Concentrating an N2 laser beam on various materials

Laser beams interact with surfaces by a variety of thermal, impulse and electrical effects. Energy coupling is considerably enhanced once surface electrical breakdown occurs. The laser heated plasma interacts then with the surface via three major interrelated damage mechanisms: thermal evaporation, ion sputtering, and unipolar arcing..., unipolar arcing is an electrical plasma-surface interaction process which leads to crater formation... many electrical micro-arcs burn between the surface and the laser heated plasma, driven by local variations of the sheath potential with the surface acting as both the cathode and anode...  Never was there a plasma evident without attendant unipolar arc craters. At low irradiance there was no other laser damage (like melting) observed, all damage was in the form of unipolar arc damage.  --  "Unipolar Arcing, A Basic Laser Damage Mechanism", Schwirzke, 1984

GLASS

A concentrated beam of UV from an N2 laser, focused on the inside surface of a glass ampoule of Ne.  A cloud of something develops around the spot, causing it to appear fuzzy, but it starts out sharp.   

Here, I nudged the tube a little during the exposure to keep the "smoke" from masking the color of the spark, so the spark appears in multiple places in the image.  A brochure said the LSI VSL-337ND-S produces a beam that can be focused to a near-diffraction limited spot better than 3 μm, with a fluence of 4.5 kJ/cm2.  I'm using an LSI VSL-337ND (no "-S" suffix).

Same thing, but beaming UV into a HeNe laser tube containing a little helium.  I think the plume's green color has to do with the glass.  Or, blue in the case of the Ne ampoule previously.  I do not see a spot unless the N2 beam is focused at the surface of a solid material, in this case glass.  

Again, but with N2 in the tube.   The tube is being held vertically; the bore is a little left of center.  The focus of the UV is at the yellow dot on the left, in front of the microscope objective.  In the center of the picture is a reflection on the HeNe bore.

WHY A PLASMA: 

Processes for creating and sustaining a plasma:

1. "Cascaded breakdown":  Solids require lower laser power than gases by two orders of magnitude, even less if the solid is transparent, due to a self-focusing effect.  If the surface is reflective, electron heating and ionization of neutral particles will be stronger at a distance of λ/4 from the surface where a maximum of the electric field forms due to the superposition of' the incoming and reflected waves.  When the solid absorbs the radiation, it evaporates and a shock is driven into the surrounding gas.

2. "Photoionization":  A photon of wavelength 337.1 nm has an energy of 3.6778 eV (5.89e-19 Joules).  The binding energy of an electron in helium's outer shell is 24.6 eV, for nitrogen it's 37.3 eV, and for Ne, 21.6 eV.   The gasses I could find with the lowest ionization energies were Ar at 15.7 eV, Xe with 12.1 eV, H with 13.6 eV, and Kr at 14.1 eV.  So, 337.1 nm will not ionize any of these gasses.  There is such a thing as multiphoton ionization, where more than one photon can gang up on the electron, but that requires a stronger laser.

3. "Thermal runaway":    Absorption of laser radiation by the electrons in the blast from the surface results in the heating of the vapor and gas.  Thermal energy then creates even more free electrons.

4. "Inverse bremsstrahlung" is the opposite of the blue glow you see when you look down into the pool of water covering a research reactor; instead of emitting a photon, an electron absorbs a photon, and is accelerated by the additional energy.   This effect is roughly proportional to the cube of the wavelength, rather than the photon energy, so the first two processes are more important for UV lasers.

WHY A SPARK:  

Electrons accelerated by one of the processes above become displaced from the ions, consequently forming an electric field.  When the electric field gets strong enough, about 10 Ampere according to "Plasma Physics: An Introductory Course" edited by R. O. Dendy, then "unipolar arcing" occurs, meaning that both the cathode and anode are at the same solid surface.

WHY A NOISE:

The shock wave moves out from the focus at the speed of sound or greater.

METAL

Observation of arcing damage on the surfaces of laser irradiated metal targets show a pattern of interference rings [Schwirzke, "Laser Induced Unipolar Arcing"].

It seems that my N2 beam cannot directly ionize a gas, but can heat a surface fast enough to vaporize it.   The free electrons in the vapor ejecta should then be able to be multiplied by the other processes mentioned.

The dark spots seen here are where the beam has ablated the coating on the surface of this small HeNe laser tube's cathode, resulting in the arc seen in the previous picture.

Aluminum foil.

This is the back side of the foil.  I started the exposure after the first pulse, before the foil was pierced.  It takes about 10 pulses to break through.

The "L"-shaped holes in the foil through a microscope.

PLASTIC

This is a disposable acrylate cuvette filled with saline.   Above is taken from the side.  And, the second picture is from above.  One can see, in both the lens and the glowing gas that the UV beam is a fat "L"-shape.

Graphite was coated on the same piece of PMMA as used above.  It easily makes a sparking noise like metal.  I moved the piece around from test to test, hence all the burn spots.