We report on the fabrication of Nd:YCOB () optical waveguides by using ion irradiation at an ultralow fluence of . The confocal microphotoluminescence investigation on the produced waveguides has shown the well-preserved fluorescence features within the guiding layer with respect to the bulks. Under the optical pump at wavelength of , continuous wave waveguide lasers at have been generated at room temperature with a high slope efficiency of .
© 2011 Optical Society of America
Neodymium-doped yttrium calcium oxyborate ( or Nd:YCOB) is an excellent self- frequency-doubling (SFD) crystal with a combination of photoluminescence features of the ions and the nonlinear properties of YCOB matrix, which makes it very promising for laser diode directly pumped visible lasers [1, 2]. By using waveguide technology a unique platform of multifunctional applications could be realized through highly compact on-chip circuits with small sizes . Waveguide laser devices are intriguing components for diverse photonic applications owing to the enhanced optical gain as well as the excellent performances, such as low lasing threshold [4, 5]. Particularly, based on the SFD crystals, it is possible to facilitate the integration of the intracavity visible waveguide lasers that do not require the additional frequency conversion through a nonlinear crystal waveguide [6, 7]. In addition, the traditional laser plus nonlinear waveguide configuration (e.g., hybrid system) seems to be quite difficult to achieve high overlap coupling due to the diverse properties (e.g., refractive indices, physical densities) of the laser and nonlinear crystals for waveguide fabrication . In this sense, the Nd:YCOB waveguide could be a suitable platform to host the intracavity for the laser generation of infrared light and synchronously realize the nonlinear process in the same structure. Nevertheless, the first significant step to realize the SFD waveguide lasers is to obtain the fundamental frequency laser generation, e.g., at wavelength of .
A few techniques have been utilized to fabricate waveguides in optical crystals, however, only the “physical” methods, such as ultrafast laser writing  and energetic ion beam implantation/irradiation [10, 11] are applicable to Nd:YCOB due to its stable chemical properties. As of yet, there is no report on the Nd:YCOB waveguide lasers. The normal ion implantation technique, which has been widely used to produce waveguiding structures in a broad range of optical materials, creates negative refractive index layers (so-called “optical barriers”) at the end of ion range through the nuclear collision correlated damages. For the swift heavy ion irradiation (with energy higher than ), the refractive index of the substrates is modified by the electronic excitation induced damages, which mainly happens during most path of the incident ions’ trajectory via the impact of amorphous or highly defective nanotracks from a single ion or the overlap of a few ions [12, 13, 14, 15, 16, 17, 18, 19]. In addition, the required fluences for swift ions are considerably lower than those of the normal ion implantation, which saves the time for the waveguide fabrication . This technique has been successfully applied to produce waveguides in a few optical crystals, such as [12, 13, 19], Nd:YAG [20, 21], , and . Particularly, the swift heavy ion irradiated Nd:GdCOB (belongs to same family of Nd:YCOB) waveguides have shown excellent nonlinear properties for green laser generation . In this Letter, we report, to our best knowledge for the first time, on the fabrication of Nd:YCOB waveguides by using swift ion irradiation, and the continuous wave (cw) waveguide lasers generation at a wavelength of .
The Nd:YCOB (doped by ions with concentration of ) wafer was cut along the direction to satisfy the fundamental wave to second harmonic generation. It was with size and optically polished. One sample surface of was irradiated with ions by using the facility at the Institute of Modern Physics, Chinese Academy of Sciences. The accelerating energy was set at and the fluence was at . In order to slow down the incident ions, a stopper foil of aluminum was placed in front of the sample. The ion current density was kept less than to avoid additional charging and heating effect on the sample. According to our calculation by SRIM (Stopping and Range of Ions in Matter) code , the practical irradiation energy reaching on the sample surface was .
Figure 1a shows the microscope image of the ion irradiated Nd:YCOB sample. It can be clearly seen that the ion beam modified region of the Nd:YCOB is with thickness , which is in good agreement with the mean projected range of the ions in the Nd:YCOB crystal calculated by the SRIM 2010 code. It should be noted that we did not observe any dark modes by using the m-line technique through the prism coupler (Metricon 2010). However, by using an end-face coupling system (at ), we found clear guiding modes [e.g., see TM mode profile in Fig. 1b] of a multimode waveguide structure, which suggests the configuration of the waveguide structure is a “buried” layer and must be of positive index changes with respect with the unmodified bulk. The maximum index alternation of the waveguide layer was determined to be , which was estimated from the measurement of the NA of the planar waveguide . This technique has been proved to be especially successfully to obtain the refractive index alternations of multimode waveguides.
We used a fiber-coupled confocal microscope (Olympus BX-41) to investigate the microphotoluminescence (μ-PL) properties of the Nd:YCOB waveguides. The excitation laser was focused onto the cross section by using a microscope objective with NA . And the backscattered fluorescence emission signals were collected with the same objective, after passing through a series of filters and a confocal pinhole, were collected by a fiber-coupled spectrometer (SPEX500M, USA). The sample was mounted on an XY motorized stage with a high spatial resolution of .
Figure 2a shows the room-temperature PL emission spectrum of the ions in Nd:YCOB crystal correlated to the transition channel. We focused on the emission line in order to obtain the detailed modification of the ion beams on the fluorescence properties of the waveguides. Figures 2b, 2c, 2d depict the spatial dependence of the emitted intensity, peak position, and line width (FWHM) of the emission line, respectively. As one can see, the intensity of fluorescence signals decreases by only 10% in the waveguide with respect to the bulk, which means the majority of the PL active features has been preserved, without clear quenching. Nevertheless, the peak position shifts by , and the emission line is broadened by maximally , suggesting the obvious modification of the Nd:YCOB fluorescence emission properties.
The laser operation experiment was performed by using an end-face coupling system at room temperature. The end faces of the Nd:YCOB sample were placed closely between two dielectric mirrors to construct the Fabry–Perot lasing cavity (the input one with transmission of 98% at and reflectivity at and the output one with reflectivity at and at , respectively). The pump beam from a Ti:sapphire cw laser (Coherent 110) was focused into the cavity by using a convex lens (with focus length of ), and the emission waveguide laser at was collected with a microscope objective and imaged by an infrared CCD camera.
Figure 3 shows the room-temperature laser emission spectrum of the Nd:YCOB waveguide. The emission line is centered at , which corresponds to the main fluorescence transition of channel of ions. The FWHM of the emission line is , which clearly demonstrates the realization of waveguide laser emission.
Figure 4 depicts the output waveguide laser power as a function of the absorbed pump power at . Based on this data, one could determine that the pump threshold () for laser generation is , and the slope efficiency (Φ) is as high as 67.9%. The measured maximum output power is at pump power of , corresponding to an optical-to-optical conversion efficiency of 42%. The excellent lasing performance of the Nd:YCOB waveguides suggests that the swift ion irradiation does not affect the active features of material, which opens up an exciting possibility for further SFD lasing of the waveguides.
In conclusion, we have reported on the fabrication of an Nd:YCOB planar waveguide by using swift ion irradiation. The fluorescence properties of the bulk materials have been well preserved in the waveguides, although clear modification happens after the irradiation. The cw waveguide laser in Nd:YCOB has been realized for the first time, which exhibits excellent performances at oscillations. Future work would be performed on the achievement of the SFD laser from the swift heavy ion irradiated Nd:YCOB waveguides.
The work is supported by the National Natural Science Foundation of China (NSFC) (10925524) and the 973 Project of China (2010CB832906 and 2010CB832902).
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