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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 22 — Nov. 4, 2013
  • pp: 27238–27245
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Nanowatt threshold, alumina sensitized neodymium laser integrated on silicon

Ashley J. Maker and Andrea M. Armani  »View Author Affiliations


Optics Express, Vol. 21, Issue 22, pp. 27238-27245 (2013)
http://dx.doi.org/10.1364/OE.21.027238


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Abstract

Low threshold lasers based on rare-earth elements have enabled numerous scientific discoveries and innovations in industry. However, pushing the threshold into the sub-microwatt regime has been stymied by a fundamental material phenomenon. Specifically, rare earth dopants form clusters which quench emission and reduce efficiency. Here, we fabricate resonant cavity lasers from neodymium-doped silica films containing alumina. The alumina prevents the clustering of the Neodymium, enabling the lasers to achieve thresholds of 530nanoWatts at room temperature.

© 2013 Optical Society of America

1. Introduction

High efficiency, low threshold fiber lasers based on rare-earth metal doped glasses have enabled numerous industries and applications, ranging from defense to biological investigations [1

1. O. S. Wolfbeis, “Fiber-optic chemical sensors and biosensors,” Anal. Chem. 78(12), 3859–3874 (2006). [CrossRef] [PubMed]

3

3. X. Bevenot, A. Trouillet, C. Veillas, H. Gagnaire, and M. Clement, “Hydrogen leak detection using an optical fibre sensor for aerospace applications,” Sensor Actuat. Biol. Chem. 67, 57–67 (2000).

]. While there are several motivations for designing an optically-pumped laser system based on a rare earth element, two reasons stand out. First, using a single pump source, it is possible to generate a wide range of emission wavelengths due to the multiple excitation and decay pathways of rare earth ions. For example, when neodymium is pumped near 800nm, lasing near 900-940nm, 1050-1150nm, and ~1330nm can be observed [4

4. J. A. Koningstein and J. E. Geusic, “Energy levels + crystal-field calculations of neodymium in yttrium aluminum garnet,” Phys. Rev. 136(3A), A711–A716 (1964). [CrossRef]

6

6. C. Bartolacci, M. Laroche, T. Robin, B. Cadier, S. Girard, and H. Gilles, “Effects of ions clustering in Nd3+/Al3+-codoped double-clad fiber laser operating near 930 nm,” Appl. Phys. B 98(2-3), 317–322 (2010). [CrossRef]

]. Second, rare earth ions have very large absorption cross sections for excitation. Because the laser threshold is inversely proportional to the gain coefficient, they are an ideal gain medium.

However, the field of optoelectronics is moving towards the development of integrated photonic devices, in which all of the individual components are fabricated directly on a silicon substrate. This approach requires the transformation of fiber laser systems into integrated microscale devices. One straightforward approach is to use integrated waveguides in lieu of optical fiber [5

5. J. Yang, K. van Dalfsen, K. Worhoff, F. Ay, and M. Pollnau, “High-gain Al2O3:Nd3+ channel waveguide amplifiers at 880 nm, 1060 nm, and 1330 nm,” Appl. Phys. B 101(1–2), 119–127 (2010). [CrossRef]

, 7

7. J. Bonar, J. A. Bebbington, J. S. Aitchison, G. D. Maxwell, and B. J. Ainslie, “Aerosol doped Nd planar silica wave-guide laser,” Electron. Lett. 31(2), 99–100 (1995). [CrossRef]

, 8

8. T. Kitagawa, K. Hattori, Y. Hibino, and Y. Ohmori, “Neodymium-doped silica-based planar wave-guide lasers,” J. Lightwave Technol. 12(3), 436–442 (1994). [CrossRef]

]. However, the optical loss of integrated waveguides, which affects the laser threshold, is significantly higher than optical fiber.

The threshold power (Pthres) is defined as the power at which the signal loss equals the gain. As such, in cavity-based rare-earth lasers, it is dependent on the balance between the gain concentration and the cavity Q, and a simple pair of expressions can be written to establish the lasing condition. The first expression defines the balance between the Q of the cavity (cavity loss) and the dopant concentration (NT) [9

9. Y. Li, G. Vienne, X. Jiang, X. Pan, X. Liu, P. Gu, and L. Tong, “Modeling rare-earth doped microfiber ring lasers,” Opt. Express 14(16), 7073–7086 (2006). [CrossRef] [PubMed]

, 10

10. E. R. Giles and E. Desurvire, “Modeling erbium-doped fiber amplifiers,” J. Lightwave Tech. 9(2), 271–283 (1991). [CrossRef]

]:
αp<ΓsσseNT.
(3)
where Γs is the overlap factor which describes the interaction strength between the optical intensity and the rare earth distribution in the material, and σsa(σse) describes the optical absorption (emission) cross sections of the rare earth material used. Using Eq. (3), a lower bound on the Q of the signal can be determined (using Q = 2πn/λαp):

Q>2πnλΓsσseNT
(4)

From these conditions, an expression for the threshold power can be written [9

9. Y. Li, G. Vienne, X. Jiang, X. Pan, X. Liu, P. Gu, and L. Tong, “Modeling rare-earth doped microfiber ring lasers,” Opt. Express 14(16), 7073–7086 (2006). [CrossRef] [PubMed]

]:

Pth=1Γphνpσsa1τσsaσse+αpΓsσseNT1αpΓsσseNT1EA
(5)

where A is the cross section of the resonator. One important aspect of this expression is that the quality factor appears in both the enhancement factor (E) and in the cavity loss (αp) terms. However, as seen in Eqs. (1) and (5), both terms are also dependent on other factors, such as coupling and gain concentration. As such, with low dopant concentrations or by operating in the critical coupling regime, the dependence of Pthres on Q can be simplified to P~Q−2; however, in other regimes of operation, the behavior can be significantly more complex.

Given the relations in Eqs. (1)(5), high Q devices containing dopants with large absorption cross sections are desirable for laser applications. For example, in a silica cavity with a quality factor (Q) of 100 million, the increase in circulating intensity is approximately 100,000 times. Using this approach, lasing thresholds as low as tens to hundreds of nanowatts have been achieved in rare earth-doped spheres [11

11. V. Lefèvre-Seguin, “Whispering-gallery mode lasers with doped silica microspheres,” Opt. Mater. 11(2–3), 153–165 (1999). [CrossRef]

, 12

12. V. Sandoghdar, F. Treussart, J. Hare, V. Lefèvre-Seguin, J. Raimond, and S. Haroche, “Very low threshold whispering-gallery-mode microsphere laser,” Phys. Rev. A 54(3), R1777–R1780 (1996). [CrossRef] [PubMed]

]. However, silica microspheres are not integrated on silicon wafers; therefore, they face many of the same challenges as fiber lasers.

Combining the high Q factors of the microsphere cavity and the requisite on-chip fabrication, silica toroid resonators are an especially promising platform for integrated rare earth lasers [13

13. D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Ultra-high-Q toroid microcavity on a chip,” Nature 421(6926), 925–928 (2003). [CrossRef] [PubMed]

]. Recent research has demonstrated rare earth-doped silica toroid microlasers with microwatt thresholds. For example, a 4.2 μW lasing threshold has been achieved in a ytterbium and erbium codoped silica toroid [14

14. H. S. Hsu, C. Cai, and A. M. Armani, “Ultra-low-threshold Er:Yb sol-gel microlaser on silicon,” Opt. Express 17(25), 23265–23271 (2009). [CrossRef] [PubMed]

], and a neodymium-doped toroid integrated on silicon recently demonstrated a 69µW lasing threshold [15

15. J. T. Lin, Y. X. Xu, J. X. Song, B. Zeng, F. He, H. L. Xu, K. Sugioka, W. Fang, and Y. Cheng, “Low-threshold whispering-gallery-mode microlasers fabricated in a Nd:glass substrate by three-dimensional femtosecond laser micromachining,” Opt. Lett. 38(9), 1458–1460 (2013). [CrossRef] [PubMed]

]. While these results are promising, integrated silica devices have not been able to achieve sub-microwatt lasing thresholds. One reason for this limitation is the limited solubility of rare earth dopants in silica. This causes the dopants to form clusters which quench emission, even at moderate concentrations [16

16. W. V. Moreshead, J. L. R. Nogues, and R. H. Krabill, “Preparation, processing, and fluorescence characteristics of neodymium-doped silica glass prepared by the sol-gel process,” J. Non-Cryst. Solids 121(1–3), 267–272 (1990). [CrossRef]

].

One solution for reducing dopant clustering and improving laser performance is to incorporate metal ions, such as aluminum, into the silica host matrix. Aluminum ions have been observed to selectively coordinate around neodymium ions, preventing clustering [6

6. C. Bartolacci, M. Laroche, T. Robin, B. Cadier, S. Girard, and H. Gilles, “Effects of ions clustering in Nd3+/Al3+-codoped double-clad fiber laser operating near 930 nm,” Appl. Phys. B 98(2-3), 317–322 (2010). [CrossRef]

, 17

17. B. Wilhelm, V. Romano, and H. P. Weber, “Fluorescence lifetime enhancement of Nd3+-doped sol-gel glasses by Al-codoping and CO2-laser processing,” J. Non-Cryst. Solids 328(1–3), 192–198 (2003). [CrossRef]

19

19. A. J. Berry and T. A. King, “Characterization of doped sol-gel derived silica hosts for use in tunable glass lasers,” J. Phys. D Appl. Phys. 22(10), 1419–1422 (1989). [CrossRef]

]. In particular, the Nd3+ ions tend to coordinate favorably with Al-O bonds in the silica matrix. This stabilizes the Nd3+ ions and disperses them more uniformly throughout the silica [20

20. T. Fujiyama, T. Yokoyama, M. Hori, and M. Sasaki, “Silica glass doped with Nd and Al prepared by the sol-gel method - change in the state of aluminum in the formation process,” J. Non-Cryst. Solids 135(2–3), 198–203 (1991). [CrossRef]

]. As a result of this coordination and the decrease in clustering, the fluorescence behavior of Nd3+ ions improves significantly [18

18. I. M. Thomas, S. A. Payne, and G. D. Wilke, “Optical-properties and laser demonstration of Nd-doped sol-gel silica glasses,” J. Non-Cryst. Solids 151(3), 183–194 (1992). [CrossRef]

21

21. E. J. A. Pope and J. D. Mackenzie, “Nd-doped silica glass. 1. Structural evolution in the sol-gel state,” J. Non-Cryst. Solids 106(1–3), 236–241 (1988). [CrossRef]

].

In the present work, we demonstrate an alumina-sensitized neodymium-doped silica toroid microlaser integrated on a silicon wafer (Fig. 1(a)
Fig. 1 Toroid microlaser and characterization setup. a) Scanning electron micrograph of as-fabricated toroid resonator. b) Schematic of characterization setup. A tapered fiber couples pump laser light into the resonator. The output is split and sent to a photodetector and optical spectrum analyzer, enabling simultaneous analysis of the transmission spectra and emitted light. c) PovRay rendering of tapered fiber and toroid. d) The tapered fiber must couple both the 765nm pump light and the emitted light near 900-940nm and 1050-1150nm.
). Specifically, we synthesize a series of silica sol-gels which are co-doped with 0.1 mol% Nd3+ and 0 to 2 mol% alumina. Then, we fabricate toroid microlaser devices from the films and characterize the dependence of lasing threshold and emission behavior on the alumina concentration. Since the alumina reduces clustering and creates a more favorable local environment around the Nd3+ ions, the lasing threshold and efficiency are significantly enhanced, enabling sub-microwatt lasing thresholds in an integrated silica device.

2. Material synthesis and device fabrication

The Nd3+ and alumina-doped silica is synthesized using the following sol-gel method, in which the silica precursor tetraethoxysilane (TEOS) undergoes an acid catalyzed hydrolysis-condensation reaction. Tetraethoxysilane (TEOS, Alfa Aesar, 99.999%), aluminum isopropoxide (Sigma Aldrich, 99.99%), ethanol, deionized water, and nitric acid (HNO3, EMD, 68%), are combined in a glass vial, stirring 5 minutes between each addition. The molar ratios of (TEOS: ethanol:water:HNO3) are set to (1:4:4:0.05), and the molar ratio of aluminum isopropoxide is varied between 0 to 0.02 to control the resulting concentration of alumina. After the addition of HNO3, the mixture is stirred for 1 hour at 70°C, allowing the TEOS and aluminum isopropoxide to undergo hydrolysis and condensation reactions. While the sol gel is stirring, neodymium nitrate (Alfa Aesar, 99.99%) is dissolved in deionized water in a 1:10 molar ratio. When the TEOS solution has finished stirring, the neodymium solution is added to the TEOS solution and stirred for 30 minutes. Afterwards, the solution is aged for 60 hours in ambient conditions.

Silica films are created by spin-coating the aged sol gels onto bare silicon wafers at 7000rpm for 30 seconds. The coated wafers are dried on a 75°C hot plate for 5 minutes, and then annealed in a tube furnace at 900°C for 2 hours with a ramp rate of 1°C/minute. This process creates a 350nm thick film, and is repeated once more to achieve a ~700nm thick film, as measured by ellipsometry. Once the annealing steps are complete, 40 μm diameter toroidal microcavity devices are fabricated from the sol gel silica films using a combination of photolithography and etching processes followed by a laser reflow step (Fig. 1(a)) [13

13. D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Ultra-high-Q toroid microcavity on a chip,” Nature 421(6926), 925–928 (2003). [CrossRef] [PubMed]

, 22

22. A. J. Maker and A. M. Armani, “Fabrication of silica ultra high quality factor microresonators,” J. Visualized Exp. 65, 4164 (2012).

].

3. Device and material characterization

To characterize the quality factor and the lasing behavior of the toroid lasers, we couple light into the cavity from a 765-781nm tunable narrow linewidth laser (Velocity series, Newport) using a tapered optical fiber waveguide (Fig. 1(b)1(d)). The tapered fiber’s output containing both the pump light and the toroid laser’s emitted light is sent to a 90:10 splitter. The 90% end goes to an optical spectrum analyzer (Agilent 86142B) and the remaining 10% to a detector and high speed digitizer/oscilloscope (National Instruments). This allows simultaneous monitoring of the toroid’s emitted light on the OSA and the amount of light coupled into the toroid laser on the digitizer/oscilloscope.

To measure the resonators’ quality factors (Q), the transmission spectrum is recorded using the oscilloscope and is fit to a Lorentzian. The linewidth is determined from the full-width-half-maximum of the fit, and from this value, the Q of the device is determined (Q = λ/δλ). During the Q measurement, the scan range and rate of the laser are optimized to minimize any non-linear effects which might distort the linewidth (δλ). In laser characterization experiments, the toroid laser’s emission intensity is measured with different amounts of power coupled into the toroid. Then, the emission versus coupled power is plotted and a line is fit to the data. The lasing threshold is given by the x-intercept of the line, while the laser’s slope efficiency is given by the slope of the line.

4. Effects of alumina concentration on quality factor and microlaser performance

When pumped with the tunable laser near ~780nm, all of the neodymium-doped toroid lasers showed emission peaks in the 900-940nm range and 1050-1150nm range (Fig. 3(a)
Fig. 3 Effects of alumina on lasing wavelength. a) Representative OSA spectra for a sample containing 1 mol% alumina, showing lasing near 900-940nm and 1050-1100nm. b) As the alumina concentration increases, more Nd ions are surrounded by alumina and the lasing wavelength range blue shifts.
). These emitted wavelengths correspond to stimulated emission from the 4F5/2 energy level to the 4I9/2 and 4I11/2 levels, respectively [4

4. J. A. Koningstein and J. E. Geusic, “Energy levels + crystal-field calculations of neodymium in yttrium aluminum garnet,” Phys. Rev. 136(3A), A711–A716 (1964). [CrossRef]

, 5

5. J. Yang, K. van Dalfsen, K. Worhoff, F. Ay, and M. Pollnau, “High-gain Al2O3:Nd3+ channel waveguide amplifiers at 880 nm, 1060 nm, and 1330 nm,” Appl. Phys. B 101(1–2), 119–127 (2010). [CrossRef]

]. It is important to note that the number of observed lasing peaks (hence, the number of lasing modes) depends on many factors, including the coupling, input power, dopant concentration, device materials, and geometry. This balance is also evident in Eq. (5). Therefore, by optimizing these parameters, either single mode or multimode lasing can be achieved.

As the alumina concentration increases, the lasing wavelength range shifts from ~940nm and ~1080-1160nm in alumina-free microlasers to ~900nm and ~1050-1130nm in silica containing 2 mol% alumina (Fig. 3(b)). At greater alumina concentrations, more neodymium ions are coordinated with Al-O bonds instead of Si-O bonds. When coordinated with alumina, the Nd3+ ions have reduced clustering, quenching and nonradiative decay compared to plain silica [18

18. I. M. Thomas, S. A. Payne, and G. D. Wilke, “Optical-properties and laser demonstration of Nd-doped sol-gel silica glasses,” J. Non-Cryst. Solids 151(3), 183–194 (1992). [CrossRef]

, 21

21. E. J. A. Pope and J. D. Mackenzie, “Nd-doped silica glass. 1. Structural evolution in the sol-gel state,” J. Non-Cryst. Solids 106(1–3), 236–241 (1988). [CrossRef]

, 24

24. S. Mathur, M. Veith, H. Shen, S. Hufner, and M. H. Jilavi, “Structural and optical properties of NdAlO3 nanocrystals embedded in an Al2O3 matrix,” Chem. Mater. 14(2), 568–582 (2002). [CrossRef]

]. In addition, the alumina has a lower phonon energy than the sol gel silica, causing the Nd3+ emission to blue shift slightly [18

18. I. M. Thomas, S. A. Payne, and G. D. Wilke, “Optical-properties and laser demonstration of Nd-doped sol-gel silica glasses,” J. Non-Cryst. Solids 151(3), 183–194 (1992). [CrossRef]

, 21

21. E. J. A. Pope and J. D. Mackenzie, “Nd-doped silica glass. 1. Structural evolution in the sol-gel state,” J. Non-Cryst. Solids 106(1–3), 236–241 (1988). [CrossRef]

, 24

24. S. Mathur, M. Veith, H. Shen, S. Hufner, and M. H. Jilavi, “Structural and optical properties of NdAlO3 nanocrystals embedded in an Al2O3 matrix,” Chem. Mater. 14(2), 568–582 (2002). [CrossRef]

]. Therefore, the observed lasing at slightly higher energy wavelengths further indicates that the neodymium ions become surrounded and enhanced by alumina.

Due to the lasing threshold’s dependence on Q, directly plotting threshold versus the alumina concentration can lead to a misinterpretation of the results. One method for disambiguating this behavior is to normalize the threshold by the quality factor of the device (Eq. (5)). While the dependence due to the enhancement in excitation efficiency (E) can be easily isolated in Eq. (5) and normalized from the data, the dependence on cavity loss is more challenging to extricate from the other terms, particularly when high dopant concentrations are used. In these situations, normalizing by Q, and not Q2, is appropriate. When this analysis is performed, a clear trend emerges. Specifically, the lasing threshold/Q decreases linearly with increasing alumina concentration (Fig. 5(a)
Fig. 5 Effects of alumina on lasing performance. As alumina concentration increases, the efficiency increases (a) and the lasing threshold decreases (b), due to alumina’s declustering and sensitizing effects.
). Additionally, the lasing threshold/Q ratio decreases over 5-fold as the alumina concentration increases, significantly enhancing the device performance.

In addition to decreasing the lasing threshold, increasing the alumina concentration greatly improves the slope efficiency of the toroid microlasers. As the alumina concentration is increased from 0 to 2 mol%, the slope efficiency of the toroid laser increases up to 29-fold. This efficiency increase is linear with alumina concentration over this range (Fig. 5(b)). The simultaneous improvement of lasing threshold and slope efficiency is notable, as typically improving one performance metric comes at the cost of worsening the other.

It is also important to mention the slope efficiency values in Fig. 5(b) could be further improved by optimizing the coupling of the laser’s emitted light back into the tapered fiber or employing an alternative method for extracting the emission. As shown previously in Fig. 1(d), the tapered fiber in the present work simultaneously coupled the ~780nm pump light into the toroid and coupled the ~900-1150nm Nd3+ lasing out of the toroid. As a result, the coupling efficiency over this entire wavelength range was not ideal, and the slope efficiency values appear lower.

5. Conclusion

In conclusion, we have successfully fabricated toroid microlasers from neodymium-doped silica. The addition of alumina to the Nd3+-doped silica reduces clustering of Nd3+ dopant, causing emission at slightly shorter wavelengths, up to a 29-fold increase in slope efficiency, and enabling sub-microwatt lasing thresholds to be achieved in a lasing device which is integrated on silicon. Therefore, these efficient and ultra-low threshold alumina-enhanced lasers will benefit applications in integrated optics and communications. Since the absorption of water is especially low at the ~800nm pump and ~1064nm emission wavelengths of neodymium [25

25. G. M. Hale and M. R. Querry, “Optical-constants of water in 200-nm to 200-mum wavelength region,” Appl. Opt. 12(3), 555–563 (1973). [CrossRef] [PubMed]

], these devices will find useful applications in biosensing as well [26

26. A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan, and K. J. Vahala, “Label-free, single-molecule detection with optical microcavities,” Science 317(5839), 783–787 (2007). [CrossRef] [PubMed]

29

29. A. J. Maker and A. M. Armani, “Heterodyned toroidal microlaser sensor,” Appl. Phys. Lett. 103(12), 123302 (2013). [CrossRef]

].

Acknowledgments

References and links

1.

O. S. Wolfbeis, “Fiber-optic chemical sensors and biosensors,” Anal. Chem. 78(12), 3859–3874 (2006). [CrossRef] [PubMed]

2.

S. D. Jackson and A. Lauto, “Diode-pumped fiber lasers: A new clinical tool?” Lasers Surg. Med. 30(3), 184–190 (2002). [CrossRef] [PubMed]

3.

X. Bevenot, A. Trouillet, C. Veillas, H. Gagnaire, and M. Clement, “Hydrogen leak detection using an optical fibre sensor for aerospace applications,” Sensor Actuat. Biol. Chem. 67, 57–67 (2000).

4.

J. A. Koningstein and J. E. Geusic, “Energy levels + crystal-field calculations of neodymium in yttrium aluminum garnet,” Phys. Rev. 136(3A), A711–A716 (1964). [CrossRef]

5.

J. Yang, K. van Dalfsen, K. Worhoff, F. Ay, and M. Pollnau, “High-gain Al2O3:Nd3+ channel waveguide amplifiers at 880 nm, 1060 nm, and 1330 nm,” Appl. Phys. B 101(1–2), 119–127 (2010). [CrossRef]

6.

C. Bartolacci, M. Laroche, T. Robin, B. Cadier, S. Girard, and H. Gilles, “Effects of ions clustering in Nd3+/Al3+-codoped double-clad fiber laser operating near 930 nm,” Appl. Phys. B 98(2-3), 317–322 (2010). [CrossRef]

7.

J. Bonar, J. A. Bebbington, J. S. Aitchison, G. D. Maxwell, and B. J. Ainslie, “Aerosol doped Nd planar silica wave-guide laser,” Electron. Lett. 31(2), 99–100 (1995). [CrossRef]

8.

T. Kitagawa, K. Hattori, Y. Hibino, and Y. Ohmori, “Neodymium-doped silica-based planar wave-guide lasers,” J. Lightwave Technol. 12(3), 436–442 (1994). [CrossRef]

9.

Y. Li, G. Vienne, X. Jiang, X. Pan, X. Liu, P. Gu, and L. Tong, “Modeling rare-earth doped microfiber ring lasers,” Opt. Express 14(16), 7073–7086 (2006). [CrossRef] [PubMed]

10.

E. R. Giles and E. Desurvire, “Modeling erbium-doped fiber amplifiers,” J. Lightwave Tech. 9(2), 271–283 (1991). [CrossRef]

11.

V. Lefèvre-Seguin, “Whispering-gallery mode lasers with doped silica microspheres,” Opt. Mater. 11(2–3), 153–165 (1999). [CrossRef]

12.

V. Sandoghdar, F. Treussart, J. Hare, V. Lefèvre-Seguin, J. Raimond, and S. Haroche, “Very low threshold whispering-gallery-mode microsphere laser,” Phys. Rev. A 54(3), R1777–R1780 (1996). [CrossRef] [PubMed]

13.

D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Ultra-high-Q toroid microcavity on a chip,” Nature 421(6926), 925–928 (2003). [CrossRef] [PubMed]

14.

H. S. Hsu, C. Cai, and A. M. Armani, “Ultra-low-threshold Er:Yb sol-gel microlaser on silicon,” Opt. Express 17(25), 23265–23271 (2009). [CrossRef] [PubMed]

15.

J. T. Lin, Y. X. Xu, J. X. Song, B. Zeng, F. He, H. L. Xu, K. Sugioka, W. Fang, and Y. Cheng, “Low-threshold whispering-gallery-mode microlasers fabricated in a Nd:glass substrate by three-dimensional femtosecond laser micromachining,” Opt. Lett. 38(9), 1458–1460 (2013). [CrossRef] [PubMed]

16.

W. V. Moreshead, J. L. R. Nogues, and R. H. Krabill, “Preparation, processing, and fluorescence characteristics of neodymium-doped silica glass prepared by the sol-gel process,” J. Non-Cryst. Solids 121(1–3), 267–272 (1990). [CrossRef]

17.

B. Wilhelm, V. Romano, and H. P. Weber, “Fluorescence lifetime enhancement of Nd3+-doped sol-gel glasses by Al-codoping and CO2-laser processing,” J. Non-Cryst. Solids 328(1–3), 192–198 (2003). [CrossRef]

18.

I. M. Thomas, S. A. Payne, and G. D. Wilke, “Optical-properties and laser demonstration of Nd-doped sol-gel silica glasses,” J. Non-Cryst. Solids 151(3), 183–194 (1992). [CrossRef]

19.

A. J. Berry and T. A. King, “Characterization of doped sol-gel derived silica hosts for use in tunable glass lasers,” J. Phys. D Appl. Phys. 22(10), 1419–1422 (1989). [CrossRef]

20.

T. Fujiyama, T. Yokoyama, M. Hori, and M. Sasaki, “Silica glass doped with Nd and Al prepared by the sol-gel method - change in the state of aluminum in the formation process,” J. Non-Cryst. Solids 135(2–3), 198–203 (1991). [CrossRef]

21.

E. J. A. Pope and J. D. Mackenzie, “Nd-doped silica glass. 1. Structural evolution in the sol-gel state,” J. Non-Cryst. Solids 106(1–3), 236–241 (1988). [CrossRef]

22.

A. J. Maker and A. M. Armani, “Fabrication of silica ultra high quality factor microresonators,” J. Visualized Exp. 65, 4164 (2012).

23.

E. D. Palik, Handbook of Optical Constants of Solids (Elsevier, 1985).

24.

S. Mathur, M. Veith, H. Shen, S. Hufner, and M. H. Jilavi, “Structural and optical properties of NdAlO3 nanocrystals embedded in an Al2O3 matrix,” Chem. Mater. 14(2), 568–582 (2002). [CrossRef]

25.

G. M. Hale and M. R. Querry, “Optical-constants of water in 200-nm to 200-mum wavelength region,” Appl. Opt. 12(3), 555–563 (1973). [CrossRef] [PubMed]

26.

A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan, and K. J. Vahala, “Label-free, single-molecule detection with optical microcavities,” Science 317(5839), 783–787 (2007). [CrossRef] [PubMed]

27.

H. K. Hunt and A. M. Armani, “Label-free biological and chemical sensors,” Nanoscale 2(9), 1544–1559 (2010). [CrossRef] [PubMed]

28.

M. S. Luchansky and R. C. Bailey, “High-Q optical sensors for chemical and biological analysis,” Anal. Chem. 84(2), 793–821 (2012). [CrossRef] [PubMed]

29.

A. J. Maker and A. M. Armani, “Heterodyned toroidal microlaser sensor,” Appl. Phys. Lett. 103(12), 123302 (2013). [CrossRef]

OCIS Codes
(130.0130) Integrated optics : Integrated optics
(140.3530) Lasers and laser optics : Lasers, neodymium
(140.4780) Lasers and laser optics : Optical resonators
(160.5690) Materials : Rare-earth-doped materials
(160.6060) Materials : Solgel

ToC Category:
Integrated Optics

History
Original Manuscript: September 3, 2013
Revised Manuscript: October 24, 2013
Manuscript Accepted: October 25, 2013
Published: November 1, 2013

Citation
Ashley J. Maker and Andrea M. Armani, "Nanowatt threshold, alumina sensitized neodymium laser integrated on silicon," Opt. Express 21, 27238-27245 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-22-27238


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