Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: DC-powered Fe3+:sapphire Maser and its Sensitivity to Ultraviolet Light   The zero-field Fe3+:sapphire whispering-gallery-mode maser oscillator
exhibits several alluring features: Its output is many orders of magnitude
brighter than that of an active hydrogen maser and thus far less degraded by
spontaneous-emission (Schawlow-Townes) and/or receiving-amplifier noise. Its
oscillator loop is confined to a piece of mono-crystalline rock bolted into a
metal can. Its quiet amplification combined with high resonator Q provide the
ingredients for exceptionally low phase noise. We here concentrate on novelties
addressing the fundamental conundrums and technical challenges that impede
progress. (1) Roasting: The "mase-ability" of sapphire depends significantly on
the chemical conditions under which it is grown and heat-treated. We provide
some fresh details and nuances here. (2) Simplification: This paper obviates
the need for a Ka-band synthesizer: it describes how a 31.3 GHz loop
oscillator, operating on the preferred WG pump mode, incorporating Pound
locking, was built from low-cost components. (3) "Dark Matter": A Siegman-level
analysis of the experimental data determines the substitutional concentration
of Fe3+ in HEMEX to be less than a part per billion prior to roasting and up to
a few hundred ppb afterwards. Chemical assays, using different techniques
(incl. glow discharge mass spectra spectroscopy and neutron activation
analysis) consistently indicate, however, that HEMEX contains iron at
concentrations of a few parts per million. Drawing from several
forgotten-about/under-appreciated papers, this substantial discrepancy is
addressed. (4) Excitons: Towards providing a new means of controlling the
Fe3+:sapph. system, a cryogenic sapphire ring was illuminated, whilst masing,
with UV light at wavelengths corresponding to known electronic and
charge-transfer (thus valence-altering) transitions. Preliminary experiments
are reported.

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The categories of this paper are: astro-ph
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The categories of this paper are: stat
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The categories of this paper are: hep-ex
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The categories of this paper are: math
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-th
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The categories of this paper are: eess
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The categories of this paper are: cs
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The categories of this paper are: gr-qc
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The categories of this paper are: funct-an
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The categories of this paper are: acc-phys
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The categories of this paper are: solv-int
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The categories of this paper are: chao-dyn
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The categories of this paper are: q-fin
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The categories of this paper are: hep-ph
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The categories of this paper are: chem-ph