Laser Second Harmonic Generation Research Articles

Angular non-critical phase-matching second harmonic generation with the Ba3(ZnB5O10)PO4 crystal

The angular non-critical phase-matching (A-NCPM) second-harmonic-generation (SHG) properties of Ba3(ZnB5O10)PO4 (BZBP) crystal were investigated. Based on the small signal solution formula of SHG, the effective nonlinear coefficient deff along z-axis was corrected to 1.43 p.m./V. Combining the theoretical SHG PM conditions, BZBP crystal can be achieved type-I A-NCPM with the fundamental wavelength of 750 nm on y-axis, and 1012 nm on z-axis. Using an OPO laser as pump source, the BZBP crystal could realize the 506 nm and 375 nm SHG lasers with the conversion efficiencies of 35 % and 45.5 % respectively. The acquired results demonstrated that BZBP is a promising NLO crystal for A-NCPM frequency conversion applications in the output spectral regions of Ti:sapphire laser and near-infrared lasers.

Structure, Microstructure, and Properties of Modified Ceramics (Na,Sr)0.5Bi0.5TiO3

Single-phase ceramic samples of new compositions (Na1 – хSrх)0.5Bi0.5TiO3 (x = 0–0.5), including those modified by additives of SiO2 and ZnO oxides, have been obtained by solid-phase synthesis. The crystal structure and microstructure of these samples, as well as their dielectric, nonlinear optical, and local piezoelectric properties, have been studied. The formation of a perovskite-type phase with a pseudocubic unit cell in all synthesized samples and an increase in the cell volume as a result of partial substitution of perovskite structure cations are established. A decrease in the temperature of ferroelectric phase transitions (confirmed by the methods of dielectric spectroscopy and laser second-harmonic generation) to the tetragonal paraelectric phase is revealed. Remanent piezoelectric hysteresis loops are obtained for the synthesized samples in the polarization switching mode; this result confirms the occurrence of ferroelectric polarization switching.

A bond-free PPLN thin film ridge waveguide

Lithium niobate thin film waveguide is attractive in nonlinear frequency conversion, especially in the second harmonic generation (SHG). However, the conventional fabrication method of lithium niobate thin film waveguide requires expensive bonding equipment and the prepared film is easy to peel off. In this paper, a novel fabrication method of PPLN thin film ridge waveguide with a trapezoidal substrate is presented, based on glue bonding. The PPLN waveguide together with its trapezoidal substrate is surrounded with low refractive glue to modulate the vertical refractive index gradient along the waveguide length, which was proved to be useful for broadband SHG with a theoretical bandwidth above 25 nm by changing the substrate thickness from 3 μm to 17 μm. The proposed structure was verified by using a C-band ASE laser as the pump source, where the SHG laser bandwidth is about 11 nm. The power of SHG is 2.55 μw, and the normalized conversion efficiency is 10.5 %/W∙cm2. This work is beneficial to simplify the fabrication process of lithium niobate thin film ridge waveguide, and has certain promotion significance for photonic chip integration.

A systematic study of PPLN length dependence in intracavity SHG

In this paper, a systematic study of the relationship between nonlinear crystal length and intracavity second-harmonic generation (SHG) using MgO-doped periodically-poled lithium niobate (MgO:PPLN) is presented. The experimental results demonstrate a relationship between the maximum SHG power generated and the full-width at half maximum (FWHM) of the crystal’s temperature tuning curve to the length of the nonlinear optical crystal. It was shown that maximum SHG power increases rapidly with the increase of MgO:PPLN length, reaching a saturation length (~ 2 mm), which is much shorter than that predicted by the single-pass SHG theory. This saturation length of the MgO:PPLN crystal is almost independent on 808 nm pump power for typical powers used in continuous wave intracavity SHG lasers. In addition to this saturation effect, a broadening effect was also observed, the FWHM of the temperature tuning curve was shown to have a larger FWHM than that predicted by the single-pass SHG theory for MgO:PPLN shorter than the saturation length. This work has the benefit of allowing engineers to optimize nonlinear crystal length when developing intracavity SHG based diode-pumped solid state (DPSS) lasers.

Noncentrosymmetric Tetrel Pnictides RuSi4P4 and IrSi3P3: Nonlinear Optical Materials with Outstanding Laser Damage Threshold

AbstractNoncentrosymmetric (NCS) tetrel pnictides have recently generated interest as nonlinear optical (NLO) materials due to their second harmonic generation (SHG) activity and large laser damage threshold (LDT). Herein nonmetal‐rich silicon phosphides RuSi4P4 and IrSi3P3 are synthesized and characterized. Their crystal structures are reinvestigated using single crystal X‐ray diffraction and 29Si and 31P magic angle spinning NMR. In agreement with previous report RuSi4P4 crystallizes in NCS space group P1, while IrSi3P3 is found to crystallize in NCS space group Cm, in contrast with the previously reported space group C2. A combination of DFT calculations and diffuse reflectance measurements reveals RuSi4P4 and IrSi3P3 to be wide bandgap (Eg) semiconductors, Eg = 1.9 and 1.8 eV, respectively. RuSi4P4 and IrSi3P3 outperform the current state‐of‐the‐art infrared SHG material, AgGaS2, both in SHG activity and laser inducer damage threshold. Due to the combination of high thermal stabilities (up to 1373 K), wide bandgaps (≈2 eV), NCS crystal structures, strong SHG responses, and large LDT values, RuSi4P4 and IrSi3P3 are promising candidates for longer wavelength NLO materials.

Beam Control in an Intracavity Frequency-Doubling Semiconductor Disk Laser

In this paper, we have demonstrated a 1.3 W green laser using a V-shaped intracavity frequency doubling 1036 nm semiconductor disk laser. The beam quality of the fundamental and second harmonic generation (SHG) laser is investigated. It has been found that the output lasers at the fundamental and SHG wavelength both suffer from reduced beam quality along with power scaling. The measured beam profile is elliptical under high power operation. In order to improve the SHG laser beam, an intracavity aperture is employed to control the mode characteristics of fundamental frequency light. By fine-tuning the aperture, a nearly circular beam profile with improving beam quality and brightness is realized.

Ungerminated Rice Grains Observed by Femtosecond Pulse Laser Second-Harmonic Generation Microscopy.

As a demonstration that second-order nonlinear optical microscopy is a powerful tool for rice grain science, we observed second-harmonic generation (SHG) images of amylose-free glutinous rice and amylose-containing nonglutinous rice grains. The images obtained from SHG microscopy and photographs of the iodine-stained starch granules indicate that the distribution of starch types in the embryo-facing endosperm region (EFR) depends on the type of rice and suggests that glucose, maltose, or both are localized on the testa side of the embryo. In the testa side of the embryo, crystallized glucose or maltose are judged to be detected by SHG. These monosaccharides and disaccharides play an important role, as they trigger energy in the initial stage of germination. These results confirm SHG microscopy is a good method to monitor the distribution of such sugars and amylopectin in the embryo and its neighboring regions of rice grains.

Second-order nonlinear optical microscopy of spider silk

Asymmetric $\rm \beta$-sheet protein structures in spider silk should induce nonlinear optical interaction such as second harmonic generation (SHG) which is experimentally observed for a radial line and dragline spider silk by using an imaging femtosecond laser SHG microscope. By comparing different spider silks, we found that the SHG signal correlates with the existence of the protein $\rm \beta$-sheets. Measurements of the polarization dependence of SHG from the dragline indicated that the $\rm \beta$-sheet has a nonlinear response depending on the direction of the incident electric field. We propose a model of what orientation the $\rm \beta$-sheet takes in spider silk.

Laser second-harmonic generation from an overdense plasma slab

AbstractA s-polarized short-pulse laser impinged obliquely on an overdense plasma slab is shown to produce very significant second harmonic in the direction of specular reflection and transmission. The laser induces a non-linear current on electrons, which is curl free. However, with sharp plasma boundary, it gives rise to electromagnetic radiation at the second harmonic. Our formalism includes multiple reflections of the incident and second-harmonic waves from both the front and rear surfaces. The present work includes finiteness of the slab. The normalized second-harmonic amplitude acquires a sharp peak at some specific angle of incidence for a particular set of parameters dependent on thickness of the slab and plasma density.

Second Harmonic Generation (SHG) Anisotropy Imaging for Evaluation of Dysplasia in Barrettʼs Esophagus

Introduction: Barrett's without dysplasia (WD) and with dysplasia either low grade (LGD) or high grade (HGD) have different prognosis and management strategies. The standard mechanism for histologic diagnosis is tedious and can miss areas of interest. SHG laser is a coherent nonlinear process, where two lower energy photons are up-converted to twice the incident frequency (half the wavelength) of an excitation laser. SHG has the ability to directly visualize the collagen assembly in the extracellular matrix (ECM), which can be related to pathologic effect. Previous studies have demonstrated this effect on multiple tissues including differentiating between malignant and non-malignant colon polyp tissue. We postulate that SHG could be utilized in the evaluation of Barrett's tissue to differentiate between nondysplastic and dysplastic Barrett's tissue. Methods: A total of 9 de-identified H&E slides were obtained through IRB exempt protocol. This included 3 slides of Barrett's WD, 3 slides of LGD, and 3 slides of HGD. SHG laser was used to obtain 10 different images for WD and LGD, and 9 images for HGD at constant average power of excitation light (20 milliwatts) at 800 nm. Forward propagating SHG signal was collected. Images were taken with the analyzing polarizer in the parallel and orthogonal positions. Intensities of parallel (Ipar) and orthogonal (Iorth) components were measured from the collagen fibers aligned at 90 degrees relative to the polarization of excitation light. The anisotropy parameter was calculated: β= (Ipar - Iorth)/(Ipar + 2 Iorth). Results: The mean β for Barrett's tissue types, as well as Kruskall-Wallis mean rank (p=0.028) are seen in table 1. Barrett's WD had the highest β score, indicating the lowest proportion of non-aligned fibers. Post Hoc analysis using Mann-Whitney U (p=0.33) is shown in table 2. Conclusion: Samples of Barrett's WD had the highest β values, indicating the most aligned fibers. LGD and HGD showed lower β values compared to WD indicating more misaligned fiber. Statistical analysis shows significant difference between non-dysplastic and dysplastic groups in terms of anisotropy. This suggests that SHG can distinguish between non-dysplastic and dysplastic Barrett's tissue. More studies especially on live tissue need to be done to confirm these findings. SHG could be incorporated into the endoscope light source and utilized to supplement and enhance traditional pathology.Table 1: Mean β and Mean Rank for Anisotropy parameter β for Barrett's WD, low grade dysplasia, and high grade dysplasiaTable 2: Post Hoc analysis of Anisotropy parameter β between Barrett's WD and HGD

Generation of 15W femtosecond laser pulse from a Kerr-lens mode-locked Yb:YAG thin-disk oscillator**Project supported by the National Basic Research Program of China (Grant No. 2013CB922402), the National Major Instrument Program of China (Grant No. 2012YQ120047), and the National Natural Science Foundation of China (Grant Nos. 11434016 and 61210017).

We demonstrated a robust power-scalable Kerr-lens mode-locked (KLM) operation based on a Yb:YAG thin-disk oscillator. 15-W, 272-fs pulses were realized at a repetition rate of 86.7 MHz with an additional Kerr medium and a 2.5 mm hard aperture in the cavity. 247-fs pulses with an average power of 11 W could also be obtained by using a 2.4 mm hard aperture. Based on this shorter pulse, high efficient second-harmonic generation (SHG) was performed with a 1.7-mm-long LiB3O5 (LBO) crystal. The SHG laser power was up to 5 W with the power fluctuation RMS of 1% measured over one hour.

First-Principles Evaluation of the Alkali and/or Alkaline Earth Beryllium Borates in Deep Ultraviolet Nonlinear Optical Applications

The prospects of all known noncentrosymmetric alkali and/or alkaline earth beryllium borates for nonlinear optical (NLO) generation in the deep-ultraviolet (DUV) spectral region are evaluated by first-principles studies. It is shown that all these crystals possess large enough energy band gaps and relatively strong NLO responses that satisfy the requirement for DUV second-harmonic generation (SHG). However, the practical DUV SHG capabilities of these crystals are mostly limited by the phase-matching condition, as determined not only by the optical birefringence but also by the refractive-index dispersion that is explicitly emphasized for the first time in this work. Therefore, although the K2Be2BO3F family and NaBeB3O6 both have a large birefringence, Δn ≈ 0.08, at a wavelength of 400 nm, the DUV SHG lasers can only be produced in the former but cannot be fulfilled in the latter due to its large refractive-index dispersion in the DUV region. The crystal structural features for large birefringence and the electronic structure origins for different refractive-index dispersion behaviors are discussed. The elucidation of the structure–property relationship presents a useful guide to the exploration of new NLO crystals that can be practically applied for DUV SHG.

Surface plasma wave assisted second harmonic generation of laser over a metal film

Second harmonic generation of laser mode converted surface plasma wave (SPW) over a corrugated metal film is studied. The laser, impinged on the metal film, under attenuated total reflection configuration, excites SPW over the metal–vacuum interface. The excited SPW extends over a much wider surface area than the laser spot cross-section. It exerts a second harmonic ponderomotive force on metal electrons, imparting them velocity that beats with the surface ripple to produce a nonlinear current, driving resonant second harmonic surface plasma wave.

Angular Dependence of the Second Harmonic Generation Induced by Femtosecond Laser Irradiation in Silica-Based Glasses: Variation with Writing Speed and Pulse Energy

To control second harmonic generation (SHG) in silica-based glasses is crucial for fabricating photonic devices, such as frequency doubling waveguides. Here, we investigated SHG of laser induced nonlinear optical crystals in silica-based glasses, according to writing speed and pulse energy. We observed two regions with different probing laser polarization angular dependence: a) a well-defined cosine-like curve with period of 180° at low pulse energy (0.8 μJ) whatever the writing speed or at high pulse energy (1.4 μJ) with high writing speed (25 μm/s). This is accounted for by a well-defined texture for the nano crystals with their polar axis oriented perpendicular to the writing laser polarization; and b) a double cosine-like curve revealing a second texture of the crystals at high pulse energy (1.4 μJ) with low writing speed (5 μm/s) and with the polar axis oriented closer parallel to the writing laser polarization. Therefore, a SHG dependence on probing laser polarization angle may show high contrast by a correct choice of the writing speed and pulse energy. These results pave the way for elaboration of nonlinear optical devices.

Precise, motion-free polarization control in Second Harmonic Generation microscopy using a liquid crystal modulator in the infinity space

Second Harmonic Generation (SHG) microscopy coupled with polarization analysis has great potential for use in tissue characterization, as molecular and supramolecular structural details can be extracted. Such measurements are difficult to perform quickly and accurately. Here we present a new method that uses a liquid crystal modulator (LCM) located in the infinity space of a SHG laser scanning microscope that allows the generation of any desired linear or circular polarization state. As the device contains no moving parts, polarization can be rotated accurately and faster than by manual or motorized control. The performance in terms of polarization purity was validated using Stokes vector polarimetry, and found to have minimal residual polarization ellipticity. SHG polarization imaging characteristics were validated against well-characterized specimens having cylindrical and/or linear symmetries. The LCM has a small footprint and can be implemented easily in any standard microscope and is cost effective relative to other technologies.

Dual-wavelength, continuous-wave Yb:YAG laser for high-resolution photothermal common-path interferometry

We present a continuous-wave (CW) intracavity frequency-doubled Yb:YAG laser providing 1030 and 515nm output simultaneously. This laser system was designed for photothermal common-path interferometry to measure spatially resolved profiles of the linear absorption in dielectric media and coatings for visible or infrared light as well as of the nonlinear absorption for the combination of both. A Z-shape laser cavity was designed, providing a beam waist in which an LBO crystal was located for effective second-harmonic generation (SHG). Suitable frequency conversion parameters and cavity configurations were discussed to achieve the optimal performance of a diode-pumped CW SHG laser. A 12.4W 1030nm laser and 5.4W 515nm laser were developed simultaneously in our experiment.

The generation of a continuous-wave Nd:YVO4/LBO laser at 543 nm by direct in-band diode pumping at 888 nm

We report the generation of a green laser at 543 nm by intracavity frequency doubling of the continuous-wave (cw) laser operation of a 1086 nm Nd:YVO4 laser under 888 nm diode pumping into the emitting level 4F3/2. An LiB3O5 (LBO) crystal, cut for critical type I phase matching at room temperature, is used for the laser second-harmonic generation. At an incident pump power of 17.8 W, as high as 4.53 W cw output power at 543 nm is achieved. The optical-to-optical conversion efficiency is up to 25.4%, and the fluctuation of the green output power is better than 2.3% in a 30 min period.

Linear and nonlinear optical studies of γ-glycine single crystals from ammonium carbonate for optoelectronic applications

Single crystals of γ-glycine were grown by solvent evaporation method from a mixture of aqueous solutions of glycine and ammonium carbonate at ambient temperature for the first time. Powder XRD, CHN analysis, Differential scanning calorimetry analysis, Fourier transform infrared spectroscopic techniques were employed to characterize the crystal. The lattice parameters were calculated and they agree well with the reported values. The UV cut off of γ-glycine is below 300nm and has a wide transparency window, which is suitable for second harmonic generation of laser in the green region. Nonlinear optical property of γ-glycine crystal were studied using Q switched Nd: YAG laser. The second harmonic generation conversion efficiency of γ-glycine is 1.85 times that of KDP.

High power 4.65 μm single-wavelength laser by second-harmonic generation of pulsed TEA CO2 laser in AgGaSe2 and ZnGeP2

A high power 4.65 μm single-wavelength laser by second-harmonic generation (SHG) of TEA CO2 laser pulses in silver gallium selenide (AgGaSe2) and zinc germanium phosphide (ZnGeP2) crystals is reported. Experimental results show that the average output power of SHG laser is not only restricted by the damage threshold of the nonlinear crystals, but also limited by the irradiated power of fundamental-wave laser depending on the operating repetition-rate. It is found that ZnGeP2 can withstand higher 9.3 μm laser irradiation intensity than AgGaSe2. As a result, using a parallel array stacked by seven ZnGeP2 crystals, an average power of 20.3 W 4.65 μm laser is obtained at 250 Hz. To the best of our knowledge, it is the highest output power for SHG of CO2 laser by far.

Crystal growth and characterization of semiorganic single crystals of l-histidine family for NLO applications

Single crystals of l-histidine hydrochloride monohydrate and tartaric acid mixed l-histidine hydrochloride monohydrate have been grown by slow evaporation solution growth technique from appropriate mixtures of respective chemicals. Single crystal and powder X-ray diffraction analyses, Fourier transform infrared, nuclear magnetic resonance spectral analysis, thermo-gravimetric (TG), differential thermal analysis (DTA) have been employed to characterize the as-grown crystals. It is observed to be a wide transparency window for both the crystals from 300 to 1000 nm, which is suitable for second harmonic generation of laser in the blue region. Nonlinear optical characteristics have been studied using Q switched Nd: YAG laser ( λ=1064 nm). The second harmonic generation conversion efficiency of the grown crystal shows the suitability for frequency conversion applications.