A novel quasi-phase-matching frequency doubling technique

A new quasi-phase-matching technique for efficient second-harmonic generation is reported. It is based on the spatial periodic modulation of the light intensity along the propagation direction, rather than the conventional spatial periodic modulation of the nonlinear optical coefficients. It can be realized by using a novel dual-channel waveguide frequency doubler structure for the desired light intensity distribution. This dual-channel waveguide device has major advantages including very small beam size, high light intensity within long nonlinear-waveguide interaction length, highly efficient second-harmonic generation, ease in fabrication of the nonlinear channel waveguides without any spatially periodic poling, and low waveguide propagation losses. The new quasi-phase-matching technique can also be applied to third-harmonic generation and other nonlinear optics processes.     

A Doubly Resonant Photonic-Crystal Microcavity for Second-Harmonic Generation

A two-dimensional, photonic-crystal microcavity is proposed in order to achieve efficient second-harmonic generation in the third optical communication window. The simultaneous resonance conditions of the pump beam at the fundamental frequency and second-harmonic field generated inside the structure with a defect provides a considerable enhancement of the conversion efficiency. In the configuration we propose, a transverse-magnetic (TM) polarized beam resonating at the fundamental frequency generates a second-harmonic field corresponding to a transverse-electric (TE) polarized resonant mode. The design of this doubly resonant microcavity is carried out by a linear analysis to search for the resonance frequencies and calculate their field distributions. The nonlinear analysis of the second-harmonic generation is performed using a dispersive finite-difference time-domain code.     

Second-harmonic generation in three-well and bound-to-continuum GaAs-based quantum-cascade lasers

We present efficient second-harmonic generation in state of the art GaAs-based quantum-cascade lasers with nonlinear output powers up to 100 μW. The nonlinear output was significantly improved by applying an AlGaAs waveguide structure, which guides both, the fundamental and nonlinear light. We further show the influence of the doping in the active region on the nonlinear light generation by comparing two similar structures with different doping levels. PACS 42.55.Px; 42.65.Ky; 81.05.Ea     

Nonreciprocal frequency doubling of electromagnetic waves through double resonance and Bragg reflection in photonic crystals

A computational study of the uni-directional second-harmonic generation in a one-dimensional dual photonic crystal structure made of GaAs, AlAs and SiO₂ with quadratic optical nonlinearity and material dispersion is presented. The computational approach uses a shooting method to solve nonlinear wave equations for coupled fundamental and second-harmonic fields and the invariant imbedding method to obtain the linear transmittance and group index spectra. The dual structure consists of two substructures, the conversion structure creating a strongly enhanced second-harmonic signal and the filter structure blocking the fundamental frequency field by Bragg reflection while permitting the passage of the second-harmonic field. The conversion structure is built with an elementary cell consisting of four sublayers whose thicknesses are systematically varied. Doubly resonant second-harmonic generation with very high conversion efficiency is achieved for light incident from the conversion structure side by choosing the geometrical parameters of the elementary cell optimally and controlling the band structure. A new mechanism to enhance second-harmonic generation by controlling the energy flow between the fundamental frequency and second-harmonic fields has also been found.     

Second harmonic generation in a strontium barium niobate crystal with a random domain structure

The phenomenon of the second-harmonic (SH) generation in a strontium barium niobate (SBN) crystal in directions not obeying the conditions of phase matching for spatially homogeneous media has been studied. The appearance of the SH signal is interpreted as resulting from the quasi-phase-matching nonlinear wave interaction due to the random domain structure of the SBN crystal containing acicular ferroelectric microdomains. A theoretical model describing the SH generation in such two-dimensional, spatially inhomogeneous nonlinear optical structures is proposed. An analysis of the second-harmonic emission directivity patterns shows that the main contribution to the noncollinear SH generation in SBN is due to near-surface domains extending to a depth of up to several dozen microns.     

Resonant second-order nonlinear optical processes in quantum cascade lasers

We demonstrate an efficient intracavity nonlinear interaction of laser modes in a specially adapted quantum cascade laser. A two-wavelength quantum cascade laser structure emitting at wavelengths of 7.1 and 9.5 micrometer included cascaded resonant optical intersubband transitions in an intracavity configuration leading to resonantly enhanced sum-frequency and second-harmonic generation at wavelengths of 4.1, 3.6, and 4.7 micrometer respectively. Laser peak optical powers of 60 and 80 mW resulted in 30 nW of sum-frequency signal and 10-15 nW of second-harmonic signal, both in good agreement with theoretical calculations.     

On the theory of second-harmonic generation in 2D nonlinear photonic crystals with arbitrary domain structures

Theoretical analysis is given to second-harmonic generation in 2D nonlinear photonic crystals of rectangular symmetry with rectangular domains. Formulas for the nonlinear response of the crystal with an arbitrary domain structure are derived within the approximation of a given field. A possibility of calculating time profiles of second-harmonic pulses down to the femtosecond range is demonstrated.     

Nonlinear optical processes in domain structures of strontium tetraborate

The random quasi-phase-matching in irregular domain structures of strontium tetraborate has been investigated for nonlinear optical generation at 266 nm. The factor of increasing second-harmonic generation efficiency due to random quasi-phase-matching is determined. The spectral dependences of the second-harmonic generation are calculated. It is shown that its efficiency can be optimized at a fixed pump wavelength by rotating the domain structure.     

Enhancement and inhibition of second-harmonic generation and absorption in a negative index cavity

We study second-harmonic generation in a negative-index material cavity. The transmission spectrum shows a bandgap between the electric and magnetic plasma frequencies. The nonlinear process is made efficient by local phase-matching conditions between a forward-propagating pump and a backward-propagating second-harmonic signal. By simultaneously exciting the cavity with counterpropagating pulses, and by varying their relative phase difference, one is able to enhance or inhibit linear absorption and the second-harmonic conversion efficiency.     

Efficient collinear fourth-harmonic generation by two-channel multistep cascading in a single two-dimensional nonlinear photonic crystal

We investigate efficient fourth-harmonic generation in a single two-dimensional (2D) quadratically nonlinear photonic crystal. We propose a novel parametric process that starts with phase-matched generation of a pair of symmetric second-harmonic waves, which then interact to produce a fourth-harmonic wave that is collinear to the fundamental. We show that this process is more efficient than conventional fourth-harmonic-generation schemes by a factor that reaches 4 at low intensities and discuss how to design and optimize the nonlinear 2D photonic crystals that are implemented in LiNbO(3) and LiTaO(3) .     

Multiple Čerenkov second-harmonic waves in a two-dimensional nonlinear photonic structure

We report simultaneous generation of multiple conical second-harmonic waves in a two-dimensional nonlinear photonic structure when illuminated by two overlapping noncollinear fundamental beams. We show that this phenomenon is caused by the nonlinear ?erenkov radiation emitted due to the interaction of photons from each constituent fundamental beam as well as the virtual one propagating along the bisector of the two beams. In addition, by studying the asymmetric geometry of the interaction, we uniquely verify the effects of reciprocal vectors on the ?erenkov-type second-harmonic generation in nonlinear photonic structures.     

Magnetic-Field-Induced Optical Second-Harmonic Generation Study of Co/Pt and Co/Ta Interfaces

Magnetic properties of an interface between cobalt and platinum or tantalum nanolayers have been studied by the optical second-harmonic generation and nonlinear magneto-optical Kerr effect methods. It has been shown that a high sensitivity of the second-harmonic generation method makes it possible to determine the orientation of the easy magnetization axis in the plane of a polycrystalline structure without measurement of the magnetic field dependence of second-harmonic generation. The comparison of the field dependences of magnetic-field-induced second-harmonic generation with the linear magneto-optical effect indicates the difference in the processes of magnetization reversal in Co/Pt and Co/Ta interfaces and the bulk of the cobalt film. In particular, a new linear in magnetization effect has been observed in the second harmonic that is symmetry-forbidden for uniformly magnetized structures.     

Integrated-grating-induced control of second-harmonic beams in frequency-doubling crystals

We report a novel type of integrated nonlinear photonic device for controlling the generation of several second-harmonic beams. Two-dimensional diffraction gratings are recorded with femtosecond laser pulses at the entrance surface of a frequency-doubling crystal. This periodic spatial modulation of the material surface induces noncollinear propagation of the fundamental input signal in the crystal. By slightly changing the angle of incidence of the seed beam, collinear and noncollinear phase matching can be achieved between different diffraction orders, in this way allowing the efficient generation of several second-harmonic beams.     

Standing-wave nonlinear optics in an integrated semiconductor microcavity

We present a concept of standing-wave optical frequency conversion in dispersive microcavities theoretically and experimentally, allowing efficient ultracompact nonlinear photonics. We developed a time-dependent model, incorporating the dispersion into the structure of the spatial cavity modes, where the conversion efficiency is enhanced by the optimization of a nonlinear cavity mode overlap. We designed and fabricated integrated double-resonance semiconductor microcavities for standing-wave second-harmonic generation. The measured efficiency exhibits a significant maximum near the cavity resonance owing to the intracavity power enhancement and the dispersion-induced wavelength detuning effect on the mode overlap, in good agreement with our theoretical predictions.     

Phase correction in double-pass quasi-phase-matched second-harmonic generation with a wedged crystal

Compensation for dispersive elements is necessary for efficient multiple-pass and intracavity nonlinear frequency-conversion devices. We describe the use of a wedged quasi-phase-matched crystal to compensate for the phase shifts introduced by mirrors in such devices, taking advantage of the periodic variation in the relative phases of the interacting waves in a quasi-phase-matching grating. A representative double-pass second-harmonic generation experiment with a 5-cm -long periodically poled lithium niobate crystal showed the expected conversion efficiency enhancement.     

Second-harmonic generation spectroscopy on?organic?nanofibers

Optical second-harmonic generation upon nanosecond laser irradiation of nanofibers grown from nonsymmetrically functionalized para-quarter-phenylene molecules on mica surfaces is investigated in the spectral region around electronic resonances. Strong multiphoton excited luminescence appears together with the second-harmonic signal in the spectra. It is demonstrated how the two components can be separated by analyzing emission spectra recorded over a broad range of wavelengths around the second-harmonic energy. Spectra of the absolute values of the second-harmonic signals are obtained for three differently functionalized molecules. Together with supplementary measurements of the structure of the nanofiber films this forms the basis for estimation of the second-harmonic susceptibility of the fiber materials. Values of the nonlinear susceptibilities are obtained, which are comparable to those of well-known inorganic nonlinear optical crystals.     

Diode laser frequency doubling in a ppMgO:LN ridge waveguide: influence of structural imperfection, optical absorption and heat generation

In this work, we investigate experimentally and theoretically limitations of highly efficient high-power single-pass second-harmonic generation process in a quasi-phase matched magnesium oxide-doped lithium niobate ridge waveguide due to structural imperfections, optical absorption and resulting heat generation. With the distributed Bragg reflector diode laser emitting at 1,061?nm, applied in the presented bench-top experiment, a coupling efficiency into the ridge waveguide nonlinear device of 86?% is realized. An internal normalized conversion efficiency of 375?%?(Wcm ${}^2$ ) ${}^{-1}$ is determined. A maximum second-harmonic power of 386?mW is reached at a corresponding opto-optical and electro-optical conversion efficiency of 38 and 9.5?%, respectively. A saturation of the conversion efficiency at a high power level as well as its dependency on the nonlinear device structure inhomogeneity are confirmed experimentally. With a theoretical model incorporating geometrical inhomogeneity, linear and nonlinear absorption and resulting heat load the second-harmonic generation in a ridge waveguide can be simulated realistically. Increased values of nonlinear absorption coefficients, higher than until now reported, lead to a very good agreement between theoretical and experimental results. Moreover, according to the simulation, structural inhomogeneities can enhance the SHG conversion efficiency at high output powers.     

Cascaded waveguide phase-matching arrangement

We propose a new method of phase matching in nonlinear waveguides, using the higher-order waveguide mode as an intermediary for efficient second-harmonic or difference-frequency generation, and show that high conversion efficiencies can be achieved with this scheme.     

Optical-damage-free guided second-harmonic generation in 1% MgO-doped stoichiometric lithium tantalate

Room-temperature cw second-harmonic generation from telecom wavelengths, with 30% W(-1) cm(-2) efficiency and second-harmonic power levels up to 41 mW, was achieved in buried waveguides fabricated by reverse-proton exchange in 1% MgO-doped stoichiometric lithium tantalate without any evidence of optical damage. The technology proves suitable for the realization of efficient nonlinear frequency converters and all-optical devices.     

Continuous-wave single-frequency 532 nm laser source emitting 130 W into the fundamental transversal mode

The nonlinear effect of second-harmonic generation is an efficient way to realize high-power green laser sources. But when scaling up the harmonic power, many setups reported in the literature have been limited by conversion efficiency degradation or the fundamental laser power. Here we report on the generation of 134 W of cw laser light at a wavelength of 532 nm from a fundamental power of 149 W by second-harmonic generation in an external optical resonator comprising a lithium triborate crystal. The external conversion efficiency was 90%. The harmonic light consisted of a single spectral line. At least 97% of it was emitted into the fundamental transversal mode.