periodic spurious poles of the whole system. Then, the effective identification of the real modal parameters of the structure is achieved by comparing the system poles identified via conventional TRF. Finally, the feasibility and robustness of the proposed method were verified using a calculation example with four-degrees-of-freedom system. In addition, the modal parameters of a structure under periodic excitation were effectively identified by taking a pumping station as an example, and the results show that the method accurately identified the structural modal parameters when the excitation contained periodic components, which has wider prospects for technical applications.

Effects of dispersion and pumping scheme on soliton-comb regimes in χ (2) microresonators

Monochromatic pumping of χ ( 2 ) microresonators is promising for frequency comb generation. Two main generation schemes refer to pumping into the phase-matched first harmonic (FH) and second harmonic (SH)—FH and SH pumping. The phase matching can be natural or artificial (employing periodic poling). Also, minimization of the FH and SH group velocity difference (temporal walk-off) is needed for effective comb generation. So far, perspective soliton-comb regimes were predicted for SH pumping in lithium niobate (LN)-based resonators at a special poling. Here we explore, analytically and numerically, the possibilities for realizing the χ ( 2 ) comb generation for the remaining options of pumping as applied to LN microresonators. This includes the FH pumping case for the pump wavelength λ p ≃ 2698 n m in radially poled resonators and also FH and SH pumping schemes at natural phase matching, λ p ≃ 1664 and 832 nm. It is found that the nonlinear regimes are substantially different and less efficient here as compared to the primary SH pumping case. This is caused by a profound difference between the FH and SH excitation schemes and also by different combinations of signs of the dispersion coefficients for FH and SH resonator modes.

Phase-locked degenerate backward wave optical parametric oscillator

We report a self-phase-locked and degenerate backward wave optical parametric oscillator (BWOPO) generating a counterpropagating signal and an idler in the spectral band of optical communication. The device is based on a periodically poled Rb-doped KTiOPO4 crystal with a poling period of 433 nm. To the best of our knowledge, this is the shortest quasi-phase matching period reported in the bulk of a ferroelectric crystal. The spectral and coherence properties of the BWOPO at degeneracy are investigated. We observe a single pass conversion efficiency of 40%. At degeneracy, the counterpropagating signal and idler generate phase-matched sum-frequency generation in both directions. We confirm the phase-locked state at degeneracy by interfering the pump with the frequency-doubled backward wave. The phase-locked state is stable during the entire duration of the experiment without active stabilization.