An electric field sorted out the excited molecules
Townes’s lecture describes an ammonia beam passing through a nonuniform electric field. The field focused excited molecules toward a cavity and pushed lower-state molecules away. The resonant transition therefore received more upper-state molecules than a thermal gas would supply.
The inversion transition lies near 23.87 gigahertz. The working 1954 apparatus was the work of Gordon and Zeiger as well as Townes. Basov and Prokhorov developed related ideas in quantum electronics independently, and the 1964 prize recognized this broader foundation.
Prepare the particles so emission beats absorption
A resonant field can stimulate downward transitions and take energy from the molecular beam. Lower-state molecules can absorb from the same field. Net gain depends on the populations, the transition strengths and the interaction time; stimulated emission by itself is not enough.
A stimulated photon is not just any photon. Einstein showed in 1916 that an atom recoils one way when it absorbs light and the opposite way when light stimulates it to emit, so the new photon travels with the light that triggered it. In the wave picture the field drives the atom, and the light the atom gives back joins that field with the same frequency, phase and polarization. Spontaneous photons leave in random directions and add almost nothing to the beam.
When each energy level contains one internal state, net gain tracks the excess upper population. Some levels contain several distinct internal states at the same energy. The useful comparison then accounts for how particles fill those states and how strongly each transition couples to the field. Pumping or state selection replenishes the particles able to emit.
Shining the transition's own light on a two-level gas cannot create an upper-state excess. The light pushes particles down as often as it lifts them, so it can only bleach the gas until it is nearly transparent. Inversion needs another route.
Optical materials face the same population problem through different routes. Ruby emits into its normally occupied ground state, so pumping must move a substantial fraction of particles out of that state to overcome absorption. In a four-level route, the laser transition ends in a state that empties quickly. Fewer particles remain there to absorb the laser light. Both the supply of excitation and the emptying of the absorbing state matter.
How much pumping is enough follows from simple rate equations. In a three-level material such as ruby, more than half of all particles must leave the ground state before the material stops absorbing; below that pump rate, even a long-lived upper state only makes the material less opaque. A four-level material gains from the first pumped particle, but only while its lower laser state empties faster than the upper state decays. If it empties more slowly, particles pile up there and the gain dies soon after the pump turns on.
A passive cavity stores light supplied by an external drive
A passive optical cavity separates the job of feedback from the job of excited matter. Imagine two mirrors that each reflect ninety-five percent of the incident power. A little light enters, returns after reflection, and joins light admitted on later trips. At a resonant frequency the returning waves arrive in step and build a strong circulating field. Light leaking back through the entrance can cancel the wave reflected directly from that mirror. With identical mirrors, no internal absorption and a perfectly matched incident beam, the steady transmitted power can equal the incident power. Multiplying two single-pass transmission fractions misses the repeated interference.
The large circulating power represents energy accumulated from the external drive. Turn that drive off and a fixed fraction escapes or is absorbed on every round trip, so stored energy decays exponentially. A lossier cavity forgets the drive sooner. Internal absorption can also take some of the incoming power even when reflection cancels. Sustained maser oscillation adds what this passive comparison lacks: excited molecules continually replace the energy leaving the field.
Feedback sets a threshold for oscillation
A cavity sends part of the field back for another pass. Oscillation can start when small-signal gain makes up for the round-trip loss at one of the cavity’s allowed frequencies. The intensity condition below is a schematic optical analogue. It assumes uniform gain and given mirror power reflectivities, and it is not fitted to the ammonia apparatus. The gain and loss coefficients have units of inverse length, the length is the one-way cavity length and the reflectivities are fractions of power.
Above threshold, depletion and saturation change the gain, so an ever-growing exponential does not describe steady output. Which frequency oscillates also depends on the cavity and the gain profile.
Sources: [1]
The growing field needs a seed
The threshold says what happens to light already present. An unseeded optical cavity illustrates how the first light can arrive: an excited particle emits on its own into a field pattern supported by the cavity. This spontaneous emission supplies a tiny seed. Below threshold most seeds disappear through the losses; above it a seed can grow rapidly. Waiting for a seed and amplifying it are distinct parts of startup.
The waiting time varies from one start to the next because those emissions occur at random. A mean intensity curve hides that randomness. Work on stochastic laser dynamics, including André, Mørk and Wubs's 2020 study, follows discrete emissions, absorption, pumping and cavity losses. It separates the mean growth of light from fluctuations in how many photons occupy the cavity.
Why the output settles
As the field grows, it stimulates more excited particles to release energy. The usable population falls, and so does the gain. A steady oscillator settles where that saturated gain just replaces the loss, while fresh excited particles keep arriving. This gives a steady output instead of unlimited exponential growth.
An optical cavity allows many frequencies, evenly spaced like the teeth of a comb, and spontaneous emission seeds every one of them. When each excited particle can amplify any of these frequencies, all of them draw on the same supply. The frequency nearest the peak of the gain grows fastest. Once it pulls the gain down to its own loss, every other frequency has slightly less gain than loss and fades back to a faint glow. Even a tiny gain advantage is enough to decide the winner.
In a two-mirror cavity, light running both ways forms a standing wave, with fixed bright and dark bands along the beam. Excited particles in the dark bands are left unused, and another frequency whose bright bands fall there can still grow. Several frequencies can therefore keep running together. A ring cavity in which the light circulates one way has no such bands.
An optical version also shows why neither a perfect reflector nor an almost transparent output mirror is automatically best. Trapping nearly all the light leaves little useful output. Letting too much escape raises the gain needed for oscillation. The balance depends on the available pumping and internal loss.
Maser and laser share a principle but have separate histories
Reaching optical frequencies needed materials, pumping and resonators suited to much higher frequencies. Maiman demonstrated a working ruby laser in 1960. Crediting that device to the three 1964 laureates would blur the line between founding proposals and one particular experimental realization.
An optical resonator also has to choose a beam shape. In 1961 Fox and Li calculated light making many trips between two mirrors of finite size. The field settled into fixed patterns, and each pattern lost a different share past the mirror edges. Curved mirrors hold a beam near the axis only for some spacings; in 1966 Kogelnik and Li set out that rule and the Gaussian beam shapes that fit such a cavity. Put a small opening in the cavity and the wider patterns lose more, so only the narrowest is left.
A narrow output spectrum does not, by itself, establish every quantum property of the light. The Glauber article asks what coincidence measurements can add.
Find the state selector in Townes’s diagram
Read Fig. 1 of Townes’s lecture and follow the molecular beam to the cavity output. The state selector supplies a nonthermal population; the cavity lets that population repeatedly exchange energy with the microwave field.
Sources: [1]