Wave Second Harmonic Generation Research Articles

Efficient Second-Harmonic Generation in Thin-Film Lithium Tantalate Through Modal Phase-Matching

Lithium tantalate (LT) exhibits nonlinear optical properties that are comparable to those of lithium niobate (LN), yet the former surpasses the latter in several respects. These include an enhanced optical damage threshold, a wider transparency range, and lower birefringence. Consequently, LT is an excellent material for optical frequency conversion applications. In this study, we have devised a novel device based on thin-film lithium tantalate (TFLT) for the efficient generation of second-harmonic waves. The design employs modal phase-matching (MPM), which circumvents the intricacies of conventional poling techniques, and attains a normalised conversion efficiency of 120% W−1cm−2. In order to address the challenges presented by higher-order modes, a mode converter with an insertion loss of less than 0.1 dB has been developed, thereby ensuring the efficient utilisation of the second harmonic. This study not only demonstrates the potential of TFLT for high-performance SHG, but also promotes the development of integrated nonlinear TFLT platforms.

Efficient Simultaneous Second Harmonic Generation and Dispersive Wave Generation in Lithium Niobate Thin Film

AbstractLithium niobate thin film (LNTF) is a promising platform for ultra‐low loss nonlinear integrated photonics. Here, the simultaneous generation of second harmonic wave (SHW) and dispersive wave (DW) are demonstrated in a single LNTF under the pump of a femtosecond pulse laser, with a conversion efficiency exceeding 25%. The second harmonic generation (SHG) uses the modal phase matching mechanism based on the second‐order nonlinear effect, while the DW generation is based on the perturbations of soliton dynamics caused by self‐phase modulation and higher‐order dispersion. Notably, significant and symmetrical SHW and DW patterns are observed, which exhibit strong spatial dispersion properties. A comprehensive analysis of the phase‐matching conditions are conducted for SHG and DW generation and provide a clear elucidation of the spectral properties of different regions of the emitted light patterns. Additionally, the evolution of the pump light in LNTF is thoroughly investigated, and the solutions of the generalized Schrödinger equation are in good agreement with these experimental results. This work sheds new light on the rich physics of nonlinear optical interactions on LNTF, and by utilizing the synergistic effect of second‐order and third‐order nonlinear effects, this study anticipates achieving efficient and high energy on‐chip broadband frequency conversion and supercontinuum generation across octaves.

Resonant generation of propagating second-harmonic spin waves in nano-waveguides

Generation of second-harmonic waves is one of the universal nonlinear phenomena that have found numerous technical applications in many modern technologies, in particular, in photonics. This phenomenon also has great potential in the field of magnonics, which considers the use of spin waves in magnetic nanostructures to implement wave-based signal processing and computing. However, due to the strong frequency dependence of the phase velocity of spin waves, resonant phase-matched generation of second-harmonic spin waves has not yet been achieved in practice. Here, we show experimentally that such a process can be realized using a combination of different modes of nano-sized spin-wave waveguides based on low-damping magnetic insulators. We demonstrate that our approach enables efficient spatially-extended energy transfer between interacting waves, which can be controlled by the intensity of the initial wave and the static magnetic field.

Auto-pump-depleted stable single-longitudinal-mode 1.5 μm source with the assistance of SHG.

To realize a stable single-longitudinal-mode (SLM) 1550-nm light source for the generation of non-classical states, a ring auto-pump-depleted singly resonant optical parametric oscillator (SRO) with the assistance of second-harmonic-wave generation (SHG) is designed and built in this Letter. A magnesium oxide doped periodically polarized lithium niobate (MgO:PPLN) crystal and a lithium triborate (LBO) crystal are employed as the optical parametric downconversion (OPDC) and SHG crystals, respectively. Especially, the introduced SHG can firstly increase the loss difference between the lasing and non-lasing modes so that the dual-mode or multi-mode coupling in the achieved SRO can be effectively eliminated and the stable SLM operation is achieved. At the same time, the SHG will automatically adjust the output coupling efficiency of SRO, so as to achieve efficient conversion efficiency and auto-pump depletion of SRO. In addition, due to the SHG, it is easy to achieve the low-intensity noise multi-wavelength output for the stable SLM SRO. As a result, the output powers of the SLM 1550 nm and 775 nm are up to 4.05 W and 3.25 W, respectively, and the total optical conversion of the built SRO can achieve 45.58%. The presented method paves a way to develop a compact stable SLM multi-wavelength SRO, and the obtained SRO is further beneficial to develop compact continuous-variable non-classical light fields.

Damage detection method based on continuous wavelet transform of second harmonic Lamb waves

ABSTRACT The nonlinear Lamb waves are sensitive to closed micro cracks and have a great potential for the detection of damage to prevent the catastrophic failure of entire structure. A five-stage closed micro crack localisation method based on time of flight (ToF) of the second harmonic Lamb waves is proposed in this research. The first step is frequency analysis of the received signals to ensure the generation of second harmonic wave. The second step is to extract second harmonic wave component in frequency range. The third step is to obtain wave signals in time domain. The fourth step is to extract ToF of second harmonic wave. The fifth step is to locate damage with ToFs. The method provides an effective strategy based on wavelet transform to extract the second harmonic Lamb waves from the dynamic response signals for the detection of closed micro crack. The numerical and experimental results indicate that the proposed method can accurately locate the closed micro crack in plate-like metallic and composite structures.

Second Harmonic Generation by ultra-fast Lasers in Plasma.

The interaction of laser with plasma gives rise to several novel relativistic nonlinear optical effects. Higher harmonic generation is an important nonlinear effect. High power ultrafast lasers uses chirped pulse amplification technique to reduce duration of pulse and hence produces pulses of extremely high power densities which in turn oscillates electrons in the medium. The velocity of oscillating electron beats with applied frequency giving rise to higher order harmonics. This paper presents the idea of generation of second harmonic waves with the ultra-fast lasers in plasma, phase matching conditions of second harmonic generated by self modulation under paraxial approximations and its application in imaging microscopy.

Numerical simulation of nonlinear second harmonic wave generation by the finite difference frequency domain method

Nonlinear second harmonic wave generation (SHG) has been thoroughly examined in one dimension both analytically and numerically. Recently, the application of advanced domain poling techniques enabled the fabrication of two-dimensional (2D) patterns of the sign of the nonlinear coefficient in certain nonlinear crystals, such as LiNbO3 and LiTaO3. This method can be used to achieve quasi phase matching in SHG and hence amplification of the second harmonic fields in 2D. In this study we present a true vectorial numerical method for the simulation of SHG by extending the finite difference frequency domain method (FDFD). Our nonlinear method (NL-FDFD) operates directly on the electromagnetic fields, uses two meshes for the simulation (for ω and 2ω fields), and handles the nonlinear coupling as an interaction between the two meshes. Final field distributions can be obtained by a small number of iteration steps. NL-FDFD can be applied in arbitrarily structured linear media with an arbitrarily structured χ(2) component both in the small conversion efficiency and the pump depleted cases.

One-dimensional particle simulation of wave propagation and generation of second harmonic waves in a composite of plasma and metamaterial

One-dimensional electromagnetic particle simulations are performed with the combination of the overdense plasma and the negative-permeability state (metamaterial). Transverse electromagnetic waves enter the plasma-metamaterial composite, and the waves propagate in it with a negative phase velocity and a positive group velocity. We confirm that the plasma-metamaterial composite has a negative refractive index. The dispersion relation of the composite has the propagation band under the plasma frequency, and we confirm this band by theoretical calculations with the kinetic effects and by particle simulations. When the phase velocity approaches the thermal velocity of the electrons, we find the damping of the transverse wave in the propagation band. The propagating transverse waves generate the propagating second harmonic waves throughout the composite. The intensities of the second harmonic waves are enhanced because the fundamental frequency waves are efficiently injected into the overdense plasma with the metamaterial since the refractive index is not imaginary but real with negative values.

Dynamical centrosymmetry breaking — A novel mechanism for second harmonic generation in graphene

We discover an unusual phenomenon that occurs when a graphene monolayer is illuminated by a short and intense pulse at normal incidence. Due to the pulse-induced oscillations of the Dirac cones, a dynamical breaking of the layer’s centrosymmetry takes place, leading to the generation of second harmonic waves. We prove that this result can only be found by using the full Dirac equation and show that the widely used semiconductor Bloch equations fail to reproduce this and some other important physics of graphene. Our results open new windows in the understanding of nonlinear light-matter interactions in a wide variety of new 2D materials with a gapped or ungapped Dirac-like dispersion.

Magnetic-field-induced nonlinear optical responses in inversion symmetric Dirac semimetals

We show that, under the effect of an external magnetic field, a photogalvanic effect and the generation of second harmonic wave can be induced in inversion-symmetric and time reversal invariant Dirac semimetals. The mechanism responsible of these non linear optical responses is the magnetochiral effect. The origin of this magnetochiral effect is the band bending of the dispersion relation in real Dirac semimetals. Some observable consequences of this phenomenon are the appearance of a dc current on the surface of the system when it is irradiated with linearly polarized light or a rotation of the polarization plane of the reflected second harmonic wave.

Experimental investigation of nonlinear regular wave transformation over a submerged step: Harmonic generation and wave height modulation

In this study, a series of flume experiments were conducted to investigate the harmonic generation and wave height modulation (i.e., recursion) of nonlinear waves passing over a submerged step. A total of 35 experimental conditions were considered, including several different incident amplitudes/periods and bottom configurations, i.e., four step widths and three step heights (or three submergence depths). The waveforms across a 7.8-m-long segment were recorded over a period of 21.8s using the non-intrusive imaging system. The measured surface elevations were further analyzed to reveal the amplitude variations of harmonic frequencies based on the 2D Fast Fourier and 1D Morlet wavelet transforms. A set of parameters was proposed to depict the generation of second-harmonic waves at the step crest. Overall, it was found that the most influencing factors for harmonic generation are the Ursell number of incident waves and relative height of the obstacle while the relative step width yields an insignificant impact. The wave height modulation (i.e., recursion) of second harmonic is affected mainly by the wavelengths of its free and bound components, which are accurately estimated from the Morlet wavelet transform analysis and third-order Stokes dispersion relation. The amplitude of primary wave exhibits a different recursion pattern due to the generation of the third, forth, and higher harmonics. Based on the concept of average energy flux, the recursion phenomenon of harmonics is further clarified.

The generation of second harmonic waves in an isotropic solid with quadratic nonlinearity under the presence of a stress-free boundary

This research examines the interaction of an incident longitudinal wave with a stress-free boundary, determining all the second harmonic waves generated at this boundary and in the bulk. The nonlinear equations of motion are solved using a perturbation method to obtain the results for both the resonant and non-resonant reflected waves. In the case of resonance, it is shown that the amplitudes of the second harmonic waves linearly depend on propagation distance. The second harmonic waves generated for six different incidence angles, including normal incidence, are then numerically calculated. The presented framework can be used to analyze various situations and enables experimentalists to design their measurement setups. The results provide physical insights into the behavior of second harmonics before and after reflection from a stress-free boundary, and are important when interpreting experimentally obtained material nonlinearity parameters.

Harmonic generation by reflecting internal waves

The generation of internal gravity waves by tidal flow over topography is an important oceanic process that redistributes tidal energy in the ocean. Internal waves reflect from boundaries, creating harmonics and mixing. We use laboratory experiments and two-dimensional numerical simulations of the Navier–Stokes equations to determine the value of the topographic slope that gives the most intense generation of second harmonic waves in the reflection process. The results from our experiments and simulations agree well but differ markedly from theoretical predictions by S. A. Thorpe [“On the reflection of a train of finite amplitude waves from a uniform slope,” J. Fluid Mech. 178, 279 (1987)] and A. Tabaei et al. [“Nonlinear effects in reflecting and colliding internal wave beams,” J. Fluid Mech. 526, 217 (2005)], except for nearly inviscid, weakly nonlinear flow. However, even for weakly nonlinear flow (where the Dauxois–Young amplitude parameter value is only 0.01), we find that the ratio of the reflected wave number to the incoming wave number is very different from the prediction of weakly nonlinear theory. Further, we observe that for incident beams with a wide range of angles, frequencies, and intensities, the second harmonic beam produced in reflection has a maximum intensity when its width is the same as the width of the incident beam. This observation yields a prediction for the angle corresponding to the maximum in second harmonic intensity that is in excellent accord with our results from experiments and numerical simulations.

Continuous wave second-harmonic generation using domain-disordered quasi-phase matching waveguides

Second-harmonic generation in domain-disordered quasi-phase-matched GaAs/AlGaAs superlattice-core waveguides was demonstrated using a continuous wave fundamental source. Output second-harmonic powers of up to 1.6 μW were measured when on a Fabry-Pérot resonance peak. Temperature-related bistable behavior was observed in both the fundamental and second-harmonic output when tuning either the input power or input wavelength.

Characterization of femtosecond pulses via transverse second-harmonic generation in random nonlinear media

We study, both experimentally and theoretically, the process of second-harmonic generation by two noncollinear beams in ferroelectric crystals with a disordered distribution of ferroelectric domains. We show that this parametric process results in generation of second-harmonic wave in the direction transverse to the propagation of the fundamental beam. We demonstrate how this effect can be used for the femtosecond pulse characterization enabling the estimation of both width and chirp of the pulse.

Highly efficient and compact laser-diode end-pumped Q-switched Nd:YVO4/KTP red laser

A laser-diode end-pumped acousto-optic (AO) Q-switched Nd:YVO 4/KTP red laser by using a plano-concave cavity was demonstrated for the first time. This linear cavity configuration could guarantee not only moderate intracavity focusing on KTP crystal but also good beam quality. Under the absorbed pump power of 14.8 W, the maximum average output power at 671 nm was obtained to be 1.37 W at the repetition frequency of 15 kHz, with the corresponding optical conversion efficiency of 9.3% and the pulse width of 33 ns. The energy of a single pulse and corresponding peak power are estimated to be 91.3 μJ and 2.77 kW, respectively. The rate equations are also numerically solved by introducing the nonlinear loss resulting from generation of second-harmonic wave.

Mono- and multi-layer langmuir-blodgett films of maleimide polymers possessing nonlinear optical-active side chains

A copolymer P[OSA-MI] was synthesized by copolymerization of its corresponding monomers,N-phenyl maleimide (MI) and 2-octen-1-ylsuccinic anhydride (OSA). The polymer (poly[2-[1-(2,5-dioxo-1-phenylpyrrolidin-3-ylmethyl)heptyl]-succinic acid 4-(2-{ethyl-[4-(4-nitrophen-ylazo) phenyl]amino}ethyl) ester]) P[DR1MA-MI] was obtained from the reaction of P[OSA-MI] with 2-[4-(4-nitrophenylazo)-N-ethylphenylamino] ethanol (DR1). A stable monolayer of P[DR1MA-MI] was formed by spreading the solution of the polymer in chloroform. In Y-type Langmuir-Blodgett (LB) films prepared using this Langmuir-Blodgett method, the second harmonic waves generated from adjacent monolayers canceled each other out. In X- and Z-type LB films, the second harmonic intensity increased upon increasing the number of monolayers, but this increase was somewhat smaller than predicted by the square law. This phenomenon is due to defects or imperfect alignment of the dipoles in the LB film. The generation of second harmonic waves from Y-type LB films having an even number of monolayers supports this argument. The degree of imperfection seemed to increase as the number of layers increased. The second-order nonlinear optical properties of spin-cast films of these polymers were also measured. The largest second harmonic coefficient of the poled P[DR1MA-MI] film coated on a glass plate was 19 pm/V.

Large enhancement in quasiphase matched second-harmonic generation efficiency by an external temperature gradient

We show that for continuous wave second-harmonic generation using quasiphase matching in periodically segmented waveguides, thermal loading caused by the fundamental and the generated beams degrades the quasiphase matching condition which corrupts the spectral quality of the second-harmonic field. We also show that a carefully designed external temperature gradient leads to an enhancement of the conversion efficiency and control of the spectral line shape of the output field. Compared to earlier work, our simple scheme yields an enhancement of the second-harmonic conversion efficiency by a factor of 2–4 depending on the input fundamental power.

Second-harmonic generation device with integrated periodically domain-inverted regions and distributed Bragg reflector in a LiNbO3 channel waveguide

We propose a new structure of LiNbO3 guided wave second-harmonic generation device, in which the fundamental light beam is sufficiently confined by two distributed Bragg reflectors (DBRs) formed at input and output ports. The 40% increase in the conversion compared with a conventional device without DBR was achieved under a quasi-phase matching condition.

Optimum acoustic wave second-harmonic generation in piezo-electric semiconductors

For metals that become superconducting, it is possible in principle to evaluate the electronic specific heat in the normally conducting state from magnetic measurements only. If the metal can be obtained sufficiently pure both physically and chemically, the transition normal: super state in a magnetic field should be thermodynamically reversible, as was first shown by Meissner and Ochsenfeld. Indeed in the transition metals the latter contribution is predominant, since due to their tighter binding the d-electrons show a much narrower band than the s-electrons and so have a greater number of energy levels per unit energy range. Measurements of ce and hence of N(E*) for each series of transition metals is of interest, therefore, in order to obtain some estimate of the filling of the d bands. It is of interest to note that when similar corrections are made to the free electron evaluation of the electronic paramagnetic susceptibility, it is found by Pines that there is not a similar cancelation.