The invention of the laser has resulted in many innovations, and the device has become
ubiquitous. However, the maser, which amplifies microwave radiation rather than visible
light, has not had as large an impact, despite being instrumental in the laser's birth.
The maser's relative obscurity has mainly been due to the inconvenience of the operating
conditions needed for its various realizations: atomic and free-electron masers require
vacuum chambers and pumping; and solid-state masers, although they excel as low-noise
amplifiers and are occasionally incorporated in ultrastable oscillators, typically
require cryogenic refrigeration. Most realizations of masers also require strong magnets,
magnetic shielding or both. Overcoming these various obstacles would pave the way
for improvements such as more-sensitive chemical assays, more-precise determinations
of biomolecular structure and function, and more-accurate medical diagnostics (including
tomography) based on enhanced magnetic resonance spectrometers incorporating maser
amplifiers and oscillators. Here we report the experimental demonstration of a solid-state
maser operating at room temperature in pulsed mode. It works on a laboratory bench,
in air, in the terrestrial magnetic field and amplifies at around 1.45 gigahertz.
In contrast to the cryogenic ruby maser, in our maser the gain medium is an organic
mixed molecular crystal, p-terphenyl doped with pentacene, the latter being photo-excited
by yellow light. The maser's pumping mechanism exploits spin-selective molecular intersystem
crossing into pentacene's triplet ground state. When configured as an oscillator,
the solid-state maser's measured output power of around -10 decibel milliwatts is
approximately 100 million times greater than that of an atomic hydrogen maser, which
oscillates at a similar frequency (about 1.42 gigahertz). By exploiting the high levels
of spin polarization readily generated by intersystem crossing in photo-excited pentacene
and other aromatic molecules, this new type of maser seems to be capable of amplifying
with a residual noise temperature far below room temperature.