Second harmonic generation in periodically poled lithium niobate waveguides with stitching errors

Periodically poled lithium niobate (PPLN) waveguides are a proven and popular means for efficient wavelength conversion. However, conventional PPLN waveguides typically have small mode field diameters (MFD) (≲6 µm) or significant insertion and/or propagation losses, limiting their ability to operate at multi-watt power levels. In this work we utilise zinc indiffused PPLN ridge waveguides that have a larger MFD, favourable pump/SHG modal overlap, and low insertion losses. Here for the first time, we have demonstrated continuous wave (CW) spectral narrowing from a PPLN waveguide, both with high efficiency and multi-watt second harmonic generation (SHG). 2.5 W of 780 nm has been produced by SHG of an amplified 1560 nm telecom laser with a device efficiency of 58% in a 4.0-cm long ridge waveguide. We have modelled conversion efficiency and applied experimentally measured waveguide parameters to show excellent agreement to the SHG spectra. Spectral narrowing of the full width half maximum (FWHM) of 35.7% has been measured as the nonlinear drive is increased. This work demonstrates that single-pass, multi-watt, CW SHG at 780 nm is feasible from our PPLN waveguide in the large conversion regime.

In this paper, we investigated the second harmonic (SH) and the third harmonic (TH) generations in nanorods and nanotubes. Nonlinear free and bound electrons responses of metal combined with the third-order nonlinearity of dielectric core were utilized to study the nonlinear response of nanotubes made of noble metals such as gold. The impacts of the dielectric constant of core medium, thickness of metal shell, and size of nanotube on the SH and TH conversion efficiency are reported. The physical interpretations behind the results are discussed. Finally, we have shown that enhancement of the SH and TH generations are more noticeable and adjustable by varying the core-shell parameters compared to those of nanorods.

In this paper, noise characteristics of second-harmonic generation (SHG) in periodically poled lithium niobate (PPLN) using the quasi-phase-matching (QPM) technique are studied experimentally. In the experiment, a 0.78-mm second-harmonic (SH) wave was generated when a 1.55-mm fundamental wave passed through a PPLN crystal (bulk or annealed proton-exchanged waveguide). The fundamental and SH waves were then separated through a beam splitter and sent to two photodetectors, respectively. The time-domain and frequency-domain characteristics of the fundamental and SH waves were analyzed. By using the pump-probe method, the noise characteristics were studied further when 532-nm irradiation light co-propagated with the 1.55-mm fundamental light in the PPLN crystals. It is found that for the bulk and waveguides of PPLN crystals, the SH wave has a higher relative noise level than the corresponding fundamental wave. For the same fundamental wave, the SH wave has lower noise in a bulk crystal than in a waveguide, and in MgO-doped PPLN than in undoped PPLN. In addition, the photorefractive effect incurred by the irradiation light can influence the SHG noise.

Although tightly focused input beams for efficient second harmonic (SH) generation (SHG) are usually avoided because of material damage, this is exactly the condition needed for the generation of spatial solitons (QSS) based on quadratic (χ(2)) nonlinearities.[1,2] These QSS’es are mutually self-trapped fundamental (FW) and harmonic (SH) optical beams in which the mutual beam narrowing due to photon exchange incipient in up and down conversion balances diffraction. The generation of quadratic solitons significantly affects the SHG process, for example by placing an upper limit on the amount of fundamental that can be converted to SH, by increasing the bandwidth of the process etc.[3] Of particular interest are periodically poled ferroelectric media which are an important limit of non-critical phase-matching. In fact, periodically poled LiNbO3 (PPLN) has been shown to have the lowest soliton threshold of 1.5 GW/cm2 reported to date [4]. That pioneering work established the existence of quadratic solitons in periodically poled media but neither this work, or any other work on QSSes has addressed the impact of QSS on the SHG tuning curves.

We theoretically study the gain-assisted double plasmonic resonances to enhance second harmonic generation (SHG) in a centrosymmetric multilayered silver-dielectric-gold-dielectric (SDGD) nanostructure. Introducing gain media into the dielectric layers can not only compensate the dissipation and lead to giant amplification of surface plasmons (SPs), but also excite local quadrupolar plasmon which can boost SHG by mode matching. Specifically, as the quadrupolar mode dominates SHG in our nanostructure, under the mode matching condition, the intensity of second harmonic near-field can be enhanced by 4.43 × 102 and 1.21 × 105 times when the super-resonance is matched only at the second harmonic (SH) frequency or fundamental frequency, respectively. Moreover, the intensity of SHG near-field is enhanced by as high as 6.55 × 107 times when the nanostructure is tuned to double super-resonances at both fundamental and SH frequencies. The findings in this work have potential applications in the design of nanosensors and nanolasers.

We theoretically study how the intensity of second harmonic generation (SHG) for a sheet of two-dimensional (2D) material is controlled by an underlying layered structure. By utilizing the transfer matrix method with the inclusion of a nonlinear sheet current to describe the response of the 2D material, an explicit expression for the intensity of upward propagating second harmonic (SH) light is obtained, and the effects of the layered structure can be identified by a structure factor β, defined as the ratio of SH intensity from such a structure to that from a freely suspended 2D material. Our results show that the influence of a layered structure on the SHG intensity arises from interference effects of both the fundamental light and the SH light; the value of the structure factor is 0 ≤ β ≤ 64. Furthermore, when the incident light is pulsed, the interference effects are partially canceled due to the existence of many wave vectors and frequencies, and the cancellation becomes severe for thick films, small beam spots, and short pulses. For a specific structure of 2D material/dielectric film/substrate, the thickness of the dielectric film can effectively tune the value of β in an interval [βmin, βmax], and detailed discussions are performed for the thicknesses when these two extreme values can be obtained. When there is optical loss or the substrate is not perfectly reflective, the extreme value of βmax or βmin cannot reach 64 or 0. A large βmax requires two conditions to be fulfilled: (1) the substrate should be highly reflective, and (2) the refractive indices of the dielectric film at the fundamental and the SH frequencies should differ. Our results indicate how practical substrate structures can be used to achieve high SH signals, and the simple expression we give for the SH enhancement will be useful in characterizing the nonlinear susceptibility of 2D materials.

Second Harmonic Generation (SHG) has become a powerful tool for studying the surfaces of liquids and other centric systems [1], but due to the symmetry rule no Second Harmonic (SH) can be produced from their bulk. It is then of great interest that a number of recent experimental studies have shown strong SHG from randomly oriented suspension of finely fragmented purple membranes of Halobacterium halobium contain Bacteriorhodopsin (BR) [2 ], which belongs to the family of retinal proteins. The understanding of the nature of the SH signal itself still constitutes an obscure matter. Most of the mechanisms proposed for the description of this effect, have focused on the hyper-Rayleigh incoherent scattering, but some of them suggest also coherent nonlinear contributions into the SH signal.

We have found that the resonance energy of optical second-harmonic (SH) generation from Au films on NaCl(100) depends strongly on the film thickness. The resonance peak of the SH intensity was found at $2\ensuremath{\Elzxh}\ensuremath{\omega}=3.2\mathrm{eV}$ for the island film of mean thickness 0.4 nm, while it appeared at $2\ensuremath{\Elzxh}\ensuremath{\omega}=2.4\mathrm{eV}$ for the continuous film of thickness 40 nm. We suggest that the SH intensity peak observed at $2\ensuremath{\Elzxh}\ensuremath{\omega}=3.2\mathrm{eV}$ originates from a resonant transition from occupied to unoccupied electronic levels of the (100) surface on top of the Au islands. The SH intensity peak observed at $2\ensuremath{\Elzxh}\ensuremath{\omega}=2.4\mathrm{eV}$ for the film thickness of 40 nm originates from the roughness-induced linear dielectric property of the film.

The visibility of inverted domain structures using the second harmonic generation (SHG) microscope is discussed based upon the three-wave coupling equations. Without reference second harmonic (SH) waves (non-interference case), the SHG intensity at a domain boundary decreases steeply along the perpendicular direction to the domain wall, which is observed as a dark line. Thus the non-interference SHG microscope reveals the location of domain boundaries but not the domain polarity which the SHG interference microscope can do. In the case of periodically poled domain structures, the clear observation criterion is expressed with the wave-number offset between the fundamental and SH waves.

Short-period domain patterning by ion beam irradiation in lithium niobate waveguides produced by soft proton exchange

The temporal and spectral characteristics of femtosecond second-harmonic-generation (SHG) in quasi-phase-matched (QPM) periodically poled lithium niobate (PPLN) are investigated experimentally and theoretically. In the experiments, we studied SHG in PPLN waveguides with different device design and fabrication conditions by adopting ~100-fs optical pulses with different peak powers in the 1.55-μm band. Large phase modulation and spectral broadening of the fundamental wave, and a shift of the centre frequency of the second-harmonic wave were observed when the peak power of the fundamental wave was high enough. Saturation of the conversion efficiency was also observed, which depends on fundamental pulse width and peak power. These phenomena could be attributed to nonlinear phase accumulation under large group-velocity mismatch. In the theory, evolutions of pulse waveforms and optical spectra were studied for both the fundamental and second-harmonic pulses by solving the coupled-mode equations. The simulation results are in good agreement with the experimental data.

The changes of the morphology during heating and the degree of crystallinity of dry and hydrated starch granules are investigated using second harmonic generation (SHG) based Stokes polarimetry. A spatial distribution of various polarization parameters, such as the degree of polarization (DOP), the degree of linear polarization (DOLP), and the degree of circular polarization (DOCP) are extracted and compared with the two dimensional second harmonic (SH) Stokes images of starch granules. The SH signal from hydrated and dry starch on heating differed significantly in DOLP and DOCP values, indicating that hydrated starch has a greater degree of ultrastructural amylopectin disorder. The detail of denaturation and the phase transition of hydrated starch demonstrate the significant influence of thermal processing.

Based on vector diffraction theory, an analytical integral representation to calculate the field gradient of focused electromagnetic fields is derived by using the differential recursion formula between the Bessel functions with different order. Within the phenomenological model for second harmonic generation (SHG) of centrosymmetric material, the second harmonic (SH) response of a single centrosymmetric spherical particle excited by a focused beam under different values of the NA is investigated theoretically. The results show that, with increasing NA, the SH radiation pattern of the surface response hardly changes. For the larger value of the NA, because the elements of the field gradient are almost the same order of magnitude and the relative magnitudes of Ey and Ez increase, the bulk response related to the parameters δ′ and ζ become remarkable.

The grating-assisted nonlinear directional couplers are investigated by using the singular perturbation technique with multiple space scales. One of the waveguides of the nonlinear directional couplers has second order nonlinear cladding whose surface has a grating profile. The period of the grating is determined to satisfy the quasi-phase matching condition between the fundamental wave and second harmonic (SH) wave. From the solvability condition to have nontrivial solutions for each order, the coupled-mode equations between fundamental and SH waves are derived. In the numerical examples, lithium niobate (LiNbO/sub 3/) is chosen as second-order nonlinear medium. After the coupled-mode equations are examined numerically, it is found that the SH wave separates from the fundamental wave due to the difference of the coupling length between the fundamental and SH waves.

This paper demonstrates that the spatial distribution of electronic states of an arsenic ion implanted Si(111) substrate can be observed by using a wavelength tunable second harmonic (SH) microscope in a wide photon energy range from 2ℏω=1.96to5.19eV. The contrast in the SH intensity images between the As-doped area and the nondoped area depends greatly on the SH photon energy. For 2ℏω>3eV, optical second harmonic generation (SHG) from the nondoped area was stronger than from the doped area, and the contrast was reversed for 2ℏω⩽2.33eV. The contrast in the SH intensity images was considerably different from that in the linear optical reflection images, indicating that spectroscopic SH microscopy can provide different informations on electronic levels from that associated with the linear optical response. It is suggested that the larger SH intensity from the nondoped area for 2ℏω>3eV results from the resonant SHG enhancement effect associated with the bulk Si E1 (3.4eV) and E2 (4.3eV) gaps. In the case of the doped area, a small resonant enhancement of the SH intensity was observed around 2ℏω=2.33eV. This resonance may result from an energy level created by the ion implantation.