Bandwidth Of Second Harmonic Generation Research Articles

Growth and Temperature Properties of the Phase-Matching Angle of 23% Deuterated ADP Crystal.

Theoretical analysis indicated that partially deuterated ammonium dihydrogen phosphate (DADP) crystal with a deuterium content of 23% could realize spectral noncritical phase-matching (S-NCPM) for type-I frequency doubling of an Nd:glass laser. To explore the temperature dependence of the phase matching (PM) angle of the second harmonic generation (SHG) process and the output SHG bandwidth of DADP crystal at 1.053 μm, we used the point-seed rapid-growth method to grow targeted DADP crystal with 23% deuterium content. Experimental results indicated that the grown DADP crystal had high quality and large dimensions (7 × 6 × 6 cm3). Using a femtosecond OPO laser as a tunable light source, the temperature dependence of the PM angle of the SHG process in DADP crystal at 1.053 μm was investigated. The PM angle changed linearly with temperature, as predicted by the theoretical calculation. In addition, under the condition of higher temperature, broad bandwidths of the second harmonic of DADP crystal were still observed. These results provide excellent guidance and reference value for the application of wavelength insensitive phase-matched second harmonic generation in partially deuterated DADP.

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.

Ultrabroadband second-harmonic generation based on a dispersion-engineered lithium niobate on insulator waveguide

Second-harmonic generation (SHG) is widely used for wavelength conversions in various applications including multi-wavelength lasers, imaging, and ultrafast optics. However, it remains a long-standing challenge to achieve both a broad SHG bandwidth and high efficiency due to the medium dispersion. Here, we propose an ultrabroadband efficient SHG on a lithium niobate on insulator (LNOI) platform by dispersion engineering. The waveguide cross section with two V-shaped grooves is carefully designed to minimize first- and second-order dispersion, which is feasible for current fabrication techniques. Due to the flattened dispersion, a record-high, to our knowledge, bandwidth-length product ( Δ λ ⋅ L ) of 2444 µ m 2 is achieved in a periodically poled 1.64-cm-long LNOI waveguide, which is four times the previously reported result. Such a scheme can also be generalized to SHG waveguides on other material platforms. Our results provide a new strategy of dispersion engineering for integrated photonics, and may be helpful for applications including frequency doubling, supercontinuum generation, and optical frequency combs.

Second-harmonic and sum-frequency generation based on birefringence phase matching of BaMgF4 crystal.

BaMgF4 is a ferroelectric nonlinear crystal with a very wide transparency window ranging from 125 nm to 13µm of the wavelength. Therefore, it is a candidate material to generate ultraviolet or deep ultraviolet laser, which is very important in lithography, semiconductor manufacturing, and advanced instrument development. Here, the second-order birefringence phase-matching processes of the BaMgF4 crystal were studied, including second-harmonic generation (SHG) and sum-frequency generation (SFG). In the experiments, we measured the phase-matching angle, nonlinear frequency conversion efficiency, and angle bandwidth of SHG and SFG processes of BaMgF4 crystal, which are in well agreement with the theoretical calculations. This study may promote the research of nonlinear optical process of BaMgF4 crystal and also the further development of all-solid-state vacuum ultraviolet lasers.

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.

Wideband second harmonic generation using multiple tilted gratings in a planar waveguide

In this paper, we propose a device based on planar waveguides with multiple slanted quasi-phase-matching gratings to achieve a significant increase in the bandwidth of second-harmonic generation. We show that an appropriate device design can significantly flatten the efficiency of second-harmonic generation as a function of input pump frequency. The design using slanted gratings allows for the fundamental and second harmonic to be noncollinear and thus makes the collection of the generated wave easy. Numerical simulations are performed in a potassium titanyl phosphate (KTP) planar waveguide, and we show the analysis and results for increased bandwidth using two, three, and four gratings, which can be easily extended to more gratings without any loss of generality.

Broadband second-harmonic generation in APPLN with group-velocity matching

In this paper, we present a method to increase the acceptance bandwidth of second harmonic generation (SHG) with 5 mol% MgO-doped aperiodically poled lithium niobate (APPLN) when the conditions of quasi-phase-matching and group-velocity matching are satisfied simultaneously. The APPLN, with a length of 10 mm, has 3333 uniform domains, with the polarization directions of each domain optimized by a genetic algorithm, to obtain the profile of nonlinear coefficients in the communication band. By adjusting the positions and quantities of the fundamental wavelengths appropriately, the evolution of bandwidths is investigated theoretically. The simulation results show that the acceptance bandwidth of SHG is approximately 243.3 nm for type-I (o+o→e) interaction around the wavelength of zero-group-velocity dispersion in crystal, which is enhanced by 4.5 times that of periodic structures.

Broadband Second-Harmonic Generation with Group-Velocity Matching in 5mol%MgO-doped Aperiodic Poled Lithium Niobate

In this paper, we design an aperiodic poled lithium niobate by using genetic algorithm to increase the acceptance bandwidth of second harmonic generation in 5mol% MgO-doped with group-velocity matching, and an acceptance bandwidth of 243.3nm is obtained finally, which is enhanced to be 4.5 times of that in periodic structure.

Broadband second harmonic generation in an imperfect nonlinear photonic crystal with random defects

In this paper, we study broadband second harmonic generation (SHG) in an imperfect nonlinear photonic crystal in which defects are introduced with random lengths. We show that the efficient SHG output is obtained when the length of each defect varies near certain specialized values. The bandwidth of the SHG output broadens with the increasing randomness of defect length. Moreover, the SHG bandwidth is nearly unaffected only when the total length of the whole structure is long enough. The disordered structure also exhibits good tolerance to the fabrication error, which provides a way to control SHG intensity and bandwidth separately.

Type-I frequency-doubling characteristics of high-power, ultrafast fiber laser in thick BIBO crystal

We report on experimental realization of optimum focusing condition for type-I second-harmonic generation (SHG) of high-power, ultrafast laser in "thick" nonlinear crystal. Using single-pass, frequency doubling of a 5 W Yb-fiber laser of pulse width ~260 fs at repetition rate of 78 MHz in a 5-mm-long bismuth triborate (BIBO) crystal we observed that the optimum focusing condition is more dependent on the birefringence of the crystal than its group-velocity mismatch (GVM). A theoretical fit to our experimental results reveals that even in the presence of GVM, the optimum focusing condition matches the theoretical model of Boyd and Kleinman, predicted for continuous-wave and long-pulse SHG. Using a focusing factor of ξ=1.16 close to the estimated optimum value of ξ=1.72 for our experimental conditions, we generated 2.25 W of green radiation of pulse width 176 fs with single-pass conversion efficiency as high as 46.5%. Our study also verifies the effect of pulse narrowing and broadening of angular phase-matching bandwidth of SHG at tighter focusing. This study signifies the advantage of SHG in "thick" crystal in controlling SH-pulse width by changing the focusing lens while accessing high conversion efficiency and broad angular phase-matching bandwidth.

Designing photonic quasi-crystal fibers of various folds: onto optimization of efficiency and bandwidth of second harmonic generation.

We design photonic quasi-crystal fibers (PQFs) of six-, eight-, ten-, and twelve-folds for determining the optimized efficiency as well as the bandwidth of second harmonic generation (SHG). We report a maximum SHG relative efficiency of 941.36% W⁻¹ cm⁻² for a twelve-fold PQF of 2 μm pitch. The detailed numerical results reveal that, while the relative efficiency increases appreciably, the phase-matching bandwidth increases marginally, as and when the number of folds increases. As the primary interest of this work is to enhance the relative efficiency, we focus our analysis with a twelve-fold PQF for which the efficiency turns a maximum. In line with the practical feasibility of poling, we keep the pitch at 7 μm and report an optimized relative efficiency and phase-matching bandwidth as 95.28% W⁻¹ cm⁻² and 50.51 nm.cm, respectively.

Analysis of acceptable spectral windows of quadratic cascaded nonlinear processes in a periodically poled lithium niobate waveguide

We report a systematic and comparative study of the acceptance bandwidths of two cascaded quadratic nonlinear processes in periodically poled lithium niobate waveguides, namely cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) and cascaded sum-frequency generation and difference-frequency generation (cSFG/DFG). We first theoretically and experimentally study the acceptance bandwidths of both the individual second-harmonic generation (SHG) and sum-frequency generation (SFG) processes in the continuous wave (CW) and pulsed-pump regimes. Our results show that the SHG bandwidth is approximately half that of the SFG process in the CW regime, whereas the SHG acceptance bandwidth can approach the CW SFG bandwidth limit when pulsed-pump is used. As a consequence we conclude that the tuning bandwidths of both cascaded processes should be similar in the pulsed pump regime once the pump pulse bandwidths approach that of SFG (i.e. the cSHG/DFG bandwidth is not limited by the CW SHG bandwidth). We confirm that this is the case experimentally.

Origin and effect of high-order dispersion in ultrashort pulse multiphoton microscopy in the 10 fs regime

Short pulses can induce high nonlinear excitation, and thus they should be favorable for use in multiphoton microscopy. However, the large spectral dispersion can easily destroy the advantages of the ultrashort pulse if there is no compensation. The group delay dispersion (GDD), third-order dispersion, and their effects on the intensity and bandwidth of second-harmonic generation (SHG) signal were analyzed. We found that the prism pair used for compensating the GDD of the two-photon microscope actually introduces significant negative high-order dispersion (HOD), which dramatically narrowed down the two-photon absorption probability for ultrashort pulses. We also investigated the SHG signal after GDD and HOD compensation for different pulse durations. Without HOD compensation, the SHG efficiency dropped significantly for a pulse duration below 20 fs. We experimentally compared the SHG and two-photon excited fluorescence (TPEF) signal intensity for 11 fs versus 50 fs pulses, a pulse duration close to that commonly used in conventional multiphoton microscopy. The result suggested that after adaptive phase compensation, the 11fs pulse can yield a 3.2- to 6.0-fold TPEF intensity and a 5.1-fold SHG intensity, compared to 50 fs pulses.

Broadcast wavelength conversion based on cascaded (2) nonlinearity in MgO-doped periodically poled LiNbO3

The first experimental demonstration of broadcast wavelength conversion is reported, based on the cascaded second-order nonlinearity in 5% MgO-doped periodically poled LiNbO3 with type I quasi-phase matching. A broad second-harmonic generation bandwidth up to 27 nm was obtained. Using the device, a free three-channel converter with no crosstalk in the communication band was demonstrated.

SHG in Bragg reflection waveguides for enhanced bandwidth applications

A novel scheme for second harmonic generation in asymmetric Bragg reflection waveguides is presented for enhancing the bandwidth of non-linear interaction. The strong dispersion exhibited by the Bragg reflection waveguide is exploited for countering the rapidly falling efficiency as one moves away from the phase-matching wavelength. The role of the periodic cladding on the efficiency and bandwidth of second harmonic generation in a lithium niobate based Bragg reflection waveguide is analyzed. Such an idea could be used in other parametric processes for enhancing the bandwidth and for easing fabrication tolerances.