Quasi-phase Matched Second Harmonic Generation Research Articles

Broadband second-harmonic generation based on orientation-patterned II-VI crystals

Broadband second-harmonic generation (SHG) has important applications in many fields, such as femtosecond laser technology, optical communication, and biomedicine. The broadband SHG has been implemented in some crystals; however, it is not clear for forward and backward broadband quasi-phase-matching (QPM) SHGs in orientation-patterned Ⅱ-Ⅵ crystals. In this paper, the characters of orientation-patterned ZnS (OP-ZnS), orientation-patterned ZnSe (OP-ZnSe), and orientation-patterned CdTe (OP-CdTe) are, respectively, studied, including the forward broadband SHG and backward broadband SHG bandwidths at different temperatures. In addition, the influences of temperature on the forward broadband SHG bandwidth are investigated. The forward broadband QPM SHGs of OP-ZnS, OP-ZnSe, and OP-CdTe are obtained, and the backward broadband QPM SHGs cannot be obtained. The influence of temperature on the forward bandwidth is small.

Quasi-phase matching waveguides on lithium niobate and KTP for nonlinear frequency conversion: A comparison

Established waveguide fabrication technologies on lithium niobate (LN) and potassium titanyl phosphate (KTP) were revisited, and a comparative analysis of their performance for type-0 quasi-phase matched second-harmonic generation at 1.55 µm was provided based on literature data and our simulations. This analysis aided identifying gaps where the waveguide performance is below the theoretical predictions, and the possible reasons are discussed. It provides the reader with a roadmap for choosing the most appropriate waveguide type and material choice between LN and KTP for desired performance of targeted applications.

Broadband second harmonic generation in aperiodic nonlinear photonic crystals: 1D projection from 2D Vogel sunflower spiral array

The structure of the sunflower head can be used to realize broadband applications in optics. However, this 2D structure, with a ring-shaped reciprocal space, only has a normalized Fourier coefficient peak around 0.033, which leads to a relatively low conversion efficiency and may restrict its applications. We tried to maintain its broadband features while with larger Fourier coefficients by structure dimension reduction. We obtained an aperiodic 1D structure from a 2D Vogel sunflower spiral array by a cut-and-projection method. Workable reciprocal vector bands were found with this 1D structure in the vicinity of a pre-set central wavelength λ0 = 1.4 µm, and its peak Fourier coefficients can be 5–7 times as large as the original 2D structure. With this, we investigated broadband quasi-phase matching (QPM) second harmonic generation (SHG) in samples with different reversed ratios D. To illustrate in more detail, three samples were closely examined with D = 0.4, 0.5, and 0.6. Bandwidths of these three samples for first-order QPM SHG are 90, 70, and 30 nm, respectively, with a fundamental wave in the vicinity of λ0 = 1.4 µm. The exact SHG solution of coupled-wave equations was used in the evaluation of conversion efficiencies. Calculations showed broadband high conversion efficiency.

Multiple quasi-phase-matched second-harmonic generation in phase reversal optical superlattice structure

Domain-engineered quasi-phase-matching (QPM) devices are known for its versatility and ability to tune the nonlinear optical frequency conversion process. In this paper, a simple approach is presented to generate multiple quasi-phase-matched second-harmonic generation (SHG) in the phase reversal optical superlattice (PROS) structure. Theoretical studies are carried out by simulation based on the domain reversed lithium niobate QPM devices. The nature of the generated multiple SHG spectra is analyzed when the phase reversal (PR) domains are distributed at equal and unequal intervals along the length of the device. The distribution of phase reversal domains at equal intervals is limited to the generation of dual SHG peaks irrespective of its number. On the contrary, we could generate equal-intensity multiple SHG peaks with PR domains distribution at unequal interval. Using this scheme, five peaks QPM SHG are generated by distributing four PR domains in specific locations of the PROS QPM device. The dependency of the PR domain and its location in the PROS QPM device are analyzed to design desirable multi-wavelength converters.

Quasi-phase-matched second harmonic generation of UV light using AlN waveguides

As an alternative to electrically injected diodes, UV light emission can be obtained via second harmonic generation (SHG). In weakly birefringent materials such as aluminum nitride (AlN), the phase matching of the driving and second harmonic waves can be achieved by the quasi-phase-matching (QPM) technique, where the polarity of the material is periodically changed commensurate with the coherence wavelength. QPM also allows the use of the highest nonlinear susceptibility, and therefore, higher conversion efficiencies are possible. In this work, the QPM SHG of UV light in AlN lateral polar structure-based waveguides is demonstrated. The peak intensity of the frequency doubled laser light was measured at 344 nm and 472 nm wavelengths, in agreement with dispersion-based theoretical predictions. These results confirm the potential of III-nitride-based lateral polar structures for quasi-phase-matched nonlinear optics and for frequency doubling media for UV light generation.

First-Order Forward and Backward Quasi-Phase Matched Second Harmonic Generation in Silicon-Organic Hybrid Structures

We propose and design a high-efficiency forward and backward second harmonic generation (SHG) in nonlinear polymer filled silicon slot waveguide known as silicon-organic hybrid (SOH) waveguides. The spatial distribution of second-order nonlinearity, χ(2) , of waveguide is altered by periodically poling of nonlinear polymer to achieve quasi-phase matching (QPM) between fundamental frequency and second harmonic waves. The simulations show that first-order QPM SHG with high conversion efficiency of η ≍ 7.2% can be achieved in the optimized structure with relatively small input power of 25 mW. This level of improvement is the result of high confinement of SOH waveguide as well as possible fabrication of very small period length in silicon photonics which could not be realized in traditional nonlinear materials such as LiNbO3. Our simulation results show ∼17% efficiency with only 25 mW input power for forward SHG being comparable to the other platforms even with smaller device length and lower pump power. We expect this device will open up its application in the fields of all-optical signal processing, optical (mirrorless) parametric amplification, low-noise detection of mid-IR signal in near-IR windows, and mid-IR sources.

Broadband quasi-phase matching in a MgO:PPLN thin film

Future quantum information networks operated on telecom channels require qubit transfer between different wavelengths while preserving quantum coherence and entanglement. Qubit transfer is a nonlinear optical process, but currently the types of atoms used for quantum information processing and storage are limited by the narrow bandwidth of upconversion available. Here we present the first experimental demonstration of broadband and high-efficiency quasi-phase matching second-harmonic generation (SHG) in a chip-scale periodically poled lithium niobate thin film. We achieve a large bandwidth of up to 2 THz for SHG by satisfying quasi-phase matching and group-velocity matching simultaneously. Furthermore, by changing the film thickness, the central wavelength of the quasi-phase matching SHG bandwidth can be modulated from 2.70 μm to 1.44 μm. The reconfigurable quasi-phase matching lithium niobate thin film provides a significant on-chip integrated platform for photonics and quantum optics.

Theoretical study on broadband second harmonic generation in periodically and aperiodically poled KTP

In this paper, the dispersion property of KTP is analyzed to calculate the group-velocity matching (GVM) wavelength at a given temperature. Then, first-order quasi-phase-matching (QPM) broadband second harmonic generation (SHG) for two different types of interaction is studied at the calculated wavelength in periodically poled KTP (PPKTP). To broaden the acceptance bandwidth of QPM SHG, a structure of aperiodically poled KTP (APPKTP) is designed using a genetic algorithm with the QPM and GVM conditions satisfied simultaneously. Next, the broadband property of APPKTP is studied. Research shows that, for type-I QPM, the bandwidth at the communication band is ~1.9 nm in a 10 mm long PPKTP crystal with a set temperature of 22 °C, while a wide acceptance bandwidth of 73.9 nm can be obtained at the same waveband for type-II QPM, with the conversion efficiency decreasing to 4.64%. In addition, the maximum available acceptance bandwidths are ~9.3 nm for type-I QPM and 417.1 nm for type-II QPM, using the APPKTP designed above. The tradeoff between bandwidth and conversion efficiency is also discussed.

Engineering phase shifter domains for multiple QPM using simulated annealing algorithm

We have utilized the general algorithm of simulated annealing (SA) to engineer the phase shifter domains in a quasi-phase-matching (QPM) device to generate multiple frequency conversion. SA is an algorithm generally used to find the global maxima or minima in a given random function. Here, we have utilized this algorithm to generate multiple QPM second harmonic generation (SHG) by distributing phase shifters suitably. In general, phase shifters are distributed in a QPM device with some specific profile along the length to generate multiple QPM SHG. Using the SA algorithm, the location of these phase shifters can be easily identified to have the desired multiple QPM with higher conversion efficiency. The methodology to generate the desired multiple QPM SHG using the SA algorithm has been discussed in detail.

Effect of field modulation on the quasi-phase-matching for second harmonic generation in a two-dimensional nonlinear photonic crystal

Second harmonic generation (SHG) in a two-dimensional (2D) nonlinear photonic crystal (NPC) with finite width along [Formula: see text]-direction that is embedded in air is investigated, without adopting the traditional approximations such as a plane-wave approximation (PWA) and slowly varying amplitude approximation (SVAA). The so-called quasi-phase-matching (QPM) and the corresponding SHG conversion efficiency can be modulated significantly by the field of fundamental wave (FW). It is assumed that the incident light, along [Formula: see text]-direction, is normally launched upon the surface of the sample, and QPM for different directions is investigated. It is found that the QPM shows significant differences, compared with the traditional QPM along the two different directions: in the direction of finite width of the sample, the peak value of SHG conversion efficiency is deviated from the traditional case and it gets to its peak values when the transmittance resonance occurs. However, in the other direction, the deviation from the traditional QPM arises from the field modulation of the second harmonic wave (SHW) and in this direction, it is investigated that the full width at half maximum of QPM is much wider than that in the direction of finite width of the sample. These results can be used to provide a theoretical guidance for achieving QPM SHG.

Thermo-optic dispersion formula for the ordinary wave in 5 mol% MgO doped LiNbO3 and its application to temperature insensitive second-harmonic generation

We report the high accuracy thermo-optic dispersion formula for the ordinary wave of 5mol% MgO doped congruent LiNbO3 (MgO:LiNbO3), which reproduces well our experimental data for the temperature-dependent birefringent phase-matching (BPM) and quasi-phase-matching (QPM) conditions with the oo–e, oo–o, and oe–o interactions in the 0.41–3.7μm range. In addition, we found the temperature insensitive quasi-phase-matched second-harmonic generation (QPM/SHG) points exist in periodically poled MgO:LiNbO3 (MgO:PPLN) with the oo–o and oe–o interactions for the first time.

All-optical isolator under arbitrary linearly polarized fundamental wave in an optical superlattice

We theoretically investigate an all-optical isolator under arbitrary linearly polarized fundamental wave (FW) input in an optical superlattice (OSL). The scheme is based on simultaneously phase matching the first-order Type I (oo-e) quasi-phase-matching (QPM) second-harmonic generation (SHG) process and higher-order Type 0 (ee-e) QPM SHG process in an OSL with a defect inserted in an asymmetrical position. Simulation results show that the contrast ratio of the all-optical isolator can maintain close to 1 under arbitrary linearly polarized FW. Thus, an all-optical isolator based on an OSL that is not sensitive to the direction of linear polarization can be realized. We also show that, with the defect in a strong asymmetry position, the length of the defect can be designed flexibly to maintain a high contrast ratio. Additionally, if the length of the OSL is longer, the nonreciprocal response can be realized for low optical intensities.

Fabrication of Thick Periodically-poled Lithium Niobate Crystals by Standard Electric Field Poling and Direct Bonding

We fabricated 1 mm-thick periodically-poled lithium niobate (PPLN) crystals by using a high-voltage amplifier for standard electric field poling combined with a voltage multiplier. Furthermore, two 1 mm-thick PPLNs were directly bonded to make a 2 mm-thick PPLN. The large aperture allowed broad angular tuning, and a broad spectral range of quasi-phase matched second-harmonic generation can be achieved in a single channel. High-power applications are also expected.

All optical wavelength broadcast based on simultaneous Type I QPM broadband SFG and SHG in MgO:PPLN

We experimentally demonstrate wavelength broadcast based on simultaneous Type I quasi-phase-matched (QPM) broadband sum-frequency generation (SFG) and second-harmonic generation (SHG) in 5 mol.% MgO-doped periodically poled lithium niobate (MgO:PPLN). One signal has been synchronously converted into seven different wavelengths using two pumps at a 1.5 microm band via quadratic cascaded nonlinear wavelength conversion. By selecting different pump regions, i.e., selecting different cascaded chi((2)):chi((2)) interactions, the flexible wavelength conversions with shifting from one signal to single, double, and triple channels were also demonstrated.

Acousto-optic tunable second harmonic generation in periodically poled LiNbO_3

Acousto-optic (AO) tunable second harmonic generation (SHG) was proposed in periodically poled lithium niobate (PPLN). The acoustic wave could either be induced from an external transducer or self-generated in PPLN driving with a cross-field radio frequency field. The reciprocal vector of PPLN compensates the SHG wave-vector mismatch when quasi-phase- matching (QPM) condition is satisfied, while phonons with suitable frequencies may affect it by scattering photons to different polarization state. The QPM SHG and AO polarization rotation are coupled together. Second harmonic waves' intensities, polarization states and even phases thus could be manipulated instantly through AO interaction.

Temperature-Dependent Sellmeier Equation for 1.0 mol% Mg-Doped Stoichiometric Lithium Tantalate

We propose new coefficients for the Sellmeier equations of 1.0 mo1% Mg-doped stoichiometric LiTaO3. The extraordinary refractive indices for the wavelength range 0.5–4.0μm and for temperature 30–170°C are based on measured data derived from quasi-phase-matched (QPM) optical parametric oscillator (OPO) and second-harmonic generation (SHG) based on periodically poled Mg-doped stoichiometric LiTaO3 with a pump wavelength of 1064 nm. The corrected refractive indices are in good agreement with our experimental data obtained from QPM OPO and SHG pumped at 1064μm at different temperatures.

Nonlinear frequency conversion in two-dimensional nonlinear photonic crystals solved by a plane-wave-based transfer-matrix method

We develop the plane-wave-based transfer-matrix method, which is a formalism that has been extensively used to calculate the scattering of electromagnetic waves by usual linear photonic crystals, to solve the quasi-phase-matched (QPM) second-harmonic generation (SHG) in two-dimensional (2D) nonlinear photonic crystals. Using this method, we can calculate the conversion efficiency of high-order collinear and noncollinear QPM second-harmonics. We apply this method to evaluate the SHG conversion efficiency in the 2D periodically poled ${\text{LiNbO}}_{3}$ crystal with rectangular lattice. The high-order collinear and noncollinear QPM SHGs are observed. The wavelengths at which various orders of QPM SHGs occur and their relative conversion efficiencies predicted by the current theoretical method are in fairly good agreement with those found in experiments. This indicates that the developed method is effective and efficient in handling nonlinear light diffraction in nonlinear photonic crystals.

Ultra-broadband optical parametric generation and simultaneous RGB generation in periodically poled lithium niobate

We report on efficient collinear optical parametric generation (OPG) with gain band ranging from 1400 to 2600 nm in a 2 cm-long periodically poled lithium niobate (PPLN) crystal. Such an ultra-broad gain band was obtained by choosing the pump wavelength at 933 nm, at which the group-velocities of the signal and the idler match near the degeneracy point. High OPG efficiency was obtained by quasi-phase matching (QPM). The ultra-broadband OPG led to efficient collinear RGB generation from a single PPLN crystal at a fixed pump wavelength. The green and red beams were found to be originating from high-order QPM sum-frequency generation between the pump and selected frequencies in the OPG band, while the blue beam was high-order QPM second-harmonic generation of the pump.

Visible Micro Solid-State Lasers

We review the recent trends of visible light sources based on nonlinear frequency conversion from the Nd3+-doped micro-lasers. In order to evaluate the emission properties of 1.3, 1.0, and 0.9 in Nd3+-ion doped materials, the spectroscopic parameters were summarized. Recently appeared Nd: GdV4O has the large absorption and emission cross-sections with high thermal conductivity, as high as YAG. After the demonstration of near quantum limit highly efficient operation in Nd: GdVO4, the intra-cavity frequency doubled micro-laser was reviewed as a high power compact green laser. On the other hand, we have demonstrated the high power stable green and deep blue laser under extra-cavity frequency doubling with periodically-poled MgO: LiNbO3 (PPMgLN) for quasi-phase matched second harmonic generation (QPM-SHG). Finally, edge pumped Yb: YAG microchip laser was discussed for the high-power visible microchip lasers.

QUASI-PHASE-MATCHED SECOND-HARMONIC GENERATION IN NONLINEAR OPTICAL POLYMER WAVEGUIDE

We address issues related to quasi-phase-matched (QPM) second-harmonic generation (SHG) in nonlinear polymer waveguide for applying at optical communication. This field of research is very challenging because the high nonlinearity of polymers leads the device to operate at low pump power levels. However, key problems to this approach include difficulty of phase matching, the high propagation losses of the guided waves, and low nonlinearity contrast in the +/0 type QPM geometry. By optimizing these points on material development, waveguide design, and device fabrication, we demonstrate a QPM SHG with the best efficiency in polymer waveguide.