What do you get when you cross organic chemistry with physical chemistry? Organic laser dyes! This post has been inspired by my recent tinkering with an extremely annoying, extremely picky dye laser in the laser lab at UK.
The lab’s post-doc and I are trying to achieve at least 0.8 W of lasing at 570 nm, in order to probe a band of the PCN radical at this wavelength. The power has to be 0.8 W because the beam has to pass through a frequency doubler, for a reason I’m not quite clear on yet (it has something to do with PCN having some spectroscopic feature around 300 nm). The pump laser, which supplies the energy needed for stimulated emission, is at 532 nm. How the hell, you may ask, can we get a 570-nm beam using a 532-nm “input” beam? This is where the dyes come in.
A dye laser works by exploiting the energy gaps in the electronic states of organic molecules. The pump laser provides the “kick” that gets the molecule into an excited state. Relaxation processes, which increase the wavelength of the emitted beam relative to the input, may then occur, and finally a photon is emitted to return the molecule to the ground state. Variance in the extent of relaxation of the dye molecules produces a “spread” of lasing wavelengths, which means that dye lasers are actually wavelength-tunable! There is one wavelength for which lasing is a maximum; this corresponds to the most likely rovibronic transition for that dye molecule. But at wavelengths around this maximum the dye molecules may (and probably will) lase.
Wavelength tuning is done by adjusting the angle of a diffraction grating opposite the output coupler of the dye laser. The dye’s emitted light travels in two directions: towards the output coupler, which reflects some light back to encourage stimulated emission and transmits the rest, and towards a diffraction grating, which is a full reflector that ensures that only light of the desired wavelength makes it back to the dye cell. By adjusting the angle of the diffraction grating, you change the wavelength that is allowed to make it back to the dye after emission (everything else gets scattered inside the laser cavity). If the wavelength is in the dye’s “range,” and everything is aligned right, you’ll see a bright spot of laser light among the rest of the spontaneously emitted light.
Spontaneous emission is bad. Why? Because it’s not laser light! It contains contributions from the whole tuning range of the dye, which is a big no-no. It’s not coherent, it’s not intense, it’s not the right wavelength…it just sucks. The thing is, we have lost the ability to make our laser lase using the ONLY dye that has a lasing maximum near 570 nm.
We tried pumping the laser with 532 nm, but had to resort to a mixture of rhodamine 590 and 610 that gave too little output power. So we decided to pump with 355 nm, the advantage being that rhodamine 590 has a lasing maximum at 574 nm when pumped with 355-nm light (the pumping wavelength affects a dye’s lasing maximum, by the way). But changing the pump laser has thrown everything out of alignment! We can get DCM to lase, but no other dye works (including R590)!
I went rock climbing today…I wonder if the post-doc has made any progress…updates to follow!