Magnesium-doped Lithium Niobate Research Articles

Optical Image Self-Convolution Based on Second-Harmonic Generation With a Single Spatial Light Modulator

By combining optical Fourier transform with second-harmonic generation (SHG) from infrared to visible region in a periodically poled Magnesium-doped Lithium Niobate (PPLN) crystal, we present a simple yet efficient experiment to revisit the optical implementation of image self-convolution. In contrast to previous scheme, here the main feature is the holographic gratings we prepare by a phase-only spatial light modulator that enables a deeper understanding on the centrosymmetry for intensity images, and particularly, the conjugate symmetry for both intensity and phase images. We manifest both cases by observing the real-time autocorrelation peaks that appear gradually in the output SHG signals. As our optical scheme possesses the inherent capacity of parallel processing, it will find potential in pattern recognition, particularly, for infrared remote sensing.

THz polariton laser using an intracavity Mg:LiNbO3 crystal with protective Teflon coating.

An enhancement in the performance of a THz polariton laser based on an intracavity magnesium-doped lithium niobate crystal (Mg:LiNbO3) in surface-emitted (SE) configuration is demonstrated resulting from the deposition of a protective Teflon coating on the total internal reflection surface of the crystal. In this cavity geometry the resonating fields undergo total internal reflection (TIR) inside the lithium niobate, and laser damage to that surface can be a limiting factor in performance. The protective layer prevents laser damage to the crystal surface, enabling higher pump power, yielding higher THz output power and wider frequency tuning range. With the unprotected crystal, narrow-band THz output tunable from 1.50 to 2.81 THz was produced, with maximum average output power of 20.1 µW at 1.76 THz for 4 W diode pump power (limited by laser damage to the crystal). With the Teflon coating, no laser damage to the crystal was observed, and the system produced narrow-band THz output tunable from 1.46 to 3.84 THz, with maximum average output power of 56.8 µW at 1.76 THz for 6.5 W diode pump power. This is the highest average output power and the highest diode-to-terahertz conversion efficiency ever reported for an intracavity terahertz polariton laser.

Integration of polarization-multiplexing and phase-shifting in nanometric two dimensional self-mixing measurement.

Integration of phase manipulation and polarization multiplexing was introduced to self-mixing interferometry (SMI) for high-sensitive measurement. Light polarizations were used to increase measuring path number and predict manifold merits for potential applications. Laser source was studied as a microwave-photonic resonator optically-injected by double reflected lights on a two-feedback-factor analytical model. Independent external paths exploited magnesium-oxide doped lithium niobate crystals at perpendicular polarizations to transfer interferometric phases into amplitudes of harmonics. Theoretical resolutions reached angstrom level. By integrating two techniques, this SMI outperformed the conventional single-path SMIs by simultaneous dual-targets measurement on single laser tube with high sensitivity and low speckle noise. In experimental demonstration, by nonlinear filtering method, a custom-made phase-resolved algorithm real-time figured out instantaneous two-dimensional displacements with nanometer resolution. Experimental comparisons to lock-in technique and a commercial Ploytec-5000 laser Doppler velocity meter validated this two-path SMI in micron range without optical cross-talk. Moreover, accuracy subjected to slewing rates of crystals could be flexibly adjusted.

3.8-μm Mid-Infrared Laser Quasi-Synchronously Pumped by a MOPA Structured Picosecond Yb Fiber Amplifier With Multi-pulse Operation

A picosecond pulsed mid-infrared laser with multi-pulse operation is experimentally demonstrated. This mid-infrared laser was composed of a fiber laser pumped optical parametric oscillator (OPO). The pump source of this OPO was a master oscillator power amplification-structured picosecond fiber amplifier with pulse bunch output, which was further used to quasi-synchronously pump a periodically poled magnesium-oxide doped lithium niobate crystal based OPO to obtain 3.8-μm wavelength output with picosecond multi-pulse operation. The pump-idler conversion efficiency, the signal spectrum, and the signal pulse bunch are presented, respectively.

Low energy electron imaging of domains and domain walls in magnesium-doped lithium niobate.

The understanding of domain structures, specifically domain walls, currently attracts a significant attention in the field of (multi)-ferroic materials. In this article, we analyze contrast formation in full field electron microscopy applied to domains and domain walls in the uniaxial ferroelectric lithium niobate, which presents a large 3.8 eV band gap and for which conductive domain walls have been reported. We show that the transition from Mirror Electron Microscopy (MEM – electrons reflected) to Low Energy Electron Microscopy (LEEM – electrons backscattered) gives rise to a robust contrast between domains with upwards (Pup) and downwards (Pdown) polarization, and provides a measure of the difference in surface potential between the domains. We demonstrate that out-of-focus conditions of imaging produce contrast inversion, due to image distortion induced by charged surfaces, and also carry information on the polarization direction in the domains. Finally, we show that the intensity profile at domain walls provides experimental evidence for a local stray, lateral electric field.

Longitudinally polarized single-cycle terahertz pulses generated with high electric field strengths

We demonstrate the generation of single-cycle longitudinally polarized terahertz pulses with field amplitudes in excess of 11 kV/cm using the interferometric recombination of two linearly polarized terahertz beams. High field strength transversely polarized pulses were generated by optical rectification in a matched pair of magnesium-oxide doped stoichiometric lithium niobate (MgO:SLN) crystals with a reversal in the χ333(2) orientation. The discontinuity in χ333(2) produces a polarity flip in the transverse field; the longitudinal field produced as a consequence of the transverse field discontinuity was measured in the far-field. Both the spatial and temporal profiles of the measured longitudinally polarized terahertz radiation were consistent with the propagation of the transverse discontinuity.

Optimization of the idler wavelength tunable cascaded optical parametric oscillator based on chirp-assisted aperiodically poled lithium niobate crystal**Project supported by the National Natural Science Foundation of China (Grant No. 61505236), the Innovation Program of Shanghai Institute of Technical Physics, China (Grant No. CX-2), and the Program of Shanghai Subject Chief Scientist, China (Grant No. 14XD1404000).

We present the numerical results for the optimization of the pump-to-idler conversion efficiencies of nanosecond idler wavelength tunable cascaded optical parametric oscillators (OPO) in different wavelength tuning ranges, where the primary signals from the OPO process are recycled to enhance the pump-to-idler conversion efficiencies via the simultaneous difference frequency generation (DFG) process by monolithic aperiodically poled, magnesium oxide doped lithium niobate (APMgLN) crystals. The APMgLN crystals are designed with different chirp parameters for the DFG process to broaden their thermal acceptance bandwidths to different extents. The idler wavelength tuning of the cascaded OPO is realized by changing the temperature of the designed APMgLN crystal and the cascaded oscillation is achieved in a single pump pass singly resonant linear cavity. The pump-to-idler conversion efficiencies with respect to the pump pulse duration and ratio of OPO coefficient to DFG coefficient are calculated by numerically solving the coupled wave equations. The optimal working conditions of the tunable cascaded OPOs pumped by pulses with energies of 350 μJ and 700 μJ are compared to obtain the general rules of optimization. It is concluded that the optimization becomes the interplay between the ratio of OPO coefficient to DFG coefficient and the pump pulse duration when the idler wavelength tuning range and the pump pulse energy are fixed. Besides, higher pump pulse energy is beneficial for reaching higher optimal pump-to-idler conversion efficiency as long as the APMgLN crystal is optimized according to this pump condition. To the best of our knowledge, this is the first numerical analysis of idler wavelength tunable cascaded OPOs based on chirp-assisted APMgLN crystals.

Modeling skin cooling using optical windows and cryogens during laser induced hyperthermia in a multilayer vascularized tissue

This article deals with the spatial and the temporal evolution of tissue temperature during skin surface cooled laser induced hyperthermia. Three different skin surface cooling methodologies viz., optical window contact cooling, cryogenic spray cooling and cryogen cooled optical window contact cooling are considered. Sapphire, yttrium aluminum garnet, lithium tantalate, and magnesium oxide doped lithium niobate are the considered optical windows. The cryogens considered are liquid CO2 and R1234yf. Heat transfer in the multilayer skin tissue embedded with thermally significant blood vessels pairs is modeled using the Pennes and Weinbaum–Jiji bioheat equations. Weinbaum–Jiji bioheat equation is used for the vascularized tissue. Laser transport in the tissue is modeled using the radiative transfer equation. Axial and radial (skin surface) temperature distributions for different combinations of optical windows and cryogens are analyzed. Liquid CO2 cooled yttrium aluminum garnet is found to be the best surface cooling mechanism.

Field induced modification of defect complexes in magnesium-doped lithium niobate

Dielectric constant, thermally stimulated depolarization currents (TSDC), and conductivity of undoped and 5% Mg-doped LiNbO3 single crystals between −100 °C and 200 °C have been investigated. A Debye-like dielectric relaxation with an activation energy of 135 meV is observed in the Mg-doped material, but not in undoped crystals. On heating this relaxation disappears near 140 °C and does not reappear after cooling. Anomalies observed in TSDC around this temperature are attributed to the motion of lithium vacancies, in agreement with conductivity measurements. It is proposed that in thermal equilibrium the electrons from the MgLi• donors are trapped in (4MgLi•+4VLi′) defect complexes. High-temperature poling breaks these defect complexes. The transition of the liberated electrons between the MgLi• donor centers and the NbNb forming the conduction band gives rise to the observed dielectric relaxation.

Nanoscale and macroscopic electrical ac transport along conductive domain walls in lithium niobate single crystals

The electrical impedance properties of UV-illuminated (λ = 310 nm) charged, conductive domain walls (CDWs) in 5 mol% magnesium-doped lithium niobate (LNO) single crystals are investigated on the nm-length scale using nanoimpedance microscopy (NIM) as well as by comparing the macroscopically measured complex impedance response between multi- and single-domain LNO samples. Similar to the case of dc conductivity, a higher conductivity of domain walls (DWs) compared to the bulk insulating matrix was observed. The contrast between DWs and bulk is most pronounced at lower frequencies (f 200 Hz) due to the large bulk capacitance at higher frequencies. Moreover, the simultaneous application of both an ac and dc bias results in an increased real part of the ac DW current. Also, equivalent circuits accurately describing both the domain and CDW contributions were developed; as a result we are able to analyze and quantify the complex dielectric conductive behavior of both bulk and CDWs in LNO within the framework of the mixed conduction model. Hopping of excited charge carriers along the CDWs was identified as the dominant charge transport process.

Terahertz wave parametric amplifier

The importance of terahertz (THz) wave techniques has been demonstrated in various fields, and the range of applications is now expanding rapidly. However, the practical implementation of THz science to solve the real-world problems is restricted due to the lack not only of convenient high power THz wave emitters and sensitive detectors but also of efficient quasi-optical active devices such as amplifiers. In this work, we demonstrate the direct amplification of THz waves in room temperature using magnesium oxide-doped lithium niobate (MgO:LiNbO3) crystals as the nonlinear gain medium. The input THz wave is injected as a seed beam along with the pump beam into the nonlinear crystal and it is amplified by the optical parametric process. We report gain in excess of 30 dB with an input THz pulse energy of less than 1 pJ. We believe that this demonstration will contribute to the convenience and further applicability of THz frequency techniques.

Octave-spanning coherent mid-IR generation via adiabatic difference frequency conversion

We demonstrate efficient downconversion of a near-IR broadband optical parametric chirped pulse amplifier (OPCPA) pulse to a 1.1-octave-spanning mid-IR pulse (measured at -10 dB of peak) via a single nonlinearly and adiabatically chirped quasi-phase-matching grating in magnesium oxide doped lithium niobate. We report a spectrum spanning from 2 to 5 μm and obtained by near full photon number conversion of μJ-energy OPCPA pulses spanning 680-870 nm mixed with a narrowband 1047-nm pulse. The conversion process is shown to be robust for various input broadband OPA pulses and suitable for post-amplification conversion for many near-IR systems.

A fiber laser pumped dual-wavelength mid-infrared optical parametric oscillator based on aperiodically poled magnesium oxide doped lithium niobate

We report a fiber laser pumped dual-wavelength mid-infrared optical parameter oscillator (OPO) based on an aperiodic optical superlattice (AOS) structure. An aperiodically poled MgO-doped lithium niobate wafer was designed and fabricated as the nonlinear crystal for the OPO. Using a linearly polarized pulsed Yb fiber laser at 1.064 μm as the pump source, mid-infrared emissions at 3.3 and 3.8 μm were obtained simultaneously from the OPO. The total output power at the idler wavelengths of 3.3 and 3.8 μm reached 3.07 W under the maximum pump power of 27 W. The pump-to-idler conversion efficiency was 11.4% with a ratio of about 1.2:1 for 3.3 μm and 3.8 μm.

The effect of an optical pump on the absorption coefficient of magnesium-doped near-stoichiometric lithium niobate in terahertz range

The absorption coefficient of magnesium-doped near-stoichiometric lithium niobate crystal is measured by terahertz time-domain spectroscopy in a frequency range of 0.2 THz–0.9 THz at room temperature. The absorption coefficient is modulated by external optical pump fields. Experimental results show that the absorption coefficient of near-SLN:Mg crystal is approximately in a range of 22 cm−1–35 cm−1 in a frequency range of 0.2 THz–0.9 THz and tunable up to nearly 15%. Further theoretical analysis reveals that the variation of absorption coefficient is related to the number of light-induced carriers, domain reversal process, and OH− absorption in this crystal.

High Power Efficient 3.81 $\mu{\rm m}$ Emission From a Fiber Laser Pumped Aperiodically Poled Cascaded Lithium Niobate

We experimentally demonstrated a high power mid-infrared optical parametric oscillator that was pumped by a linearly polarized, pulsed Ytterbium-doped fiber laser. Cascaded parametric conversion was realized to obtain a higher conversion efficiency from the pump of 1.06 μm to the expected idler ~ 3.8 μm. A monolithic aperiodically poled magnesium-oxide doped lithium niobate was designed to compensate the phase mismatches of the nonlinear conversions from the pump to idler and the primary signal to idler simultaneously. The highest idler output power of 4.35 W at 3.81 μm was obtained under the pump power of 24.8 W at 1063.5 nm with pump-to-idler conversion efficiency of 17.5% and slope efficiency of 21.3%.

Diode laser frequency doubling in a ppMgO:LN ridge waveguide: influence of structural imperfection, optical absorption and heat generation

In this work, we investigate experimentally and theoretically limitations of highly efficient high-power single-pass second-harmonic generation process in a quasi-phase matched magnesium oxide-doped lithium niobate ridge waveguide due to structural imperfections, optical absorption and resulting heat generation. With the distributed Bragg reflector diode laser emitting at 1,061 nm, applied in the presented bench-top experiment, a coupling efficiency into the ridge waveguide nonlinear device of 86 % is realized. An internal normalized conversion efficiency of 375 % (Wcm $${}^2$$ ) $${}^{-1}$$ is determined. A maximum second-harmonic power of 386 mW is reached at a corresponding opto-optical and electro-optical conversion efficiency of 38 and 9.5 %, respectively. A saturation of the conversion efficiency at a high power level as well as its dependency on the nonlinear device structure inhomogeneity are confirmed experimentally. With a theoretical model incorporating geometrical inhomogeneity, linear and nonlinear absorption and resulting heat load the second-harmonic generation in a ridge waveguide can be simulated realistically. Increased values of nonlinear absorption coefficients, higher than until now reported, lead to a very good agreement between theoretical and experimental results. Moreover, according to the simulation, structural inhomogeneities can enhance the SHG conversion efficiency at high output powers.

Influence of the Ferroelectric Nature of Lithium Niobate to Drive Photocatalytic Dye Decolorization under Artificial Solar Light

Photocatalytic decolorization of acid black 1 (a.k.a. amido black 10B) and rhodamine b was investigated over powders of lithium niobate or lithium niobate doped with iron to form n-type material doped or p-type magnesium doped lithium niobate. In all cases, photostimulation was performed using simulated solar illumination. The rate of decolorization of the dye solutions was found to be fastest over p-type material than with the undoped powder, with the n-type proving least effective. The change in rate was attributed to changes in the majority carrier, associated with the dopant altering the ratio of reactive species (holes and electrons). We also show that the surface depolarization field associated with a ferroelectric material alters the surface chemistry by changing the Stern and inner Helmholtz plane due to the interaction of catalyst surface charge and the polar nature of solvated species. The spatial separation of REDOX reactions in ferroelectric powders positively influences the proportion of reac...

Pyroelectrically induced photorefractive damage in magnesium-doped lithium niobate crystals

Beam distortion due to photorefraction limits the usability of lithium niobate (LiNbO3) crystals for frequency conversion applications. To prevent beam distortion in LiNbO3, 5 mol.% magnesium-doped LiNbO3 (MgO:LN) is usually used. However, we show that strong beam distortion of green laser light can occur within seconds in MgO:LN, starting at light intensity levels in the 100 mW/cm2 regime, if the crystal is heated by several degrees Celsius during or before illumination. Beam distortion does not occur in undoped congruent LiNbO3 (CLN) under the same conditions. We show that the pyroelectric effect together with an elevated photoconductivity compared to that of CLN causes this beam distortion and that this effect also influences frequency conversion experiments in the infrared even if no external heating is applied.

Electrical conductivity and asymmetric material changes upon irradiation of Mg-doped lithium niobate crystals with low-mass, high-energy ions

Radiation damage in magnesium-doped lithium niobate crystals, created by low-mass, high-energy ions which have transmitted the entire crystal thickness, leads to an enhanced electrical dark conductivity as well as an enhanced photoconductivity. Experimental results on the electrical properties after ion exposure are given, and an asymmetric dependence of the conductivity as well as refractive index changes on the irradiation geometry with respect to the ferroelectric axis is revealed.

Efficient parametric conversion from 106 to 38 μm by an aperiodically poled cascaded lithium niobate

We experimentally demonstrated an efficient optical parametric oscillator (OPO) with high parametric conversion from 1.0645 to 3.8 μm. An aperiodically poled magnesium oxide doped lithium niobate wafer was designed and fabricated as the nonlinear crystal of the OPO. A linearly polarized acousto-optic Q-switched Nd:YVO4 laser was used as the pump source. High pump-to-idler conversion efficiency of 18.5% was achieved with a slope efficiency of up to 21.5%. When compared with a periodically poled channel fabricated on the same wafer, under the condition of output coupler optimized for the periodically poled lithium niobate based OPO, an improvement of slope efficiency by 28.3% from 15.2% to 19.5% and total efficiency by 12.5% from 13.6% to 15.3% under the highest pump power of 11 W was realized for the pump-to-idler conversion.