Efficient laser emissions at 106 μm of swift heavy ion irradiated Nd:YCOB waveguides
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Efficient laser emissions at 1:06 μm of swift heavy ion irradiated Nd:YCOB waveguides Yingying Ren, 1 Ningning Dong, 1 Yuechen Jia, 1 Lilong Pang, 2 Zhiguang Wang, 2 Qingming Lu, 3 and Feng Chen 1, * 1 School of Physics, State Key Laboratory of Crystal Materials and Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Jinan 250100, China 2 Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 73000, China 3 School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China *Corresponding author: [email protected] Received September 20, 2011; accepted October 11, 2011; posted October 19, 2011 (Doc. ID 155033); published November 22, 2011 We report on the fabrication of Nd:YCOB (Nd:YCa 4 OðBO 3 Þ 3 ) optical waveguides by using 170 MeV Ar 8þ ion irradia- tion at an ultralow fluence of 2 × 10 12 ions=cm 2 . The confocal microphotoluminescence investigation on the pro- duced waveguides has shown the well-preserved fluorescence features within the guiding layer with respect to the bulks. Under the optical pump at wavelength of 810 nm, continuous wave waveguide lasers at 1061:2 nm have been generated at room temperature with a high slope efficiency of ∼67:9%. © 2011 Optical Society of America OCIS codes: 230.7390, 140.3390, 130.3120. Neodymium-doped yttrium calcium oxyborate (Nd:YCa 4 OðBO 3 Þ 3 or Nd:YCOB) is an excellent self- frequency-doubling (SFD) crystal with a combination of photoluminescence features of the Nd 3þ ions and the nonlinear properties of YCOB matrix, which makes it very promising for laser diode directly pumped visible la- sers [1,2]. By using waveguide technology a unique plat- form of multifunctional applications could be realized through highly compact on-chip circuits with small sizes [3]. 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 tradi- tional laser plus nonlinear waveguide configuration (e.g., Nd:YVO 4 þ KTP hybrid system) seems to be quite diffi- cult to achieve high overlap coupling due to the diverse properties (e.g., refractive indices, physical densities) of the laser and nonlinear crystals for waveguide fabrica- tion [8]. 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. Neverthe- less, the first significant step to realize the SFD wave- guide lasers is to obtain the fundamental frequency laser generation, e.g., at wavelength of 1:06 μm. A few techniques have been utilized to fabricate wave- guides in optical crystals, however, only the “physical” methods, such as ultrafast laser writing [9] 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 la- sers. The normal ion implantation technique, which has been widely used to produce waveguiding structures in a broad range of optical materials, creates negative refrac- tive 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 1 MeV=amu), the refractive index of the substrates is modified by the electronic excitation in- duced damages, which mainly happens during most path of the incident ions’ trajectory via the impact of amor- phous or highly defective nanotracks from a single ion or the overlap of a few ions [12–19]. In addition, the re- quired fluences for swift ions are considerably lower than those of the normal ion implantation, which saves the time for the waveguide fabrication [12]. This technique has been successfully applied to produce waveguides in a few optical crystals, such as LiNbO 3 [12,13,19], Nd:YAG [20,21], KGdðWO 4 Þ 2 [22], and K 1-x Li x Ta 1-y Nb y O 3 [23]. Particularly, the swift heavy ion irradiated Nd:GdCOB (belongs to same family of Nd:YCOB) waveguides have shown excellent nonlinear properties for green laser gen- eration [24]. In this Letter, we report, to our best knowl- edge for the first time, on the fabrication of Nd:YCOB waveguides by using swift Ar 8þ ion irradiation, and the continuous wave (cw) waveguide lasers generation at a wavelength of 1:06 μm. The Nd:YCOB (doped by Nd 3þ ions with concentration of 5 at:%) wafer was cut along the direction to satisfy the 1061 nm fundamental wave to 531 nm second harmonic generation. It was with 5 mm × 5 mm × 2 mm size and op- tically polished. One sample surface of 5 mm × 5 mm was irradiated with Ar 8þ ions by using the facility at the In- stitute of Modern Physics, Chinese Academy of Sciences. The accelerating energy was set at 880 MeV and the flu- ence was at 2 × 10 12 cm -2 . In order to slow down the in- cident ions, a stopper foil of aluminum was placed in front of the sample. The ion current density was kept less than 30 nA=cm 2 to avoid additional charging and heating effect on the sample. According to our calculation by SRIM (Stopping and Range of Ions in Matter) code [25], the practical irradiation energy reaching on the sample surface was 170 MeV. Figure 1(a) shows the microscope image of the 170 MeV Ar 8þ ion irradiated Nd:YCOB sample. It can be clearly seen that the ion beam modified region of the Nd: YCOB is with thickness w ¼ 33:8 μm, which is in good agreement with the mean projected range of the 170 MeV Ar 8þ ions in the Nd:YCOB crystal calculated by the SRIM 2010 code. It should be noted that we did not observe any December 1, 2011 / Vol. 36, No. 23 / OPTICS LETTERS 4521 0146-9592/11/234521-03$15.00/0 © 2011 Optical Society of America
Transcript of Efficient laser emissions at 106 μm of swift heavy ion irradiated Nd:YCOB waveguides
Efficient laser emissions at 1:06 μm of swift heavyion irradiated Nd:YCOB waveguides
Yingying Ren,1 Ningning Dong,1 Yuechen Jia,1 Lilong Pang,2 Zhiguang Wang,2 Qingming Lu,3 and Feng Chen1,*1School of Physics, State Key Laboratory of Crystal Materials and Key Laboratory of Particle Physics
and Particle Irradiation (MOE), Shandong University, Jinan 250100, China2Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 73000, China
3School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China*Corresponding author: [email protected]
Received September 20, 2011; accepted October 11, 2011;posted October 19, 2011 (Doc. ID 155033); published November 22, 2011
We report on the fabrication of Nd:YCOB (Nd:YCa4OðBO3Þ3) optical waveguides by using 170MeV Ar8þ ion irradia-tion at an ultralow fluence of 2 × 1012 ions=cm2. The confocal microphotoluminescence investigation on the pro-duced waveguides has shown the well-preserved fluorescence features within the guiding layer with respect tothe bulks. Under the optical pump at wavelength of 810nm, continuous wave waveguide lasers at 1061:2 nm havebeen generated at room temperature with a high slope efficiency of ∼67:9%. © 2011 Optical Society of AmericaOCIS codes: 230.7390, 140.3390, 130.3120.
Neodymium-doped yttrium calcium oxyborate(Nd:YCa4OðBO3Þ3 or Nd:YCOB) is an excellent self-frequency-doubling (SFD) crystal with a combination ofphotoluminescence features of the Nd3þ ions and thenonlinear properties of YCOB matrix, which makes itvery promising for laser diode directly pumped visible la-sers [1,2]. By using waveguide technology a unique plat-form of multifunctional applications could be realizedthrough highly compact on-chip circuits with small sizes[3]. Waveguide laser devices are intriguing componentsfor diverse photonic applications owing to the enhancedoptical gain as well as the excellent performances, suchas low lasing threshold [4,5]. Particularly, based on theSFD crystals, it is possible to facilitate the integrationof the intracavity visible waveguide lasers that do notrequire the additional frequency conversion through anonlinear crystal waveguide [6,7]. In addition, the tradi-tional laser plus nonlinear waveguide configuration (e.g.,Nd:YVO4 þ KTP hybrid system) seems to be quite diffi-cult to achieve high overlap coupling due to the diverseproperties (e.g., refractive indices, physical densities) ofthe laser and nonlinear crystals for waveguide fabrica-tion [8]. In this sense, the Nd:YCOB waveguide could bea suitable platform to host the intracavity for the lasergeneration of infrared light and synchronously realizethe nonlinear process in the same structure. Neverthe-less, the first significant step to realize the SFD wave-guide lasers is to obtain the fundamental frequencylaser generation, e.g., at wavelength of 1:06 μm.A few techniques have been utilized to fabricate wave-
guides in optical crystals, however, only the “physical”methods, such as ultrafast laser writing [9] and energeticion beam implantation/irradiation [10,11] are applicableto Nd:YCOB due to its stable chemical properties. Asof yet, there is no report on the Nd:YCOB waveguide la-sers. The normal ion implantation technique, which hasbeen widely used to produce waveguiding structures in abroad range of optical materials, creates negative refrac-tive index layers (so-called “optical barriers”) at the endof ion range through the nuclear collision correlateddamages. For the swift heavy ion irradiation (with energyhigher than 1MeV=amu), the refractive index of the
substrates is modified by the electronic excitation in-duced damages, which mainly happens during most pathof the incident ions’ trajectory via the impact of amor-phous or highly defective nanotracks from a single ionor the overlap of a few ions [12–19]. In addition, the re-quired fluences for swift ions are considerably lower thanthose of the normal ion implantation, which saves thetime for the waveguide fabrication [12]. This techniquehas been successfully applied to produce waveguides ina few optical crystals, such as LiNbO3 [12,13,19], Nd:YAG[20,21], KGdðWO4Þ2 [22], and K1−xLixTa1−yNbyO3 [23].Particularly, the swift heavy ion irradiated Nd:GdCOB(belongs to same family of Nd:YCOB) waveguides haveshown excellent nonlinear properties for green laser gen-eration [24]. In this Letter, we report, to our best knowl-edge for the first time, on the fabrication of Nd:YCOBwaveguides by using swift Ar8þ ion irradiation, and thecontinuous wave (cw) waveguide lasers generation at awavelength of 1:06 μm.
The Nd:YCOB (doped by Nd3þ ions with concentrationof 5 at:%) wafer was cut along the direction to satisfy the1061 nm fundamental wave to 531 nm second harmonicgeneration. It was with 5mm × 5mm × 2mm size and op-tically polished. One sample surface of 5mm × 5mm wasirradiated with Ar8þ ions by using the facility at the In-stitute of Modern Physics, Chinese Academy of Sciences.The accelerating energy was set at 880MeV and the flu-ence was at 2 × 1012 cm−2. In order to slow down the in-cident ions, a stopper foil of aluminum was placed infront of the sample. The ion current density was kept lessthan 30 nA=cm2 to avoid additional charging and heatingeffect on the sample. According to our calculation bySRIM (Stopping and Range of Ions in Matter) code [25],the practical irradiation energy reaching on the samplesurface was 170MeV.
Figure 1(a) shows the microscope image of the170MeV Ar8þ ion irradiated Nd:YCOB sample. It can beclearly seen that the ion beam modified region of the Nd:YCOB is with thickness w ¼ 33:8 μm, which is in goodagreement with the mean projected range of the 170MeVAr8þ ions in the Nd:YCOB crystal calculated by the SRIM2010 code. It should be noted that we did not observe any
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dark modes by using the m-line technique through theprism coupler (Metricon 2010). However, by using anend-face coupling system (at 632:8 nm), we found clearguiding modes [e.g., see TM mode profile in Fig. 1(b)] ofa multimode waveguide structure, which suggests theconfiguration of the waveguide structure is a “buried”layer and must be of positive index changes with respectwith the unmodified bulk. The maximum index alterna-tion of the waveguide layer was determined to be∼3 × 10−3, which was estimated from the measurement ofthe NA of the planar waveguide [26]. This technique hasbeen proved to be especially successfully to obtain therefractive index alternations of multimode waveguides.We used a fiber-coupled confocal microscope (Olym-
pus BX-41) to investigate the microphotoluminescence(μ-PL) properties of the Nd:YCOB waveguides. The488 nm excitation laser was focused onto the crosssection by using a 100× microscope objective with NAN:A: ¼ 0:95. And the backscattered Nd3þ fluorescenceemission signals were collected with the same objective,after passing through a series of filters and a confocalpinhole, were collected by a fiber-coupled spectrometer(SPEX500M, USA). The sample was mounted on an XYmotorized stage with a high spatial resolution of 100 nm.Figure 2(a) shows the room-temperature PL emission
spectrum of the Nd3þ ions in Nd:YCOB crystal correlatedto the 4F3=2 →
4I9=2 transition channel. We focused on the881:6 nm emission line in order to obtain the detailedmodification of the Ar8þ ion beams on the fluorescenceproperties of the waveguides. Figures 2(b)–2(d) depictthe spatial dependence of the emitted intensity, peak po-sition, and line width (FWHM) of the 881:6 nm emissionline, respectively. As one can see, the intensity of fluor-escence signals decreases by only 10% in the waveguidewith respect to the bulk, which means the majority ofthe PL active features has been preserved, without clearquenching. Nevertheless, the peak position shifts by0:7 cm−1, and the emission line is broadened by maxi-mally 4 cm−1, suggesting the obvious modification of theNd: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 clo-sely between two dielectric mirrors to construct theFabry–Perot lasing cavity (the input one with transmis-sion of 98% at 810 nm and reflectivity >99% at 1:06 μmand the output one with reflectivity >99% at 808 nm and∼95% at 1:06 μm, respectively). The 810 nm pump beamfrom a Ti:sapphire cw laser (Coherent 110) was focused
into the cavity by using a convex lens (with focus lengthof 25mm), and the emission waveguide laser at ∼1:06 μmwas collected with a 20× microscope objective and im-aged by an infrared CCD camera.
Figure 3 shows the room-temperature laser emissionspectrum of the Nd:YCOB waveguide. The emission lineis centered at 1061:2nm, which corresponds to the mainfluorescence transition of 4F3=2 →
4I11=2 channel of Nd3þions. The FWHM of the emission line is ∼1nm, whichclearly demonstrates the realization of waveguide laseremission.
Figure 4 depicts the output 1061:2nm waveguide laserpower as a function of the absorbed pump power at810 nm. Based on this data, one could determine that thepump threshold (Pth) for laser generation is ∼31:5mW,
Fig. 1. (Color online) (a) Microscope image of the Ar8þ ionirradiated Nd:YCOB sample, and (b) the measured near-field in-tensity of the TM mode.
Fig. 2. (Color online) (a) Room-temperature luminescenceemission spectrum correlated to Nd3þ ions at 4F3=2 →
4I9=2 tran-sition of the Nd:YCOB crystal, the spatial dependence of the(b) emitted intensity, (c) spectral shift, and (d) emission width(at FWHM) of the 881:6nm emission line.
Fig. 3. (Color online) Laser emission spectrum from the170MeV Ar8þ irradiated Nd:YCOB planar waveguide. The insetshows the laser modal profile (TE0) at 1061:2 nm.
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and the slope efficiency (Φ) is as high as 67.9%. Themeasured maximum output power is ∼35mW at pumppower of 83mW, corresponding to an optical-to-opticalconversion efficiency of 42%. The excellent lasing perfor-mance of the Nd:YCOB waveguides suggests that theswift Ar8þ ion irradiation does not affect the active fea-tures of material, which opens up an exciting possibilityfor further SFD lasing of the waveguides.In conclusion, we have reported on the fabrication of
an Nd:YCOB planar waveguide by using swift Ar8þ ionirradiation. The fluorescence properties of the bulk ma-terials have been well preserved in the waveguides,although clear modification happens after the irradiation.The cw waveguide laser in Nd:YCOB has been realizedfor the first time, which exhibits excellent performancesat 1:06 μm oscillations. Future work would be performedon the achievement of the SFD laser from the swift heavyion irradiated Nd:YCOB waveguides.
The work is supported by the National Natural ScienceFoundation of China (NSFC) (10925524) and the 973 Pro-ject of China (2010CB832906 and 2010CB832902).
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Fig. 4. (Color online) Output laser power at 1061:2nm as afunction of absorbed pump power at 810nm obtained fromthe Nd:YCOB waveguide.
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