Quasi phase matching in two-dimensional nonlinear photonic crystals

We analyze quasi-phase-matched (QPM) conversion efficiency of the five possible types of periodic two-dimensional nonlinear structures: Hexagonal, square, rectangular, centered-rectangular, and oblique. The frequency conversion efficiency, as a function of the two-dimensional quasi-phase-matching order, is determined for the general case. Furthermore, it is demonstrated for two basic feasible motifs, a circular motif and a rectangular motif. This enables to determine the optimal motif dimensions for achieving the highest conversion efficiency. We find that a rectangular motif is more efficient than a circular motif for quasi-phase-matched processes that rely on a single reciprocal lattice vector (RLV), and that under optimal choice of motif dimensions, it converges into a one-dimensional periodic structure. In addition, in a few specific cases we found that higher order QPM can be significantly more efficient than lower order QPM.     

On‐axis shaping of second‐harmonic beams

We report complete spatial shaping (both phase and amplitude) of the second‐harmonic beam generated in a nonlinear photonic crystal. Using a collinear second‐order process in a nonlinear computer generated hologram imprinted on the crystal, the desired beam is generated on‐axis and in the near field. This enables compact and efficient one‐dimensional beam shaping in comparison to previously demonstrated off‐axis Fourier holograms. We experimentally demonstrate the second‐harmonic generation of high‐order Hermite–Gauss, top hats and arbitrary skyline‐shaped beams.

     

Parameters for efficient growth of second harmonic field in nonlinear photonic crystals

The ultrashort pulse propagation and nonlinear second harmonic generation under the undepleted pump approximation in a quadratic nonlinear photonic crystal (NPC) structure is theoretically investigated and the optimized parameters for high second harmonic generation conversion efficiency are extracted. The transfer matrix method is used for the numerical formulation for oblique angle of incidence. A unique set of material combination GaInP/InAlP is selected as alternating nonlinear and linear layers. The NPC parameters like incident angle and layer thickness are manipulated to obtain the exact phase matching using double resonance condition for a fixed number of layers with known experimental material parameters.     

On‐chip single photon sources using planar photonic crystals and single quantum dots

We review the basic light‐matter interactions and optical properties of chip‐based single photon sources, that are enabled by integrating single quantum dots with planar photonic crystals. A theoretical framework is presented that allows one to connect to a wide range of quantum light propagation effects in a physically intuitive and straightforward way. We focus on the important mechanisms of enhanced spontaneous emission, and efficient photon extraction, using all‐integrated photonic crystal components including waveguides, cavities, quantum dots and output couplers. The limitations, challenges, and exciting prospects of developing on‐chip quantum light sources using integrated photonic crystal structures are discussed.     

Composite‐Assisted Phase‐Matching: Efficient Wavelength Conversion in Nonlinear Optical Composite Materials Containing Metal Nanoparticles

A novel phase‐matching scheme which is based on the dispersion compensation in the nonlinear optical composite materials containing metal nanoparticles is proposed. Anomalous dispersion originating from the plasmon resonance in metal nanoparticles compensates the dispersion of the host nonlinear material, leading to the perfect phase‐matching and high efficiency of nonlinear optical wavelength conversion. The effectiveness of this approach is theoretically demonstrated, taking third‐order nonlinear processes such as the direct third‐harmonic generation and four‐wave mixing in ZnO composites containing silica‐core–silver‐shell nanoparticles as examples. The results show that with the proposed phase‐matching scheme, unprecedentedly high conversion efficiency can be obtained compared with preceding results in third‐order nonlinear optical solid‐state materials.     

准相位匹配周期极化钼酸钆制备及倍频效应

根据准相位匹配原理，对钼酸钆晶体用于准相位匹配倍频过程的周期性畴结构进行了设计，当入射基频光为l.064pm时，所需的钼酸钆晶体畴结构周期为5．54μm。利用外电场极化，当外电场为2．5kV／mm时，钼酸钆晶体的畴结构反转时间tc=500～600μs。当外电场脉冲幅值为2．5kV／mm、脉冲延续时间t=0．6～0．8tc时，在钼酸钆晶体中制备得到了周期畴结构。当Nd：YAG纳秒激光器输出准连续功率为7～10mW的l.064μm激光通过具有周期畴结构的样品时，获得了0．532μm的倍频光输出。     

Phase‐matching properties of BaGa4S7 and BaGa4Se7: Wide‐bandgap nonlinear crystals for the mid‐infrared

Biaxial BaGa₄S₇ and BaGa₄Se₇ crystals transparent in the mid‐IR have been grown by the Bridgman–Stockbarger technique in sufficiently large sizes and with good optical quality to measure the refractive indices and analyze phase‐matching properties. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Angular‐ and polarization‐independent structural colors based on 1D photonic crystals

Wide‐angle, polarization‐independent structural reflective colors from both directions based on a one‐dimensional photonic crystal are demonstrated. Our device produces a distinct and saturated color with high angular tolerant performance up to ±70° for any polarization state of an incident light wave, which is highly desirable for a broad range of research areas. Moreover, the purity of the color and luminous intensity of the proposed device are improved as compared to conventional colorant‐based color filters and colloidal glasses. The present approach may have the potential to replace existing color filters and pigments and pave the way for various applications, including color displays and image sensor technologies.

     

Low‐wavelength emission of Nd‐doped lasers

This paper gives an overview of the results obtained with diode‐pumped Neodymium‐doped crystals operating below 900 nm. Operation at such low wavelengths requires considering the strong thermal population of the lower level of the laser transition. Based on a theoretical study and simulations, the paper presents the challenges related to the design of these three‐level lasers. Experimental results are given with Nd:YAG and Nd:vanadate crystals. It is explained how to deal with the line competition with emission at 946 nm or 912 nm. Finally, intracavity second‐harmonic generation is presented. The output powers reach a few hundred mW at wavelengths below 450 nm. Hence, the paper demonstrates the potential of Nd‐doped diode‐pumped solid‐state lasers for applications in the blue range, in replacement of gas lasers such as helium‐cadmium lasers.     

Opportunities for wavelength conversion with on‐chip diamond ring resonators

Recent progress in the fabrication of high‐quality synthetic diamond and of diamond waveguide structures has enabled photonics researchers to start exploiting the unique optical properties of diamond for various applications. In this article the promise of on‐chip diamond ring resonators for wavelength conversion based on Kerr and Raman‐resonant four‐wave mixing is numerically demonstrated. After examining to what extent both dispersion‐engineered phase‐matching and “automatic” quasi‐phase‐matching can be established in diamond ring converters, it is shown that such a “double‐matching” approach can yield high conversion efficiencies for a wide range of wavelengths in the near‐infrared/mid‐infrared domain, as well as in the ultraviolet/visible domain.     

Fabrication of three‐dimensional photonic crystals in quantum‐dot‐based materials

Controlling spontaneous emission (SE) is of fundamental importance to a diverse range of photonic applications including but not limited to quantum optics, low power displays, solar energy harvesting and optical communications. Characterized by photonic bandgap (PBG) property, three‐dimensional (3D) photonic crystals (PCs) have emerged as a promising synthetic material, which can manipulate photons in much the same way as a semiconductor does to electrons. Emission tunable nanocrystal quantum dots (QDs) are ideal point sources to be embedded into 3D PCs towards active devices. The challenge however lies in the combination of QDs with 3D PCs without degradation of their emission properties. Polymer materials stand out for this purpose due to their flexibility of incorporating active materials. Combining the versatile multi‐photon 3D micro‐fabrication techniques, active 3D PCs have been fabricated in polymer‐QD composites with demonstrated control of SE from QDs. With this milestone novel miniaturized photonic devices can thus be envisaged.     

4H‐SiC: a new nonlinear material for midinfrared lasers

Nonlinear optical (NLO) frequency conversion is commonly used for generating midinfrared (MIR) lasers that offer light sources for a variety of applications. However, the low laser damage thresholds of NLO crystals used so far seriously limit the output power of MIR lasers. Here, a new nonlinear material 4H‐SiC is demonstrated for producing MIR laser. Broadband MIR radiation ranging from 3.90 to 5.60 μm is generated in 4H‐SiC by phase‐matched difference‐frequency generation for the first time. The results may open a door to practically utilize wide‐bandgap semiconductors with high laser damage thresholds as NLO materials for high power output of MIR lasers.     

Quantum‐optical analogies using photonic structures

Engineered photonic waveguides have provided in the past decade an extremely rich laboratory tool to visualize with optical waves the classic analogues of a wide variety of coherent quantum phenomena encountered in atomic, molecular or condensed‐matter physics. As compared to quantum systems, optics offers the rather unique advantage of a direct mapping of the wave function evolution in coordinate space by simple fluorescence imaging or scanning tunneling optical microscopy techniques. In this contribution recent theoretical and experimental advances in the field of quantum‐optical analogies are reviewed. Special attention is devoted to some relevant optical analogies based on the use of curved photonic structures, including: coherent destruction of tunneling in driven bistable potentials; coherent population transfer and adiabatic passage in laser‐driven multilevel atomic systems; quantum decay control and Zeno dynamics; electronic Bloch oscillations and Zener tunneling, Anderson localization and dynamic localization in crystalline potentials.     

Nonlinear optics in spheres: from second harmonic scattering to quasi‐phase matched generation in whispering gallery modes

Over the last fifteen years, a series of theoretical and experimental investigations have demonstrated the usefulness of circular geometries to tailor second‐order nonlinear optical effects. However, until recently, such effects have remained rather weak, calling for their enhancement. In parallel, developments in the field of high quality factor spherical or ring resonators have shown that many different types of light‐matter interactions can be dramatically amplified when light is coupled in the whispering gallery modes of such resonators. In high‐quality spherical micro‐resonators, close to one million interactions can occur between a nonlinear molecule and a circulating light pulse. Recent research on nonlinear optics in spherical geometry is reviewed, from micrometer‐size spheres to whispering gallery mode resonators.     

Nonlinear photonic waveguides for on‐chip optical pulse compression

All‐optical signal processing on nonlinear photonic chips is a burgeoning field. These processes include light generation, optical regeneration and pulse metrology. Nonlinear photonic chips offer the benefits of small footprints, significantly larger nonlinear parameters and flexibility in generating dispersion. The nonlinear compression of optical pulses relies on a delicate balance of a material's nonlinearity and optical dispersion. Recent developments in dispersion engineering on a chip are proving to be key enablers of high‐efficiency integrated optical pulse compression. We review the recent advances made in optical pulse compression based on nonlinear photonic chips, as well as the future outlook and challenges that remain to be solved.

     

Tunable nonlinear beam shaping by non‐collinear interactions

A method is proposed for nonlinear beam shaping, employing a non‐collinear quasi phase‐matched interaction in a crystal whose nonlinear coefficient is encoded by a computer generated hologram pattern. In this method the same axis is used for both satisfying the phase‐matching requirements and encoding the holographic information, the result is a single shaped beam in the generated frequency. This allows to shape beams in one‐dimension using a very simple method to fabricate patterned nonlinear crystals and to shape beams in two‐dimensions with high conversion efficiency. The one‐dimensional case is experimentally demonstrated by converting a fundamental Gaussian beam into Hermite‐Gaussian beams at the second harmonic in a KTiOPO₄ crystal. The two‐dimensional case is demonstrated by generating Hermite‐Gaussian and Laguerre‐Gaussian beams in a stoichiometric lithium tantalate crystal. The suggested scheme enables broad wavelength tuning by simply tilting the crystal.     

The generation and utilisation of half‐cycle cut‐offs in high harmonic spectra

High‐order harmonic spectra are composed of a coherent sum of half‐cycle emissions, the cut‐off energy of which depend sensitively on different sub‐cycle portions of the driving laser field. By selecting the correct focal geometry the half‐cycle cut‐off emissions can be preferentially selected over the lower energy plateau emissions through phase matching, such that they form macroscopic half‐cycle cut‐off features in the far‐field spectrum. The energy of these macroscopic half‐cycle cut‐offs can then be used to retrieve the waveform of the driving laser field. The processes through which these macroscopic half‐cycle cut‐offs are formed and their applications, both for measuring the laser waveform and the generation of wavelength tunable isolated attosecond pulses, are reviewed in detail.     

Slow and fast light: basic concepts and recent advancements based on nonlinear wave‐mixing processes

After a review of the basic concepts of slow and fast light, recent advancements based on nonlinear wave‐mixing processes are described. As a nonlinear medium, the authors focus on a liquid crystal light valve showing that it allows obtaining a large control of the group delay, with a maximum fractional delay of 1, and a deceleration of light pulses down to group velocities as small as 0.2 mm/s. A theoretical model accompanies the observations and accounts for them in the general framework of two‐wave mixing in the light valve. At the end, a high‐sensitivity interferometer is presented as an example of slow light applications.     

Electro‐optical interferometric microscopy of periodic and aperiodic ferroelectric structures

The ferroelectric domain structures of periodically poled KTiOPO₄ and two‐dimensional short range ordered poled LiNbO₃ crystals are determined non‐invasively by interferometric measurements of the electro‐optically induced phase retardation. Owing to the sign reversal of the electro‐optical coefficients upon domain inversion, a π phase shift is observed for the inverted domains. The microscopic setup provides diffraction‐limited spatial resolution allowing us to reveal the nonlinear and electro‐optical modulation patterns in ferroelectric crystals in a non‐destructive manner and to determine the poling period, duty cycle and short‐range order as well as detect local defects in the domain structure. Conversely, knowing the ferroelectric domain structure, one can use electro‐optical microscopy so as to infer the distribution of the electric field therein.