1. Toward a Continuous-Wave Solid para-Hydrogen Raman Laser for Molecular Spectroscopy Applications William R. Evans Benjamin J. McCall Takamasa Momose Department of Physics University of Illinois at Urbana-Champaign Departments of Chemistry, Astronomy and Physics University of Illinois at Urbana-Champaign Department of Chemistry The University of British Columbia

2. Outline Motivation for the Project Stimulated Raman Scattering Hydrogen as a Raman Medium Measuring the Index of Refraction of Solid para-Hydrogen Current Progress Toward a cw Solid para-Hydrogen Raman Laser in the Visible Future Plans and Summary

3. Mid-Infrared Spectroscopy Many Attractive Targets in 5 – 10 μm Range Few Available Laser Sources  Astrochemistry need sub-MHz linewidth  Need for cw sources

4. Possible Solution: Solid para-H 2 Raman Laser Transparent for most of 100 nm to 10 μm Enormous frequency shift  4155.2 cm -1 in gas  4149.7 cm -1 in solid M. Fushitani, S. Kuma, Y. Miyamoto, H. Katsuki, T. Wakabayashi, T. Momose, and A.F. Vilesov, Optics Letters, 28, 1, 37 (2003) M. Mengel, B.P. Winnewisser, and M. Winnewisser, Canadian Journal of Physics, 78, 317 (2000)

5. Brief Review of Raman Scattering Pump photon scatters inelastically with an atom  Redshifted to a “Stokes” photon.

6. Stimulated Raman Scattering Two-photon process Incoming Stokes stimulates transition Outgoing photons emitted coherently Efficiency of SRS depends on intracavity power and Raman gain coefficient

7. Previous Work with Hydrogen GasSolid Pulsed Continuous

8. Previous Work with Hydrogen GasSolid 1980, 1986: pulsed dye laser tunable from 1-10 μm 0.2 cm -1 linewidth Pulsed Continuous A. DeMartino, R. Frey, and F. Pradere, IEEE J. QUANT. ELEC., VOL. QE-16, 11 (1980) P. Rabinowitz, B. N. Perry, and N. Levinos, IEEE J. Quantum Electron. 22, 797 (1986)

9. Previous Work with Hydrogen GasSolid 1980, 1986: pulsed dye laser tunable from 1-10 μm 0.2 cm -1 linewidth 2003 – 2004: pulsed Nd:YAG, OPO Raman in both solid and liquid tunable from 4.4-8 μm 0.3 – 0.4 cm -1 linewidth Pulsed Continuous M. Fushitani, S. Kuma, Y. Miyamoto, H. Katsuki, T. Wakabayashi, T. Momose, and A.F. Vilesov, Optics Letters, 28, 1, 37 (2003) B.J. McCall, A.J. Huneycutt, R.J. Saykally, C.M. Lindsay, T. Oka, M. Fushitani, Y. Miyamoto, and T. Momose, Applied Physics Letters, 82, 9, 1350 (2003) K.E. Kuyanov, T. Momose, and A.F. Vilesov, Applied Optics, 43, 32, 6023 (2004)

10. Previous Work with Hydrogen GasSolid 1980, 1986: pulsed dye laser tunable from 1-10 μm 0.2 cm -1 linewidth 2003 – 2004: pulsed Nd:YAG, OPO Raman in both solid and liquid tunable from 4.4-8 μm 0.3 – 0.4 cm -1 linewidth 1998 – 2002: 1-2 mW pump threshold 4 kHz linewidth doubly-resonant cavity F ~ 50,000 Pulsed Continuous J.K. Brasseur, K.S. Repasky, and J.L. Carlsten, Optics Letters, 23, 5, 367 (1998) J.K. Brasseur, P.A. Roos, K.S. Repasky, and J.L. Carlsten, Journal of the Optical Society of America B, 16, 8, 1305 (1999) L.S. Meng, K.S. Repasky, P.A. Roos, and J.L. Carlsten, Optics Letters, 25, 7, 472 (2000) L.S. Meng, P.A. Roos, K.S. Repasky, and J.L. Carlsten, Optics Letters, 26, 7, 426 (2001) (and others)

11. Previous Work with Hydrogen GasSolid 1980, 1986: pulsed dye laser tunable from 1-10 μm 0.2 cm -1 linewidth 2003 – 2004: pulsed Nd:YAG, OPO Raman in both solid and liquid tunable from 4.4-8 μm 0.3 – 0.4 cm -1 linewidth 1998 – 2002: 1-2 mW pump threshold 4 kHz linewidth doubly-resonant cavity F ~ 50,000 Pulsed Continuous

12. Tradeoff Want:  Narrow linewidth  Need cw laser  Not high complexity  Lower finesse cavity Drawbacks:  CW pump lasers have lower maximum power  Lower finesse cavity means less power buildup in the cavity  Both of these factors make lasing more difficult Tradeoff:  Need high Raman gain coefficient  Solid para-H 2

13. Solid para-H 2 Raman Gain Coefficient H 2 Gasp-H 2 Crystal n2.50×10 20 cm -3 2.64×10 22 cm -3 Γ300 MHz8.4 MHz M. Katsuragawa and K. Hakuta, Optics Letters, 25, 3, 177 (2000) Solid para-H 2 Raman gain coefficient measured by Katsuragawa and Hakuta  ~ 7,000x that of gaseous hydrogen Narrow linewidth because solid para-H 2 is a quantum crystal

14. Index of Refraction of Solid para-H 2 To design a solid para-H 2 Raman laser, we needed to know the index of refraction of solid para-H 2. Surprisingly, this quantity had never been reported before. M. Perera, et al, Optics Letters, 36, 6, 840 (March 15, 2011)

15. Index of Refraction of Solid para-H 2 Measurement Setup Incoming laser is refracted at slanted window. By measuring the exit angle of the laser, we can determine the index of the solid para-H 2 using Snell’s law. M. Perera, et al, Optics Letters, 36, 6, 840 (March 15, 2011) Stainless steel cell OFHC copper connected to cold head Sapphire windows Laser

16. Index of Refraction of Solid para-H 2 Results M. Perera, et al, Optics Letters, 36, 6, 840 (March 15, 2011)

17. Solid para-H 2 Raman Laser Experimental Setup With the index of refraction of solid para-H 2 in hand, we can design the setup for our laser. 1 st Stage of Project:  Raman shifting in the visible: 514 nm  654 nm  Multi-mode pump laser  Singly-resonant cavity (only building up Stokes radiation)  No active cavity locking

18. Solid para-H 2 Raman Laser Experimental Setup

21. Interfaces designed to be at Brewster’s angle.  Minimize reflective scattering losses inside cavity. Because n p-H2 is greater than 1, the windows on the cell need to be wedged. Solid para-H 2 Raman Laser Cell Design

22. Solid para-H 2 Raman Laser Experimental Setup

24. Solid para-H 2 Raman Laser Cavity Design Specialty coated cavity mirrors  1 m Radius of Curvature  High transmitting at pump wavelength (T ~ 98% at 514 nm)  High reflecting at Stokes wavelength (R = 99.5% at 654 nm) Cavity length 50 cm Diffraction grating used to separate pump beam from Stokes beam after the cavity

25. Solid para-H 2 Raman Laser Current Progress Solid para-H 2 crystal grown in new cell Pump laser through the crystal  windows properly aligned Raman output within the next few weeks

26. Solid para-H 2 Raman Laser Future Plans Active cavity locking Single-mode pump laser Doubly-resonant cavity Raman lasing in the infrared

27. Summary Solid para-H 2 is an attractive material for use as a Raman gain medium. A fully-optimized solid para-H 2 Raman laser could potentially provide the first widely tunable laser source for ultra high resolution spectroscopy in the 5-10 μm range. We have made the first ever measurements of the index of refraction of solid para-H 2. We have successfully designed and built a system that should be able to achieve Stokes output using solid para-H 2 within the next few weeks.

28. Acknowledgments Benjamin McCall Takamasa Momose Manori Perera Michael Porambo Heather Hanson Preston Buscay Kristin Evans The McCall Research Group