Second-order Nonlinear Materials Research Articles

Submicrometer periodic poling of lithium niobate thin films with bipolar preconditioning pulses

Periodically poled second-order nonlinear materials with submicrometer periods are important for the development of quasi-phase matched backward-wave nonlinear optical processes. Interactions involving counter-propagating waves exhibit many unique properties and enable devices such as backward second harmonic generators, mirrorless optical parametric oscillators, and narrow-band quantum entangled photon sources. Fabrication of dense ferroelectric domain gratings in lithium niobate remains challenging, however, due to lateral domain spreading and merging. Here, we report submicrometer periodic poling of ion-sliced x-cut magnesium oxide doped lithium niobate thin films. Electric-field poling is performed using multiple bipolar preconditioning pulses that improve the poling yield and domain uniformity. The internal field is found to decrease with each preconditioning poling cycle. The poled domains are characterized by piezoresponse force microscopy. A fundamental period of 747 nm is achieved.

Harmonic optical tomography of nonlinear structures

Second-harmonic generation microscopy is a valuable label-free modality for imaging non-centrosymmetric structures and has important biomedical applications from live-cell imaging to cancer diagnosis. Conventional second-harmonic generation microscopy measures intensity signals that originate from tightly focused laser beams, preventing researchers from solving the scattering inverse problem for second-order nonlinear materials. Here, we present harmonic optical tomography (HOT) as a novel modality for imaging microscopic, nonlinear and inhomogeneous objects. The HOT principle of operation relies on inter-ferometrically measuring the complex harmonic field and using a scattering inverse model to reconstruct the three-dimensional distribution of harmonophores. HOT enables strong axial sectioning via the momentum conservation of spatially and temporally broadband fields. We illustrate the HOT operation with experiments and reconstructions on a beta-barium borate crystal and various biological specimens. Although our results involve second-order nonlinear materials, we show that this approach applies to any coherent nonlinear process.

Transient Second-Order Nonlinear Media: Breaking the Spatial Symmetry in the Time Domain via Hot-Electron Transfer

Second-order optical effects are essential to the active control of light and the generation of new spectral components. The inversion symmetry, however, prevents achieving a bulk χ^{(2)} response, limiting the portfolio of the second-order nonlinear materials. Here, we demonstrate subpicosecond conversion of a statically passive dielectric to a transient second-order nonlinear medium upon the ultrafast transfer of hot electrons. Induced by an optical switching signal, the amorphous dielectric with vanishing intrinsic χ^{(2)} develops dynamically tunable second-order nonlinear responses. By taking the second-harmonic generation as an example, we show that breaking the inversion symmetry through hot-electron dynamics can be leveraged to address the critical need for all-optical control of second-order nonlinearities in nanophotonics. Our approach can be generically adopted in a variety of material and device platforms, offering a new class of complex nonlinear media with promising potentials for all-optical information processing.

Synthesis, growth, structure analysis and performance characterization of DOST crystal: a nonlinear functional material with a wide range of applications

A new non-centrosymmetric organic ion nonlinear optical crystal, DOST.

Second harmonic generation in monolithic lithium niobate metasurfaces.

Second-order nonlinear metasurfaces have proven their ability to efficiently convert the frequency of incident signals over subwavelength thickness. However, the availability of second-order nonlinear materials for such metasurfaces has so far been limited to III-V semiconductors, which have low transparency in the visible and impose constraints on the excitation geometries due to the lack of diagonal second-order susceptibility components. Here we propose a new design concept for second-order nonlinear metasurfaces on a monolithic substrate, which is not limited by the availability of thin crystalline films and can be applied to any non-centrosymmetric material. We exemplify this concept in a monolithic Lithium Niobate metasurface with cylinder-shaped corrugations for enhanced field confinement. By optimizing the geometrical parameters, we show enhanced second harmonic generation from a near-infrared pump beam with conversion efficiency above 10-5 using 1 GW/cm2 pump intensity. Our approach enables new opportunities for practical designs of generic metasurfaces for nonlinear and quantum light sources.

Azonia aromatic heterocycles as a new acceptor unit in D-π-A+vs D-A+ nonlinear optical chromophores

A comparison of D-π-A+ and D-A+ cationic chromophores based on the quinolizinium system as new acceptor units is reported along with the results of studies into their linear and non-linear optical properties and electrochemical data. Experimental and theoretical data show that quinolizinium-based chromophores may provide a new generation of second-order non-linear materials with enhanced performance. The first hyperpolarizabilities were measured by Hyper-Rayleigh scattering experiments and the experimental data are supported by a theoretical analysis. In some chromophores the absence of a bridge (D-A+) between the donor and acceptor fragments enhances the NLO properties and the single crystal structure of such a material has been determined by X-ray diffraction.

Bulk second-harmonic generation from thermally evaporated indium selenide thin films.

We investigate bulk second-order nonlinear optical properties of amorphous indium selenide thin films fabricated by thermal evaporation. Such films are shown to exhibit strong and photostable second-harmonic generation (SHG). We report strong thickness dependence of the second-harmonic signals as characterized by the Maker-fringe method. The absolute value of the nonlinear susceptibility tensor of the film is addressed by analyzing the interference of SHG signals from the film and the glass substrate. The value of the joint non-diagonal component of the susceptibility is found to be 4 pm/V, which is comparable to that of widely used second-order nonlinear materials.

Parametric down-conversion photon-pair source on a nanophotonic chip

Quantum-photonic chips, which integrate quantum light sources alongside active and passive optical elements, as well as single-photon detectors, show great potential for photonic quantum information processing and quantum technology. Mature semiconductor nanofabrication processes allow for scaling such photonic integrated circuits to on-chip networks of increasing complexity. Second-order nonlinear materials are the method of choice for generating photonic quantum states in the overwhelming majority of linear optic experiments using bulk components, but integration with waveguide circuitry on a nanophotonic chip proved to be challenging. Here, we demonstrate such an on-chip parametric down-conversion source of photon pairs based on second-order nonlinearity in an aluminum-nitride microring resonator. We show the potential of our source for quantum information processing by measuring the high visibility anti-bunching of heralded single photons with nearly ideal state purity. Our down-conversion source yields measured coincidence rates of 80 Hz, which implies MHz generation rates of correlated photon pairs. Low noise performance is demonstrated by measuring high coincidence-to-accidental ratios. The generated photon pairs are spectrally far separated from the pump field, providing great potential for realizing sufficient on-chip filtering and monolithic integration of quantum light sources, waveguide circuits and single-photon detectors.

Polar phase transitions and physical properties in fresnoite A2TiSi2O8 (A= Ba, Sr) by first principles calculations

Polar phase transitions of fresnoites, Ba2TiSi2O8 (BTS) and Sr2TiSi2O8 (STS) have been comparatively analyzed by the first principles calculations. We show that both BTS and STS have a polar structure with the space group P4bm as a ground state, and there is a fictitious phase transition in the tetragonal space group from the nonpolar P4/mbm meta-stable phase to the polar P4bm phase. From the analyses of the two atomic structures, we find that a noticeable issue in the phase transition is bond length changes of Si–O and Ti–O which break the inversion symmetry, resulting that one of vertices in the edge-shared Si–O and Ti–O polyhedron is detached in the polar phase. The structural phase transition between the polar and the nonpolar states are discussed in terms of electronic structures and structural symmetry mode analyses. We evaluate the size of spontaneous polarizations of BTS and STS in the polar P4bm phases, and the correlation analysis shows significant contributions of the detached polyhedrons to the strong polar property. We also show second harmonic generation susceptibilities of BTS and STS as a candidate for second-order nonlinear optics materials. Our quantitative studies can provide full understandings of atomic and electronic mechanisms of their polar phase and nonlinear optical properties.

Direct generation of optical frequency combs in χ(2) nonlinear cavities

Abstract Quadratic nonlinear processes are currently exploited for frequency comb transfer and extension from the visible and near infrared regions to other spectral ranges where direct comb generation cannot be accomplished. However, frequency comb generation has been directly observed in continuously pumped quadratic nonlinear crystals placed inside an optical cavity. At the same time, an introductory theoretical description of the phenomenon has been provided, showing a remarkable analogy with the dynamics of third-order Kerr microresonators. Here, we give an overview of our recent work on χ(2) frequency comb generation. Furthermore, we generalize the preliminary three-wave spectral model to a many-mode comb and present a stability analysis of different cavity field regimes. Although our work is a very early stage, it lays the groundwork for a novel class of highly efficient and versatile frequency comb synthesizers based on second-order nonlinear materials.

Image restoration based on phase conjugation in second-order nonlinear materials under reflection configuration.

We have demonstrated the recovery of the distorted images by a phase plate to nearly their original qualities under a refection configuration. This is made possible by utilizing difference-frequency generation in a bulk KTiOPO(4) (KTP) crystal and an AFB-KTP crystal composite. In such a geometry, both input and phase-conjugated waves propagate through the distorted phase plate for a round trip. They are reflected by the object, which can be far away from the imaging system, and captured by the cameras. Therefore, such a configuration is much more practical for eventually deploying our imaging system for realizing practical applications. We have demonstrated the advantages of the imaging system such as polarization insensitivity and broadband capability.

Frequency comb generation in quadratic nonlinear media

We experimentally demonstrate and theoretically explain the onset of optical frequency combs in a simple cavity-enhanced second-harmonic-generation system, exploiting second-order nonlinear interactions. Two combs are simultaneously generated around the fundamental pump frequency, with a spectral bandwidth up to about 10 nm, and its second harmonic. We observe different regimes of generation, depending on the phase-matching condition for second-harmonic generation. Moreover, we develop an elemental model which provides a deep physical insight into the observed dynamics. Despite the different underlying physical mechanism, the proposed model is remarkably similar to the description of third-order effects in microresonators, revealing a potential variety of new effects to be explored and laying the groundwork for a novel class of highly efficient and versatile frequency comb synthesizers based on second-order nonlinear materials.

Single-photon blockade in doubly resonant nanocavities with second-order nonlinearity

We propose the use of nanostructured photonic nanocavities made of second-order nonlinear materials as prospective passive devices to generate strongly sub-Poissonian light via single-photon blockade of an input coherent field. The simplest scheme is based on the requirement that the nanocavity be doubly resonant, i.e. possess cavity modes with good spatial overlap at both the fundamental and second-harmonic frequencies. We discuss feasibility of this scheme with state-of-the art nanofabrication technology, and the possibility to use it as a passive single-photon source on-demand.

Engineering of effective second-order nonlinearity in uniform and chirped gratings

The dependence of second harmonic generation conversion efficiency resulting from the position of poled regions of the second-order nonlinearity in uniform and chirped gratings in ferroelectric materials has been studied analytically and numerically. The displacement of the poled region’s position from a specific location in second-order nonlinear materials can introduce a wavelength shift in a uniform grating’s second harmonic intensity peak and strongly influences the bandwidth and ripple in the harmonic conversion response of chirped gratings. We propose that the poled regions should be located at specific positions within the single period to minimize the ripples and achieve a desired nonlinearity function, and also to significantly improve tolerance to fabrication errors.

Quasi-phase matched nonlinear media: Progress towards nonlinear optical engineering

Nonlinear optics has been enabled by the invention of the laser 50 years ago, and since then both technologies evolved together complementing and enabling each other. In this paper we attempt to briefly overview the historic development of second-order nonlinear materials and try to discern the trends which might dominate developments in the future. The advent of methods of quasi-phase matching and added degrees of freedom in managing three-wave mixing processes pushed the field of nonlinear optics into the realm of engineering. This trend, we believe will continue in the future. In this text we give a short description of design principles of one-dimensional and two-dimensional quasi-phase matched structures and emphasize the increased functionality afforded by such structures. We also overview the nonlinear media where engineering of nonlinear interactions has been demonstrated with the primary focus on ferroelectric and semiconductor nonlinear crystals.

Modeling of ultrabroadband and single-cycle phenomena in anisotropic quadratic crystals

We present a comprehensive model to describe the propagation of single-cycle and broadband optical pulses in anisotropic, dispersive, and nonlinear materials. Two nonlinear coupled wave equations describe the dynamics and interactions of optical pulses in uniaxial second-order nonlinear materials. The equations are first order in the propagation coordinate and are valid for arbitrarily wide pulse bandwidth, providing an accurate modeling of the evolution of ultrabroadband pulses also when the separation into different coupled frequency components is not possible or not profitable. We exploit this model to simulate recently observed femtosecond single-cycle multiterahertz transients in gallium selenide and to predict harmonic generation and spectral broadening in the visible and mid-infrared in lithium niobate.

Unidirectional ring-laser operation using sum-frequency mixing

A technique enforcing unidirectional operation of ring lasers is proposed and demonstrated. The approach relies on sum-frequency mixing between a single-pass laser and one of the two counterpropagating intracavity fields of the ring laser. Sum-frequency mixing introduces a parametric loss for the intracavity field copropagating with the single-pass field, effectively generating a loss difference between the copropagating and counterpropagating intracavity fields. This loss mechanism ensures stable unidirectional lasing. The approach is generic and can be implemented at any desired lasing wavelength where lossless second-order nonlinear materials are available. Numerical modeling and experimental demonstration of parametric-induced unidirectional operation of a diode-pumped solid-state 1342nm cw ring laser are presented.

Semiconductor lasers enable advances in nonlinear biophotonics

The successful demonstration of two-photon excitation fluorescence imaging of biotissues,1 such as mouse kidney and brain cells, has spurred development of a variety of novel multiphoton imaging (MPI) technologies. For example, using near-IR optical pulses, MPI can peer deep into tissues for noninvasive profiling. Moreover, because the fluorescence occurs only at the focal point of IR optical pulses, very long-time imaging is possible. This minimizes the consumption of fluorescent material. Many MPI technologies depend on high peak-power (over a kilowatt) ultrashort optical pulse sources to efficiently induce various multiphoton, nonlinear optical effects inside biospecimens. But most current sources are mode-locked solid-state lasers, which are bulky, expensive, and require maintenance. Increasing the practical appeal of MPI will require ‘real-world’ alternatives. Semiconductor laser diodes (LDs) have enjoyed substantial success as dependable, low-cost, high-performance light sources in information and communication technologies. In addition to their usefulness as mass-produced IT devices, LDs also have potential applications in scientific and technological measurement systems. Here we report evidence of their relevance to nonlinear bioimaging. In our research, LDs serve as key, easy-tooperate devices for generating stable picosecond optical pulses. A subsequent increase in optical power using low-noise and low-nonlinear-effect amplifiers raises the optical pulse peak power to more than a kilowatt, making it suitable for MPI. Because of the high peak power, efficient wavelength conversions using second-order nonlinear materials (e.g., ferroelectric optical crystals) and third-order materials (e.g., optical fibers, liquids) are also possible. Figure 1 presents the basic concept of our optical pulse source in schematic form. Figure 1. Schematic for generating kilowatt peak power picosecond optical pulses using a semiconductor laser oscillator. LD: laser diode.

Microwave Photonic Signal Detection Using Phase-Matched Optical Rectification in an AlGaAs Waveguide

We demonstrate a novel approach for detecting microwave-modulated optical signals through velocity-matched optical rectification in second-order optical nonlinear materials. Optical intensity modulation is down-converted to microwave frequency (5-20 GHz) using a traveling-wave GaAs electrooptic modulator as an optical square-law detector. Phase matching between microwave and optical waves is achieved through a velocity-matched slow-wave asymmetric coplanar-strip electrode structure.

Pseudospectral time-domain methods for modeling optical wave propagation in second-order nonlinear materials

Frequency conversion in second-order nonlinear materials is sensitive to the phase velocities of interacting optical waves. Accurate modeling of such problems with the finite-difference time-domain method requires extremely fine grid resolutions to minimize numerical dispersion errors. We propose an alternative approach based on a pseudospectral time-domain (PSTD) method for solving the nonlinear Maxwell’s equations. Low-dispersion PSTD schemes with second- and fourth-order time stepping are developed and investigated. Benchmark simulations of second-harmonic generation (SHG) demonstrate that the PSTD schemes offer significant improvements in computational efficiency and accuracy. We demonstrate use of these schemes by modeling SHG in a nonlinear grating illuminated at an oblique angle, where phase matching is achieved in two dimensions.