KTP Crystal Research Articles

Formation of submicron stripe domain ensembles during polarization reversal in Rb doped KTP crystal covered by dielectric layer

The formation of self-organized quasi-regular domain structure in Rb:KTP crystals covered by photoresist layer was studied. The instability of domain wall shape resulting in formation of domain streamers on the wall was revealed. The streamers structure appeared on Z+ polar surface and grew in polar direction. The minimal obtained structure period was 1 µm and the structure length reached 50 μm. The width of the stripe domains between the neighboring streamers went down to 100 nm and the resulting aspect ratio was 500. The obtained effect is very promising for the submicron periodical poling in the crystals of KTP family.

Effect of Atomic Oxygen and Electron Irradiation on the Electronic Structure of KTiOPO4 Crystal

AbstractKTiOPO4 (KTP) crystals are prepared by the top‐seeded solution growth method. Effects of space irradiation on KTP crystal structure have been investigated using Space Tribology in‐situ Analysis System: KTP crystal (001) surfaces were irradiated by atomic oxygen (AO) and electron (EL), and the electronic structure was analyzed using X‐ray photoelectron spectroscopy (XPS). The AO has strong oxidizing property which results in decreases in the intensity of C 1s peak. The content of P increases after AO/EL irradiation, while the content of Ti decreases after AO irradiation. The reduced ΔBE (O─P) shows stronger ionicity of P─O bonds after AO/EL irradiation compared with pristine KTP crystal. The deconvolution of O 1s photoemission line reveals that peaks at 530.8–531.0 and 530.1 eV are assigned to O─P and O─Ti bonds, respectively. Additionally, variations in the trend of O─P bond and O─Ti bond contents are resembled to that of P and Ti after AO/EL irradiation.

µJ-level multi-cycle terahertz generation in a periodically poled Rb:KTP crystal.

We demonstrate multi-cycle terahertz (MC-THz) generation in a 15.5 mm long periodically poled rubidium (Rb)-doped potassium titanyl phosphate (Rb:PPKTP) crystal with a poling period of 300 µm. By cryogenically cooling the crystal to 77 K, up to 0.72 µJ terahertz energy is obtained at a frequency of 0.5 THz with a 3 GHz bandwidth. A maximum internal optical-to-terahertz conversion efficiency of 0.16% is achieved, which is comparable with results achieved using periodically poled lithium niobate crystal. Neither photorefractive effects nor damage was observed with up to 900mJ/cm2, showing the great potential of Rb:PPKTP for multi-millijoule-level MC-THz generation.

Terahertz Optical Properties of KTiOPO4 Crystal in the Temperature Range of (−192)–150 °C

This paper presents the results of an experimental study of the optical properties of highly resistive monocrystals of potassium titanyl phosphate (KTiOPO4, KTP) in the frequency range of 0.2–1 THz and the temperature range of (−192)–150 °C. The dispersion of the refractive indices is approximated in the form of Sellmeier equations. The results show that the temperature dependence of the Sellmeier coefficients for all three principal optical axes is close to linear and, most likely, does not experience an extremum in the vicinity of the activation temperatures of the cationic conductivity of the KTP crystal at (−73)–(−23) °C. Weak frequency dependence of an optical axis direction angle VZ in the range of 0.2–1 THz is confirmed. However, the change in VZ with temperature is three times higher than reported before.

High-Power High-Repetition-Rate Tunable Yellow Light Generation by an Intracavity-Frequency-Doubled Singly Resonant Optical Parametric Oscillator

An efficient intracavity-frequency-doubled singly resonant optical parametric oscillator (SR-OPO), pumped by a high-maturity 532 nm laser, tunable in the yellow wavelength region is demonstrated. An elaborate V-shaped idler cavity design is employed for a potassium titanyl phosphate (KTP) OPO nonlinear crystal to achieve a 5.06 W idler output at 1151.2 nm at a 10 kHz pulse repetition frequency. Using a second KTP crystal to intracavity frequency double the circulating idler wave, a 2.37 W yellow output at 575.6 nm is generated. High optical conversion efficiency (25.4% and 11.2%), 0.5 nm spectral linewidth, and 32 ns pulse duration of the 575.6 nm yellow output are achieved.

Sub-Wavelength THz Imaging of the Domains in Periodically Poled Crystals Through Optical Rectification

By focusing a femtosecond laser beam onto the lateral face of a periodically poled KTP crystal, we generate a THz pulse through optical rectification of the laser pulse. The THz signal is recorded by means of a common THz time-domain technique, which allows us to obtain both the magnitude and sign of the THz waveform, and then the magnitude and phase of its spectral components thanks to a numerical Fourier-transform. By moving the laser beam along the crystal, we record a THz signal that renders for the alternative orientation of the crystal domains, with a lateral resolution as good as 10 μm, whatever the THz wavelength. This demonstrates the potential of the Optical Rectification TeraHertz Imaging (ORTI) technique to produce sub-wavelength THz images.

Digital laser for on-demand intracavity selective excitation of second harmonic higher-order modes.

In this article, we demonstrate selective excitation of second harmonic higher-order modes inside a diode end-pumped solid-state laser resonator that comprises of a nonlinear potassium titanyl phosphate (KTP) crystal and a digitally addressed holographic end-mirror in a form of a reflective phase-only spatial light modulator (SLM). The emitted second harmonic higher-order modes at 532 nm are generated by an intracavity nonlinear KTP crystal that is pumped by high-order fundamental modes operating at 1064 nm. The fundamental modes are digitally controlled by displaying a computer-generated hologram in the form of a grey-scale image to the SLM screen for on-demand high-order modes. The phase matching of the fundamental mode to the generated frequency-doubled mode is achieved by controlling the phase of the digital hologram to either achieve a high or quasi-degree of orbital angular momentum conservation. We show that we can intracavity generate frequency-doubled high-order Laguerre-Gaussian modes and Hermit-Gaussian modes that are either quasi or fully reproducible in the far-field. To the best of our knowledge, this is the first laser to generate frequency-doubled on-demand higher-order modes inside the cavity at the visible (green) wavelength of 532 nm.

13.7-W 588-nm Yellow Laser Generation by Frequency Doubling of 885-nm Side-Pumped Nd: YAG-YVO4 Intracavity Raman Laser

An actively Q-switched all-solid-state YVO4 Raman laser, which used a 885-nm side-pumped Nd:YAG laser as the fundamental wavelength and delivered yellow output, is demonstrated. An in-band pump scheme is adopted to mitigate the thermal effects in the laser crystal and a Z-shaped laser cavity is used to compensate thermal lensing further. By intracavity frequency doubling of the Stokes wave with KTP crystal, 13.7-W yellow output at a wavelength of 588 nm, with a pulse repletion frequency of 10 kHz, was obtained under a diode pump power of 233 W. Additionally, single-pulse energy was 1.37 mJ with the pulse-width of 12.5 ns, corresponding to a peak power of 110 kW.

Remote Analysis of Methane Concentration in the Atmosphere with an IR Lidar System in the 3300–3430 nm Spectral Range

A differential absorption lidar (DIAL) system based on optical parametric oscillators (OPO) with nonlinear KTA and KTP crystals is designed. The crystals allow laser radiation tuning in the IR wavelength region. A series of experiments on remote monitoring of methane along a horizontal surface sounding path in the 3300–3430 nm spectral range was carried out. Based on the experimental results, the CH4 concentrations are retrieved along a surface 800-m path in the spectral range under study with a spatial resolution of 100 m.

Increasing the conversion efficiency of KTP crystal using the pump-power technique

In this study, the effect of pumping power on the conversion efficiency of nonlinear crystal (KTP) was investigated using laser pump-power technique. The results showed that the higher the pumping power values, the greater the conversion efficiency (η) and, as the crystal thickness increases within limitations, the energy conversion efficiency increases at delay time of (0.333 ns) and at room temperature. Efficiency of 80% at length of KTP crystal (L = 1.75 X 10-3 m) and Pin = 28MW, and also, compare the experimental results with numerical results by using MATLAB program.

72.3 W 1064 nm Nd:YAG Single Crystal Fiber Laser and Intracavity Double Frequency Generation

As a bridge between crystal and fiber, the single crystal fiber (SCF) can combine the advantages of the two materials and show great potential in high-power lasers. In this paper, we select a Nd:YAG SCF as the gain medium in a diode-end-pumped linear cavity. The Nd:YAG SCF with the dimensions of ∅ = 1 mm × 50 mm was manufactured using the micro-pulling-down technique. At an incident pump power of 153.0 W, the maximum output power of the 1064 nm laser was 72.3 W. To the best of our knowledge, this is the highest power obtained in Nd:YAG SCF lasers. In addition, using a second-type phase-matched KTP crystal, the 10 W continuous wave green laser was obtained with an intracavity double-frequency structure.

Studying the Effect of Laser Pump Pulse Energy and Delay Time on Conversion Efficiency of KTP

In this study, the effect of increasing pump pulse energy and delay time on the energy conversion efficiency of the Potassium Titanyl Phosphate (KTP) crystal at room temperature was investigated. It was found that the higher the pump pulse the greater the efficiency at a certain value of the delay time. Moreover, at the delay time 3.524ns, we found that the efficiency of the conversion of energy increases from 0.0112 to 0.0159. We also observed that the lower delay time between the pump and the probe pulses leads to increase the rate of energy conversion efficiency of the KTP crystal, where the reaches up to 3, which is higher than the value recorded in the absence of a pump pulse. The highest value of the rate of energy conversion efficiency recorded in this study was 0.0159. This work was achieved at room temperature.

Cavity-enhanced sum-frequency generation of blue light with near-unity conversion efficiency.

We report on double-resonant highly efficient sum-frequency generation in the blue range. The system consists of a 10-mm-long periodically poled KTP crystal placed in a double-resonant bow-tie cavity and pumped by a fiber laser at 1064.5 nm and a Ti:sapphire laser at 849.2 nm. An optical power of 375 mW at 472.4 nm in a TEM00 mode was generated with pump powers of 250 mW at 849.2 nm and 200 mW at 1064.5 nm coupled into the double-resonant ring resonator with 88% mode-matching. The resulting internal conversion efficiency of 95(±3)% of the photons mode-matched to the cavity constitutes, to the best of our knowledge, the highest overall achieved quantum conversion efficiency using continuous-wave pumping. Very high conversion efficiency is rendered possible due to very low intracavity loss on the level of 0.3% and high nonlinear conversion coefficient up to 0.045(0.015) W-1. Power stability measurements performed over one hour show a stability of 0.8%. The generated blue light can be tuned within 5 nm around the center wavelength of 472.4 nm, limited by the phase-matching of our nonlinear crystal. This can however be expanded to cover the entire blue spectrum (420 nm to 510 nm) by proper choice of nonlinear crystals and pump lasers. Our experimental results agree very well with analytical and numerical simulations taking into account cavity impedance matching and depletion of the pump fields.

Tunable dual-color terahertz wave parametric oscillator based on KTP crystal.

A tunable dual-color KTP terahertz (THz) wave parametric oscillator (TPO) pumped by a dual-wavelength laser was proposed in this Letter. Theoretical analysis denotes that the emission of a tunable dual-color THz wave can be achieved by the simultaneous stimulated polariton scattering processes from multiple A1-symmetry phonon modes of the KTP crystal. The tunable dual-color THz wave emitted from KTP TPO was demonstrated in our experiment, where the THz frequencies simultaneously tuned from 3.15THz to 11.63THz and from 1.47THz to 6.03THz with some gaps. The maximum dual-color THz output energy of 1.31μJ was obtained under the THz frequencies of 5.94THz and 4.42THz. Moreover, at a certain phase-matching angle, the THz output energies for the two frequencies were independent, which means that the dual-color THz wave emission with any energy ratio can be achieved by adjusting the pump energy ratio between a dual-wavelength laser.

Wave-Function Engineering for Spectrally Uncorrelated Biphotons in the Telecommunication Band Based on a Machine-Learning Framework

Indistinguishable single photons are key ingredient for a plethora of quantum information processing applications ranging from quantum communications to photonic quantum computing. A mainstream platform to produce indistinguishable single photons over a wide spectral range is based on biphoton generation through spontaneous parametric down-conversion (SPDC) in nonlinear crystals. The purity of the SPDC biphotons, however, is limited by their spectral correlations. Here, we present a design recipe, based on a machine-learning framework, for the engineering of biphoton joint spectrum amplitudes over a wide spectral range. By customizing the poling profile of the KTiOPO$_4$ (KTP) crystal, we show, numerically, that spectral purities of 99.22%, 99.99%, and 99.82% can be achieved, respectively, in the 1310-nm, 1550-nm, and 1600-nm bands after applying a moderate 8-nm filter. The machine-learning framework thus enables the generation of near-indistinguishable single photons over the entire telecommunication band without resorting to KTP crystal's group-velocity-matching wavelength window near 1582 nm.

Surface behavior in potassium titanyl phosphate crystal irradiated by swift Kr8+ ions

Potassium titanyl phosphate (KTiOPO4, KTP) possesses superior properties in several optical applications. A KTP crystal is irradiated by swift Kr8 + ions with high energies and low fluences. The stopping and range of ions in matter code is used to simulate the energy deposition process of swift Kr8 + ions in the irradiated crystal. The electronic and nuclear stopping power profiles of swift Kr8 + ions irradiated on the KTP crystal are simulated, and the micro-Raman spectrum of the irradiated layer at different depths is measured by a confocal micro-Raman system. The results show that the high electronic energy loss can cause lattice damage along the ion trajectory, whereas the nuclear energy loss causes damage at the end of the ion track. The optical transmission properties of the irradiated KTP crystal are researched at visible and near-infrared bands.

High conversion efficiency and extra-cavity sum frequency 355nm UV laser output

Currently, the common method to obtain ultraviolet laser is an all-solid-state laser combined with nonlinear frequency conversion technology, which generates three, four or five harmonics outside or inside the resonator cavity through the frequency conversion of near-infrared light waves around 1.0μm. Ultraviolet light sources are mainly obtained in two bands of 355mm and 266nm. Ultraviolet laser is characterized by high photon energy, short wavelength, cold treatment and good focusing performance, be widely used in the fields of medicine and biology, industrial microprocessing, atomic/analytical spectroscopy and chemical dynamics, environmental detection and atmospheric detection. In this article, we put forward with two sum frequency crystals to obtain UV laser, based on the method of theoretical analysis and experiment validation, Nd:YAG crystal was used as a gain medium to generate 1064 nm laser, double frequency generated 532 nm green laser by KTP crystal, 1064 nm and 532 nm throughed two sum frequency crystals and frequency conversion outside the cavity. The High conversion efficiency and high peak power 355nm ultraviolet light output is realized.

Generation of ∼100 kW narrow-line far-infrared radiation from a KTP off-axis THz parametric oscillator

In the far-infrared spectrum between 20 and 60 μm, the free-electron laser (FEL) is the only wavelength-tunable coherent radiation source capable of generating kilowatt to megawatt peak powers with a linewidth of the order of 1%. Here, we report the detection of >70 kW radiation power at about 52 μm in a 45 kW radiation power for the far-infrared radiation. With 63% coupling efficiency of the silicon prism atop the KTP crystal, the measured >70 and >45 kW far-infrared radiation correspond to >111 and >71 kW powers extracted from the KTP crystal of the seeded off-axis THz parametric oscillator. The radiation source accomplished in this work has great potential to become a tabletop and economical alternative for the bulky and expensive far-infrared FELs in national facilities.

Development of Near/Mid IR differential absorption OPO lidar system for sensing of atmospheric gases

The differential absorption lidar system based on optical parametric oscillators with nonlinear KTA and KTP crystals which allow laser radiation tuning both in the near and in the middle IR spectral region is described. Lidar echo signals have been experimentally recorded in the 1.8–2.5 and 3–4 μm wavelength ranges. The results of H2O and CO2 profile measurements along the horizontal sounding path are presented.

High temperature molten flux growth, structural and optical characteristics of KTiOPO4:Ho and KTiOPO4:Er single crystals

KTiOPO4:Ho and KTiOPO4:Er single crystals are grown by a top-seeded solution growth method from potassium phosphate and MoO3 flux. The crystalline phase, packing structure and chemical composition of the crystals are analyzed by X-ray diffraction (XRD), energy dispersive spectrum (EDS). The doping of Ho and Er induced strain in the crystal is determined from Williamson–Hall relation. The bonding between titanium and oxygen of Ho and Er doping in the KTP crystal is changing the pseudo-symmetrical deviation in the lattice. The bonding of O and Ti decreases with doping Ho and Er and Ti3+ content in the crystal. The bonding, distortion of the TiO6-TiO6 chain and change in inter-atomic distances by erbium and holmium doping are assessed by Raman spectroscopy and low temperature ESR spectroscopy which evidences the low concentration of Ti3+ presence in the crystal. The KTiOPO4:Ho and KTiOPO4:Er single crystal shows second harmonic generation properties which shows that the efficiency ratio of ηKTiOPO4:Ho/η KTiOPO4 = 0.63 and ηKTiOPO4:Er/η KTiOPO4 = 0.80.