Study Notes: Ammonia Maser

The first "laser" was created using ammonia molecules (NH3).

http://boson.physics.sc.edu/~gothe/730-F12/talks/henderson-1.pdf

It generated an electromagnetic wave of energy in the microwave spectrum, so it is technically a "maser" (Microwave Amplification by Stimulated Emission of Radiation) not a higher energy "laser" (Light Amplification by Stimulated Emission of Radiation), which emits waves in the optical spectrum.

http://bio100.class.uic.edu/lectures/visiblelight.jpg

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Requires some physics to understand what's going on...

Quantum Mechanics with Basic Field Theory by Bipin R. Desai. 2010. ISBN: 978-0-511-69134-8 (ebook) 

Ammonia molecules have a high and low energy state. We can use an electric field to separate out the high energy ammonia, and allow them to drop to low energy state, thereby emitting radiation. This radiated energy is very small...

13.2.5 Ammonia maser
In Chapter 8 we considered the energy eigenstates of the ammonia molecule NH3 treated
as a one-dimensional bound-state problem with a symmetric potential, V(x). We found
that there are two lowest eigenstates, a symmetric state ( (ψS) with eigenvalue ES and an
antisymmetric state ( (ψA), with eigenvalue EA such that EA > ES, with their difference
given by EA − ES = 2δ (13.179)
where δ is a very small quantity for normal configurations of the potentials.

If we subject the molecule to an electric field E then a dipole is generated as the electrons
and the nucleus are stretched apart under the influence of the field.

In practice the ammonia beam will contain an equal mixture of ( (ψ A) and ( (ψ S). But prior to entering the apparatus and being subjected to the oscillating electric field, it is made to pass through a nonhomogeneous time-independent electric field in order that ( (ψ A) and ( (ψ S) are separated.

the sign of the force is different between the states ( (ψA) and ( (ψS) , these two types of
particles will be deflected differently, much the same way as in the Stern–Gerlach experiment for spin-up and spin-down particles. This is then the basic mechanism of separating the two states.

... but by 'stimulating' the ammonia with the optimal frequency in an varying electric field, a significant wave of radiation - the maser - can be created. Here's how.

Using a harmonic electric field, there is a natural cycle of energy absorption and emission. When the device is tuned the right frequency, the high energy ammonia can pass through the electric field over the complete duration of emission cycle, expending as much energy as possible into the maser.

Consider the case when the ammonia molecules in the state ( (ψ A) enter an apparatus subjected to a harmonically varying electric field during the emission cycle. If the frequency ω of the oscillating electric field is tuned to the level difference, 2δ, of the molecule then ((ψ A) will give up energy to the radiation field and convert to( (ψ S) .We note that the transition ((ψ A) → ( (ψ S) will also happen naturally through the tunneling of the middle barrier; that is, a spontaneous emission that has a rate which is much smaller than the “stimulated” emission we are considering here.

In practice, then, after the separation has been achieved, the pure ( (ψA) beam enters a
microwave cavity that has the dimensions adjusted so that the beam spends exactly the
same time as the emission cycle (t = π!/2ηE). The microwave is tuned to the energy
difference, EA − ES, in order that the entering state ( (ψA) gives out all the energy to the
radiation energy, which then gains in strength.

This mechanism is the essence of the maser, which is the acronym for microwave
amplification by simulated emission of radiation

Here's a more detailed description of the absorption and emission cycle of a harmonic electric field. During the emission cycle, the field urges the ammonia to drop to a lower energy state and thereby release its energy into the maser.

(i) During the interval between t = 0 and t = π/2γ , c1(t) becomes smaller and the state ((ψ0 1) gets depleted as the system absorbs energy from the external interaction. The system achieves full absorption at t = π/2γ , as the higher energy level ( (ψ0 2) gets fully populated and c1 (t) = 0, c2 (t) = 1. This interval corresponds to the so called “absorption cycle.”

(ii) From t = π/2γ to t = π/γ , the cycle reverses, c2 (t) becomes smaller as the system gives up excess energy from the upper level to the external potential while it descends down to the lower level ( (ψ0 1). At t = π/γ we have, once again, c1 (t) = 1, c2 (t) = 0. This is called the “emission cycle.”

(iii) This absorption–emission cycle continues indefinitely.

(iv) The maximum value (= 1) of the above amplitude is achieved at ω = ω21, while at ω = ω21 ± 2γ the amplitude reaches half the maximum value. The quantity 4γ is called the full width at half-maximum (see Fig. 13.5). The absorption–emission cycle exists away from the resonance but one never achieves full absorption (c2(t) = 1) or full emission (c1(t) = 1)

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Naturally, what Science Fiction nerds really care about is whether lasers can really be turned into powerful weapons of gleeful destruction. Probably not. The U.S. tried to create a painful laser that could be used for non-lethal crowd control, and results were unimpressive.

"High-Power Microwave Weapons Start to Look Like Dead-End: Despite 50 years of research on high-power microwaves, the U.S. military has yet to produce a usable weapon"

"For some Pentagon officials, the demonstration in October 2007 must have seemed like a dream come true — an opportunity to blast reporters with a beam of energy that causes searing pain.

The event in Quantico, Virginia, was to be a rare public showing for the US Air Force's Active Denial System: a prototype non-lethal crowd-control weapon that emits a beam of microwaves at 95 gigahertz. Radiation at that frequency penetrates less than half a millimeter into the skin, so the beam was supposed to deliver an intense burning sensation to anyone in its path, forcing them to move away, but without, in theory, causing permanent damage.

However, the day of the test was cold and rainy. The water droplets in the air did what moisture always does: they absorbed the microwaves. And when some of the reporters volunteered to expose themselves to the attenuated beam, they found that on such a raw day, the warmth was very pleasant."

It's awesome to imagine a laser in the fiction as a continuous beam of annihilating red energy ripping through tanks and bunkers, but in reality you would prefer to fire pulses of energy. It's just not that powerful.

https://www.extremetech.com/extreme/153224-the-science-of-beam-weapons/2
Even in the much more foreseeable future of strategic, platform-mounted lasers, weaponizing light requires the use of pulses. The first reason for this, again, is power, but just as important is the mechanism by which laser damage their targets:

when a strong enough laser hits a surface, say the wing of a drone, the surface layer will (should) sublimate — that is, go directly from a solid to a gas — and fill the space around the target with a beam-scattering cloud of vaporized metal.

You have to wait a while, maybe ten or fifteen microseconds, for that cloud to disburse, or else waste energy while it scatters your beam all over creation. Once the tiny cloud has puffed away, we can send a second pulse, then a third, and so on.

Basically, a puff of water moisture counters your laser.

"Your attack has been rendered harmless. It is, however, quite pretty." - Saprazzan vizier
Magic: the Gathering card from Merfolk expansion, Mercadian Masques.

So don't bring a laser to a fight on a humid day.