We report on the growth and laser operation of a fiber-shaped single crystal of Pr3+:LiLuF4 (Pr:LLF) grown by the micro-pulling-down (µ-PD) method. We checked the optical quality of the crystal by measuring its spectroscopic features, and by observing the profile of a laser beam transmitted through it. Using an InGaN laser diode as pump source, we achieved continuous-wave laser emission in the orange, red, and deep red regions, with slope efficiencies comparable with or exceeding previous reports on this material. To the best of our knowledge, this work represents the first observation of visible laser emissions from a crystal grown by the µ-PD technique.
© 2016 Optical Society of America
Visible lasers nowadays have ubiquitous applications in many fields of our society, including electronics, data storage, bloodless surgery, entertainment, and of course scientific research, in which they have contributed to spectroscopy and manipulation of new materials, precision metrology, holographic techniques, and astrophysical experiments. Visible solid-state lasers based on trivalent praseodymium ions (Pr3+) have been studied over the years, because of their absorption bands in the blue region, suitable for InGaN-based laser diodes, and of the wide range of wavelengths in which they can emit . Pr3+ ions are sensitive to the structural properties of the embedding crystals, because they potentially allow excited state absorptions and nonradiative relaxations that should be suppressed . These processes are better reduced in fluoride crystals, although laser experiments have been conducted successfully also in oxide hosts [3,4]. Laser emissions with Pr3+-doped fluorides have been reported so far in cyan, green, orange, red, and deep red regions [5–9].
Many applications of visible lasers require miniaturized systems, so it is often necessary to reduce the sizes of active crystals. Having a smaller gain medium also allows a more effective cooling, resulting in enhanced power scalability of the laser source. This size reduction could be particularly beneficial for transportable optical lattice clocks based on Sr atoms, which can be pumped by Pr3+-based lasers [10, 11]. Fluoride crystals for laser applications are usually grown by the Czochralski method (CZ), which has achieved remarkable results [12, 13]. However, with this technique it is difficult to prepare tiny samples, both because of the large sizes of the grown boules, and because of the fragility of fluorides, which prevents efficient mechanical processing of the crystals. In contrast, micro-pulling-down (µ-PD) is a technique that directly enables to grow fiber-shaped single crystals, with a diameter as low as 500 µm, which cannot be obtained from large boules. Currently, µ-PD is one of the most promising methods to build miniaturized or integrated laser devices that operate at wavelengths unreachable by laser diodes. This technique also decreases the minimum amount of raw materials needed to perform a growth, resulting in a reduction of time and cost required [14,15].
Infrared continuous-wave laser emissions have been reported in µ-PD grown crystals, both in oxides and fluorides doped with various rare earth ions [16–19]. Pulsed ultraviolet laser operation has also been obtained in a µ-PD grown fluoride crystal . However, none of these materials was suitable for visible lasers until now. In this work, we report on the first observation, to the best of our knowledge, of visible laser emissions from a crystal grown by the µ-PD method. We chose LiLuF4 (LLF) as host, because of the remarkable performances already demonstrated in fiber crystals doped with holmium trivalent ions , and we employed Pr3+ ions as dopant. Spectroscopic properties were measured to assess the quality of this crystal and to compare it with other samples grown by the CZ method. We achieved continuous-wave laser operation in the orange, red, and deep red regions, with performances comparable or higher than previous reports on this material.
2. Crystal growth and properties
We performed two growths of Pr3+:LiLuF4 (Pr:LLF) fiber-like single crystals, both with a doping concentration of 1% at. of Pr3+ ions in the melt. LLF has a tetragonal lattice with the scheelite structure, and is isomorphic to the well known LiYF4. Its unit cell contains four molecules, with sizes a = 5.119 Å and c = 10.511 Å, and its symmetry group is I41/a. In doped crystals, rare earth trivalent ions substitute lutetium at the S4 site, with coordination number equal to 8 .
Both growths were carried out in a custom-made µ-PD furnace located in our laboratories. This machine employs a RF heating system to increase temperature of a glass-like carbon crucible, shaped like a cylinder ending with a cone, with a 1.3 mm wide aperture on the bottom. Inner volume of the crucible was about 4 cm3 and was loaded with 5N pure powders of LiF3, LuF3, and PrF3, in the proper quantities. Total weight of the powders was 3 grams for each growth, demonstrating the possibility to produce laser crystals even with very low amounts of starting materials. During the growths, temperature was set to about 1120 K, while the growing crystal was pulled down at about 1 mm/h. A high purity platinum wire was employed as a seed to start the growth. While easier to implement, this method of seeding does not allow to select beforehand the lattice orientation of the grown crystal.
Two different monocrystalline fibers were grown with the same doping concentration. The first fiber was about 80 mm long, while the second fiber was about 40 mm long. Both fibers had a diameter of about 1.6 mm, almost constant through the whole length. Figure 1 shows a picture of the second fiber. It can be seen that the crystal had a rough appearance on the outside, due to precipitation of exceeding LiF during the cooling phase . Despite that, the inner part of both fibers was transparent and free of cracks, bubbles, and other visible defects. We checked the monocrystalline structure of the fibers using a X-rays Laue diffractometer. This measurement also identified the orientation of the crystallographic c-axis in the fibers. We determined an angle of about 11 degrees between the c-axis of LLF and the growth direction in the first fiber. The same angle was about 63 degrees in the second fiber crystal. Multiple cylindrical samples were carved from both the fibers, polished on both circular facets, and employed for subsequent structural, optical, and spectroscopical measurements.
We tested the optical quality of the fiber crystals by measuring the distortions introduced in a laser beam that crossed a 3.9 mm long sample, carved from the second growth. We selected the TEM00 mode of a He-Ne laser by using a pinhole and a microscope objective, as detailed in . The probe beam was unpolarized. Figure 2 shows the beam profile before and after the propagation through the sample, acquired with a DataRay WinCamD-UCD15 CCD beam imaging camera. It can be seen that the propagation though the crystal does not introduce any significant distortions in the laser beam, confirming the high optical quality of these fibers.
3. Spectroscopic characterization
3.1. Absorption spectra
We performed a complete spectroscopic characterization of the fiber crystals by acquiring polarized absorption in the blue region, emission spectra in the visible region, and fluorescence decay time of the 3P0 level of Pr3+ ions. Absorption spectra of a sample carved from the second growth were acquired with a Varian Cary 500 spectrophotometer, at room temperature, between 430 and 490 nm, with a resolution of 0.15 nm. We measured absorption for light parallel polarized along the optic axis of LLF (π) and orthogonal to it (σ).
Pr3+ ions also have an absorption band in the orange region, between 560 and 620 nm, that can reduce laser performances at these wavelengths . Absorption in the other areas of the visible spectrum is negligible. We verified that the absorption spectra of our sample in the orange region matched the data reported for CZ grown Pr:LLF crystals .
Results of these measurements are shown in Fig. 3, in comparison with data acquired on a previous growth of Pr:LLF, performed by the CZ method. Absorption spectra of the CZ sample are scaled by a factor of two, to avoid superimposition of the curves and improve readability of the plots. Both polarized spectra show the same peaks and features for samples grown by the two techniques, indicating the high quality of the present crystal. Data show that this material can be efficiently pumped in the blue region, in a range accessible by InGaN laser diodes. In particular, there is an absorption peak at 444 nm for input light polarized parallel to the c-axis, corresponding to a transition from the ground state to the 3P2 level of Pr3+ ions.
3.2. Emission spectra
Steady-state fluorescence of the Pr:LLF fiber crystal was sampled with a monochromator, using a diffraction grating with 1200 g/mm, and a Hamamatsu R943-02 photomultiplier as detector. For these measurement, we employed a sample carved from the first growth. An InGaN laser diode was used as pump source, tuned at 444 nm and focused on the sample with a 10 cm lens. The blue diode was oriented to pump the crystal in π polarization. Fluorescence was focused on the monochromator with a 7.5 cm lens, placed at a right angle to the pump beam. Light from the sample was chopped and the resulting signal was acquired with a lock-in amplifier.
We measured the emissions of Pr3+ ions decaying from the 3P0 level, looking specifically in the orange, red, and deep red regions. Fluorescence from the fiber crystal was sampled at room temperature, between 590 and 740 nm, with a resolution of 0.13 nm, for both available polarizations. Figure 4 shows the emission spectra of this fiber crystal and of a CZ grown Pr:LLF sample, for both polarizations. Fluorescence spectra of the CZ sample are scaled by a factor of two for clarity, as in the plots of the absorption spectra. It can be seen that emission spectra for the two growth methods are very similar to each other.
3.3. Fluorescence decay time
Fluorescence decay time of the 3P0 level was measured with the same experimental setup employed to acquire steady-state emissions, but using a pulsed, frequency-doubled, Ti:sapphire laser as pump source. Excitation wavelength was set at about 480 nm, with a pulse duration of 30 ns and a repetition rate of 10 Hz. For this measurement, we employed a sample carved from the second growth. We determined the concentration of Pr3+ ions in this sample via transmission measurements, referring to the absorption cross section reported in , resulting in a 0.3% at. doping level.
We obtained a fluorescence decay time of (42.0±0.5) µs, comparable with previous reports of CZ grown Pr:LLF . It is worth to note that the CZ grown crystal was 0.19% at. doped, while our sample was more doped. Despite the increased concentration, which should shrink the 3P0 lifetime , we observed an equivalent decay time, remarking the good structural properties of this µ-PD grown sample.
4. Laser experiments
4.1. Experimental setup
Laser experiments were conducted in a hemispherical resonant cavity, using the setup reported in Fig. 5. The input plane mirror (M1) was anti-reflective coated (T > 99%) for blue light, from 420 to 520 nm, and highly reflective coated (R > 99%) between 590 and 750 nm. The output mirror (M2) was plane-concave, with a radius of curvature approximately equal to the distance between the mirrors, which was adjusted with a translation stage. We used alternatively several output mirrors, with different highly reflective coatings and radii of curvature, to select each time a specific emission line of Pr3+. The output mirrors employed for the orange region had a radius of curvature of 100 mm, while all the other output mirrors had a radius of 50 mm.
A multimode InGaN blue laser diode (LD) was used as pump source, adjusted to emit at 444 nm, and collimated with a high-numerical-aperture lens (CL). Emission from this diode was linearly polarized and strongly asymmetric. Astigmatism of the pump beam was reduced using a pair of cylindrical lenses (CYL). A λ/2-waveplate (HW-1) and a polarizing beam splitter cube (PBS) were used to continuously tune the power of pump laser. The maximum amount of power incident to the cavity never exceeded 700 mW. We employed a second λ/2-waveplate (HW-2) to align the linearly polarized pump beam with the c-axis of the sample. A 30 mm achromatic lens (FL) focused the blue laser into the active crystal. We measured the pump beam profile after the lens with the DataRay CCD camera, obtaining a mean pump spot size of about 25 µm in the waist. We estimated an average pump beam diameter of 60 µm through the whole active medium.
For laser experiments, we employed a 3.9 mm long Pr:LLF fiber crystal sample, carved from the second growth. The fraction of pump power absorbed by this sample in a single-pass was about 78% of incident power. Both circular facets of the sample were polished to reduce surface roughness, and to make them plane and parallel to each other. No coating was applied on these facets. We fixed the sample on a copper holder, cooled with recirculating water at 16 °C. Because of the circular cross section of this fiber crystal, we carved a V-shaped valley in the copper and placed the sample inside it. The crystal was then glued in this channel using a thermally conductive epoxy adhesive, to allow heat dissipation from all the volume of the fiber. The valley was 4.2 mm long to avoid scattering of the output laser beam on the copper walls. This holder was fixed on a goniometric head capable of rotating and translating the crystal along the two horizontal directions. The whole support was placed right behind the M1 mirror.
We placed a 510 nm longpass filter behind the M2 mirror to remove the fraction of pump laser transmitted through the cavity. Input and output powers were measured at the same time using two silicon-based power meters (PM). The wavelength of laser emissions was determined with an Ocean Optics HR4000 spectrophotometer, with a resolution of 1 nm. Polarization of the output laser beams was checked with an adjustable Glan-Thompson polarizer.
4.2. Results and discussion
We achieved continuous-wave laser operation with four transitions in the orange, red, and deep red regions. For each line, we measured the output laser power as a function of the amount of pump power absorbed by the sample, employing various output mirrors when available. Output wavelength (λout), polarization with respect to LLF axis (Eout), transmittance of the M2 mirror used (TM2), slope efficiency (ηabs), absorbed power threshold (Pthr), and maximum output power (Pmax) are reported in Table 1 for all the observed emissions. We achieved maximum slope efficiencies of 5.9% at 604 nm, 16.1% at 607 nm, 30.4% at 640 nm, and 28.3% at 722 nm. Output powers for the best performing configurations in the three regions are shown in Fig. 6(a).
As far as we know, this work is also the first report of laser emission at 604 nm from Pr:LLF. This wavelength is more affected by absorption from the ground state to the 1D2 level of Pr3+ ions . Ground-state reabsorption reduced the slope efficiency at 604 nm and generally increases the difficulty of achieving laser emission at shorter wavelengths, in the orange region, with this doping ion.
We investigated the quality of the laser emissions and of the fiber crystal by measuring the beam propagation factor M2 of two laser transitions, at 607 and at 640 nm. A Coherent ModeMaster MMH-2S beam propagation analyzer was employed for this measurement. The beam propagation was studied along two orthogonal directions, not aligned with the c-axis of the sample. For each laser, we adjusted the cavity to emit at maximum output power. For the red region, we used the output mirror with transmittance of 1%. The average M2 parameter was 1.2 for the 607 nm laser, and 1.1 for the 640 nm line. Figure 6(b) shows the beam diameter of the red emission as a function of the distance to the focal plane. The inset contains an intensity beam profile for the 640 nm laser. These data confirm that, despite the small cross section of this crystal, the output laser beams have high quality and diffraction-limited Gaussian propagation, with performances similar to visible lasers obtained in CZ grown fluoride crystals [12,25].
We estimated the fraction of passive round-trip cavity losses with the Findlay-Clay  and Caird  methods, for the 640 nm laser emission, using the data listed in Table 1, acquired with five different output mirrors. We obtained a fraction of losses of 1.4% and 0.6% with the two methods, respectively. These losses are lower than what has been reported so far in other µ-PD grown crystals, employed for infrared laser applications (2.4% at 1030 nm, between 2.0% and 4.1% at 1053 nm, and 3.2% at 2067 nm [17–19]). In the wavelength regions in which the bulk absorption is negligible, propagation losses in insulating crystals are primarily caused by Rayleigh scattering from the polished facets . Because losses from Rayleigh scattering are expected to increase for decreasing wavelength, these results further emphasize the high optical and structural quality of this fiber crystal. Caird analysis also allowed to evaluate the limiting slope efficiency at 640 nm for our cavity setup, resulting in a value of 35%, close to the efficiency obtained with the 5% output mirror.
In previously reported CZ grown Pr:LLF crystals, losses ranged from 0.4% to 6% in the red region [24, 29, 30], thus the losses estimated in this sample are comparable or even lower than previously reported results. About laser performances, generally we improved the slope efficiencies and reduced the threshold powers in comparison with diode-pumped Pr:LLF crystals reported so far (12% and 122 mW at 607 nm, 38% and 208 mW at 640 nm, 24% and 133 mW at 722 nm [23,29]), with the only exception being the maximum slope efficiency of the red line. These parameters indicate that the different method of growth and the reduced cross section of this kind of crystals did not affect negatively the laser performances, and yield comparable or improved results, even in the visible region.
5. Conclusions and perspectives
We report on visible laser emissions from a Pr:LLF crystal grown by the µ-PD method. After the growth, we tested the structural and optical integrity of the crystal by checking X-rays diffractometries and transmission of a He-Ne laser. We performed a spectroscopic characterization and we obtained results comparable with a CZ grown sample of the same material. Laser performances were tested in a hemispherical resonant cavity, using a blue laser diode as pump source, in the orange, red, and deep red regions. We achieved continuous-wave laser operation at 604, 607, 640, and 720 nm. To the best of our knowledge, this is the first observation of visible laser emissions from a crystal grown by the µ-PD technique. This work is also the first report of laser emission at 604 nm from this material.
Overall, the laser parameters of this crystal were comparable or better with respect to previous reports on the same material, demonstrating that µ-PD is a suitable method to grow fluoride crystals for visible laser applications, and could sometimes be preferential because of the reduced cost and enhanced compactness achievable. We measured the beam propagation factor of the output lasers, obtaining M2 values lower than 1.2, indicating quasi-diffraction-limited output beams. In addition to that, we estimated the fraction of passive losses in the sample, obtaining much lower values than in other crystals grown by the µ-PD technique, pointing out the increased optical quality achieved in this crystal, which was needed for visible lasers.
Performances and integration capabilities of these fiber crystals could be improved by coating directly the facets of the sample, hence removing the need of external mirrors, and reducing the size of the whole laser system. Further studies could also be conducted on the methods employed to seed the µ-PD growth, to reduce the angle between the c-axis and the circular facets. For example, the use a crystalline seed should allow to preselect the lattice orientation of the grown fiber crystal. This would enhance the fraction of absorbed pump power and would allow to shrink even more the length of the laser gain medium, without the need to increase the doping concentration.
Z. Zhang acknowledges support from the Marie Curie Actions of the European Union’s 7th Framework Programme under REA no. 287252. The authors thank I. Grassini for her assistance during the preparation of the samples.
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