Enhanced third-order nonlinear optical properties of methyl orange dye-doped potassium penta borate octa hydrate (MOPPB) single crystals using CW diode laser for optical limiting applications

We report pure potassium penta borate octa hydrate (PPB) and methyl orange dye-doped potassium penta borate octa hydrate (MOPPB) for optical limiting applications. The crystals were grown by slow evaporation solution growth technique at ambient conditions. The single crystal X-ray diffraction (SXRD) evinces the orthorhombic class of the pristine and MOPPB crystals. Formation of PPB and incorporation of MO dye in PPB was confirmed through Fourier transform infrared (FTIR) study. Calculated hardness number and stiffness constant with Vicker’s Micro hardness study portrays the soft nature of the title crystals. The decrease in bandgap of MOPPB is due to the additional energy levels introduced between the valence band and conduction band which in turn increases the net polarizability of the crystal. Relative second harmonic generation (SHG) efficiency of pure PPB and MOPPB is 0.45 times and 0.40 times that of SHG output of KDP. Z-scan studies shows that both crystals show reverse saturable absorption (RSA) ascribed due to two-photon absorption (TPA) process. MOPPB [β = 1.68 × 10–5 m/W, n2 = 10.2 × 10–12 m2/W, χ(3) = 3.5 × 10–9 m2/V2] possess higher NLO coefficients than pure PPB [β = 0.98 × 10–5 m/W, n2 = 4.62 × 10–12 m2/W, χ(3) = 6.1 × 10–9 m2/V2] which assured the superiority of azo dye incorporation in PPB crystals. Pristine and MOPPB crystals depict the optical limiting (OL) behavior under continuous wave (CW) diode laser irradiation at 785 nm. Thus, the linear and nonlinear properties bestow the MOPPB single crystal as a versed material for OL devices used for the safeguard of optical components from laser damage.


Introduction
Nowadays, nonlinear optical (NLO) materials are overwhelmingly explored because of their dynamic and vibrant applications in telecommunications, optical switching, optical modulation, 3-D optical memory devices, optical limiters etc., [1].For the past three decades, these NLO materials are attracting the scientific community for the generation of short wavelength lasers using second and third harmonic generation phenomenon [2][3][4].With horizon of laser wavelength expanding very rapidly, one alarming fact of laser is that they can easily damage photosensitive components including human eyes [5].So in recent years, utilization of organic NLO materials are no longer limited to production of laser through harmonic generation, but also in addressing the solution for laser accidents [6][7][8].Laser safety devices that functions on NLO phenomenon are generally called as optical limiters which exhibits transparency at low powers and turns opaque at high incident intensities.Regardless of the input power magnitude, these optical limiters are designed to maintain irradiance, power, fluence or energy transmitted by an optical system below some specified maximum value.Fabrication and testing the performance of optical limiters made of organic, inorganic and semiorganic materials are under recent limelight.Among them, organic azo dyes are interesting as they possess broad spectral response and high nonlinear absorption coefficients.Azo dyes contain organic compounds which contain the colouring agent (N=N) possessing the functional group R-N=N-R' where R' and R are aryl groups.When these dyes are doped into the host materials, they show improved nonlinear optical absorption phenomena such as Saturable Absorption (SA), Reverse Saturable Absorption (RSA), etc., [9][10].In the choice of host material, inorganic borates such as Potassium Penta Borate octa hydrate (PPB) are potential candidate as they possess high mechanical and thermal stability along with high NLO coefficients [11][12][13][14].PPB is basically a transparent material and is highly transparent to visible region.Hence addition of organic dyes like Methyl orange (MO) can avail interesting visible absorption to the host and these change in band structure can influence the nonlinear absorption behavior of the system [15-20].Methyl Orange dyes find wide applications in light emitting diodes, organic semiconductors, photo voltaic devices, thermal printing, biology, medical fields as wound healing, photo dynamic therapy, pharmaceutical chemistry, etc., [21].Also the chromophore of MO shows excellent nonlinear optical behavior especially Reverse Saturable Absorption [9][10].The molecular structure of Methyl Orange is shown in Fig. (1).This article reports the preparation and characterization of pure (PPB) and methyl orange doped potassium penta borate (MOPPB) for the optical limiting applications for the first time.Under CW Diode laser of 785 nm wavelength excitation, both the materials exhibit optical limiting action exposing their suitability for laser safety devices.

Crystal Growth and Structural Confirmation
Commercially available Potassium Penta borate octa hydrate (B10H16K2O24) and Methyl Orange (C14H14N3NaO3S) were taken for the reaction.Saturation point at room temperature was noted to grow the pure PPB single crystals.For MOPBB the doping concentration was taken in the ratio of 1:0.05M.pH of both pure PPB and MOPPB solutions were maintained at 7 and 5, respectively.After 4 hours of vigorous stirring both the solutions were filtered and sealed with perforated sheet and left undisturbed for slow evaporation at ambient condition.Fig. 2 (a) & (b) shows the harvested pure and MO doped PPB single crystals after a period of 35 and 45 days, respectively.The pristine crystals and doped PPB were subjected to single Crystal X-ray Diffraction (XRD) studies using an ENRAF NONIUS CAD4 X-Ray Diffractometer with a MoKα radiation (λ=0.71073Å).The unit cell parameters for pure PPB and MO doped PPB are tabulated in Table 1.The obtained results were in good agreement with reported literature [22].It was also clear from the changes seen in the unit cell parameter and volume that MO entered successfully into the crystal lattice of the parent material without any change in the crystal structure.
Molecular structure and the functional group associated with grown crystals were identified by Perkin-Elmer Fourier Transform Infrared (FTIR) spectrometer.The recorded FTIR spectra of pure and MO doped PPB crystals are shown in Fig. 3 and it gives the necessary information about the molecular arrangements of the crystals.The strong and broad band associated with the stretching vibrations of the hydroxyl group with strong hydrogen bonding appears at 3444 cm -1 [23].Also, the peak at 3378 cm -1 is due to the O-H stretching vibrations [24].The C-H stretching vibration is characterized at 3062 cm -1 [25].The C-H stretching mode vibrations due to MO appear at 2669 cm -1 [18].The peak at 1844 cm -1 is due to the stretching vibrations of CO band [26].NH2 in plane deformations are characterized at 1653 cm -1 and 1434 cm -1 [27].The peak at 1358 cm -1 is due to C-CH in-plane deformations [28].The B-O stretching vibrations of BO3 is confirmed by the peak at 920 cm -1 .Moreover the wagging of N-H vibrations are confirmed from the peaks at 782cm -1 , 765 cm -1 and 734 cm -1 [29].OBO terminal bending and ring bending vibrations are assured by the peaks at 590 cm -1 and 508 cm -1 .The spectral band assignments of MO doped PPB crystals are shown in Table 2.

Mechanical Stability
Mechanical stability of the crystals was tested under Vickers Microhardness Tester (Shimadzu HMV-2).Both PPB and MOPPB crystals were subjected to three loads 25, 50 and 100 g and the corresponding hardness value were plotted.Also, the direct variation of stress and strain relating to the stiffness constant can be calculated from the Wooster's relation [30][31][32].The Vicker's Hardness number can be calculated using the formula   = 1.8544  2 / 2 .Measure of Hardness value could easily say the measure of resistance to the permanent deformation or damage.This mechanical stability of the grown bulk material depends on structure, lattice energy, binding energy, interatomic spacing and Debye temperature.Also, it is closely related to the elastic constants, yield strength and stiffness constant [22].From Fig. 4, it can be observed that the hardness number gradually increases with the applied load.Both pure PPB and MOPPB show the increase of hardness number with applied load thereby proving the Reverse Indentation Size Effect (RISE) [30].On doping PPB with MO there was a clear improvement in the hardness number.This is because, MO dye being a large molecule gets filled up in the voids of the PPB crystal structure resulting in a compact lattice.The deforming capacity of both the crystals was observed to decrease on increasing loads above 100g which resulted in crack formation.Heavier load dislocates the lattice and cracks are developed on the crystal surface.The Meyer's index for Pure PPB is 1.6 and that of MOPPB is 2. Hence, both the crystals belong to the soft nature and can be utilized for laser safety devices.

Linear optical study
The absorption and the transmission spectra of PPB and MOPPB were studied between wavelengths 200-1200 nm using Perkin-Elmer UV-visible spectrometer.The optical bandgap of both pure PPB and MOPPB crystals can be calculated by the following formula   = From the figure, the lower cut-off wavelength of pure PPB was found at 190 nm [22] due to π-π* transitions whereas MOPPB crystals have two peaks, one at 278.60 nm due to π-π* transitions and the second one at 464.65 nm due to n-π* transitions [33].This confirms the addition of host molecule in the parent material.It is known that the MO dye contains one azo group and two phenyl rings.This azo group acts as a bridge between two phenyl rings and the OH-BO groups of the borate molecule [20].This modifies the optical properties of the PPB crystals.Hence the inclusion of MO dye molecules introduces the additional energy levels between the valence band and conduction bands [20][21].Because of this the bandgap of MOPPB crystals is decreased to 5.7 eV from 5.91 eV which is for pure PPB.Hence, alteration in band structure along with change in visible absorption is observed in MOPPB crystals.

SHG Study
Kurtz-Perry Technique was equipped to study the second harmonic generation employing the Q-Switched Nd:YAG laser of wavelength 1064 nm.In the SHG Study, Potassium Dihydrogen Phosphate (KDP) was taken as a reference whose SHG signal output was 70 mV.The recorded output of pure and MO doped PPB crystals were found to be about 32 mV and 28 mV respectively.Thus, Pure PPB is 0.45 times and MO doped PPB is 0.40 times that of SHG output of KDP.The decrease in output of MO doped from that of pure PPB might be due to the self-absorption of the input 532 nm wavelength by the doped material itself [24].This can be well correlated readily by the absorption peak at 532 nm wavelength from the Fig. 5. Hence we could clearly observe that doping of MO in PPB shows some serious changes in the SHG efficiency.

Z-Scan and Optical Limiting Study
The nonlinear absorption (NLA) and nonlinear refraction (NLR) along with third order NLO susceptibility of pure PPB and MO doped PPB were determined from open aperture (OA) and closed aperture (CA) Z-scan experiment respectively.The continuous wave laser beam with 785 nm wavelength having peak power of 50 mW was focused and passed through the grown material.
The sample was moved on the sample stage from the source to the detector along the direction of beam propagation.The transmittance of the beam was recorded at far-field for various positions of the sample with and without aperture infront of the detector.Then the graph was drawn between the transmittance and position of the sample.In general, nonlinear absorption arises due to various phenomenon like saturable absorption (SA), reverse saturable absorption (RSA) that originates from two photon absorption (2PA), multiphoton absorption (MPA), excited state absorption (ESA) or free carrier absorption (FCA) [36].Here, valley-like OA curve (Fig. 7) of pure and MO doped PPB exposes the existence of reverse saturable absorption (RSA).To validate the observed nonlinearity and identify the mechanism responsible for nonlinear absorption, the experimental data were fitted with the theoretical equations given in Sheik-bahae formalism for an open aperture, , where,   is the nonlinear absorption coefficient, n is order of the nonlinear absorption (n = 1, 2, 3…,),  is the effective thickness of the sample,  0 is the incident intensity of the laser beam at the focal point and ) is the Rayleigh length [37].The theoretical plots (solid line) matched very well with the experimental fits (solid circle) for two photon absorption equation.Thus both pure and MO doped PPB crystals exhibit two-photon absorption.Here as the mode of excitation is CW, observed nonlinearity is mostly thermo-optic in origin.And the presence of near-resonant energy states close to excitation wavelength have created sequential 2PA involving thermally induced excited state absorption.
And, the closed aperture (CA) technique reveals about the negative or positive lens resulting in self-defocusing or self-focusing behaviour of the sample.Here, the prepared sample shows the self-focusing behaviour i.e. pre-focal valley followed by post-focal peak indicating the nonlinearity of the sample to be a positive lensing effect due to thermal effect.Theoretical validation of experimental data was made by fitting the theoretical normalized transmittance using the relation for closed aperture, where, ∆∅ is the phase distortion of the transmitted laser beam due to refraction.Estimated nonlinear absorption coefficient (), nonlinear refractive index (n2), third-order NLO susceptibility of pure PPB and MO doped PPB are summarized in Table 4. 2PA coefficient, nonlinear refractive index and thirdorder NLO susceptibility of MOPPB is higher than PPB which assured the superiority of azo dye incorporation in PPB crystals.
By optical limiting measurement, the critical power of the laser beam at which the nonlinearity starts to affect the transmission can be measured.Optical Limiting (OL) curves illustrated in Fig. 8 were extracted from the corresponding open aperture (OA) Z-scan data.The position-dependent fluence was calculated from the equation F(z) = (4√ln 2    where F(z) is the fluence, Ein is the incident laser energy, ₒ is the beam waist of the laser beam at the focus.The observed reverse saturable absorption and self-focusing could be attributed to the optical limiting behaviour of the pure PPB and MOPPB crystals.The onset optical limiting thershold of PPB and MOPPB is 0.843x10 -6 J/m 2 and 0.832x10 -6 J/m 2 respectively.Thus MOPPB stands superior than pure PPB for optical limiting applications which includes the safeguard of optical components from laser damages.    ) 5.9x10 -10 m 2 /V 2 13.2x10 -10 m 2 /V 2 Imaginary part NLO susceptibility, Im( (3) ) 3.4x10 -9 m 2 /V 2 5.9x10 -9 m 2 /V 2 Susceptibility( (3) ) 3.5x10 -9 m 2 /V 2 6.1x10 -9 m 2 /V 2 OL Threshold 0.843x10 -6 J/m 2 0.832x10 -6 J/m 2

Conclusions
intra molecular hydrogen bonding between the Methyl Orange dye and the OH groups in the host molecule PPB could be easily identified by the UV-Vis absorption spectroscopic studies.The recorded transmission spectra of pure PPB and MOPPB crystals are shown in Fig. 5.