Toward a Continuous-Wave

Solid para-Hydrogen Raman Laser for Molecular Spectroscopy Applications

William R. Evans

Benjamin J. McCall

Takamasa Momose

Department of PhysicsUniversity of Illinois at Urbana-Champaign

Departments of Chemistry, Astronomy and PhysicsUniversity of Illinois at Urbana-Champaign

Department of ChemistryThe University of British Columbia

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

Mid-Infrared Spectroscopy

Many Attractive Targets in 5 – 10 μm Range Few Available Laser Sources

Astrochemistry need sub-MHz linewidth Need for cw sources

Possible Solution: Solid para-H2 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)

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

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

Previous Work with Hydrogen

Gas Solid P

ulse

dC

ontin

uous

Previous Work with Hydrogen

Gas Solid1980, 1986: pulsed dye laser tunable from 1-10 μm 0.2 cm-1 linewidth P

ulse

dC

ontin

uous

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)

Previous Work with Hydrogen

Gas Solid1980, 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 linewidthP

ulse

dC

ontin

uous

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)

Previous Work with Hydrogen

Gas Solid1980, 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

Pul

sed

Con

tinuo

us

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)

Previous Work with Hydrogen

Gas Solid1980, 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

Pul

sed

Con

tinuo

us

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-H2

Solid para-H2 Raman Gain Coefficient

H2 Gas p-H2 Crystal

n 2.50×1020 cm-3 2.64×1022 cm-3

Γ 300 MHz 8.4 MHz

M. Katsuragawa and K. Hakuta, Optics Letters, 25, 3, 177 (2000)

Solid para-H2 Raman gain coefficient measured by Katsuragawa and Hakuta ~ 7,000x that of gaseous hydrogen

Narrow linewidth because solid para-H2 is a quantum crystal

Index of Refraction of Solid para-H2 To design a solid para-H2 Raman laser, we

needed to know the index of refraction of solid para-H2.

Surprisingly, this quantity had never been reported before.

M. Perera, et al, Optics Letters, 36, 6, 840 (March 15, 2011)

Index of Refraction of Solid para-H2

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-H2 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

Index of Refraction of Solid para-H2

Results

M. Perera, et al, Optics Letters, 36, 6, 840 (March 15, 2011)

Solid para-H2 Raman LaserExperimental Setup With the index of refraction of solid para-H2 in

hand, we can design the setup for our laser. 1st 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

Solid para-H2 Raman LaserExperimental Setup

Solid para-H2 Raman LaserExperimental Setup

Solid para-H2 Raman LaserExperimental Setup

Interfaces designed to be at Brewster’s angle. Minimize reflective scattering losses inside cavity.

Because np-H2 is greater than 1, the windows on the cell need to be wedged.

Solid para-H2 Raman LaserCell Design

Solid para-H2 Raman LaserExperimental Setup

Solid para-H2 Raman LaserExperimental Setup

Solid para-H2 Raman LaserCavity 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

Solid para-H2 Raman LaserCurrent Progress Solid para-H2 crystal grown

in new cell Pump laser through the

crystal windows properly aligned

Raman output within the next few weeks

Solid para-H2 Raman LaserFuture Plans Active cavity locking Single-mode pump laser Doubly-resonant cavity Raman lasing in the infrared

Summary Solid para-H2 is an attractive material for use as a

Raman gain medium. A fully-optimized solid para-H2 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-H2.

We have successfully designed and built a system that should be able to achieve Stokes output using solid para-H2 within the next few weeks.

Acknowledgments

Benjamin McCall Takamasa Momose Manori Perera Michael Porambo Heather Hanson Preston Buscay Kristin Evans

The McCall Research Group