Magnesium-doped Lithium Niobate Research Articles

Highly Efficient Ultraviolet Harmonic Generation at a Nonlinear Crystal Interface

High conversion efficiency is vital for the acquisition of UV light through nonlinear processes. Here, we investigate an alternative method to realize highly efficient second-harmonic generation (SHG) based on the interface of $\ensuremath{\beta}$ barium metaborate ($\ensuremath{\beta}\mathrm{BBO}$) and magnesium-doped lithium niobate ($\mathrm{MgO}$:LN). Theoretical simulations indicate that the SHG efficiency of our method is significantly higher than other phase-matching conditions. In the experiment, we demonstrate that higher conversion efficiency can be realized compared with only employing a single BBO crystal. Besides, the shortest output wavelength of SHG can reach 205 nm in our method. The method proposed here can open another way for the integrated ultraviolet laser and may promote the development of UV micromachining in the future.

Luminescent defect centers in magnesium-doped lithium niobate crystals

The photoluminescence of LiNbO3:Mg(0.19÷5.29 mol.% MgO) crystals obtained by various technologies has been studied in the near IR region. It showed that a contribution to the photoluminescence of the crystals was made by two main bands at 1.53 and 1.35 eV being associated with intrinsic defect centers in the crystals. The intensity of these bands, especially the band at 1.53 eV, in LiNbO3:Mg(0.19÷5.29 mol.% MgO) crystals correlated well with the concentration of hydroxyl groups in ones. Therefore, there was a complex luminescence center including OH- groups in the crystals.

Thermally assisted growth of bulk domains created by femtosecond laser in magnesium doped lithium niobate

The domain growth to the polar surfaces from the tracks created by focused NIR femtosecond pulses in the bulk of MgO-doped lithium niobate plate has been studied. The tracks have been imaged by optical and Cherenkov-type second harmonic generation microscopy. The domain appearance at the irradiated surface has been attributed to domain nucleation and growth under the action of the pyroelectric field arising during cooling after heating by pulse irradiation. The pyroelectric field produced during the sample cooling has stimulated the domain growth to both polar surfaces. The obtained knowledge can be used for development of the domain engineering.

Correlation between Infrared Absorption and Lithium Sublattice Disorder in Magnesium-Doped Lithium Niobate

Lithium niobate is a ferro- and piezoelectric material with excellent optical properties and a wide variety of applications. The defect structures of congruent and Mg-doped crystals are still under intense discussion. In this work, undoped lithium niobate and magnesium-doped lithium niobate grown from congruent melt with the addition of 0 to 9 mol% MgO were investigated by infrared absorption, establishing the dependence of the absorbance on the Mg-doping level in two bands related to OH- stretching vibrations. The absorption band at 3485 cm-1 peaks at a MgO concentration in melt of 1 mol% and vanishes for MgO concentrations above the threshold level for optical damage suppression (4.8 mol%). A corresponding peak occurs in the minimum yield of the 7Li(p,α)4He reaction during ion channeling measurements, indicating a maximum of disorder in the Li sublattice. A possible explanation for this correlation is the attribution of this absorption band to ilmenite stacking fault sequences instead of isolated NbLi antisites in undoped and low-doped material. On the other hand, the OH- absorption band at 3535 cm-1 stays weak up to the MgO concentration threshold, and then increases, hinting to a defect related to the increase of vacancies due to the lack of charge compensation.

Synchronously Intracavity-Pumped Picosecond Optical Parametric Oscillators for Sensors.

The research and development of laser systems for intracavity phase interferometry is described. These systems are based on an intracavity synchronously pumped optical parametric oscillator (OPO), enabling the generation of two trains of picosecond pulses inside a single cavity. In such a configuration, it is possible to measure the beat note frequency between two pulses and to very precisely determine the phase difference between them. The pump source is a diode-pumped passively mode-locked Nd:YVO laser. A periodically poled magnesium-doped lithium niobate crystal is used as the optical parametric oscillator crystal coupling the pump and the signal cavities. We designed a synchronously pumped OPO in a linear and ring cavity configuration allowing generation in a dual-pulse regime. By a mutual detuning of both cavity lengths, the quasi-synchronous regime of pumping was achieved and high harmonics of repetition rate frequencies were generated. Such a system can be useful for applications such as pump-probe spectroscopy or for testing telecommunication systems. We also realized the subharmonic OPO cavity as a source of two independent trains of picosecond pulses suitable for intracavity phase interferometry; we also measured the beat note signal.

Study of optical rotation quasi-phase-matching technique based on total-internal-reflection for generating highly efficient second harmonic in an electro-optic chiral material using the concept of highly multimodal nonlinear guided wave approach

The propagation and interaction of the optical radiations are analyzed using the theory of a highly multimodal nonlinear guided wave methodology instead of plane wave optics for explaining the optical rotation quasi-phase-matching technique based on total-internal-reflection. Thereby, the yielding second harmonic from a yttrium oxide-coated rectangular slab of magnesium oxide-doped lithium niobate crystal shows very high efficiency. The conversion efficiency of 2.7% has been observed by performing the computer-aided simulation and corresponds to a second harmonic wavelength of 532 nm after considering the surface roughness and absorption.

Analysis of highly efficient DFG in an electro-optic material exhibiting polarization rotation using TIR-based ORQPM technique

This paper is a numeric-analytical work for highly efficient difference-frequency generation (DFG) by the integrated effect of total-internal-reflection-based optical rotation quasi-phase-matching (ORQPM) and nonresonant quasi-phase-matching techniques in a thin yttrium oxide-coated rectangular slab of magnesium oxide-doped lithium niobate crystal. The conversion efficiency of 37.2% has been obtained by ray-optics analysis, corresponding to an idler wavelength of 1570[Formula: see text]nm. Moreover, the guided-wave approach has also been analyzed for more accurate and realistic outcomes, yielding a peak conversion efficiency of 1.64%. The impact of the influencing factors like surface roughness, absorption, and nonlinear law of reflection has also been incorporated in the computer-aided simulation for providing a pragmatic understanding of the whole study.

Widely tunable single-photon source with high spectral-purity from telecom wavelength to mid-infrared wavelength based on MgO:PPLN* *Project supported by the National Basic Research Program of China (Grant Nos. 2017YFA0303700 and 2019YFA0308700) and the National Natural Science Foundation of China (Grant Nos. 11627810 and 11690031).

By utilizing the extended phase-matching (EPM) method, we investigate the generation of single photons with high spectral-purity in a magnesium-doped periodically-poled lithium niobate (MgO:PPLN) crystal via the spontaneous parametric down-conversion (SPDC) process. By adjusting the temperature and pump wavelength, the wavelength of the single photons can be tuned from telecom to mid-infrared (MIR) wavelengths, for which the spectral-purity can be above 0.95 with high transmission filters. In experiments, we engineer a MgO:PPLN with poling period of 20.35 μm which emits the EPM photon pair centered at 1496.6 nm and 1644.0 nm and carry out the joint spectral intensity (JSI) and Glauber’s second-order self-correlation measurements to characterize the spectral purity. The results are in good agreement with the numerical simulations. Our work may provide a valuable approach for the generation of spectrally pure single photons at a wide range of wavelengths which is competent for various photonic quantum technologies.

Highly efficient second-harmonic generation in an electro-optic chiral material using total-internal-reflection-based optical rotation quasi-phase-matching technique

The present work introduces the concept of a total-internal-reflection (TIR)-based optical rotation, quasi-phase-matching (QPM) technique for generating highly efficient second-harmonic optical output in a rectangular slab of magnesium oxide-doped lithium niobate crystal coated with a thin layer of yttrium oxide. Combinational effects of TIR optical rotation QPM and fractional QPM techniques are experienced at certain bounce points inside the slab, thereby enhancing the conversion efficiency. In this analysis, the thin-film is used for controlling the phase-shifts accompanying the propagation of p- and s-polarized light at the slab-film interface during TIR. A conversion efficiency of 32%, corresponding to a second-harmonic wavelength of 532 nm has been observed using computer-aided simulation. Optical losses such as surface roughness, absorption, and interference effect due to the nonlinear law of reflection have also been incorporated.

Enhanced Ultraviolet Damage Resistance in Magnesium Doped Lithium Niobate Crystals through Zirconium Co-Doping.

MgO-doped LiNbO3 (LN:Mg) is famous for its high resistance to optical damage, but this phenomenon only occurs in visible and infrared regions, and its photorefraction is not decreased but enhanced in ultraviolet region. Here we investigated a series of ZrO2 co-doped LN:Mg (LN:Mg,Zr) regarding their ultraviolet photorefractive properties. The optical damage resistance experiment indicated that the resistance against ultraviolet damage of LN:Mg was significantly enhanced with increased ZrO2 doping concentration. Moreover, first-principles calculations manifested that the enhancement of ultraviolet damage resistance for LN:Mg,Zr was mainly determined by both the increased band gap and the reduced ultraviolet photorefractive center O2−/−. So, LN:Mg,Zr crystals would become an excellent candidate for ultraviolet nonlinear optical material.

Small-Polaron Hopping and Low-Temperature (45–225 K) Photo-Induced Transient Absorption in Magnesium-Doped Lithium Niobate

A strongly temperature-dependent photo-induced transient absorption is measured in 6.5 mol% magnesium-doped lithium niobate at temperatures ranging from 45 K to 225 K. This phenomenon is interpreted as resulting from the generation and subsequent recombination of oppositely charged small polarons. Initial two-photon absorptions generate separated oppositely charged small polarons. The existence of these small polarons is monitored by the presence of their characteristic absorption. The strongly temperature-dependent decay of this absorption occurs as series of thermally assisted hops of small polarons that facilitate their merger and ultimate recombination. Our measurements span the high-temperature regime, where small-polaron jump rates are Arrhenius and strongly dependent on temperature, and the intermediate-temperature regime, where small-polaron jump rates are non-Arrhenius and weakly dependent on temperature. Distinctively, this model provides a good representation of our data with reasonable values of its two parameters: Arrhenius small-polaron hopping’s activation energy and the material’s characteristic phonon frequency.

Auger electron spectroscopy for surface ferroelectric domain differentiation in selectively poled MgO:LiNbO3

Auger electron spectroscopy (AES) as a method to characterize the ferroelectric polarization domains in magnesium-doped lithium niobate crystals is demonstrated. Preliminary measurements on a test sample show a clearly identifiable relative shift in the energy of the Auger oxygen KLL transition peak between poled (inverted) and un-poled domains. Auger electrons detected from the negative polarization domains (-Z) have a higher energy than those from the positive domains indicating a lower ionization energy at the -Z domain surface. The degree of electron energy separation between the −Z and +Z domains was found to be dependent on proximity to the domain boundary and was potentially diminished by the accumulated charge under the incident primary beam. Polarization domain resolution is demonstrated on both the micron and millimeter scale, suggesting potential applicability of this technique to surface investigation and domain structure characterization of nonlinear optical devices such as periodically poled lithium niobate.

Submicrometer periodic poling of lithium niobate thin films with bipolar preconditioning pulses

Periodically poled second-order nonlinear materials with submicrometer periods are important for the development of quasi-phase matched backward-wave nonlinear optical processes. Interactions involving counter-propagating waves exhibit many unique properties and enable devices such as backward second harmonic generators, mirrorless optical parametric oscillators, and narrow-band quantum entangled photon sources. Fabrication of dense ferroelectric domain gratings in lithium niobate remains challenging, however, due to lateral domain spreading and merging. Here, we report submicrometer periodic poling of ion-sliced x-cut magnesium oxide doped lithium niobate thin films. Electric-field poling is performed using multiple bipolar preconditioning pulses that improve the poling yield and domain uniformity. The internal field is found to decrease with each preconditioning poling cycle. The poled domains are characterized by piezoresponse force microscopy. A fundamental period of 747 nm is achieved.

High-contrast imaging of 180° ferroelectric domains by optical microscopy using ferroelectric liquid crystals

Ferroelectric liquid crystals (FLCs) couple the direction of their spontaneous electric polarization to the direction of tilt of their optic axis. Consequently, reversal of the electric polarization by an electric field gives rise to an immediate and lasting optical response when an appropriately aligned FLC is observed between crossed polarizers, with one field direction yielding a dark image and the opposite direction yielding a bright image. Here, this peculiar electro-optic response is used to image, with high optical contrast, 180° ferroelectric domains in a crystalline substrate of magnesium-doped lithium niobate. The lithium niobate substrate contains a few domains with upward electric polarization surrounded by regions with downward electric polarization. In contrast to a reference non-chiral liquid crystal that is unable to show ferroelectric behavior due to its high symmetry, the FLC, which is used as a thin film confined between the lithium niobate substrate and an inert aligning substrate, reveals ferroelectric domains as well as their boundaries, with strong black and white contrast. The results show that FLCs can be used for non-destructive readout of domains in underlying ferroelectrics, with potential applications in, e.g., photonic devices and non-volatile ferroelectric memories.

Two-Color Quantum Correlation between Down-Conversion Beams at 1.5 and 3.3 μm from a Singly Resonant Optical Parametric Oscillator

We demonstrated a two-color quantum correlation between the down-conversion beams with a telecommunication wavelength at 1.5 μm and mid-infrared wavelength at 3.3 μm generated by a singly resonant optical parametric oscillator (SRO) operated above the pump threshold with a magnesium-oxide doped periodically-poled lithium niobate crystal in the cavity. A maximum of 1.8 dB noise reduction of the intensity difference of the twin beams was measured at the analysis frequency of 5 MHz. Based on a theoretical model for the quantum correlation between the twin beams given by a semi-classical approach, the influence of the analysis frequency and pump parameter on the quantum correlation between the twin beams was discussed theoretically and experimentally. The quantum correlation between the twin beams was degraded at the analysis frequencies above 5 MHz due to the limitation of the bandwidth of SRO cavity and was degraded at the analysis frequencies below 5 MHz due to the excess intensity noise of the pump. The two-color quantum correlated twin beams at 1.5 and 3.3 μm have potential applications in high-precision measurements beyond the shot noise level.

High-power mid-infrared picosecond pulse bunch generation through difference frequency generation

We report a high average power mid-infrared picosecond (ps) pulse bunch output by means of direct difference frequency generation (DFG) in periodically poled magnesium-doped lithium niobate between a linearly polarized ps pulse bunch Yb fiber laser and a synchronized Er fiber laser. The ps pulse bunch Yb fiber laser was composed of an all polarization maintained “figure of eight” structured mode-locked Yb fiber laser as the seed, a pulse multiplier, and two stages of Yb fiber amplifiers. The mode-locked Yb fiber laser has an output ps laser pulse at 1030 nm with a repetition rate of 16.32 MHz. The pulses were then transformed to the pulse bunches through the pulse multiplier. Within each bunch, there were 16 equally spaced pulses with pulse widths of 8.5 ps and time intervals around 300 ps. The Er fiber laser had a gain switched seed laser diode working at 1550 nm with a pulse width around 5.1 ns, which was synchronized to the mode-locked Yb fiber laser pulse bunch, and two stages of Er fiber amplifiers. Under the average power of 27.8 W of Yb fiber laser, we obtained an average output power of 3.1 W at 3.07 µm. This is, to the best of our knowledge, the highest average power mid-infrared ps pulse bunch obtained via DFG directly.

Reducing leakage current during periodic poling of ion-sliced x-cut MgO doped lithium niobate thin films

Ion-sliced lithium niobate thin films offer high optical confinement over their bulk counter parts, enabling compact and efficient nonlinear optics. Quasi-phase matching by periodic poling of thin film lithium niobate remains challenging, however, due to large leakage currents, especially in magnesium oxide doped lithium niobate, which is used in bulk to reduce photorefractive damage. Here we present fabrication and poling details of 700 nm thick x-cut magnesium oxide doped lithium niobate thin films using electric field poling. We introduce a silicon dioxide insulation layer under co-planar electrodes to reduce leakage current. Rounded tips are utilized to encourage nucleation at the center of the electrodes. Uniform domains with 7.5 µm period and 50% duty cycle are achieved. The poling characteristics are compared to bulk lithium niobate, with and without the silicon dioxide insulation layer. The domains are characterized by piezoresponse force microscopy, hydrofluoric etching, and waveguide second harmonic generation at 1550 nm wavelength in a thin film lithium niobate waveguide loaded with a silicon nitride strip.

Transient transmission oscillations in doped and undoped lithium niobate induced by near-infrared femtosecond pulses

Abstract

Picosecond near-to-mid-infrared absorption of pulse-injected small polarons in magnesium doped lithium niobate

Femtosecond-pulse-induced (Epump = 2.5 eV) picosecond infrared absorption is studied in the spectral region between 0.30 eV and 1.05 eV in LiNbO3:Mg. We find a non-instantaneous mid-infrared absorption peak in the time domain up to 1 ps and a broad-band, long-lived absorption (maximum at 0.85 eV, width ≈ 0.5 eV), for t > 1 ps. The modelling succeeds by considering small NbNb4+ electron polaron formation along the sequence: (i) two-photon injection of hot electron-hole pairs at Nb-O-octahedra, (ii) dissociation and electron cooling by electron-phonon-scattering, and (iii) electron self-localization by strong electron-phonon-coupling.

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.