Second Harmonic Generation Frequency Research Articles

Second harmonic generation in anisotropic stratified media: a generalization of the Berreman method and its application to photonic materials.

We have developed a numerical method for calculating the second-harmonic generation (SHG) generated by an anisotropic material whose optical properties present an arbitrary modulation in one dimension. The method is based on the Berreman 4 × 4 matrix formalism, which is generalized to include nonlinear optical phenomena. It can be used under oblique incidences of the input beam, and is valid even when the SHG frequency is close to photonic bands, where the usual slowly-varying-amplitude approximation breaks down. As an example of application, we have studied the SHG performance of ferroelectric and helielectric fluids. The obtained results indicate that the present procedure may contribute to improving the structural design and enlarging the variety of nonlinear optical materials for application in optical devices.

Manipulating Second Harmonic Generation in Higher‐Order Topological Photonic Crystals

AbstractHigher‐order topological photonic crystals (TPCs) have attracted much attention in the past several years since they do not obey the conventional bulk‐boundary correspondence in topological insulating phases. This characteristic offers a new dimension with which to trap and manipulate the flow of light. In this work, 2D higher‐order TPCs for engineering optical second harmonic generation (SHG) are designed. In the TPCs, a topological corner‐state is achieved and it is demonstrated that the high localization of photons at the corner‐state can significantly promote optical SHG. In addition, topological edge‐states are obtained by breaking the Dirac degeneracy point near the SHG frequency. They are also topologically protected and immune to bending resistance. Therefore, the enhanced SHG signal from the corner‐state can be manipulated precisely for transmitting through the designed interface with relatively low loss due to the topological‐protection edge waves. This design strategy combines the advantages of corner‐state and edge‐state in higher‐order TPCs, demonstrating potential applications of topological photonic devices, such as optical communication and optical computing technologies.

Asymmetric interferometric frequency resolved optical gating for complete unambiguous ultrashort pulse characterisation

We propose a modification of the second-harmonic generation frequency resolved optical gating method (SHG-FROG) utilizing asymmetric spectral interferometry (aiFrog) for complete unambiguous reconstruction of ultrashort optical pulse intensity and phase. The large amount of information encoded in the aiFrog trace allows a straightforward calculation of the fundamental pulse spectrum and a direct reconstruction of the pulse envelope and phase. A simple and robust iterative algorithm with fast convergence is developed for pulse reconstruction from the aiFrog trace, taking advantage of large data redundancy and noise averaging. Due to the asymmetry of the setup, our method is free from the known challenges of a standard SHG-FROG (direction-of-time ambiguity, almost identical FROG traces for well separated pulses with different phases and for asymmetric well separated pulses). The method is highly sensitive to the weak satellite pulse of the main pulse, which can be used to track pre- or post-pulses even without full processing of the measured aiFrog trace.

Few-cycle optical pulse characterization under phase-mismatching.

The phase-matching bandwidth of nonlinear crystal is of great significance in ultrashort laser pulse characterization. In order to satisfy the phase-matching bandwidth, ultra-thin nonlinear crystals are generally required. However, the significantly reduced conversion efficiency, as well as the machining difficulties, limits its applications. Here, we show that sufficient spectrum bandwidth response can be achieved for a thick crystal when the phase-matching wavelength is tuned outside of the spectral window of the measured pulse. By applying this phenomenon to a single-shot second-harmonic generation frequency resolved optical gating (SHG-FROG) device, we successfully characterized a few-cycle pulse using a 150µmβ-barium borate (BBO) crystal. The accuracy of the method was verified by comparing the conventional pulse retrieving approach with a 5µm BBO crystal, which has a sufficient phase-matching bandwidth.

Магнитоиндуцированный нелинейно-оптический отклик плeнок на основе нанослоeв тяжeлых и ферромагнитного металлов

We present the results of the experimental studies of magnetization-induced effects in optical second harmonic generation (SHG) from three-layer W/Co/Pt films with different thicknesses of the cobalt layer (2-10 nm) influenced by longitudinal dc magnetic field. The intensity effect in the p-polarized second harmonic that is odd in magnetization was observed, which reveals an increase in the corresponding magnetic contrast of the SHG intensity with an increase of the cobalt layer thickness from 2 nm to 10 nm. A phenomenological description of the observed effects was carried out when considering the bulk nonlinear polarization at the SHG frequency, which is proportional to the magnetization gradient in the direction normal to the structure, as well as the interface contribution, which is proportional to the square of the magnetization of the films.

Graphene-Integrated Plasmonics Metasurface for Active Controlling Artificial Second Harmonic Generation

Plasmonics has developed tremendously in recent years owing to its significant performance in enhanced nonlinear optical emission. However, plasmonic structures have predefined geometry, lacking flexibility for active control of the artificial nonlinear signal. In this paper, we numerically investigate active control of second harmonic generation (SHG) from plasmonic metasurface based on magnetic Lorentz force. The unit element of designed plasmonic metasurface consists of a gold split ring resonator and a bismuth bar, and integrated with graphene layer. By varying the Fermi energy of graphene from 0.2 eV to 1.0 eV, we achieve a 17.2 THz shift of the second harmonic frequency. In addition, it is demonstrated that the range of SHG frequency shift can be further enlarged by increasing the number of graphene layer. Our results pave the way for applications in generation of continuous laser source, optical communications and signal processing.

Triplet Sensitization by Lead Halide Perovskite Thin Films for Efficient Solid-State Photon Upconversion at Subsolar Fluxes

Summary We investigate the rubrene triplet sensitization by perovskite thin films based on methylammonium formamidinium lead triiodide (MAFA) of varying thicknesses. The power-law dependence of both the MAFA photoluminescence (PL) intensity and upconverted emission is tracked as a function of the incident power density. Bimolecular triplet-triplet annihilation (TTA) exhibits a unique power-law dependence with a slope change from quadratic-to-linear at the threshold Ith. The underlying MAFA PL power-law dependence dictates the power law of the upconverted PL: (1) below Ith, the slope of the upconverted PL is twice the value of the MAFA PL; (2) above Ith, it follows the same power law as the underlying recombination of mobile electrons and holes in the MAFA films. We find that the Ith shifts to subsolar incident laser powers when increasing the MAFA thickness above 30 nm. For the thickest MAFA film of 380 nm we find an upconversion threshold of Ith = 7.1 mW/cm2.

Pulse compression of white-light continuum generated at 1053 nm in bulk sapphire: an experimental study

The white-light continuum in optical bulk material has the potential to be used as a seed pulse in optical parametric amplifiers, which entails the compression of seed pulses to temporally overlap with pump pulses. An experimental study of the compression of the white-light continuum is reported in this article. Both chirped mirror compression and prism pair compression methods are used. The 280 fs pulses centered at 1053 nm are employed to generate a white-light continuum in a bulk sapphire plate; the spectrum of a white-light continuum spans from 650 nm to 1000 nm and its second harmonic generation (SHG) is measured. The scanning SHG frequency resolved optical gating technique was used to characterize the compression of the white-light continuum. Prism pair only compression of the (650–1000) nm spectra yielded 190 fs, with chirp mirrors added after leading to 150 fs. Spectral masking of the spectrum below 850 nm inside the prism compressor resulted in a pulse duration of ~120 fs for the 850–1000 nm spectral portion of the pulses.

X-SEA-F-SPIDER characterization of over octave spanning pulses in the infrared range.

We show a practical implementation of a pulse characterization method for sub-cycle pulse measurements in the infrared spectral range based on spectral shearing interferometry. We employ spatially-encoded arrangement filter-based spectral phase interferometry for direct electric field reconstruction with external ancila pulses (X-SEA-F-SPIDER). We show merits and limitations of the setup and an in-depth comparison to another widely used temporal characterization technique - Second-Harmonic Generation Frequency Resolved Optical Gating (SHG-FROG). The X-SEA-F-SPIDER implementation presented in this paper allows measurement of sub-cycle pulses with over one octave wide spectrum spanning the 900-2400 nm range without adding any extra dispersion due to the pulse characterization apparatus.

Frequency resolved optical gating characterization of sub-ps pulses from single-section InAs/InP quantum dash based mode-locked lasers

Mode-locking of single-section Fabry-Pérot InAs/InP edge emitting quantum dash based lasers at 1.56 µm under continuous wave operation is studied by second-harmonic generation frequency resolved optical gating. Self-starting pulses of a width down to 374 fs can be observed after external chirp compensation using standard single-mode fiber (SMF-28). Pulse compression using different lengths of SMF-28 and pulse shape as well as phase dependence on bias conditions are investigated. Consistency with stepped-heterodyne technique for advanced pulse characterization is shown.

Experimental analysis of phase conjugation properties of four-wave mixing in an SOA after probe broadening due to fibre dispersion

Experimental results are presented which analyze the phase conjugation properties of four-wave mixing signals generated due to the beating between probe pulses broadened by a length of fibre and narrow pump pulses. This results in four-wave mixing pulses significantly narrower than the injected probe pulses albeit with reduced phase conjugation properties, which are examined. The pulses are completely characterized using the second-harmonic generation frequency resolved optical gating technique. The probe pulse is initially broadened due to propagation through 40m of dispersion compensating fibre. This causes the probe pulse to be much wider than the injected pump pulse, in contrast to previously reported results. The four-wave mixing signal is therefore both wavelength converted and compressed, due to the gating properties of four-wave mixing, with respect to the initial probe signal. The phase conjugation properties of the converted signal are discussed and this signal is then passed through a second length of dispersion compensating fibre in order to further compress the pulse and to examine in more detail the phase conjugation properties.

Second harmonic generation due to quadrupole interaction in a photonic crystal slab: angle dependence and symmetry of the unit cell.

We investigate second harmonic generation (SHG) from a photonic crystal slab consisting of centrosymmetric materials. The SHG signal is observed in the transmission direction when the incident laser excites the quasiwaveguide mode. As the SHG frequency approaches the exciton level, the SHG intensity increases resonantly. When the incident angle is exactly 0, the SHG signal vanishes even if the transmission dip is excited. This fact is readily explained by a quadrupole theory based on the Lorentz oscillator model, where the source of the nonlinearity is the Lorentz force. When the unit cell in the photonic crystal lacks inversion symmetry, the SHG signal is expected even for the normal incidence. It is experimentally demonstrated for a square array of triangular semiconductor slabs.

Optical Second-Harmonic Generation at Selective Reflection Band of The SmC* Phase

The process of optical second harmonic generation (SHG) in a slab of the helically modulated SmC* material is analysed for a case when the SHG frequency is in the vicinity or inside the region of the selective reflection band. The analysis bases on the numerical solutions of the nonlinear wave equation. Various possible phase matched combinations of the optical eigen-modes are discussed and the dependence of the SHG intensity on the optical frequency and sample thickness is given.

Operation of cusptron at fundamental and harmonic cyclotron frequencies

Microwave radiation at the fundamental and harmonic electron cyclotron frequencies generated by a cusptron oscillator is reported. A low-energy, axis-rotating beam of 28-30 kV, 0.8-3.5 A, 4 mu s, and 60 pps interacts with a single RF mode, both in a circular cavity and in a six-vane circuit by the negative mass instability. In fundamental and second-harmonic frequency generation with a circular circuit, the independently excited modes are TE/sub 11/s and TE/sub 21/s, with radiation power of more than 1.8 kW and an electronic efficiency of approximately 7.5%. Employing a six-vane circuit, microwave radiation of 6.0 GHz (sixth harmonic) and 3.9 GHz (fourth harmonic) is also independently generated with more than 10.4 and 4.0 kW radiation power, respectively. Corresponding electronic efficiencies are approximately 10.0 and 9.5%. >

Small-Signal Second-Harmonic Generation by a Nonlinear Transmission Line (Comments)

In the above paper, the important problem of second-harmonic frequency generation along nonlinear transmission lines is studied. However, some remarks concerning the fundamental role of dissipation and dispersion should be given which are already known from the literature.

Nonlinear wave propagation along periodic-loaded transmission line

A high frequency transmission line periodically loaded with varactor diodes is presented to study nonlinear wave propagation. The nonlinearity and dispersion characteristics are experimentally and theoretically analysed. Experimental results on shock wave formation and harmonic frequency generation are found to be in good agreement with theoretical predictions. The influence of dispersion on second-harmonic frequency generation is particularly discussed yielding large values of the efficiency.