Does there exist a laser that is tunable over the whole visible spectrum? If so does there exist one that is solid state? This is to settle a score with my dumb workmates.
The Coherent Chameleon titanium-sapphire laser tunes over a pretty enormous range, mostly in the IR (680 to 1080 nm). With a frequency doubling crystal, it covers most of the visible (340 to 540 nm), and if you add an OPO, it can tune over the entire visible range with frequency doubling.
Supercontinuum sources cover the entire visible spectrum simultaneously, and are often operated with post-filtering if you want narrowband light.
Toptica's iChrome is a narrowband source that tunes over the entire visible by following supercontinuum generation with frequency doubling.
All these lasers are solid-state.
Now, regarding the spirit of your question. Does your bet with your dumb co-workers allow nonlinear optics like supercontinuum generation or frequency doubling? Does it allow mixtures of different gain media? Are you specifically curious about semiconductor lasers? Do you care about the coherence of the output, the bandwidth, or the brightness?
Georg is correct. The free electron laser is tunable:
Unlike gas, liquid, or solid-state lasers such as diode lasers, in which electrons are excited in bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium which moves freely through a magnetic structure, hence the term free electron.[ The free-electron laser has the widest frequency range of any laser type, and can be widely tunable, currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, to ultraviolet, to X-rays.
But it is not a lasing that can be used in a pointer or a portable method. It needs accelerated electrons. So it depends what sort of argument you have with your friend.
Today, a free-electron laser requires the use of an electron accelerator with its associated shielding, as accelerated electrons are a radiation hazard. These accelerators are typically powered by klystrons, which require a high voltage supply. The electron beam must be maintained in a vacuum which requires the use of numerous vacuum pumps along the beam path. While this equipment is bulky and expensive, free-electron lasers can achieve very high peak powers, and the tunability of FELs makes them highly desirable in several disciplines, including medical diagnosis and non-destructive testing.
For comparison, this company Radiantis sells OPOs that are tunable over almost the whole visible spectrum. OPOs are not lasers, but they are sources of coherent light which use a solid state gain medium.
Full disclosure: I do not work for and have never done business with Radiantis, but I have toured their lab once.