Highly Efficient Double Tungstate Waveguide Lasers
Highly Efficient Double Tungstate Waveguide Lasers D. Geskus, K. van Dalfsen, F. Ay, K. Wörhoff, S. Aravazhi, and M. Pollnau The potassium double tungstates KGd(WO4)2, KY(WO4)2, and KLu(WO4)2 are excellent candidates for solid-state lasers, see Ref. [1] and Refs. therein, because of their high refractive index of ~2.02.1, the large transition cross-sections of rare-earth (RE) ions doped into these hosts, the possibility to incorporate very large concentrations of RE3+ ions, reaching the stoichiometric structure KRE(WO4)2, a long inter-ionic distance of ~0.5 nm that allows for large doping concentrations without lifetime quenching, and a reasonably large thermal conductivity of ~3.3 W m-1 K-1. These advantages have been exploited to demonstrate thin-disk lasers, broadly tunable and high-energy ultrashort-pulse lasers, and low-quantum-defect lasers.
1. Lattice-matched, High-refractive-index-contrast Double Tungstate Waveguides
LASER OUTPUT POWER [mW]
We apply liquid phase epitaxy at temperatures of 920-923°C to grow RE3+-doped double tungstate thin layers onto undoped, (010)-orientated, laser-grade polished KYW substrates of 1 cm2 size. While the use of low-temperature chloride solvents can lead to 3D island nucleation and insertion defects [2], the solvent K2W2O7 results in excellent layer and interface quality and has enabled the fabrication of KYW:Yb3+ planar waveguides with a thickness in excess of 10 µm, in which the firstever laser operation of a double tungstate waveguide was demonstrated with high slope efficiencies up to 80% [3]. A breakthrough has been obtained by co-doping the active layer with optically inert Gd3+ and Lu3+ ions [4]. Since Gd3+ and Lu3+ change the lattice parameters in opposite directions, choice of the right fractions of these two ions allows for lattice matching of the RE3+-activated layer with the undoped substrate (Fig. 1, left). Besides, co-doping with large amounts of Gd3+ and Lu3+ ions significantly increases the refractive index contrast of such a co-doped layer with respect to the undoped substrate by more than an order of magnitude to ~10-2 (Fig. 1, left) [5], thereby allowing for much thinner (only a few µm-thick) single-transverse-mode waveguides, resulting in tighter pump and laser mode confinement as well as easing the requirements on microstructuring.
T = 23 %, η = 82.3 %
200
T = 20 %, η = 72.4 % T = 10 %, η = 60.2 % T = 5.0 %, η = 45.5 %
150
T = 1.7 %, η = 23.8 % 100
50
(a) 0 0
50
100
150
200
250
300
ABSORBED PUMP POWER [mW]
Fig. 1. (left) Lattice mismatch along the a axis and refractive index increase for KGdxLuyYb1-x-y(WO4)2 versus the fractions of Gd3+ and Lu3+ ions [5]; (right) laser output power from a Gd3+, Lu3+ co-doped planar waveguide as a function of absorbed pump power for various outcoupling mirror transmissions [6].
In such co-doped layers continuous-wave laser operation in a planar waveguide configuration was observed at 1025 nm [6]. For 23% output coupling, a maximum output power of 195 mW was obtained; a slope efficiency of 82.3% was derived (Fig. 1, right), which is the highest value yet reported for a planar waveguide laser to date.
2. Microstructured Channel Waveguide Lasers The fabrication of channel waveguides in bulk double tungstates by femtosecond-laser writing or ion-beam implantation [7] typically induces considerable waveguide propagation losses and leads to comparatively small refractive index changes, necessitating rather large mode sizes. The small layer thickness of our co-doped waveguides greatly facilitates microstructuring of double tungstates [8]. Exploiting this option, we fabricated ridge channel waveguides in double tungstate layers by use of standard photo-resist as a mask and Ar+ beam etching (Fig. 2, left). The obtained KGd1-xLux(WO4)2:Yb3+ channel waveguides with well-defined cross-sections of a few µm2 and excellent mode confinement allowed us to demonstrate channel waveguide lasers with 418 mW of continuous-wave output power at 1023 nm and a slope efficiency of 71% versus launched pump power at 981 nm (Fig. 2, right), which is the highest slope efficiency in single-directional operation ever reported for a dielectric channel waveguide laser [9]. The degree of output coupling was 70%.
Output Power (mW)
500 Measured performance with Tout = 70%
400
Linear fit: Slope efficiency = 71%, Threshold = 40 mW
300 200 100 0 0
100
200
300
400
500
600
700
Launched Pump Power (mW) Fig. 2. (left) SEM micrograph of a microstructured channel waveguide before overgrowth [8]; (right) inputoutput curve of the KGd1-xLuxW:Yb3+ channel waveguide laser pumped at 981 nm and lasing at 1023 nm [9].
|←Pout= ~11 mW→|
Normalized Intensity
Normalized Laser Emission
1.0 0.8 0.6 0.4 0.2 0.0 960
980
1000
1020
1040
Wavelength (nm)
1060
1.0 0.8 Pump 0.6
Laser Emission
0.4 0.2 0.0 970
975
980
985
Wavelength (nm)
Fig. 3. (left) Measured emission spectra under pumping at 930 nm when changing the angle of the reflective grating [9]; (right) spectrum recorded during lasing with the smallest attained quantum defect [9].
Also planar and channel waveguide lasers operating at the zero-phonon line at 980.6 nm were demonstrated [10]. By grating tuning in an extended cavity and pumping at 930 nm, we demonstrated laser operation from 980 nm to 1045 nm (Fig. 3, left) [9]. When pumping at 973 nm, lasing at 980 nm with an extremely low quantum defect of 0.7% was achieved [9]. This is currently the world record for any rare-earth-ion-doped laser.
3. On-chip Integrated Laser Following an approach for nano-structuring of silicon [11], deeply etched Bragg gratings were fabricated by focused ion beam (FIB) milling in KGdxLu1-x(WO4)2:Yb3+. By optimizing parameters such as dose per area, dwell time and pixel resolution the redeposition effects were minimized and grating structures more than 4 µm in depth with an improved sidewall angle of ~5º were achieved. An on-chip integrated laser cavity at ∼980 nm was achieved by defining a FIB reflective grating and FIB polished waveguide end-facet (Fig. 4, left). With this cavity, an integrated waveguide laser was demonstrated in KGdxLu1-xW:Yb3+ (Fig. 4, right) [12].
Fig. 4. (left) Schematic of experimental setup configuration used for laser experiments [12]; (right) measured (dots) and modeled (line) performance of a laser cavity involving a 12-µm-wide grating structure [12].
4. Tm-doped Double Tungstate Waveguide Lasers Also doping of double tungstate thin films with Tm3+ ions is very promising and has resulted in similarly astounding laser performance. High-quality monoclinic planar waveguide crystals of Tmdoped KY(WO4)2 were grown. For the first time, planar waveguide lasing in the 2-µm spectral range was demonstrated in a double tungstate waveguide. The maximum achieved fundamental-mode continuous-wave output power was 32 mW [13]. Meanwhile, we have generated significantly higher output powers and excellent slope efficiencies in co-doped channel waveguides (numbers to be published) [14].
Collaborations 1. Advanced Photonics Laboratory, Institute of Imaging and Applied Optics, Ecole Polytechnique Fédérale de Lausanne, Switzerland. 2. Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy, Berlin, Germany. 3. Departamento Física Aplicada, Facultad de Ciencias, Universidad de Cantabria, Santander, Spain. 4. Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, Switzerland. 5. Optoelectronics Research Centre, University of Southampton, United Kingdom. 6. Low Temperature Division, MESA+ Institute for Nanotechnology, University of Twente.
Funding 1. NWO Vici Personal Grant “Integrated optical structures: From active thin films to micro/nanocavities to ultrashort-pulse lasers” (2006-2011). 2. Senter-Novem / STW – IOP Photonic Devices Project “On-chip integrated NH3 human gas sensor” (2009-2013).
References [1] M. Pollnau, Y.E. Romanyuk, F. Gardillou, C.N. Borca, U. Griebner, S. Rivier, and V. Petrov, "Double tungstate lasers: From bulk toward on-chip integrated waveguide devices", IEEE J. Select. Topics Quantum Electron. 13 (3), 661-671 (2007). Invited Paper. [2] Y.E. Romanyuk, I. Utke, D. Ehrentraut, V. Apostolopoulos, M. Pollnau, S. García-Revilla, and R. Valiente, "Lowtemperature liquid phase epitaxy and optical waveguiding of rare-earth-ion doped KY(WO4)2 thin layers", J. Cryst. Growth 269 (2-4), 377-384 (2004). [3] Y.E. Romanyuk, C.N. Borca, M. Pollnau, S. Rivier, V. Petrov, and U. Griebner, "Yb-doped KY(WO4)2 planar waveguide laser", Opt. Lett. 31 (1), 53-55 (2006). [4] F. Gardillou, Y.E. Romanyuk, C.N. Borca, R.P. Salathé, and M. Pollnau, "Lu, Gd co-doped KY(WO4)2:Yb epitaxial layers: Towards integrated optics based on KY(WO4)2", Opt. Lett. 32 (5), 488-490 (2007). [5] S. Aravazhi, D. Geskus, M. van Dalfsen, S. Harkema, K. Hametner, D. Günther, S. Garcia-Blanco, and M. Pollnau, "Lattice-matched, high-refractive-index-contrast, and highly RE-doped (RE3+ = Yb3+ or Tm3+) single-crystalline KGdxLuyY1-x-y(WO4)2 waveguides", submitted (2011). [6] D. Geskus, S. Aravazhi, E. Bernhardi, C. Grivas, S. Harkema, K. Hametner, D. Günther, K. Wörhoff, and M. Pollnau, "Low-threshold, highly efficient Gd3+, Lu3+ co-doped KY(WO4)2:Yb3+ planar waveguide lasers", Laser Phys. Lett. 6 (11), 800-805 (2009). [7] C.N. Borca, V. Apostolopoulos, F. Gardillou, H.G. Limberger, M. Pollnau, and R.P. Salathé, "Buried channel waveguides in Yb-doped KY(WO4)2 crystals fabricated by femtosecond laser irradiation", Appl. Surf. Sci. 253 (19), 8300-8303 (2007). [8] D. Geskus, S. Aravazhi, C. Grivas, K. Wörhoff, and M. Pollnau, "Microstructured KY(WO4)2:Gd3+, Lu3+, Yb3+ channel waveguide laser", Opt. Express 18 (9), 8853-8858 (2010). [9] D. Geskus, S. Aravazhi, K. Wörhoff, and M. Pollnau, "High-power, broadly tunable, and low-quantum-defect KGd1-xLux(WO4)2:Yb3+ channel waveguide lasers", Opt. Express 18 (25), 26107-26112 (2010). [10] D. Geskus, E.H. Bernhardi, S. Aravazhi, nd M. Pollnau, "KY(WO4)2:Gd3+,Lu3+,Yb3+ planar and channel waveguide lasers at the zero-phonon line", submitted (2011). [11] W.C.L. Hopman, F. Ay, W. Hu, V.J. Gadgil, L. Kuipers, M. Pollnau, and R.M. de Ridder, "Focused ion beam scan routine, dwell time and dose optimizations for submicrometre period planar photonic crystal components and stamps in silicon", Nanotechnol. 18 (19), 195305 (2007). [12] F. Ay, I. de la Rosa, D. Geskus, S. Aravazhi, and M. Pollnau, "Deep Bragg grating cavities in KY(WO4)2 waveguides fabricated by focused-ion-beam milling", submitted (2010). [13] S. Rivier, X. Mateos, V. Petrov, U. Griebner, Y.E. Romanyuk, C.N. Borca, F. Gardillou, and M. Pollnau, "Tm:KY(WO4)2 waveguide laser", Opt. Express 15 (9), 5885-5892 (2007). [14] K. van Dalfsen, S. Aravazhi, D. Geskus, K. Wörhoff, and M. Pollnau, "High-power Tm-doped double tungstate channel waveguide laser", submitted (2011).
1. Lattice-matched, High-refractive-index-contrast Double Tungstate Waveguides
LASER OUTPUT POWER [mW]
We apply liquid phase epitaxy at temperatures of 920-923°C to grow RE3+-doped double tungstate thin layers onto undoped, (010)-orientated, laser-grade polished KYW substrates of 1 cm2 size. While the use of low-temperature chloride solvents can lead to 3D island nucleation and insertion defects [2], the solvent K2W2O7 results in excellent layer and interface quality and has enabled the fabrication of KYW:Yb3+ planar waveguides with a thickness in excess of 10 µm, in which the firstever laser operation of a double tungstate waveguide was demonstrated with high slope efficiencies up to 80% [3]. A breakthrough has been obtained by co-doping the active layer with optically inert Gd3+ and Lu3+ ions [4]. Since Gd3+ and Lu3+ change the lattice parameters in opposite directions, choice of the right fractions of these two ions allows for lattice matching of the RE3+-activated layer with the undoped substrate (Fig. 1, left). Besides, co-doping with large amounts of Gd3+ and Lu3+ ions significantly increases the refractive index contrast of such a co-doped layer with respect to the undoped substrate by more than an order of magnitude to ~10-2 (Fig. 1, left) [5], thereby allowing for much thinner (only a few µm-thick) single-transverse-mode waveguides, resulting in tighter pump and laser mode confinement as well as easing the requirements on microstructuring.
T = 23 %, η = 82.3 %
200
T = 20 %, η = 72.4 % T = 10 %, η = 60.2 % T = 5.0 %, η = 45.5 %
150
T = 1.7 %, η = 23.8 % 100
50
(a) 0 0
50
100
150
200
250
300
ABSORBED PUMP POWER [mW]
Fig. 1. (left) Lattice mismatch along the a axis and refractive index increase for KGdxLuyYb1-x-y(WO4)2 versus the fractions of Gd3+ and Lu3+ ions [5]; (right) laser output power from a Gd3+, Lu3+ co-doped planar waveguide as a function of absorbed pump power for various outcoupling mirror transmissions [6].
In such co-doped layers continuous-wave laser operation in a planar waveguide configuration was observed at 1025 nm [6]. For 23% output coupling, a maximum output power of 195 mW was obtained; a slope efficiency of 82.3% was derived (Fig. 1, right), which is the highest value yet reported for a planar waveguide laser to date.
2. Microstructured Channel Waveguide Lasers The fabrication of channel waveguides in bulk double tungstates by femtosecond-laser writing or ion-beam implantation [7] typically induces considerable waveguide propagation losses and leads to comparatively small refractive index changes, necessitating rather large mode sizes. The small layer thickness of our co-doped waveguides greatly facilitates microstructuring of double tungstates [8]. Exploiting this option, we fabricated ridge channel waveguides in double tungstate layers by use of standard photo-resist as a mask and Ar+ beam etching (Fig. 2, left). The obtained KGd1-xLux(WO4)2:Yb3+ channel waveguides with well-defined cross-sections of a few µm2 and excellent mode confinement allowed us to demonstrate channel waveguide lasers with 418 mW of continuous-wave output power at 1023 nm and a slope efficiency of 71% versus launched pump power at 981 nm (Fig. 2, right), which is the highest slope efficiency in single-directional operation ever reported for a dielectric channel waveguide laser [9]. The degree of output coupling was 70%.
Output Power (mW)
500 Measured performance with Tout = 70%
400
Linear fit: Slope efficiency = 71%, Threshold = 40 mW
300 200 100 0 0
100
200
300
400
500
600
700
Launched Pump Power (mW) Fig. 2. (left) SEM micrograph of a microstructured channel waveguide before overgrowth [8]; (right) inputoutput curve of the KGd1-xLuxW:Yb3+ channel waveguide laser pumped at 981 nm and lasing at 1023 nm [9].
|←Pout= ~11 mW→|
Normalized Intensity
Normalized Laser Emission
1.0 0.8 0.6 0.4 0.2 0.0 960
980
1000
1020
1040
Wavelength (nm)
1060
1.0 0.8 Pump 0.6
Laser Emission
0.4 0.2 0.0 970
975
980
985
Wavelength (nm)
Fig. 3. (left) Measured emission spectra under pumping at 930 nm when changing the angle of the reflective grating [9]; (right) spectrum recorded during lasing with the smallest attained quantum defect [9].
Also planar and channel waveguide lasers operating at the zero-phonon line at 980.6 nm were demonstrated [10]. By grating tuning in an extended cavity and pumping at 930 nm, we demonstrated laser operation from 980 nm to 1045 nm (Fig. 3, left) [9]. When pumping at 973 nm, lasing at 980 nm with an extremely low quantum defect of 0.7% was achieved [9]. This is currently the world record for any rare-earth-ion-doped laser.
3. On-chip Integrated Laser Following an approach for nano-structuring of silicon [11], deeply etched Bragg gratings were fabricated by focused ion beam (FIB) milling in KGdxLu1-x(WO4)2:Yb3+. By optimizing parameters such as dose per area, dwell time and pixel resolution the redeposition effects were minimized and grating structures more than 4 µm in depth with an improved sidewall angle of ~5º were achieved. An on-chip integrated laser cavity at ∼980 nm was achieved by defining a FIB reflective grating and FIB polished waveguide end-facet (Fig. 4, left). With this cavity, an integrated waveguide laser was demonstrated in KGdxLu1-xW:Yb3+ (Fig. 4, right) [12].
Fig. 4. (left) Schematic of experimental setup configuration used for laser experiments [12]; (right) measured (dots) and modeled (line) performance of a laser cavity involving a 12-µm-wide grating structure [12].
4. Tm-doped Double Tungstate Waveguide Lasers Also doping of double tungstate thin films with Tm3+ ions is very promising and has resulted in similarly astounding laser performance. High-quality monoclinic planar waveguide crystals of Tmdoped KY(WO4)2 were grown. For the first time, planar waveguide lasing in the 2-µm spectral range was demonstrated in a double tungstate waveguide. The maximum achieved fundamental-mode continuous-wave output power was 32 mW [13]. Meanwhile, we have generated significantly higher output powers and excellent slope efficiencies in co-doped channel waveguides (numbers to be published) [14].
Collaborations 1. Advanced Photonics Laboratory, Institute of Imaging and Applied Optics, Ecole Polytechnique Fédérale de Lausanne, Switzerland. 2. Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy, Berlin, Germany. 3. Departamento Física Aplicada, Facultad de Ciencias, Universidad de Cantabria, Santander, Spain. 4. Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, Switzerland. 5. Optoelectronics Research Centre, University of Southampton, United Kingdom. 6. Low Temperature Division, MESA+ Institute for Nanotechnology, University of Twente.
Funding 1. NWO Vici Personal Grant “Integrated optical structures: From active thin films to micro/nanocavities to ultrashort-pulse lasers” (2006-2011). 2. Senter-Novem / STW – IOP Photonic Devices Project “On-chip integrated NH3 human gas sensor” (2009-2013).
References [1] M. Pollnau, Y.E. Romanyuk, F. Gardillou, C.N. Borca, U. Griebner, S. Rivier, and V. Petrov, "Double tungstate lasers: From bulk toward on-chip integrated waveguide devices", IEEE J. Select. Topics Quantum Electron. 13 (3), 661-671 (2007). Invited Paper. [2] Y.E. Romanyuk, I. Utke, D. Ehrentraut, V. Apostolopoulos, M. Pollnau, S. García-Revilla, and R. Valiente, "Lowtemperature liquid phase epitaxy and optical waveguiding of rare-earth-ion doped KY(WO4)2 thin layers", J. Cryst. Growth 269 (2-4), 377-384 (2004). [3] Y.E. Romanyuk, C.N. Borca, M. Pollnau, S. Rivier, V. Petrov, and U. Griebner, "Yb-doped KY(WO4)2 planar waveguide laser", Opt. Lett. 31 (1), 53-55 (2006). [4] F. Gardillou, Y.E. Romanyuk, C.N. Borca, R.P. Salathé, and M. Pollnau, "Lu, Gd co-doped KY(WO4)2:Yb epitaxial layers: Towards integrated optics based on KY(WO4)2", Opt. Lett. 32 (5), 488-490 (2007). [5] S. Aravazhi, D. Geskus, M. van Dalfsen, S. Harkema, K. Hametner, D. Günther, S. Garcia-Blanco, and M. Pollnau, "Lattice-matched, high-refractive-index-contrast, and highly RE-doped (RE3+ = Yb3+ or Tm3+) single-crystalline KGdxLuyY1-x-y(WO4)2 waveguides", submitted (2011). [6] D. Geskus, S. Aravazhi, E. Bernhardi, C. Grivas, S. Harkema, K. Hametner, D. Günther, K. Wörhoff, and M. Pollnau, "Low-threshold, highly efficient Gd3+, Lu3+ co-doped KY(WO4)2:Yb3+ planar waveguide lasers", Laser Phys. Lett. 6 (11), 800-805 (2009). [7] C.N. Borca, V. Apostolopoulos, F. Gardillou, H.G. Limberger, M. Pollnau, and R.P. Salathé, "Buried channel waveguides in Yb-doped KY(WO4)2 crystals fabricated by femtosecond laser irradiation", Appl. Surf. Sci. 253 (19), 8300-8303 (2007). [8] D. Geskus, S. Aravazhi, C. Grivas, K. Wörhoff, and M. Pollnau, "Microstructured KY(WO4)2:Gd3+, Lu3+, Yb3+ channel waveguide laser", Opt. Express 18 (9), 8853-8858 (2010). [9] D. Geskus, S. Aravazhi, K. Wörhoff, and M. Pollnau, "High-power, broadly tunable, and low-quantum-defect KGd1-xLux(WO4)2:Yb3+ channel waveguide lasers", Opt. Express 18 (25), 26107-26112 (2010). [10] D. Geskus, E.H. Bernhardi, S. Aravazhi, nd M. Pollnau, "KY(WO4)2:Gd3+,Lu3+,Yb3+ planar and channel waveguide lasers at the zero-phonon line", submitted (2011). [11] W.C.L. Hopman, F. Ay, W. Hu, V.J. Gadgil, L. Kuipers, M. Pollnau, and R.M. de Ridder, "Focused ion beam scan routine, dwell time and dose optimizations for submicrometre period planar photonic crystal components and stamps in silicon", Nanotechnol. 18 (19), 195305 (2007). [12] F. Ay, I. de la Rosa, D. Geskus, S. Aravazhi, and M. Pollnau, "Deep Bragg grating cavities in KY(WO4)2 waveguides fabricated by focused-ion-beam milling", submitted (2010). [13] S. Rivier, X. Mateos, V. Petrov, U. Griebner, Y.E. Romanyuk, C.N. Borca, F. Gardillou, and M. Pollnau, "Tm:KY(WO4)2 waveguide laser", Opt. Express 15 (9), 5885-5892 (2007). [14] K. van Dalfsen, S. Aravazhi, D. Geskus, K. Wörhoff, and M. Pollnau, "High-power Tm-doped double tungstate channel waveguide laser", submitted (2011).
