1. Introduction

Nd ion doped materials are mature laser media and are continuously attracting industrial interests. Nd:YVO₄ is one of those due to its low pump threshold, large emission crosssection, good mechanical, physical and chemical properties. Its absorption band centers at ~800 nm, which allows for direct diode pumping. However, the fluorescence bandwidth of such materials is about 1 nm, which limits the pulse width to be longer than a picosecond. On the other hand, Nd:glass has shown a fluorescence bandwidth of 20–30 nm, because it is glass phase and the spectrum is inhomogeneously broadened. It has been demonstrated to produce tens of femtosecond laser pulses with Nd:glass [1]. However, the low thermal conductivity of Nd:glass prevents it from being used in high average power operation.

On the other hand, diode pumped 1-micron wavelength laser materials, such as Yb ion doped materials, are also very attractive for its much broader bandwidth than that of Nd ion doped ones. There have a lot of successful demonstrations for producing 100 fs pulses from those materials, for example, from Yb:YVO₄ [2]. However, the small emission cross-section and quasi-three level systems make Yb doped materials difficult to be incorporated to the real world femtosecond lasers.

Still people are favoring Nd:YVO₄ because of its superior properties like complete 4-level system, high thermal conductivity, relatively short upper level life-time, with respect to Yb doped materials, which results in a lower threshold, the ability of continuous wave mode locking against Q-switching instability, and higher output power. If the emission spectrum of Nd:YVO₄ can be broadened to be similar to that of Nd:glass, it will be more attractive than Yb doped materials in sub-ps pulse generation.

Photo-induced structural change with femtosecond lasers has been extensively investigated in amorphous materials. One of the successful demonstrations is the writing of waveguide in transparent materials [3]. It is now well recognized that femtosecond lasers can modify the refractive index (possibly increase the index) of glasses in micrometer scale, allowing fabrication of optical waveguides and integrated devices in three dimensions. Chan et al conducted the spectroscopic study on glass exposed to femtosecond laser pulses [4]. After exposure of the sample to femtosecond laser beam, they observed a broad fluorescence band, in addition to the Raman scattering, and the fluorescence intensity increases with increasing the pulse energy.

Femtosecond laser pulse processing ion doped materials in glass phase was reported by several groups [5,6]. However, the spectroscopic study has not been particularly conducted, and only the the realization of waveguide laser was of their interest. For crystalline materials, though the spectroscopic study has not been conducted, to the best of our knowledge, the structural study on crystal quartz has been reported [7–9]. For example, after the single nano-joule femtosecond pulse exposure, Goreli et al found in crystal quartz that, the irradiated cores were changed to amorphous, while the overlay of pulses leads to partial recrystallization of the inner part. Both single and overlapping micro-joule pulses result in a complex defective crystalline structure with an amorphous core inside [9].

From the above experiments, we imagine that if an active crystal, like Nd:YVO₄, is exposed to femtosecond laser pulses, the fluorescence emission band of the ions could be expanded. In order to verify this idea, we conducted the following experiment. The experiment demonstrated that the emission spectrum of Nd:YVO₄ crystal was expanded indeed to maximum 4 times of the original after femtosecond pulses processing.

2. Experimental setup and results

The procedure of the experiment is very much similar to that of writing waveguide in a transparent medium, as shown in Fig.1.

 

Fig. 1. The schematic of the crystal processing. The procedure is very much like the waveguide writing.

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The femtosecond laser pulses were provided by a commercial femtosecond laser workstation (UMW-2110i, Clark- MXR Inc.). The central wavelength of the pulses is 780 nm and the pulse duration is 150-fs. The laser machine was operated at a repetition rate of 1 kHz and the pulse energy was adjustable and was controlled with an on-line polarization beam splitter, and a half-wave plate. The Nd:YVO₄ crystal was a 3×3 mm squared with a thickness of 1 mm and was clipped in a 3-dimensional translation stage. The laser beam was focused by an objective lens (OM 5×/0.14 NA) inside the crystal. The stage was moved from the bottom of the crystal vertically so that the laser beam scans across the crystal to form vertical channels. To avoid the surface damage, the scan was started at 0.2 mm above the bottom and ended at 0.2 mm under the surface. The scanning speed was 0.2 mm/s. Unlike in waveguide writing, the processed area should be as large as possible. To find the proper pulse energy for the channel writing, we divided the 3×3 mm square equally into 9 small squares and processed each square with the pulse energy of 1.8µJ, 2.3µJ, and 2.4µJ respectively. The vertical channels are uniformly distributed within the processed square of about 0.4×0.4mm and the channel separation was 20 µm.

 

Fig. 2. Surface of the Nd:YVO₄ crystal. The properly processed area is on the right side of the (damaged) spot arrays; the spot at the top right corner is a punched marker for identifying the processed area.

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We examined the surface morphology of the crystal with a microscope. We found that in the total 9 squares, only 3 were clearly seen damaged (for the pulse energy >2.4 µJ). Figure 2 shows the crystal surface of the Nd:YVO₄ of one square for the processed pulse energy of 2.3µJ, beside the damaged spot arrays. A damage spot on the top right is also punched by high energy pulse as a marker for locating the processed area that is not visible by microscope.

Then we measured the emission spectrum of the crystal. The crystal was pumped with an 808 nm LD, and the emission was collected by a concave mirror and focused on the fiber tip of the spectrometer. Because the pump beam was focused to a diameter of 100 µm on to the processed but undamaged area of crystal, the measured emission of was an average of at least 16 channels (4×4). Figure 3 shows the typical emission spectrum for before and after exposure to the femtosecond pulses. It can be seen that the emission spectrum of Nd:YVO₄ is expanded after the femtosecond pulse processing from about 1.5 nm to about 6.3 nm (FWHM), and the peak wavelength was red-shifted by about 0.6nm.

 

Fig. 3. Spectrum comparison before and after the femtosecond pulse processing. The spectrum is broadened after the laser pulse processing.

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3. Discussion

The mechanism of spectrum broadening and the “red shift” have not been clearly understood. The attempted explanation could be the multi-photon ionization induced recrystallization and disordering of the crystal. The processing laser wavelength is 780 nm, which is a little away from the absorption peak wavelength of the Nd:YVO₄ crystal (808nm). Both single-photon and multi-photon absorption process could happen during the femtosecond laser pulse irradiation. The local crystal melts and recrystallizes. A previous study on femtosecond laser pulse treated quartz demonstrated that there are three regions in the processed crystals for µJ level of the pulse energy [9]. The core of the channel is amorphous, surrounded by defected crystalline and complex strain regions. In any case, the crystal lattice becomes disordered [10]. As a result, the local crystal fields surrounding the Nd ions are changed, which causes an inhomogeneous broadening of the emission spectrum, like in the Nd:glass. A problem for laser oscillation is the scattering loss and the uniformity. Since we do not see the damage on the crystal surface, the scattering loss may be negligible. Unlike in the waveguide laser, where the smoothness of waveguide channel is the key to the laser oscillation, a clear interface between the core and the cladding is not necessary here. The diameter of the amorphous cores was measured (from the damage spot) to be 5 µm, and the lateral dimension of the affected zone was estimated to be 20 µm. Then the affected zones can be considered as connected, although we do not know whether the phase transition zone (from the crystal to amorphous) can cause loss or not. To ensure the uniformity, a repeated brushingup procedure should be applied which shall produce a small uniform amorphous zone as for the laser gain. The decreased thermal conductivity in the amorphous region may be another concern for the laser oscillation. However, if the processed volume is small enough, the fast heat transition to the crystalline should still happen that alleviates thermal lensing problem.

4. Conclusions

It is concluded that the emission spectrum of Nd:YVO₄ crystal can be expanded to about four times broader than that of the original by femtosecond laser pulse processing. The possible reason is that the local crystalline Nd:YVO₄ is melt and disordered. This experiment opens the possibility of sub-picosecond pulse generation directly from Nd:YVO₄ lasers. Of course in this preliminary experiment, the uniformity is far more from required for laser operation. Better control of the laser pulse energy and dense processing scan are necessary for achieving this goal.