I've heard of frequency doubling but is wavelength doubling a thing? If so, how efficient?
Frequency doubling divides the wavelength in half: http://www.rp-photonics.com/frequency_doubling.html
The inverse process also works, and is usually called parametric down conversion:
Your green laser pointer is an example of frequency doubling/second harmonic generation:
See the schematic diagram on the right hand side - enlarge, and follow the red beam into the crystal which generates the green beam.
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