Proton beam writing of Nd:GGG crystalsas new waveguide laser sources

Yicun Yao,1 Ningning Dong,1 Feng Chen,1,* Sudheer Kumar Vanga,2 and Andrew Anthony Bettiol21School of Physics, State Key Laboratory of Crystal Materials and Key Laboratory of Particle Physics and

Particle Irradiation, Shandong University, Jinan 250100, China2Centre for Ion Beam Applications, Department of Physics, National University of Singapore 117542, Singapore

*Corresponding author: drfchen@sdu.edu.cn

Received September 14, 2011; revised September 21, 2011; accepted September 27, 2011;posted September 28, 2011 (Doc. ID 154563); published October 20, 2011

Focused proton beam writing has been utilized to fabricate optical channel waveguides in Nd:GGG crystals. The1MeV proton beam irradiation creates a local modified regionwith positive refractive index changes at the end of theproton trajectory, in which the channel waveguide could confine the light field in a symmetric way. Room-temperature laser emission has been achieved at 1063:7 nm, with absorbed pump power of 61mW (at 808nm).The obtained slope efficiency of the Nd:GGG waveguide laser system is as high as 66%, which is, to our best knowl-edge, the highest value for integrated lasers from ion beam processed channel waveguide systems. © 2011 OpticalSociety of AmericaOCIS codes: 230.7380, 160.3380, 140.3570.

Optical waveguides confine light propagation in verysmall volumes, in which the light intensities reach veryhigh levels compared to in bulk [1]. As a result, laserperformance in waveguides may possess a few advan-tages with respect to that in bulk, such as low lasingthreshold and high efficiency. The compact size togetherwith these features make laser devices on waveguidingstructures especially useful in integrated photonic sys-tems [2]. In practice, benefiting from the confinementof two-dimensional geometry, channel waveguides offersuperior restriction of light by enhancing the inside op-tical intensities for higher efficiencies [3]. Ion implanta-tion is a powerful technique for modification of variousmaterials; particularly, it has shown wide feasibility inversatile optical materials [4–7]. By using the implanta-tion of normal-size ion beams (typically with a beamdiameter of 1mm), channel waveguides have been fabri-cated with patterned stripe microstructures as implanta-tion masks. This solution is successful in many examplesfor channel waveguide production; however, the qualityof the stripe masking does influence the final modal prop-erties of the waveguides and limit the applications offabricating more precise devices.Recently, proton beam writing (PBW) emerged as a

powerful tool in the micromanufacturing of diverse ma-terials [8]. Proton beams with energy of 1–3MeV are fo-cused to micro- or submicrometric scales, which issuitable to achieve a direct three-dimensional microma-chining of materials [9]. PBW has been successfully usedto produce channel waveguides in a few materials, in-cluding organic polymers, glasses, silicon, and diamond[10–14]. More recently, PBW has been applied to producechannel waveguides in laser crystals, i.e., Nd:YAG, andcw lasers with highly symmetric modes have been rea-lized from the waveguides [15–17]. This opens a way touse this powerful tool to construct more intriguing guid-ing structures to achieve diverse applications. Particu-larly, novel, highly efficient integrated light sourcesmay be obtained through the PBW channel waveguidesin laser crystals.

Neodymium doped gadolinium gallium garnet(Nd3þ:Gd3Ga5O12 or Nd:GGG) is one of the most excel-lent gain media for solid state lasers, and has attractedmuch attention owing to its advantageous features, suchas good thermal conductivities, higher separation coeffi-cient of Nd3þ, and fewer growth defects [18]. However,normal chemical methods such as metal ion thermalin-diffusion and ion exchange, cannot be applied toNd:GGG because of its stable chemical properties. Asof yet, channel waveguides have been fabricated in anNd:GGG crystal only by “physical” techniques, such asmasked ion implantation and femtosecond (fs) laserinscription, and the waveguide lasers were realized[19–21]. In this Letter, we use PBW to fabricate highlysymmetric channel waveguides in Nd:GGG and reporton waveguide laser emissions at 1063:7 nm at roomtemperature.

The optically polished Nd:GGG crystal (doped by1 at:% Nd3þ ions) used in this work was cut into waferswith dimensions of 10mm × 5mm × 1:5mm. The 1MeVPBW takes place on a 10mm × 5 mm surface of the sam-ple, with the 5mm direction as the writing direction. Theprocess is simulated by using the software SRIM 2010code [22]. Modification of the incident protons is throughthe created defects. The defect concentration profile (de-fect per atom, DPA) and Hþ ion distribution are shown inFig. 1. As we can see, the defects maintain at a very lowlevel during the first 8 μm of the protons’ track, while itreaches a peak at depth of ∼8:7 μm. Nevertheless, themaximum DPA, located at the end of Hþ trajectory, wasas low as 0.0026, which means the lattices of the Nd:GGGcrystal are well preserved even in the mostly disorderedregions.

The PBW process is carried out utilizing the facility atthe Centre for Ion Beam Applications, National Univer-sity of Singapore. The 1MeV proton beam is focused tohave a diameter of 0:5 μm. During the writing process,perpendicular scanning of the proton beam over a dis-tance of 4 μm is also taking place, to ensure an appropri-ate width of the formed waveguide; see Fig. 2(a) for a

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schematic of the fabrication process. The writing fluenceachieved is at 2 × 1016 protons=cm2.Figure 2(b) shows the image on the cross section of the

sample under polarized light, using a metalloscope (CarlZeiss, Axio Imager). One can clearly see the light guidinginside the crystal, which is located at a depth of 8:6 μmbelow the sample surface. This distance is consistentwith the mean projected range of 1MeV protons inNd:GGG crystal from the SRIM calculation. It confirmsthat the PBW channel waveguide in Nd:GGG is buriedat the end of the ions’ track (i.e., in the so-called nucleardamage region), which is similar to proton-beam-writtenNd:YAG channel waveguides.We have performed the end-face coupling experiment

at 633 nm. The near-field intensity distribution of the out-put light is shown in Fig. 3(b). As one can see, the lightmode is highly symmetric. To obtain the refractive indexmodulation induced by the incident proton beams, weapply a simple technique to measure the NA of the wave-guide [23]. By using the following formula:

Δn ¼ sin2Θm

2n; ð1Þ

with Θm as the maximum incident angular deflection atwhich no guiding light occurs, and n as the refractiveindex of the substrate, we estimate the maximum in-crease of the refractive index to be ∼1:2 × 10−3 (the re-fractive index of the substrate measured to be 1.965,using the m-line technique). According to this measure-ment, we reconstruct the refractive index modulation,as shown in Fig. 3(a). Based on this distribution, we si-mulate the light propagation in the waveguide with thefinite-difference beam propagation method by using thecommercial software BeamPROP and obtain the modalprofile of the TM00 guiding mode [Fig. 3(c)]. As one cansee from Figs. 3(a) and 3(b), there is a reasonable agree-ment between the calculated modal profile and the mea-sured one, which suggests that the reconstruction of therefractive index distribution is successful. We also deter-mine the propagation loss of the waveguide, by using theFabry–Perot method [24], to be ∼4:3 dB=cm. The propa-gation loss is expected to be further reduced by optimiz-ing the writing process, also by extra annealing steps,as has already been done in the PBW Nd:YAG channelwaveguides [17].

To achieve waveguide lasers from the microscale reso-nant cavities, we arrange another end-face couplingsystem with a tunable Ti:sapphire cw laser (Coherent110) as the pumping source. During the experiment,the Nd:GGG sample is adhered with two mirrors on bothend facets to construct a Fabry–Perot cavity. A mirrorwith transmission of 98% at 808 nm and reflectivity of>99% at 1064 nm serves as the input one, and a mirrorwith reflectivity >99% at 808 nm and ∼95% at 1064 nmis the output one, respectively. A convex lens (with focuslength of 25mm) is used to focus the 808 nm pumpinglight beam into the waveguide cavity, and the emittedwaveguide laser is gathered with a 20× microscope ob-jective lens and imaged by an infrared CCD camera.The emission spectrum of the waveguide lasers is char-acterized by a spectrometer. Figure 4 shows the mea-sured emission spectrum. The center of the emissionpeak is located at 1063:7nm, which corresponds to the4F3=2 →

4I11=2 transition of the Nd3þ ions. The FWHM

Fig. 1. (Color online) Defects (DPA, solid line) and Hþ con-centration (dashed line) as a function of the ion penetrationdepth in Nd:GGG from SRIM calculation. The spatial peak offsetbetween the defect and Hþ concentration profiles is 0:1 μm.

Fig. 2. (Color online) (a) Schematic of the PBW processingof Nd:GGG channel waveguides and (b) metalloscope imageof the cross section of the waveguide sample.

Fig. 3. (Color online) (a) Reconstructed refractive indexchange distribution of the Nd:GGG channel waveguide at633 nm,with the index of the substrate to be 1.965; (b) measuredand (c) calculated modal profile of the TM00 fundamental modeof the waveguide.

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of the emission line is ∼0:6nm, which clearly demon-strates the realization of waveguide laser emission.Figure 5 depicts the output cw waveguide laser power

as a function of the absorbed pump power for the PBWNd:GGG channel waveguide. From this figure, one candetermine that the pump threshold is ∼61mW, while theslope efficiency here reaches a value as high as 66%,which is, to our best knowledge, the highest value forintegrated lasers from ion beam processed channelwaveguide systems. The maximum output power wasmeasured to be 21mW at pump power of 91mW, corre-sponding to an optical-to-optical convention efficiency of23%. Compared with the fs-laser-written Nd:GGG wave-guide laser system, the PBWwaveguide laser system pos-sesses higher slope efficiency and optical conversionefficiency, but at the same time, it is also with a higherlasing threshold [21].In conclusion, we have demonstrated the fabrication of

the buried channel waveguide in a Nd:GGG crystal byusing the PBW technique. By irradiation of the 1MeV fo-cused proton beam at the fluence of 2 × 1016 protons=cm2,refractive index increaseof 0.0012with respect to the bulkhas been induced at the end of the Hþ trajectory. The cwwaveguide lasers at 1063:7nmhave been realizedwith theoptical pump at 808 nm at room temperature, reaching ahigh slope efficiency of ∼66%.

The work is supported by the National Natural ScienceFoundation of China (NSFC) (no. 10925524), the Promo-tive Research Fund for Excellent Young and Middle-AgedScientists of Shandong Province, China (BS2009CL003),and the 973 Project (no. 2010CB832906) of China.

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Fig. 4. (Color online) Emission spectrum of the PBW Nd:GGGwaveguide laser. Inset, image of the output laser mode (TM00).

Fig. 5. (Color online) Measured output waveguide laser poweras a function of the absorbed pump power (balls). The greensolid line shows the linear fit of the experimental data.

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