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 birth1, 2. The maser’s relative obscurity has mainly been due to the inconvenience of the operating conditions needed for its various realizations: atomic3 and free-electron4 masers require vacuum chambers and pumping; and solid-state masers5, although they excel as low-noise amplifiers6 and are occasionally incorporated in ultrastable oscillators7, 8, 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 spectrometers9 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 maser6, 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 crossing10 into pentacene’s triplet ground state11, 12. 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 maser3, 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.
Figures at a glance
Figure 1: Maser pumping scheme (Jablonski diagram). 
Pentacene guest molecules within a p-terphenyl host lattice are driven from their singlet ground states, S0, into their first-excited single states, S1, by absorbing photons of yellow pump light. From S1, they predominantly undergo intersystem crossing into their triplet ground states, T1. This process is spin selective; the relative population rates11, 12 (splitting ratios) into the three spin sublevels of T1 are as stated (also represented by solid black circles). As can be seen, the uppermost sublevel, X, is preferentially populated, resulting in a strong initial population inversion between it and the lowest sublevel, Z, so providing the conditions for masing through the X
Z transition.
Figure 2: Anatomy of the maser. 
A crystal of p-terphenyl doped with pentacene is located in the a.c. magnetic field of the TE01δ mode of a microwave resonator and illuminated with a beam of yellow light from a pulsed dye laser. By maser action, the TE01δ mode is energized, and from this mode a signal is extracted using a magnetic coupling loop. The photograph on the right shows the resonator’s sapphire ring and the pentacene:p-terphenyl crystal slotted inside it; yellow back lighting reveals flaws within the crystal.
Figure 3: Maser response in the time domain. 
The yellow trace (scale on near left) is the pump laser’s instantaneous output power, which corresponds to an energy of ~0.5 J per pulse. The blue trace (scale on right) is the instantaneous power of the maser’s oscillation burst as monitored with a microwave receiver at a resolution bandwidth of 10 MHz centred on 1.45 GHz. The green trace (same scale on right) is our fitted model’s prediction of this instantaneous power. The red curve (scale on far left) is the predicted population ratio between the uppermost (X) and lowest (Z) maser levels in response to the optical pump in the low-signal regime.
Figure 4: Frequency response of maser action. 
The circles and associated guide line plot the measured plateau amplitude of the maser oscillation (vertical scale, obtained by down-converting the oscillation’s output to a few tens of kilohertz and observing the resultant beat on a digital storage oscilloscope; a.u., arbitrary units) on illuminating the pentacene:p-terphenyl crystal (within its sapphire ring) with a ~0.5-J pulse of yellow light, for different frequencies of the microwave resonator’s TE01δ mode (horizontal scale).
right
