1. Introduction

Two-frequency lasers operating in orthogonally polarized modes are promising devices for many applications. They have been employed successfully for precision metrology such as the measure of displacement, wave plates’ retardation or force and weight [1]. Beside the femtosecond lasers, they provide and indirect method to generate terahertz (THz) waves, the direct ones being the quantum cascade lasers, the gas lasers or the Schottky and Gunn diodes. Because they are completely safe for humans and because of their noteworthy interaction with matter: air, plastic, gasoline, paper, clothing, non polar materials… are transparent, metals are reflected, water is absorbed, gazes and plastics have specific THz spectra, the THz waves have a huge potential for non destructive imaging of concealed objects and free-space communication. Pharmaceutical and food manufacturing, polymer industry, security, medical, military, wireless communications, basic research are concerned by THz waves [2, 3].

A technique for generating THz waves from a two-frequency laser is based on photoconductive antenna fabricated from a semi-conductor whose band gap is lower than the photon energies of the two beams [4, 5]. Another technique uses difference frequency generation inside a nonlinear optical crystal oriented for adequate phase matching [6, 7]. Let us cite GaP, liNbO₃, ZnGeP₂, GaSe …

Let us turn towards two-frequency generation. Yb³⁺ lasers benefit of wavelength tuneability due to the broad emission band of this ion as a consequence of electron-phonon coupling. Then the gain bandwidth, which reaches generally several THz, can be exploited. For instance, from a KGd(WO₄)₂ crystal [8], frequency differences in the 0-3.1 THz range have been obtained. In a recent work [9] one of us has obtained about 1.6 THz frequency difference from diode-pumped lasing of the Yb³⁺:GdAl₃(BO₃)₄ laser crystal. The gain bandwidth of Nd³⁺ ion is much narrower, explaining the much lower frequency differences exhibited in literature. Let us cite two-frequency lasing from the isotropic Nd³⁺:YAG. We find 26 GHz frequency difference and 3 mW total output power in ref [10]. In ref [11], the polarization effects originate from two intra-cavity quarter-wave plates and the frequency difference can be adjusted up to 730 MHz. In ref [12], 127 GHz are obtained thanks to an intracavity electro-optically tunable etalon. However, thanks to the Stark splitting involved in the ⁴F_3/2→ ⁴I_13/2 transition near 1330 nm, a frequency difference up to 3.4 THz has been obtained in Nd³⁺:YAG but with a lone polarization of the laser lines due a Brewster-cut acousto-optic modulator [13].

We show hereafter that in this context the case of Nd³⁺-doped La₂CaB₁₀O₁₉ (LCB) laser crystal is of peculiar interest. Due to the two Nd³⁺ localisations in the La³⁺ and Ca²⁺ sites [14] characterized by two different ⁴F_3/2 level lifetimes, 92 and 43 µs respectively, and two sets of spectroscopic properties, a more complex physics than in the previously cited lasers is involved. For example room temperature selective pulsed excitations at 731 and 745.6 nm lead to laser emissions at respectively 1051.4 and 1068 nm. The present work is devoted to a lone CW laser diode excitation near 800 nm in order to explore which lasing results: which Nd³⁺ site with which wavelength and which polarization. The study leads to CW lasing for the first time in this crystal (section 2) and with the main result of simultaneous lasing at 1051.4 and 1068 nm in two orthogonal polarizations (two-frequency lasing), corresponding to an unusually large frequency difference of 4.6 THz (section 4). More, the undoped LCB being a biaxial noncentrosymmetric crystal promising in the field of frequency conversion [15, 16], the Nd³⁺ doping results in a bi-functional laser/nonlinear optical crystal from which we obtained self-frequency doubling (section 3). A general review of self-frequency conversions can be found in ref [17].

2. Lasing from the regular Nd³⁺ site

Let us exhibit first a few spectroscopic properties of the LCB:Nd crystal under excitation near 800 nm. The absorption spectrum of a Z-oriented sample in X-polarization is given in Fig. 1 . We can see that the main peak is at 799.2 nm and that it is convenient for laser diode pumping: the dash line represent the emission spectrum of the diode used in the following for the laser experiments (a 200 µm fibre coupled LIMO laser diode).

 

Fig. 1 X-polarized absorption spectrum of LCB:Nd and emission spectrum of the diode used in the laser experiment.

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Exciting a Z-oriented sample at 799.2 nm with a high resolution (0.05 cm⁻¹) pulsed dye laser from Laser Analytical Systems, we obtained the two X and Y-polarized emission spectra in Fig. 2 . A comparison with the spectroscopic data recorded in our previous study [14] near 750 nm excitation and the measured value of the fluorescence lifetime: 75 µs, lead to the conclusion that this emission is mainly du to Nd³⁺ ions located in the regular La³⁺ sites.

 

Fig. 2 X and Y-polarized emission spectra of LCB:Nd under 799.2 nm excitation, with value of the fluorescence lifetime.

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With Z-oriented LCB:Nd samples anti-reflection coated near 1050 nm, we obtained the one-frequency lasing of this emission at 1051.4 nm in X-polarization, under pumping with the LIMO laser diode (the laser line is visualized by the vertical arrow in Fig. 2). The lasing wavelength is in agreement with our previous laser results under pulsed pumping of the regular Nd³⁺ sites at 731 nm [14].

Here are the experimental details. The LCB:Nd samples were inserted inside a copper holder water-cooled near 10 °C.

The laser cavity (Fig. 3 ) was constituted with a plan dichroic input mirror highly transparent near 800 nm and highly reflective in the 1020-1100 nm range. The concave output mirrors had 2% or 4% transmission near 1060 nm with 50 mm radius of curvature or 2.5% or 4% transmission with 75 mm radius of curvature. The pump beam from the fibre coupled laser diode was focused with two 60 mm focal length achromatic doublets inside the crystal, the measured pump diameter on the crystal being 220 µm. Typical laser output powers versus input powers are represented in Fig. 3 from a 8% Nd doped sample with 6 mm X 8 mm transverse dimensions and 2.94 mm thickness.

 

Fig. 3 Experimental set-up for the laser experiments.

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The slope efficiency is reasonably high (about 30%) but we can see that from about 3.2 W pump power, the laser power drops, due to a deleterious thermal effect. To prove this effect, we have imaged with a thermal camera working in the 8-14 µm range the output face of the crystal. The values of the maximum temperature of the face are reported in Fig. 4 (right Y-axis) and we observe that the slope of the temperature evolution increases above 3 W pump power. We have experienced that the thermal effect was even more drastic with a 10% doped sample: the laser output power falls down from 2.5 W pumping. We can correlate the concentration dependence of the laser efficiency breakdown with the lifetime dependence of the ⁴F_3/2 emitting level. Its value measured at 1051.4 nm fluorescence excited at 800 nm is 84 µs for 3.6% Nd and decreases to 80 µs at 8-10% and 77 µs at 12% Nd concentration. So non radiative transitions and then concentration quenching are responsible for heating the samples at high pump powers. We can also put forward the thermal lens induced in the crystal at high temperature to explain the decreasing of the laser power at high pump power. However we did not attempt to measure its focal length and its calculation should require among others the temperature derivatives of the refractive indices.

 

Fig. 4 X-polarized laser output power at 1051.4 nm from the Z-oriented LCB:Nd 8% sample and the thermal effect.

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3. Self-frequency doubling

A 8% Nd-doped sample (4 mm X 4 mm transverse dimensions and 5.41 mm thickness) oriented for phase matching the second harmonic generation of the 1051.4 nm fundamental wave, that is to say at polar angle θ = 36.3° in the XZ principal plane (azimutal angle φ = 0°), was used in the self-doubling experiment. The input mirror was replaced by a dichroic one highly reflective at both 525 and 1050 nm and the used output mirror was highly reflective near 1050 nm and transparent near 525 nm. The infrared beam escaping from the output mirror at 1051.4 nm was found to be polarized in the XZ plane while its second harmonic was Y-polarized. So the nonlinear optical process corresponds to type I ee-o interaction. The second harmonic beam power reached up to about 100 mW (Fig. 5 ). The residual infrared power is also shown in Fig. 5. The samples with higher Nd concentrations (10 and 12%) revealed to be less efficient for self-frequency doubling, due to deleterious thermal effects.

 

Fig. 5 Self-frequency doubling at 525.5 nm from the phase matched oriented LCB:Nd 8% sample.

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4. Two-frequency lasing from the two inequivalent Nd³⁺ sites

In this section we used a series of X-oriented samples with 6, 8 and 10% Nd³⁺ doping. The 6% doped sample was excited first along the X-axis with the high resolution pulsed dye laser at 799.2 nm as in section 2, that is to say at the maximum of its absorption. The obtained Y and Z-polarized emission spectra are given in Fig. 6 (a) . The more intense fluorescence occurs at 1051.4 nm in Y-polarization, with a measured lifetime of 87 µs, corresponding to Nd³⁺ ions located in the regular La positions. Then lasing from CW laser diode pumping as in section 2 is expected to occur at this wavelength and in Y-polarization.

 

Fig. 6 Emission spectra of the 6% Nd-doped LCB sample under 799.2 nm excitation (a) and 800.5 nm excitation (b), with values of fluorescence lifetimes.

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The laser cavity and in particular its two mirrors were similar than for the Z-oriented samples. Pumping up to about 3 W led effectively to lasing at 1051.4 nm in Y-polarization and we attribute the origin of this laser emission to Nd³⁺ ions located in the regular La³⁺ site, as for Z-oriented samples. The laser output power versus the pump power is visualized in Fig. 7 in the case of the 8% doped sample (6 mm X 8 mm transverse dimensions and 3.07 mm thickness) and 2% transmission output mirror. The camera-measured temperature of the output face of the crystal is also given in. Figure 7. We can see that above 3 W pumping, the temperature slope does not increase and no deleterious thermal effect occurs. Nevertheless, the output power at 1051.4 nm starts to decrease and even stops near 6 W pumping, but with no dropping of the total output power. A polarizing cube beamsplitter was then conveniently introduced in the laser beam path after the output mirror and so an unexpected Z-polarized new output beam was revealed at 1068.7 nm wavelength above 3 W pumping (Fig. 7). Its differential slope is 65%, that is to say much higher than the Y-polarized one below 3 W pumping: only 24%.

 

Fig. 7 Two-frequency laser output power from the X-oriented LCB:Nd 8% sample.

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Let us discuss the origin of this second laser emission. From the 6% Nd-doped sample and scanning the dye laser excitation wavelength, we have found that the peaks of fluorescences at 1051.4 nm in Y-polarization and 1068.7 nm in Z-polarization are not obtained at the same excitation wavelength: the 1051.4 nm emission is best excited at 799.2 nm as expected from the absorption spectrum in Fig. 1, but the 1068.7 nm emission is best excited at 800.5 nm, corresponding to the weak shoulder at this wavelength in the absorption spectrum of Fig. 1. A further excitation at 800.5 nm led to emission spectra in Y and Z-polarizations given in Fig. 6 (b), which are very different than the ones in Fig. 6 (a), revealing a different origin. The two obtained laser lines are visualized by arrows in Fig. 6 (a) and (b). Under other excitation wavelengths except 799.2 and 800.5 nm, the emission spectra change and are a mixing of the spectra represented in Fig. 6 (a) and 6 (b) but the spectra of Fig. 6 (a) are generally predominant. The lifetime of the Z-polarized 1068.7 nm emission (close to a decaying exponential under 800.5 nm excitation) being found as short as 50 µs, we attribute its origin to Nd³⁺ ions located in Ca²⁺ positions. To be precise let us add that at room temperature the absorption and emission lines of each Nd³⁺ site are quite broad and then the time evolution of the fluorescence cannot be purely exponential due to a small admixture of the emission of the other site. Let us recall that in ref. [[14], using sums of two weighted exponentials, the lifetime of the Nd³⁺ located in Ca²⁺ positions was found to be 41 µs.

More, we can see in Fig. 7 that a range of pump around 5 W power allows the simultaneous lasing at 1051.4 and 1068 nm in two orthogonal polarizations. The spectral distribution obtained with a HR2000 Ocean Optics spectrophotometer is represented in Fig. 8 , the frequency difference between the two laser lines being close to 4.6 THz. The profile of the two-frequency laser beam was measured with a Gentec CCD camera and is given in the insert of Fig. 8.

 

Fig. 8 Emission wavelengths of the two-frequency laser and its beam profile.

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According to our spectroscopic investigations [14] we attribute the origin of the 1068.7 nm laser emission to Nd³⁺ ions located in the Ca²⁺ site. From Fig. 8, it is clear that a competition occurs between the two La and Ca-site Nd³⁺ laser emissions. The Ca-site laser emission at 1068.7 nm has a higher threshold, close to 3 W, than the La-site one, probably due to the short lifetime of the Nd³⁺ ions located in Ca-sites: 41 µs. Above 3 W pumping, as soon as the Ca-site Z-polarized Nd lasing starts, the latter one benefits of the La-site Nd³⁺ laser gain because at 1068.7 nm the Z-polarized La-site Nd³⁺ emission spectrum has a valuable contribution. So the higher the 1068 nm laser emission, the lower the 1051.4 nm laser gain. More, increasing the power of the laser diode leads to increase the diode temperature despite of the thermo-electric cooler, and then the diode emission wavelength is slightly shifted from 799.2 nm near 1 W up to 800.9 near 6 W. So the Ca-site lasing was favoured at high pump power despite of the broad width of the diode emission (about 2.5 nm at half-maximum). The Ca-site laser emission increases with a high differential slope while the La-site laser emission decreases.

5. Conclusion

In summary, we have studied in continuous wave regime under laser diode pumping near 800 nm the Nd³⁺ laser operation in the La₂CaB₁₀O₁₉ biaxial crystal. From Z-oriented samples we have obtained lasing at 1051.4 nm of Nd³⁺ ions located in the regular La³⁺ sites. Self-frequency doubling of this laser emission was obtained with samples oriented at the phase matching direction of the type I ee-o nonlinear interaction. With X-oriented samples, thanks to the existence of two La³⁺ and Ca²⁺-sites for the Nd³⁺ location, we have obtained a two-frequency laser working in dual polarization with a frequency difference unusually large of 4.6 THz in the free running regime. Spectroscopic data obtained under high resolution selective laser excitation around 800 nm reveal the existence of two inequivalent Nd³⁺ centers at the origin of the two laser emissions.