Optical channel waveguides have been produced for the first time in Nd:LGS multi-functional laser crystals by using proton implantation. The obtained good guiding performance exhibits the well-confined modal fields in the waveguiding structures. The confocal fluorescence images of the obtained waveguides have revealed that the photoluminescence properties of the Nd3+ ions have been well-preserved in the waveguide’s active volume, which suggests promising applications as multi-functional integrated laser generation elements. These images have been also used to elucidate the spatial distribution of lattice damage and distortion caused by the implantation process, which are both mainly located at the nuclear collision region.
© 2010 OSA
The single-crystalline langasite (La3Ga5SiO14 or LGS) has been realized to be one of the most intriguing piezoelectric crystals for its promising applications on acoustic wave devices [1,2]. Recently, it has been reported that LGS also possesses good electrooptic features, which could be used as Q switches for lasers . In addition, when doped with rare-earth ions, LGS becomes excellent gain medium for both continuous wave (CW) and Q-switched laser generation, so that it becomes a self-activated multi-functional laser gain medium [4,5]. In particular, LGS has been demonstrated to be an excellent laser host for Nd3+ ions, allowing for multi-watt CW laser operation under laser diode pumping . In addition, the combination of its large birefringence and broad emission bands has been recently used for self-tunable laser gain operation .
Integrated photonics deals with compact optical devices of micro- or nano- dimensions. It offers a unique platform to realize multiple functions in small-size circuits in many aspects of modern photonics and telecommunication systems [7,8]. Optical waveguides play critical roles as the basic elements in these photonic networks. The compact confinement of light propagation in waveguides of dimensions within order of microns results in very high optical intensities; consequently, some performances correlated to the bulks could be considerably enhanced in waveguiding structures [9–11]. For instance, the two wave mixing gain in photorefractive waveguides may be much higher than that the bulks , and waveguide nonlinear responses are with improved conversion efficiencies . For laser gain materials, the waveguide lasers possess lower pump thresholds and enhanced efficiencies owing to the strongly reduced active volumes [11,12]. Construction of diverse waveguiding structures on gain media is the significant first step towards realization of effective waveguide lasers. Since the first non-crystalline example in fused silica and single-crystalline material in LiNbO3 [13,14], ion implantation has been proved to be a unique method to produce guiding structures in a variety of materials, including single crystals [15–19], polycrystalline ceramics , glasses , semiconductors [22,23], and organic substrates . This technique offers accurate controlling of the refractive index profiles of the waveguides by selecting diverse ion species, energies and fluences. In addition, it does not depend on the chemical properties of the target materials, which makes it unique and applicable for many materials. In addition, confocal fluorescence imaging based on the spatial analysis of the fluorescence of Nd ions has been recently evidenced to be a powerful and sensitive tool for the detection and localization of the micro-structural modifications induced in crystalline materials during ion implantation. Indeed, confocal fluorescence imaging has been found to be particularly suitable for the study of ion implanted waveguides [25–27]. In this work, we report, for the first time to our knowledge, on the formation of optical channel waveguides in Nd:LGS crystals, and on their confocal fluorescence images which have been used to evaluate their potential use as multi-functional integrated laser sources.
The z-cut Nd:LGS (doped with 1mol% Nd3+ ions) crystals were grown by Czochralski method . They were cut to dimensions of 10(x)×10(y)×1.5(z) mm3, and optically polished. By using the standard UV lithography technique, a series of photoresist stripe channels (with thickness of ~5μm and lateral width of 50μm) have been deposited on top of one x-y surface, pointing along the x-axis of the crystal. The space separation between the two neighboring stripes was of ~10 μm (i.e., the regions between the photoresist stripe are free of masks). The proton implantation at energy of 500keV and fluence of 9.6×1016 ions/cm2 was performed at this surface (beam direction was tilted by 7° off the y-z plane to minimize channeling effect), and constructed channel waveguides in the regions between the two adjacent photoresist stripes (i.e., in the photoresist-mask-free regions). For comparison we have left 1/2 part of the sample surface without any photoresist film deposition, allows for ion implanted planar waveguide formation in this region. Figures. 1(a) and (b) illustrate schematically the waveguide fabrication process. It should be pointed out that the shape of the stripe mask cross sections is strongly wedged, resulting in trapezoidal configurations of the channel waveguide cross section [see the inset of Fig. 1(c) for the microscope image of the sample end face]. This technique has been recently successfully applied to produce channel waveguides in a few crystals . In order to improve the thermal stability and guiding properties, the sample was annealed at 260°C for 30 min in air.
We used m-lines to measure the dark-mode spectroscopy of the waveguides (via a prism coupler, Metricon 2010, USA) within index error of 0.0002, and reconstructed the 2D refractive index distributions of waveguide cross sections by considering their trapezoidal shapes through the reflectivity calculation method (RCM) . An end-face coupling arrangement was utilized to experimentally characterize the modal profiles of the guided modes. The propagation losses of the waveguides were determined by a Fabry-Perot method by slightly heating the sample for a temperature increment of ~5°C . The above guiding property measurements were taken at wavelength of 633 nm with He-Ne lasers.
The confocal fluorescence images were obtained by using an Olympus BX-41 fiber-coupled confocal microscope. The 10-mW CW radiation at 488 nm from an argon laser was focused at sample’s surface by using a 100× objective with numerical aperture N.A. = 0.95. The lateral and axial resolutions of the confocal system were estimated to be, in both cases, well below 1µm. In this configuration, the 488 nm laser radiation locally excites the Nd3+ ions from their fundamental state (4I9/2) up to the excited state (2G3/2) [25–27]. Then, the subsequent 4F3/2→4I11/2 emission generated from Nd3+ ions was collected by using the same microscope objective and, after passing thorough a confocal aperture, analyzed by a CCD camera attached to a high resolution fiber-coupled spectrometer. The sample was mounted on an XY motorized stage with a spatial resolution of 100 nm, so that it was possible to scan the 488 nm excitation spot over the waveguide’s cross section. The motorized stage and CCD were controlled with a LabSpec© software for automatic acquisition and analysis of the obtained spectral data. Two dimensional (2D) images of the Nd3+ spectral properties (i.e., emitted intensity, emission bandwidth, and energy position of the emission lines) were obtained by fitting Lorentzian line-shapes to the spectra and plotting the obtained values for each measuring point.
3. Results and discussion
Figures. 2(a) and (b) show the reconstructed 2D ordinary (no) and extraordinary (ne) index profiles at 633 nm of the Nd:LGS channel waveguides produced by 500 keV proton implantation (after annealing at 260°C for 30 min), respectively. The waveguide is surrounded by barrier walls with reduced indices. For proton implanted waveguides, such barriers are created by the nuclear collisions of the incident ions with the target atoms, resulting in nuclear damages (mainly occurring at the end of ion range). With these index mappings, we have simulated the light propagation in the channels by using a commercial software BeamPROP© based on the finite difference beam propagation method (FD-BPM) . Figures. 2(c) and (d) depict the calculated modal profiles for the quasi-TE (correlated to no) and TM (correlated to ne) fundamental modes (TE00 and TM00), respectively. To validate the index profiles, we have measured the near-field light intensity (normalized) distribution from the output face of channel waveguide [Figs. 2(e) and (f) ]. As one can see, the measured modal profiles are in a good agreement with those calculated based on the reconstructed 2D index distributions. It should be pointed out the formed channel waveguides are multimode at 633 nm as well as at 1064 nm (the expected laser wavelength). One can reduce the number of the guided modes by decreasing the index contrast between the waveguide and substrate through, for example, further annealing treatment. In addition, the propagation loss of the TE00 and TM00 modes is determined to be ~1.0 and ~1.8 dB/cm, respectively, which shows acceptable guiding properties.
Figure 3 shows the comparison of the room temperature micro-luminescence spectra of Nd3+ ions as obtained after excitation in the waveguide’s active volume (i.e., between the barrier and air) and in the bulk (several microns away from the channel waveguide). Data correspond to the 4F3/2→4I11/2 laser channel and were obtained from the post annealed structure (i.e., after annealing at 260°C for 30 min). From a first inspection it is clear that the fluorescence intensity obtained from the active volume is quite close (less than 3% different) to that obtained from the original Nd:LGS network. This fact suggests that, in the waveguide region, both the pumping efficiency and the 4F3/2 quantum yield of Nd3+ ions have not been modified by the ion implantation procedure. This means that the outstanding fluorescence properties of Nd3+ ions are not deteriorated in the active volume so that the fabricated waveguides emerge as promising integrated laser elements. From a fundamental point of view, the observed unaffected fluorescence efficiency reveals that the waveguide region is virtually free from lattice defects. This is advantageous over proton implanted waveguides in Nd:YAG ceramics in which strong fluorescence quenching has been found to owing to a large density of ion induced defects .
Although the Nd3+ fluorescence properties have been found to be almost unaltered in the waveguiding regions, they have been strongly modified in the barrier regions (i.e., nuclear stopping region). This is clearly shown in Fig. 4 , which includes the fluorescence images of the Nd:LGS waveguides obtained in terms of the spatial variation of the intensity, spectral induced shift and width of the Nd3+ emission line at around 9400 cm−1. From Fig. 4(a) it is clear that the fluorescence of Nd3+ ions retains its original properties in the waveguiding region and it is only modified in the barrier region. The refractive index barrier is accompanied by a sharp reduction in the Nd3+ fluorescence efficiency, a remarkable broadening and a slight blue-shift of fluorescence lines. All these features suggest that the nuclear collisions taking place at the end of proton tracks has caused the appearance of lattice defects that quenches the Nd3+ fluorescence. The nature of these defects is unknown at this moment and cannot be elucidated from the fluorescence maps. Previous works have evidenced the creation of nano-voids and amorphous volumes at nuclear damage region of light ion implanted crystalline materials . These defects cause an overall reduction in the Nd3+ fluorescence intensity due to the null contribution of voids as well as to the lower fluorescence efficiency of amorphous materials . In addition, it has been also known that light ion implantation in crystals leads to the appearance of lattice disruptions at the end of ion paths that could also act as fluorescence quenching centers . It should be pointed out that the strong lattice distortion at the end of ion tracks has been also found in proton implanted Nd:MgO:LiNbO3 channel waveguides [25,26]. The blue-shift is attributed to a decrease in the crystal field affecting Nd3+ ions, probably due to a modification in the inter-atomic distances. This modification can be, indeed, explained in terms of the atomic displacements caused by nuclear collisions. In the present work, the proton implantation creates defective regions at the barriers, which may result in the broadening of the emission lines and decrease of the emission intensity. In this sense, the photoluminescence mapping of the H-implanted Nd:LGS waveguides offers a clear proof to show the presence and extension of crystalline and non-crystalline (or partially amorphous) regions in one material.
We have reported for the first time on the fabrication of optical channel waveguides in Nd:LGS laser crystals by using mask-assisted proton implantation. The waveguides have shown good guiding properties (well-defined modal profiles and acceptable propagation losses) and unperturbed fluorescence properties. The confocal fluorescence images have revealed that the Nd:LGS network has not been modified in the waveguide’s active volume but only at the nuclear damage region (refractive index barrier) where a large density of defects has been induced. By applying suitable optical systems, one can expect laser oscillations in the formed Nd:LGS channel waveguides.
The work is supported by the National Natural Science Foundation of China (No. 10925524), the Program for New Century Excellent Talents for Universities, China (No. NCET-08-0331) and the 973 Project (No. 2010CB832906). D. Jaque thanks the Universidad Autónoma de Madrid, Comunidad Autónoma de Madrid and Ministerio de Ciencia e Innovación for financial support under projects MAT2007-64686, CCG08-UAM/MAT-4434 and Phama S2009/MAT-1756).
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