LiCAF Crystal Research Articles

Development of broadband ultraviolet pulsed laser using Ce:LiCAF crystal to determine SO\(_2\) gas concentration by differential absorption spectroscopy

Earth’s atmosphere is a mixture of many gases which include pollutants such as SO2, NO2, NO, and O3 that have absorption in the ultraviolet (UV) wavelength region. Therefore, the development of broadband UV laser sources that are useful in a differential absorption spectroscopy (DOAS) system for environmental research is necessary. In this research, we present the DOAS system using the Ce:LiCAF laser for determining SO2 gas concentration in the atmosphere. The Ce:LiCAF laser has a full width at half maximum of 2 nm with a wavelength range from 286 to 291 nm and a peak wavelength of 288.5 nm. The results show that the DOAS system accurately determines the gas concentration with a measurement error of 6 %. This result can serve as the basis for developing practical DOAS systems with the ability to monitor a wide range of gasses and survey many other types of pollutants.

Challenges in power scaling of Cr:LiCAF lasers: effect of passive losses.

In this work, we have investigated the continuous-wave (cw) lasing potential of thin slab-shaped Cr:LiCAF crystals with a low chromium doping level of around 1% and various lengths of 1 to 2cm. These relatively long crystals with low Cr-doping facilitate the distribution of heat load in a larger volume and could enable power scaling of Cr:LiCAF lasers. However, long crystals tend to have larger passive losses, and it is also more challenging to achieve efficient mode-matching to the low-brightness pump mode in a longer gain element, which could hinder laser performance. To explore the issue, we have performed detailed cw lasing experiments in single- and multimode diode-pumped Cr:LiCAF laser systems employing crystals with different doping and length. Our results showed that current state-of-the-art crystal growth methods provide Cr:LiCAF crystals with low enough passive losses to enable cw laser efficiencies of up to 50%, even in these long samples. The pump powers available in this study (5.35W) limited the cw powers we could achieve experimentally to 2.25W level; however, our simulations indicate that thin slab-type Cr:LiCAF crystals with low Cr-doping have the potential to achieve cw powers above 10W level.

Development of a tunable, narrow, and ultraviolet Ce:LiCAF crystal multipass laser amplifier for environmental application

In this paper, the authors present results on the development of both an oscillator and a 4-pass amplifier of ultraviolet and narrow linewidth pulses using a Ce:LiCAF crystal. By using a grating at one end of the laser cavity, the output laser wavelength of the laser can be tuned from 285 nm to 296 nm with a maximum output power of 8 mW at 288.5 nm. Moreover, amplification in a 4-pass amplifier of the Ce:LiCAF laser was investigated theoretically and experimentally. From the results, 49.0 mW amplified pulses can be obtained using a 4-pass amplifier with 7.0 mW power and 3.6 ns pulse duration at 288.5 nm injection, corresponding to an amplifier gain of 7. Furthermore, the angular scattering intensity of some common single-particle aerosols such as black carbon, brown carbon, and polluted water has been studied using the Ce:LiCAF laser. This result can serve as the basis for the multipass laser amplifier’s environmental application, particularly in identifying atmospheric particles.

Theoretical and experimental study of ultraviolet broadband laser amplification using Ce:LiCAF crystal

Trivalent cerium ion-doped lithium calcium aluminum fluoride (Ce:LiCAF) crystals have been demonstrated as efficient amplification gain media in the ultraviolet (UV) wavelength region. However, spectral characteristics in the multipass amplification process have not yet been studied theoretically and experimentally. This work investigates the spectral effects, including gain narrowing and spectral shifting using the chromatic Frantz–Nodvik model to simulate the amplification of broadband nanosecond UV pulses from a Ce:LiCAF crystal. From the simulation results, 5.5 mJ amplified pulses can be obtained using a 4-pass amplifier with seed pulses having 0.7 mJ pulse energy, 3 ns pulse duration and 288.5 nm peak wavelength. These values are in good agreement with experimental results. Seed pulses with peak wavelength of 290.0 nm and linewidth of 2.0 nm could be shifted to 289.8 nm peak wavelength and narrowed to 1.9 nm linewidth after 4-pass amplification. Accounting for the spectral effects will enable a more accurate prediction of the amplified pulses’ characteristics, thereby aiding the applications of high-power UV lasers.

Effect of doping distribution on the lasing performance of a cerium-doped lithium calcium aluminum fluoride ultraviolet laser crystal

We report multi-photon luminescence and Laser Ablation–Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) as a non-destructive and sensitive method of visualizing the concentration and distribution of activator ions doped to a crystal host, respectively. Here, LA-ICPMS is able to detect concentrations that are less than 0.1%. Using these methods and a large 110-mm diameter Czochralski method-grown cerium-doped lithium calcium aluminum fluoride (Ce3+:LiCaAlF6 or Ce:LiCAF), we observed that Ce is distributed non-uniformly with varying concentrations across the diameter of the crystal boule. We further verify quantitatively that luminescence intensity increases in proportion to doping density. Ultraviolet (UV, 290-nm wavelength) laser emissions from areas of the Ce:LiCAF crystal having different doping distributions and concentrations reveal that a uniform doping distribution leads to a higher slope efficiency, lower laser threshold and a potentially higher damage threshold even though the doping concentration is low compared to areas with high but non-uniform doping. Our results show a means of improving laser performance by aiming to achieve a uniform doping distribution; and multi-photon luminescence and LA-ICPMS are powerful tools that could aid in this process. Improved laser performance could lead to new breakthroughs in laser output energies especially in the UV region for many applications.

Tunable narrow linewidth picosecond pulses from a single grating gain-switched Ce:LiCAF laser

We report a gain-switched laser oscillator configuration for generating tunable short and narrow spectral linewidth 290 nm laser pulses from a Ce:LiCAF crystal. Continuous tuning of the laser emission over a range of 18 nm from 281 nm to 299 nm is achieved using a single grating in a low-Q short-cavity oscillator. The shortest pulse duration achieved is 450 ps at 281 nm and the narrowest spectral linewidth is 0.17 nm at 287 nm. The average output energy ranged between 0.5 mJ and 0.8 mJ, resulting to conversion efficiencies of 8%–10%, depending on the output wavelength. The simplicity of our configuration makes aligning easier as well as reduces the loss in laser output energy, thereby leading to improved laser efficiency.

Size control and luminescence properties of Eu2+:LiCaAlF6 particles prepared by femtosecond pulsed laser ablation

We have fabricated Eu2+:LiCaAlF6 (Eu:LiCAF) nanoparticles via a dry pulsed laser ablation process using femtosecond laser pulses. The particle size was controlled by adjusting the laser fluence. The crystallographic orientation and chemical composition were preserved in all particles. The typical luminescence peak of Eu:LiCAF crystals at approximately 370 nm was observed in both the PL and X-ray induced luminescence (XL) spectra. The temporal PL profiles of all deposited particles exhibited fast-decay components in addition to the slow-decay components observed in the profile of the bulk crystal. Smaller particles had shorter decay times than larger particles. The fast-decay component of particles with a mean diameter of less than 0.36 μm (∼40 ns) was at least one order of magnitude shorter than the decay time of the bulk crystal (∼1700 ns). Even in the temporal XL profile, all deposited particles showed shorter decay times than the bulk decay time. We attribute these fast-decay components to defects on the particle surface. To confirm the defect level, we investigated the thermally stimulated luminescence of the deposited particles. In addition to an emission peak at approximately 540 K, which was also observed in the spectrum of the bulk crystal, an emission peak corresponding to surface defects at approximately 700 K was observed in the spectrum of the particles.

A High Q-factor, Easy-to-use, Broadband Ce:LiCAF Laser Cavity Based on Total Internal Reflection

We report a design of a cerium-doped lithium calcium hexafluoro aluminumate (Ce:LiCaAlF\(_{6}\), Ce:LiCAF) laser resonator based on total internal reflection (TIR). This TIR configuration is established by placing the Ce:LiCAF crystal between two calcium fluoride (CaF\(_{2}\)) Pellin-Broca prisms. The laser resonator exhibits a 289-nm emission with 3.5-nm spectral linewidth and 4.4-ns pulse duration. Results show that the present configuration can be a high Q-factor, easy-to-use, broadband prism resonator suitable as a laser oscillator not only in the ultraviolet (UV) region but also down to vacuum ultraviolet (VUV) wavelengths where alignment is restricted to vacuum environments.

Crystal growth and optical properties of indium doped LiCaAlF6 scintillator single crystals

The In-doped LiCaAlF6 [In:LiCAF] single crystals were grown by the micro-pulling-down (μ-PD) method, and the phases, chemical compositions, transmittance and radioluminescence spectra were investigated. All the grown crystals showed high transparency and single phase of LiCAF without visible cracks and inclusions except for the end part of In2%:LiCAF crystal which included the impurity phase. In the radioluminescence spectra of the In:LiCAF crystals under X-ray irradiation, the emission peak around 750 nm was revealed.

Growth and radioluminescence of metal elements doped LiCaAlF6 single crystals for neutron scintillator

The ns2-type metal elements (Pb and Sn) doped LiCaAlF6 single crystals were grown by a micro-pulling-down (μ-PD) method. Pb doped LiCaAlF6 [Pb:LiCAF] crystals showed high transparency and single phase of the LiCAF structure. However, we could not obtain Sn:LiCAF crystals due to the evaporation of SnF2 during the crystal growth. There was an absorption peak around 193 nm in the transmittance spectrum of Pb:LiCAF crystal. In the radioluminescence spectrum of the Pb:LiCAF crystal under X-ray irradiation, two emission peaks around 200 and 830 nm were observed.

Comparison of the electronic band structures of LiCaAlF6 and LiSrAlF6 ultraviolet laser host media from ab initio calculations

We report the electronic structures and density of states (DOS) of perfect LiCAF and LiSAF crystals calculated from density functional theory (DFT) with local density approximation (LDA) using optimized lattice constants. DOS calculations reveal that the valence band is mainly derived from F 2p, thereby resulting to a very narrow valence band manifold. Meanwhile, the conduction band is mainly derived from Ca 4s or Sr 5s resulting to Sr having a broader band dispersion compared to Ca. Both fluoride compounds have indirect band gaps with LiCAF having a band gap of 8.02 eV and LiSAF a band gap of 7.92 eV. This is, to the best of our knowledge, the first report on the electronic structure of LiSAF calculated using DFT with LDA. Our results suggest that when doped with Ce3+, the shorter 5d-conduction band distance in Ce:LiSAF combined with the difficulty of growing high-purity crystals lead to the more pronounced excited state absorption (ESA) and solarization effect experimentally observed in Ce:LiSAF, limiting its potential as a laser material compared with Ce:LiCAF.

Numerical simulation of ultraviolet picosecond Ce:LiCAF laser emission by optimized resonator transients

We perform numerical simulations to investigate the spectro-temporal evolution of broadband ultraviolet (UV) laser emission from Ce:LiCAF, modeled as a system of two homogeneous broadened singlet states, using the resonator transient method. Results show that a short-pulse Ce:LiCAF UV laser emission can be achieved by choosing a cavity with the appropriate photon cavity lifetime and by controlling the pump energy. Numerical simulations were performed to determine the optimum Q-value and cavity length that will make the photon cavity lifetime small compared to the pump pulse duration. A single 31.5 ps laser pulse with about 10 µJ output energy can be generated in principle from a 1 mm, 1 mol % Ce3+-doped LiCAF crystal using a low-Q (30% output coupler reflectivity), short cavity (2 mm cavity length) oscillator pumped by 75 ps laser pulses and 140 µJ pump energy. With these transient cavity parameters, a slope efficiency of about 10% is feasible. This technique can be extended to other fluoride-based solid-state laser gain media.

Intra-cavity frequency-doubled Cr:LiCAF laser with 265 mW continuous-wave blue (395–405 nm) output

We describe continuous-wave (cw) intracavity frequency-doubling experiments performed with a Cr:LiCAF laser. The Cr:LiCAF crystal is home-grown and had passive losses below 0.15% per cm. The laser is pumped by two recently-developed high-brightness tapered diodes, providing a total pump power of 2W at 680nm. The Cr:LiCAF laser generated up to 585mW of cw output power around 800nm with 43% slope efficiency at an absorbed pump power of 1.4W. The low passive losses of the crystal enabled storage of up to 380W of intracavity laser power using a 0.07% transmitting output coupler, demonstrating suitability of Cr:LiCAF gain media for intracavity nonlinear conversion experiments. By performing intracavity frequency doubling with a BBO crystal, cw second-harmonic powers as high as 265mW around 400nm have been realized with optical-to-optical conversion efficiencies as high as 13.3%. To our knowledge, these are the highest cw frequency-doubled laser powers and conversion efficiencies obtained from Cr:Colquiriites to date. Moreover, obtained efficiencies are superior compared to what have been achieved with similar Ti:Sapphire systems, due to lower passive losses of Cr:LiCAF crystal. These results demonstrate the appropriateness of Cr:LiCAF gain media as a high-power tunable cw radiation generator in 375–435nm region.

Neutron detection with LiCaAlF6 scintillator doped with 3d-transition metal ions

Abstract Capability of thermal neutron detection was examined for LiCaAlF 6 (LiCAF) scintillators doped with 3d-transition metal ions. Their radioluminescence spectra were measured with an 241-Am source to simulate 6 Li(n, α) 3 H reaction. The sufficiently intense radioluminescence was observed for the Mn, Co and Cu dopants, while only a weak one was observed for Ti, V, Fe and Ni. A Mn doped LiCAF crystal, which showed the highest radioluminescence intensity, was coupled with a Si avalanche photodiode for the examination of its neutron response. It was confirmed that the average current of the photodiode clearly increased under excitation with 13.5 meV neutron flux.

Low-cost, broadly tunable (375–433 nm & 746–887 nm) Cr:LiCAF laser pumped by one single-spatial-mode diode

We describe a low-cost, compact, and efficient Cr(3+):LiCaAlF(6) (Cr:LiCAF) laser that is broadly tunable in the near infrared (746-887 nm) and blue/ultraviolet (375-433 nm) regions of the optical spectrum. The pump source is a 660 nm single-mode-diode with 145 mW of output power. A melt-grown Cr:LiCAF crystal with an extremely high figure of merit above 2000 was used as the gain medium. This enabled the construction of a high-Q-cavity with continuous wave (cw) lasing thresholds as low as 3 mW, slope efficiencies as high as 54%, and output powers up to 63 mW. The stored intracavity power levels were above 30 W. By placing a beta-barium-borate crystal at a second intracavity focus, we could obtain tunable cw blue/ultraviolet radiation with output powers up to 3.5 mW around 400 nm. When mode-locked using a saturable Bragg reflector around 800 nm, the Cr:LiCAF laser produced 95 fs pulses with average powers of 33 mW and peak powers of 3.58 kW at a repetition rate of 85.5 MHz. Extracavity second harmonic generation enables generation of femtosecond pulses around 400 nm. These results demonstrate the possibility to grow Cr:LiCAF crystals with passive losses below 0.15% per cm, which enables construction of low-cost and complexity systems that are pumped only with one low-power single-mode-diode.

Crystal Growth of Na-Co-Doped Ce:LiCaAlF6 Single Crystals and Their Optical, Scintillation, and Physical Properties

We have grown Ce- and Na-co-doped LiCaAlF6 (Ce,Na:LiCAF) single crystals with various Ce and Na concentrations by the micro-pulling-down (μ-PD) method. Physical properties such as crystallinity, transmittance, photoluminescence, scintillation, and radiation resistance were investigated to evaluate the effects of charge compensation by Na co-doping in the Ce-doped LiCaAlF6 (Ce:LiCAF) crystal. Ce2%:LiCAF, Ce1%Na1%:LiCAF, Ce1.5%Na1.5%:LiCAF, and Ce2%Na2%:LiCAF crystals with no visible cracks and inclusions were prepared. The Ce1%Na1%:LiCAF crystal showed high crystallinity comparable to the crystal grown by the Czochralski method. In the transmittance and photoluminescence spectra of all crystals, an absorption peak around 270 nm and emission peaks around 285 and 310 nm originating from Ce3+ ions were observed, respectively. With increasing Ce and Na concentrations, the light yield systematically increased, and scintillation decay times decreased. Improvement of radiation resistance accomplished by charge co...

Micro-pulling-down-method-grown Ce:LiCAF crystal for side-pumped laser amplifier

We demonstrate 290nm lasing and femtosecond amplification using a micro-pulling-down method-grown Ce:LiCaAlF6 crystal, which was excited using dual-side- and four-side-pumping schemes. These results show that the flexibility and lower cost of this growth technique as well as the multi-side pumping configurations provide good prospects for high-power ultraviolet generation that will enhance the applications of the Ce:LiCaAlF6 laser and amplifier.

Progress towards fabrication of 229Th-doped high energy band-gap crystals for use as a solid-state optical frequency reference

We have recently described a novel method for the construction of a solid-state optical frequency reference based on doping 229Th into high energy band-gap crystals [1]. Since nuclear transitions are far less sensitive to environmental conditions than atomic transitions, we have argued that the 229Th optical nuclear transition may be driven inside a host crystal resulting in an optical frequency reference with a short-term stability of 3 × 10−17 < Δf/f < 1 × 10−15 at 1 s and a systematic-limited repeatability of Δf/f ∼2 × 10−16. Improvement by 102 – 103 of the constraints on the variability of several important fundamental constants also appears possible. Here we present the results of the first phase of these experiments. Specifically, we have evaluated several high energy band-gap crystals (Th:NaYF, Th:YLF, Th:LiCAF, Na2ThF6, LiSAF) for their suitability as a crystal host by a combination of electron beam microprobe measurements, Rutherford Backscattering, and synchrotron excitation/fluorescence measurements. These measurements have shown LiCAF to be the most promising host crystal, and using a 232Th doped LiCAF crystal, we have performed a mock run of the actual experiment that will be used to search for the isomeric transition in 229Th. This data indicates that a measurement of the transition energy with a signal to noise ratio (SNR) greater than 30:1 can be achieved at the lowest expected fluorescence rate.

Amplification of Ultraviolet Femtosecond Pulse by a Micro-Pulling Down Method-Grown Ce:LiCAF Crystal in a Prismatic Cell-Type, Side-Pumping Configuration

We propose an efficient side-pumping scheme for ultraviolet (UV) femtosecond pulse amplification in micro-pulling-down (µPD) method-grown Ce:LiCAF using a prismatic cell-type configuration. By uniformly illuminating four sides of a 2-mm-diameter and 30-mm-length Brewster-cut, nearly as-grown crystal, the risk of damage is dramatically reduced. Without using a chirp pulse amplifier scheme, UV femtosecond pulse at 290 nm is amplified 4 times with no significant increase to its pulse duration and B-integral. Our results show that a laser-quality, Ce:LiCAF crystal, efficiently grown by the µPD method and pumped in a prismatic-cell type configuration paves new possibilities in providing high energy UV femtosecond pulses.

Mode-locked deep ultraviolet Ce:LiCAF laser

We report mode-locked operation of a synchronously pumped Ce:LiCAF oscillator. The laser operated in the deep UV with output radiation centered at 291 nm and a pulse duration of 6 ps. The maximum output power measured was 52 mW, with 13% slope efficiency. The Ce:LiCAF crystal has a gain bandwidth capable of supporting few-femtosecond pulses, and so our results demonstrate the potential to form a new class of ultrafast lasers operating directly at deep UV wavelengths.