photons at work photons at work
Femtosecond Laser Technology
General Properties of Lasers Laser color (frequency, wavelength) Ultraviolet, visible, infrared
Continous wave (cw) or pulsed helium neon laser, laser
Q-switch (ns), ps, fs
Beam quality (M2)
Source: Wikipedia
Polarization Coherence …
Different Lasers for Different Applications UV-laser Semiconductor fabrication Eye surgery Glass processing
Visible Metrology Every Day Life (cd-player, pointer) Confocal microscopy Cosmetic surgery
Infrared Metal processing and welding Micro machining Laser microtomy Multiphoton p microscopy py Medical applications (soft tissue cutting)
Laser -Tissue Tissue Interaction photo photodisruption
Inte ensity [W W/cm2]
10 12
10 9
10
photoablation
6
vaporisation 10
3
coagulation 10 0
photochemistry 10 -3 10 -12
10 -9
10 -6
10 -3
Interaction time [s]
0 10
10 3
Near-Infrared-Lasers Near Infrared Lasers – High Penetration Depth
A Absorpti on [µm-1]
1 -1
10 10 10
-2
-3
Hämo Hämoglobin
Melanin
Wasser
Protein
-4
10
-5
10
-6
10
diagnostic window
-7
10
100nm
1µm
Wavelength
10µm
Nonlinear (Multiphoton) Absorption
Source: Wikipedia
Nonlinear (Multiphoton) Absorption linear absorption
nonlinear absorption
Preconditions of Photodisruption for precise biomedical applications Threshold for disruption scales with pulse intensity many, many photons on the same place, at the same time
Unwanted side effects (bubbles, collateral damage) scales with pulse energy
In ntensity
photon energy is converted into heat, kinetics, chemistry
Threshold for di disruption ti Pulse Energy
Beam Diameter Pulse Width
Preconditions of Photodisruption for Precise Biomedical Applications Threshold for disruption scales with pulse intensity many, many photons on the same place, at the same time
Unwanted side effects (bubbles, collateral damage) scales with pulse energy
Intensity
photon energy is converted into heat heat, kinetics kinetics, chemistry
Threshold Th h ld for f disruption Pulse P l Pulse Energy Energy
Beam Di B Diameter t Pulse Width
Characteristics of fs Photodisruption
fs Photodisruption is: ultra precise (better than Âľm resolution)
applicable to 3D processing (not limited to the surface)
Brad Amos, Division of Structural Studies, MRC Laboratory of Molecular Biology, Cambridge
How Short is Ultrashort? 1 fs = 10-15 s
speed of light:
speed of light:
1.2 sec: Moon – Earth
100 fs: 30 µ µm
But in this short time, the laser pulses carry energies of some nJ to µJ which leads to peak power of powerplants (mega to giga watts)
Factors that minimize Energy Threshold for Photodisruption
Pulse Duration Numerical Aperture
Threshold for Optical Breakdown as a function of Pulse Duration 10000
10000
LZH measurements
Flue ence [J/cm2]
1000
num. simulation (Noack et al.)
1000
100
100
10
10
1
1
regime g for biomedical application pp 0,1 0,01
0,1
1
10
100
1000
10000
Pulse Duration [ps]
0,1 100000 1000000
Numerical Aperture w
f NA 
w f
The larger the NA, th smaller the ll the th focal f l spott volume, l the smaller the energy threshold
Two Different Concepts p in Photodisruption p Process
"High" Pulse Energy (ÂľJ) "Low" Pulse Frequency (kHz)
( ) "Low" Pulse Energy (nJ) gy (MHz) "High" Pulse Frequency
others
Rowiak
Conclusion With fs-Lasers processing of biological and other materials is possible: With ultraprecision With low or even no unwanted side effects Below the surface
With fs-lasers imaging of cells and small animals is possible with: High resolution High penetration depth No unwanted damage