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T-ray laser from water or metal films?
Simple QED allows the T-ray emission from the laser heating of free flowing 200 micron water films to be superseded by metal films thereby avoiding the disruption of the water film by shock waves and plasma explosion
Terahertz waves called T-rays have frequencies from 0.3 to 3 THz, 1 THz = 1012 Hz having wavelengths from 1000 to 100 microns. T-rays are able to pass through solids and produce images of the interiors of plastic and ceramic materials, but unlike X-rays are non-ionizing and do not damage DNA. Prior T-ray research was based on short-pulse NIR lasers irradiating thin targets of metal films and air plasmas. But T-ray emission from liquid water has proved elusive. Recently,free-
In recent years, T-ray emission has been observed when metallic surfaces and nanostructures have been illuminated by ultrafast NIR laser pulses. Multiphoton photoelectron emission is thought to be the mechanism for T-ray emission, but the sequential combination of 3 or 4 NIR photons to ionize water is highly unlikely.
Is there a simpler T-ray mechanism?
Emission of T-waves is proposed caused by the simple QED conversion of laser heat under the EM confinement of nanostructures. But simple QED differs from the complex light-matter interaction proposed by Feynman and others. In simple QED, the EM confinement is a natural consequence of nanostructures having high S/V ratios with dimensions < 100 nm. S/V stands for surface-to-volume and QM for quantum mechanics. What high S/V ratios mean is laser heat is deposited in nanostructure surfaces thereby placing internal atoms under EM confinement over nanoscale dimensions that by the Planck law of QM causes the heat capacity to vanish. Speculative multiphoton combinations of NIR photons is not required.
Lacking heat capacity, the nanostructure cannot conserve laser heat by an increase in temperature. Instead, simple QED conserves the laser heat by the emission of non-thermal T-ray radiation. The T-rays having half-wavelength λ/2 = d stand across the thickness d of the water film and so Planck energy E = hν is created having frequency, ν = (c/n) / λ = c/2nd, where the velocity of light c is corrected for its slower speed in water by the refractive index n. See diverse simple QED applications of the Planck law in nanostructures at http://www.nanoqed.org/
Water films having thickness d = 177 microns are not nanostructures with dimensions d < 100 nm. At 300 K, the Planck law shows the water atoms have almost full kT heat capacity, where k is the Boltzmann constant and T absolute temperature. Since NIR lasers create a plasma in water, the refractive index n ≈ 1 giving a T-ray wavelength λ = 2nd ≈ 354 microns and frequency v ≈ 0.85 THz. The thumbnail shows the water film at 354 microns only converts about 10% of the NIR laser heat into T-ray emission, the remainder lost in thermal heating. For comparison, the Planck law at 190 and 40 K show lower temperatures allow more efficient T-ray emission, but would require freezing the water.
Simple QED that predicts 177 micron water film produces 0.85 THz radiation having 354 micron wavelength is consistent with the peak of Curve B in Figure 2(c) of . However, the water film under high laser heating creates a positive charged plasma because of the loss of electrons by the photoelectric effect. At this point, the water film is about to undergo a Coulomb explosion because of the repulsion from the positive charged atoms of the water molecules. Moreover, to avoid the obvious difficulties with creating a freely falling water film it may be desirable to consider replacing the water with a metal film having a refractive index n in the far IR close to unity. If so, the metal version of the T-ray laser would have about the same 177 micron thickness to produce 0.85 THz T-rays. Metals are absorptive, but may require a roughened metal surface to reduce reflectivity in absorbing the NIR laser irradiation.
The idea that a water film as a target to produce T-ray emission was thought impossible based on the argument that water is such a strong THz absorber. See http://www.sciencenewsline.com/
Nanoscale metal films under NIR laser excitation are more suited to producing submicron emissions in the VIS and UV than creating T-rays with 200 micron films as thermal losses are negligible for thicknesses d < 100 nm. But otherwise, simple QED is applicable to both sub and supra micron films. In this regard, consider the UV radiation from the nanoscale thickness of air in a shock wave  of the ordinary shock tube.
QM governs heat transfer in nanostructures, and not classical physics. But the water film is at the outer limits of QM applicability making it difficult to achierve a high efficiency of converting NIR laser heat into T-rays.
The Planck law of QM gives the partition of NIR laser heating into T-rays and thermal heating.
Metal films offer more control over the T-ray emission than water and are far easier to implement.
 Q. Jin, et al., "Observation of broadband terahertz wave generation from liquid water," Appl. Phys. Lett. 111, 071103 (2017); doi: 10.1063/1.4990824.
 T. Prevenslik, See https://www.prlog.org/