Solid Hydrogen
Introduction
The converter used to convert normal hydrogen entirely to 99.9% para-hydrogen. Click for full size.
Solid hydrogen crystals possess useful properties that can be exploited for use in spectroscopy. Chief among these properties are the large vibrational frequency of the H2 molecules and the large Raman gain coefficient of crystals produced from these molecules. These characteristics make solid hydrogen an appealing gain medium for a mid-infrared continuous-wave Raman laser, which would allow our lab to access an elusive yet desirable region of the electromagnetic spectrum for molecular spectroscopy. Therefore, much study has been undertaken on solid hydrogen, including development of a method to grow and crystallize it, measurement of its index of refraction, and initial planning for a proof of concept experiment of a visible solid hydrogen Raman laser.
Para-H2 Production
Molecular hydrogen exists in two distinguishable nuclear spin states: ortho-H2 and para-H2. For Raman lasing applications, pure para-H2 must be crystallized to create the gain medium. To this end, our research group has designed and built a para-H2 converter that uses a ferric oxide catalyst at cryogenic temperatures to convert normal H2 (3:1 ortho:para) produced from a hydrogen generator to 99.99% pure para-H2. The newly purified para-H2 can then be used directly in an experiment, or stored in a Teflon-lined gas cylinder for future use with minimal back-conversion.
Manori, Mike and Bill are filling the cryostat with liquid helium in order to make a crystal of solid hydrogen.
Index of Refraction Studies
In order to design a suitable cavity for the Raman laser, the index of refraction of the solid para-H2 gain medium must be known at low temperatures and at different wavelengths. Therefore, measurements of solid hydrogen crystals were made with the aim of calculating these index values. In these measurements, fresh para-H2 was condensed (see video) and crystallized in a custom cryogenic cell mounted in a liquid He-cooled cryostat under high backing pressure. Optical windows on the cell and the cryostat enabled us to introduce laser beams of different wavelengths through the crystal. The crystal refracted the beams, and by calculating the refractive angle of these beams, we were able to determine the index of refraction for solid para-H2 at these wavelengths. We also studied the index of refraction as a function of temperature and laser polarization. These measurements aided in the subsequent step of cavity design.
Solid Hydrogen Raman Laser
Tentative conceptual diagram of cw solid hydrogen Raman laser. The solid hydrogen cell inside the cryostat converts a pump laser (green) to a Stokes-shifted beam (red). The laser cavity consists of two mirrors that highly reflect at the Stokes wavelength, but transmit at the pump wavelength. A dichroic mirror reflects the pump laser back toward the hydrogen cell, while passing the Stokes beam.
We use the measurements of the index of refraction of solid para-hydrogen to design a cavity for a solid hydrogen Raman laser. Knowing the index of refraction precisely is important in designing the laser cavity so that all interfaces can be made to be at Brewster's angle. This minimizes losses inside the cavity.
Raman shifting can be used to shift the wavelength of a laser to reach new parts of the spectrum. In a Raman process, an incoming photon scatters inelastically off of a molecule in our Raman gain medium. This causes the molecule to excite to a higher vibrational state while the photon leaves red-shifted. The original photon is a pump photon, while the red-shifted photon is called a Stokes photon. If we pump the gain medium with the pump photons, while at the same time trapping a sufficient number of Stokes photons in the medium, then the Stokes photons can stimulate the Raman process and lasing can occur. The difference in energy between the pump photons and the Stokes photons is the difference between the vibrational energy states and is a property of the material.
Solid hydrogen promises to be a great Raman gain medium because it has a higher Raman gain coefficient than about any other crystal. We will be using hydrogen's huge gain coefficient together with its large Raman shift to reach wavelengths that would be difficult to reach otherwise. Solid hydrogen has been used by many groups to Raman shift pulsed lasers, but we will be using it inside a laser cavity to create a continuous-wave Raman laser. First we will be shifting from green to red in the visible as part of a proof-of-principle. Afterwards, we will use the setup to shift from the near-infrared to the far-infrared.
Related Content
Papers
| 57 | M. Perera, B. A. Tom, Y. Miyamoto, M. W. Porambo, L. E. Moore, W. R. Evans, T. Momose and B. J. McCall "Refractive Index Measurements of Solid Parahydrogen" Optics Letters (2011), 36, 840-842. |
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| 39 | B. A. Tom, S. Bhasker, Y. Miyamoto, T. Momose, and B. J. McCall "Producing and Quantifying Enriched Para-H2" Review of Scientific Instruments (2009), 80, 016108. |
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Talks
| 99 | W. R. Evans, T. Momose and B. J. McCall "Toward a Continuous-Wave Solid Hydrogen Raman Laser for Molecular Spectroscopy Applications" Sixty-Sixth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2011. |
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| 38 | B. A. Tom, B. J. McCall, Y. Miyamoto, and T. Momose "The Index of Refraction of Solid Hydrogen" Sixty-Second International Symposium on Molecular Spectroscopy, Ohio State University, Columbus, OH, 2007. |
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Posters
| 17 | L. E. Moore, M. Perera, B. A. Tom, Y. Miyamoto, M. W. Porambo, T. Momose and B. J. McCall "Index of Refraction of Solid para-Hydrogen" 2010 WCC Undergraduate Research Symposium, University of Illinois, 2010. |
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Other Publications
| 22 | L. E. Moore "Production, Crystallization, and Raman Shifting with para-Hydrogen" B. S. Thesis, University of Illinois, 2010. |
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