MgO-doped Lithium Niobate Research Articles

Local domain inversion in MgO-doped lithium niobate by pyroelectric field-assisted femtosecond laser lithography

We explore a physical approach to invert ferroelectric domains in the volume of MgO-doped lithium niobate crystals without any external electric field. Permanent defect structures are created by focused infrared femtosecond laser pulses below the material surface along the polar axis followed by a thermal treatment. This procedure leads to an inversion of ferroelectric domains beneath and above the laser-induced filaments up to the surfaces of the crystal. All domain walls are straight and up to 800 μm long. We measure the domain width in dependence on the length of the filaments and the writing energy. The smallest achieved domain width and the domain spacing is 1 μm. We propose a model taking into account the temperature dependence of the pyroelectric field and thermally activated bulk charges to explain the mechanism of domain inversion. Our findings pave the way to all-optical printing of arbitrary ferroelectric domain structures for nonlinear photonic applications.

Tunable high-average-power optical parametric oscillators near 2 μm

We report on the development of high-average-power nanosecond and picosecond laser sources tunable near 2 μm based on optical parametric oscillators (OPOs) pumped by solid-state Nd:YAG and Yb-fiber lasers at 1.064 μm. By exploiting 50-mm-long MgO-doped lithium niobate (MgO:PPLN) as the nonlinear crystal and operating the OPO in a near-degenerate doubly resonant configuration with intracavity wavelength selection elements, we have generated tunable high-average-power radiation across 1880–2451 nm in high spectral and spatial beam quality with excellent output stability. In nanosecond operation, pumping with a Q-switched Nd:YAG laser and using an intracavity prism for spectral control, we have generated more than 2 W of average power in pulses of 10 ns duration at 80 kHz repetition rate with narrow linewidth (<3 nm), with M2<2.8, and a passive power stability better than 2.2% rms over 1 h. In picosecond operation, pumping with a mode-locked Yb-fiber laser and using a diffraction grating as the wavelength selection element, we have generated more than 5 W of average power in pulses of 20 ps at 80 MHz repetition rate with narrow bandwidth (∼2.5 nm), with M2<1.8 and a passive power stability better than 1.3% rms over 2 h. The demonstrated sources represent viable alternatives to Tm3+/Ho3+-doped solid-state and fiber lasers for the generation of high-power radiation in the ∼2 μm spectral range.

Acoustically tunable photonic band gap generation in MgO doped Lithium Niobate micro-dimension plate

Refractive index contrast generation in MgO doped LiNbO3 plate via Lamb wave perturbation for induction of acoustically tunable photonic band gap is theoretically studied. Fundamental symmetric Lamb mode is excited in micro dimension thin plate structure to produce symmetric disturbance for periodic contraction and rarefaction along the length of plate. Effect of contraction and rarefaction on production of refractive index contrast is studied using photo–elastic effect. The generated quasi static low–high optical index region in plate modifies electromagnetic wave propagation and provides photonic band gaps, which are investigated using Bloch's theorem and transfer matrix method. The induced photonic band gap shows great dependency on optical contrast and unit cell dimension created by acoustic frequencies. These acoustically tunable photonic band gaps have rewarding applications in acousto optical devices, laser and sophisticated sensing.

The Formation of Self-Organized Domain Structures at Non-Polar Cuts of Lithium Niobate as a Result of Local Switching by an SPM Tip.

We have studied experimentally the interaction of isolated needle-like domains created in an array via local switching using a biased scanning probe microscope (SPM) tip and visualized via piezoelectric force microscopy (PFM) at the non-polar cuts of MgO-doped lithium niobate (MgOLN) crystals. It has been found that the domain interaction leads to the intermittent quasiperiodic and chaotic behavior of the domain length in the array in a manner similar to that of polar cuts, but with greater spacing between the points of bias application and voltage amplitudes. It has also been found that the polarization reversal at the non-polar cuts and domain interaction significantly depend on humidity. The spatial distribution of the surface potential measured by Kelvin probe force microscopy in the vicinity of the charged domain walls revealed the decrease of the domain length as a result of the partial backswitching after pulse termination. The phase diagram of switching behavior as a function of tip voltage and spacing between the points of bias application has been plotted. The obtained results provide new insight into the problem of the domain interaction during forward growth and can provide a basis for useful application in nanodomain engineering and development of non-linear optical frequency converters, data storage, and computing devices.

Extended Sellmeier equation for the extraordinary refractive index of 5% MgO-doped congruent LiNbO3 at high temperature

MgO-doped congruent Lithium Niobate (MgO:CLN) is widely used in a variety of nonlinear optical processes, while its Sellmeier equation has only been derived from experiments under 200oC. Here we extend the temperature range for the Sellmeier equation study to as wide as 60 oC to 510 oC. This work is important to extend the operation temperature range of MgO:CLN crystal.

Optical amplifier based on an Er:MgO-doped near stoichiometric lithium niobate waveguide

We report on an optical signal amplifier based on an Er:MgO-doped near stoichiometric lithium niobate (Er:MgO:SLN) waveguide. The Er:MgO:SLN waveguide was fabricated using swift carbon ion irradiation combined with precision diamond blade dicing. Under 980 nm laser pumping, the waveguide provides a 2.13 dB/cm gain at 1536 nm, 1.49 dB/cm gain at 1552 nm, and 1.37 dB/cm gain at 1565 nm, with the pumping power of 99.5 mW. This work demonstrates the potential application of swift ion irradiated Er:MgO:SLN waveguides for the optical amplifiers in the C communication band.

Entanglement generation and entanglement concentration of two-photon four-dimensional spatial modes partially entangled Dicke state

Two-photon four-dimensional spatial modes partially entangled Dicke state can be compactly generated from six concurrent spontaneous parametric down-conversion processes by cascading poling domain structures in 5% MgO-doped poled lithium niobate bulk crystal. Entanglement concentration of the two-photon four-dimensional spatial modes partially entangled Dicke state can be realized by using quantum nondestructive detection of nonlinear Kerr medium, optical beam splitter, and quantum gate operation.

Influence of the artificial surface dielectric layer on domain patterning by ion beam in MgO-doped lithium niobate single crystals

We experimentally compared the shapes and sizes of isolated domains created by the focused ion beam irradiation in the lithium niobate crystals doped by MgO with polar surface covered by dielectric (resist) layer and those with free surface. We attributed the larger sizes of isolated domains in the samples covered by the resist layer to ion localization in the resist. We revealed a change in the domain shape as a function of increasing dose and explained it in terms of kinetic approach taking into account the modification of the surface layers by ion irradiation. We applied the obtained knowledge for 1D and 2D periodical poling using ion beam. Finally, we created the 2D square 1-μm-period array of isolated domains with radius about 300 nm and maximal depth of all domains in the array up to 100 μm as well as the 1D pattern of through stripe domains with 2-μm-period in 1-mm-thick wafer.

Periodical poling of LiNbO3:MgO by electron beam

ABSTRACTWe have studied the features of periodical poling by focused electron beam irradiation of MgO-doped lithium niobate (MgOLN) single crystalline plate covered by the resist layer. We have distinguished the main types of the domain structure depending on the irradiated dose. We have revealed that total domain area and width of the domain stripes depend linearly on the dose and irradiated stripe width. We have observed the highly regular domain structures with duty-cycle close to desirable value of 0.5 on both polar surfaces.

Watt-level mid-infrared radiation via self-seeded difference-frequency generation from a pre-chirp managed femtosecond Yb-fiber amplifier.

We obtained over 1 W average power at ∼3550 nm wavelength via self-seeded difference-frequency generation (DFG) through a 5 cm long periodically poled MgO-doped lithium niobate crystal. The pump and signal sources are derived from the identical pre-chirp managed femtosecond Yb-fiber amplifier with sub-100-fs pulse duration and 84 MHz repetition rate for simple synchronization. This result is believed to be among the highest-average-power, femtosecond mid-infrared radiation obtained via DFG.

Experimental investigations of 3 mm aperture PPLN structures

We are reporting about investigation of domestic 3 mm aperture periodically polled lithium niobate (PPLN) structures for cascaded mid-IR OPO. Wide aperture periodically poled MgO-doped lithium niobate (LiNbO3) structures at multigrating, fan-out and multi fan-out configuration were prepared at “Labfer LTD”. Laser source based on such structures can be used for special applications. Four different PPLN structures were investigated and effective aperture for effective pumping was defined.

Tunable array beam splitter with different domain-etching depth based on MgO-doped lithium niobate crystal

The Talbot effect is a self-imaging phenomenon of near-field diffraction. When a plane wave is incident on a periodic diffraction grating, the image of the grating is repeated at regular distances away from the grating plane. A Talbot array illuminator is a device that splits singular light beam into an array of beams with periodical optical intensity based on Talbot effect. LiNbO3 (LN) crystal is a kind of practicable material for a Talbot array illuminator due to its perfect optical characteristics. MgO-doped LiNbO3 (MgLN) crystal shows shorter absorption edge wavelength and higher resistance to photorefractive damage than LN. Up to now, the usefulness and simplicity of Talbot effect have still aroused the interest of many scholars.In the conventional method, a Talbot array illuminator is fabricated by using high external electric field to modulate the phase difference. However, essentially, high external electric field restricts the Talbot array illuminator to applications in optical integration and optical micro structure devices. Now we are looking forward to a new way which avoids using high external electric field.In this paper, we systematically study the two-dimensional (2D) hexagonal tunable array beam splitter, which is fabricated by domain-etching in MgLN crystal, and its fractional Talbot effect. The self-imaging phenomenon caused by Talbot effect in the Fresnel field for this phase array coherently illuminated is theoretically analyzed according to Fresnel diffraction theory. We numerically simulate the light intensity distributions of Talbot diffraction image under different values of Talbot coefficient and different values of domain-etching depth. The simulation results show that can change the array period and the structure distribution of the fractional Talbot diffraction image, and the domain-etching depth can modulate the light intensity distribution of diffraction image. Based on the numerical simulation results, the 2D hexagonal array beam splitters are fabricated with different values of domain-etching depth. The fractional Talbot diffraction images of array splitters are obtained at different values of through the optical experiments. The results show that domain-etching depth can effectively modulate the intensity distribution of diffraction image, becoming a tunable array beam splitter successfully. The experimental results agree well with the simulation results. The theoretical and experimental results show that the optimal self-image visibility can be obtained at a Talbot coefficient of 0.5 and a domain-etching depth of 0.39 m, while the duty cycle is 52%. Moreover, a good self-image pattern is also observed under thinner domain-etching depth, which is beneficial to optical integration and micro optical devices.

Quadratic nonlinear optical parameters of 7% MgO-doped LiNbO3 crystal

Pure and 7% MgO-doped lithium niobate (LiNbO3) single crystals were grown by the Czochralski technique. The shift of optical absorption edge in 7% MgO-doped crystal in direction of shorter wavelength compared to undoped crystal was observed. The second harmonic generation measurements of 7% MgO-doped LiNbO3 crystal were performed at room temperature by means of the rotational Maker fringe technique using Nd:YAG laser generating at 1064nm in picoseconds regime. Experimentally obtained value of nonlinear optical coefficient d33 for 7% MgO-doped LiNbO3 was found to be less than for undoped crystal but higher than for 5% MgO-doped. I-type phase-matched second harmonic generation was achieved and the value of phase-matched angle was calculated. High quadratic nonlinearity together with tolerance to intensive laser irradiation makes 7% MgO-doped LiNbO3 crystal interesting for application in optoelectronics.

Broadband high-power mid-IR femtosecond pulse generation from an ytterbium-doped fiber laser pumped optical parametric amplifier.

We report on a high-power periodically poled MgO-doped lithium niobate (MgO:PPLN)-based femtosecond optical parametric amplifier (OPA), featuring a spectral seamless broadband mid-infrared (MIR) output. By modifying the initial chirp and spectrum of the mode-locked seed laser, the Yb fiber pump laser exhibits a final output power of 14 W with sub-200-fs pulse duration after power amplification and compression. When the OPA was seeded with a broadband amplified spontaneous emission (ASE) source, a damage-limited 0.6 W broadband MIR radiation was experimentally obtained under the pump power of 10.15 W at 82 MHz repetition rate, corresponding to an overall OPA conversion efficiency of 32.7%. The 3 dB bandwidth of the mid-IR idler was 291.9 nm, centering at 3.34 μm.

Quasi-phase-matched frequency conversion in ridge waveguides fabricated by ion implantation and diamond dicing of MgO:LiNbO(3) crystals.

We report on the fabrication and second harmonic generation from a periodically-poled MgO-doped lithium niobate ridge waveguide within the telecommunication L-band. The ridge waveguide is fabricated by carbon ion implantation and the following diamond blade dicing method. A normalized second harmonic conversion efficiency of 20.3%W-1cm-2 was obtained with a total insertion loss of 4.3dB at wavelength of 1612.7nm. Our analysis shows that at least ~70% of the second-order nonlinearity was preserved after the implantation and thermal annealing process.

Generation of high spectral purity photon-pairs with MgO-doped periodically poled lithium niobate

We study the spectral correlation of photon pairs generated via type-II spontaneous parametric down conversion in periodically poled lithium niobate crystals. By performing Schmidt decomposition on the two-photon wavefunction, we calculate the spectral purity of the two-photon state under various pump laser characteristics and doping concentrations of MgO in lithium niobate crystals. Our results show that periodically poled 5% MgO doped lithium niobate is a good candidate to generate photon-pairs with high spectral purity at telecom wavelength.

Temperature dependent refractive index and absorption coefficient of congruent lithium niobate crystals in the terahertz range.

Optical rectification with tilted pulse fronts in lithium niobate crystals is one of the most promising methods to generate terahertz (THz) radiation. In order to achieve higher optical-to-THz energy efficiency, it is necessary to cryogenically cool the crystal not only to decrease the linear phonon absorption for the generated THz wave but also to lengthen the effective interaction length between infrared pump pulses and THz waves. However, the refractive index of lithium niobate crystal at lower temperature is not the same as that at room temperature, resulting in the necessity to re-optimize or even re-build the tilted pulse front setup. Here, we performed a temperature dependent measurement of refractive index and absorption coefficient on a 6.0 mol% MgO-doped congruent lithium niobate wafer by using a THz time-domain spectrometer (THz-TDS). When the crystal temperature was decreased from 300 K to 50 K, the refractive index of the crystal in the extraordinary polarization decreased from 5.05 to 4.88 at 0.4 THz, resulting in ~1° change for the tilt angle inside the lithium niobate crystal. The angle of incidence on the grating for the tilted pulse front setup at 1030 nm with demagnification factor of -0.5 needs to be changed by 3°. The absorption coefficient decreased by 60% at 0.4 THz. These results are crucial for designing an optimum tilted pulse front setup based on lithium niobate crystals.

Mid-infrared dual frequency comb spectroscopy based on fiber lasers for the detection of methane in ambient air

We utilize mid-infrared dual frequency comb spectroscopy for the detection of methane in ambient air. Two mid-infrared frequency comb sources based on femtosecond Er:fiber oscillators are produced through difference frequency generation with periodically poled MgO-doped lithium niobate crystals and stabilized at slightly different repetition rates at about 250 MHz. We performed dual frequency comb spectroscopy in the spectral range between 2900 cm−1 and 3150 cm−1 with 0.07 cm−1 resolution using a multipass cell of ~580 m path length, and achieved the sensitivity about 7.6 × 10−7 cm−1 with 80 ms data acquisition time. We determined the methane concentration as ~1.5 ppmv in the ambient air of the laboratory, and the detection limit as ~60 ppbv for the current setup.

Domain patterning by electron beam of MgO doped lithium niobate covered by resist

Periodical domain structuring by focused electron beam irradiation of MgO-doped lithium niobate (MgOCLN) single crystalline plate covered by resist layer was studied both experimentally and by computer simulation. The dependences of domain size on the charge dose and distance between isolated domains were measured. It has been shown that the quality of periodical domain pattern depends on the thickness of resist layer and electron energy. The experimentally obtained periodic domain structures have been divided into four types. The irradiation parameters for the most uniform patterning were obtained experimentally. It was shown by computer simulation that the space charge slightly touching the crystal surface produced the maximum value of electric field at the resist/LN interface thus resulting in the best pattern quality. The obtained knowledge allowed us to optimize the poling process and to make the periodical domain patterns in 1-mm-thick wafers with an area up to 1 × 5 mm2 and a period of 6.89 μm for green light second harmonic generation. Spatial distribution of the efficiency of light frequency conversion confirmed the high homogeneity of the tailored domain patterns.

Domain wall kinetics of lithium niobate single crystals near the hexagonal corner

A mesospheric approach based on a simple microscopic 2D Ising model in a hexagonal lattice plane is proposed to explain macroscopic “asymmetric in-out domain wall motion” observation in the (0001) plane of MgO-doped stoichiometric lithium niobate. Under application of an electric field that was higher than the conventional coercive field (Ec) to the ferroelectric crystal, a natural hexagonal domain was obtained with walls that were parallel to the Y-axis of the crystal. When a fraction of the coercive field of around 0.1Ec is applied in the reverse direction, this hexagonal domain is shrunk (moved inward) from the corner site into a shape with a corner angle of around 150° and 15° wall slopes to the Y-axis. A flipped electric field of 0.15Ec is then applied to recover the natural hexagonal shape, and the 150° corner shape changes into a flat wall with 30° slope (moved outward). The differences in corner domain shapes between inward and outward domain motion were analyzed theoretically in terms of corner and wall site energies, which are described using the domain corner angle and wall slope with respect to the crystal Y-axis, respectively. In the inward domain wall motion case, the energy levels of the evolving 150° domain corner and 15° slope walls are most competitive, and could co-exist. In the outward case, the energy levels of corners with angles &amp;gt;180° are highly stable when compared with the possible domain walls; only a flat wall with 30° slope to the Y-axis is possible during outward motion.