Today Sujal introduced Fairy Lights in Femtoseconds Project by Ochiai et al. to our group. Basically, it is a tangible holographic plasma in the mid-air or water or florescent (plates)! Their SIGGRAPH 2015 paper entitled “Fairy Lights in Femtoseconds: Aerial and Volumetric Graphics Rendered by Focused Femtosecond Laser Combined with Computational Holographic Fields” can be downloaded in arXiv.

First let’s take a look at their video with pleasant background music:

The system is evaluated according to the following measurements:

  • Energy vs Brightness
    • The experiments were conducted under energies per pulse of 0.16 to 0.55 mJ. The 30-fs laser can produce plasma from 0.2-mJ pulse energy. The cross-sectional area of the focal point is theoretically calculated to be 2 × 10-7 cm2. Then, the peak intensity is 36 PW/cm2 and surely higher than the ionized plasma threshold (> 1 PW/cm2).
  • Brightness vs pulse peak
    • It was found that a 100-fs laser can generate plasma from 0.45-mJ pulse energy. Then, the peak intensity is 24 PW/cm2 and surely higher than the ionized plasma threshold (> 1 PW/cm2). Besides, it is confirmed that the 30-fs pulse requires less energy than the 100-fs pulse to produce plasma under the same average power.
  • Simultaneously addressed voxels
    • They had 1 to 4 simultaneously addressed voxels and 5 or more voxels were not visible.
  • Skin damage
    • For exposure of under 2,000 ms (2,000 shots), only 100-μm-diameter holes appeared and there was no heat damage to the leather. For a period of longer than 2,000 ms, heat effects were observed around the holes.
  • Noise level
    • The maximum noise level was 77.2 dB SPL (The background noise level was 55.7 dB SPL) with 40-fs pulses which also radiate the brightest. This noise level was not so annoying subjectively and it is acceptable in our daily lives.

Though it is still at an early stage, there are endless potentials via this technology according to their paper.

  • Aerial displays
  • Spatial AR to real-world object
  • Aerial interaction with aerial content
  • Haptic Feedback on Aerial Images

Finally, the authors discussed the major limitation – scalability:

  • Limitations
    • Scalability in size of workspace is main concern of our project. Aerial plasma is mainly limited by the objective lens after the varifocal lens.
    • Number of voxels: “We have to develop three factors to scale up our system for daily applications; increasing power of laser source, shortening pulse width to increase peak power, and increasing scanning speed.”
    • Refresh rate: The galvano mirror is the fastest, more than 1 kHz, and the others work at less than 100 Hz. It is hence reasonable to use galvano mirror mainly.
    • Safety: There are a few reports on damage of skin by femtosecond lasers.  The result shows that there was a discontinuous expansion of the damaged area when the exposure time comes up to 2,000 ms. We can minimize the damage by keeping the exposure time less than 2,000 ms by, for example, feedback control based on detection of brighter plasma emission at the surface of the finger in contact with aerial laser plasma.