Below OB Threshold mode (B/OB)

This mode of ionization is an intermediary between OB mode and F mode. It was described only recently by A. Vogel et al. [6]. The plasma density generated in this mode can go from 0 to the critical value (i.e. OB threshold). Intensities reached inside the B/OB zone can range from MPI threshold to the OB threshold. In the visible-IR domain, B/OB mode is obtained under very tight external focusing (high NA), to avoid self-focusing (SF), and for intensities below OB threshold. In the UV regime, where OB intensity threshold is below self-focusing intensity threshold, tight focusing is not necessary. The shape of the ionization area is similar to that of the focal area of the beam, and can be very small in size (only a few microns). B/OB mode is possible only at short pulse durations, where AI's contribution to the total free electron population is very small. As the pulse duration becomes even shorter, the intensity domain where B/OB is possible becomes even wider.

The principles governing this mode of ionization are very simple. Localized plasma must be generated in predictable fashion, under the OB threshold. OB intensity threshold is strongly correlated to the input intensity only at short pulse durations (see the section on the OB mode). So one important requirement, in order to systematically avoid the OB effect, is to operate at short pulse durations. In order for the ionization to take place, MPI intensity threshold must be reached. The idea is to adjust the duration of the laser pulse so that MPI, and perhaps to a lesser extent AI, have no time to raise the plasma’s density above the critical value.

In the UV, the distinction between single-photon mode (SP) and B/OB is that for the latter MPI, SPI, and perhaps to a lesser extent AI, are operating, whereas for the former, only SPI is operating.

B/OB relies mostly on MPI processes. Therefore, it is more selective then OB in terms of which type of atom or molecule is ionized or dissociated.

The theory needed to understand the most important features of B/OB are:

• The physics of strong-(laser)field interaction with matter, to account for the plasma formation. As opposed to the OB mode, in this case the role of AI is greatly reduced, and the effects are dominated by MPI processes.

• The geometrical/linear optical theory, to account at the first approximation for the spatial intensity distribution. Non-linear propagation theory is usually invoked to account for self-focusing that occurs in experiments conducted at low NA, and to account for detailed features of plasma spatial distribution.

Applications

The applications of this photoionization mode are very diverse. It is less disruptive than OB, and also more selective in terms of which atom or molecule is ionized, dissociated, or excited. Its spatial confinement is superior to OB's, and it can be used in high precision nanoprocessing [1]: surface processing (ablation), and bulk processing (modifying the local physical properties of the material and creating patterns - optical guides, greetings, micro-channels). Undesirable thermal effects can be controlled, and even eliminated, reducing collateral damage. In biology, OB is to disruptive for the delicate organic structures at the sub-cellular level. B/OB becomes more appropriate, as the plasma density and thermal effects can be controlled, and the shock wave can be eliminated. B/OB can then be safely used for microsurgery (burning holes in the cellular membrane, targeted destruction of intracellular structures [2, 3, 4]), and DNA processing (cutting chromosomes [1], DNA damage).

B/OB can also be used to acquire information about the composition and the structure of materials at the molecular level. The information can be extracted by collecting and analyzing the ejected photoelectrons, in the case of surface studies in vacuum. The selectivity of MPI processes, and the lower plasma density, translate into a higher sensitivity then OB. The plasma generated being cooler then in the case of OB, there is no emission that can be used for remote sensing.

Control of important characteristics of the B/OB mode

Range of operation of the B/OB mode

The scope of this section is to depict the domain of laser parameters where B/OB is possible, given a certain type of material. We have to keep in mind that MPI processes are dominant in this photoionization regime as opposed to OB, where AI could play an important role in plasma formation. Hence, the nature of the MPI processes involved, and their response to the variation of laser parameters determine the response in terms of plasma density, and spatial distribution.

Relative to laser light parameters

The B/OB mode is possible within a domain of laser parameters defined by two important event: the first one is the generation of MPI processes, and the second one is the accumulation of plasma above the critical value. In short, B/OB is possible for any combination of laser parameters for which MPI processes can be sustained for a finite time, and for which the plasma density at the end of the laser pulse remains below the critical value (below OB threshold). As the laser pulse sweeps across the material, a molecule will "see" the intensity of the optoelectrical field raising, reaching a maximum, and fading back to zero, probably going through multiple picks. According to the laser parameters, and the electronic structure of the material, MPI processes can be triggered at a given moment during the laser pulse, and in certain locations. This determines the lower limit of B/OB. From this moment, and until the end of the laser pulse, MPI processes, and to a lesser degree AI, contribute to the formation of plasma. If the critical value is reached, we pass to the OB mode, where the interaction between the plasma and the optoelectrical field becomes dominant. If the OB threshold is never attained, we are in the B/OB mode. The violent Coulombian expansion, bobble and shock wave formation, plasma emission, and dramatic temperature increase are avoided in the B/OB mode. The section MPI and AI in the context of laser pulses treats about plasma formation in the context of laser pulses, and it is crucial for the understanding of the B/OB mode, and its range of operation.

Average local intensity

The B/OB mode is possible on a range of intensities going from MPI threshold to OB threshold. Even though we can talk about an intensity threshold, which is the level of intensity where MPI processes become possible, B/OB is not "announced" by a sudden effect like in the case of OB. The plasma density generated can vary continuously from zero to the critical value, as the input intensity is gradually increased. We can see this on the Figure 1 (the two graphics on the write side), which presents results of simulations made by Vogel et al. [6].

Pulse duration

As mentioned earlier, B/OB is possible only for short pulse durations, where the OB threshold is statistically well defined. However, there is no precise value for which this condition is satisfied. OB's intensity threshold becomes more and more stable (confined around a certain value) in a continuous manner, as the pulse duration is gradually diminished. A border value can be established by convention invoking pragmatic reasons, in a given experimental setting. In the Figure 2 [9] we can see that the threshold fluence is scattered around the theoretical threshold value for the 7 ns pulse, and it becomes more predictable, in the fs time regime.

Moreover, well inside B/OB's time-domain of operation, the variation of the pulse duration affects the final plasma density, as a different amount of time is available to the MPI processes to generate free charges.

Wavelength

The B/OB mode is possible on the same wavelength domain as OB - from UV to IR, but only for very short pulse durations, and under very tight external focusing in the visible-IR. As OB's intensity threshold increases when the wavelength increases, it is then clear that this parameter has an impact on the B/OB's upper intensity limit. The lower intensity limit is also affected by the wavelength, as MPI processes are wavelength sensitive.

Polarization

MPI processes are sensitive to the relative orientation between the polarization of the light (for a linearly polarized laser beam), and the orientation of molecular or lattice bounds. Hence, in ordered materials the polarization has an effect on the intensity limits of the B/OB mode. This also means that for a fixed input intensity, the plasma density at the end of the laser pulse varies with polarization (see the equivalent section in the document about OB).

The transition from linear to circular polarization can also affect the intensity limits of B/OB, as for circularly polarized light higher-order multiphoton processes are significantly inhibited.

Relative to material impurities

The B/OB mode operates in the fs time domain. As AI's contribution becomes negligible, the concentration of impurities is of little concern.

Use of lens

In the visible-IR domain, where the F mode is possible, convergent lens are necessary if one wants to avoid filamentation. Therefore, for a particular experimental setting, there is a minimum NA value of operation, where the OB intensity threshold becomes smaller then the self-focusing intensity threshold.

Control of the nature of primary photolytic species

Very little scientific literature exists on the chemical changes occurring during B/OB. In the biomedical domain, undesirable effects of multi-photon fluorescence microscopy (conducted in the IR domain), have driven the attention of scientists towards the fundamental processes that can explain the observed effects. In the case of living cells, lethal and sub-lethal damage has been inferred from the behavior of cells imaged using this technique. In some cases DNA damage has been observed [reference]. Is this a direct effect, or is it an effect induced by radical species generated during photolysis? It may even be a combination of both. The work on this subject is just beginning, and part of the drive towards a better understanding of the physico-chemical processes induced by B/OB is the use of this particular ionization mode in micro-surgery (at the sub-cellular level). In this section I will try to discuss the potential effects of the most important controllable parameters. This can constitute a guide for future studies.

The nature of the primary photolytic species depends on the nature of the material (its composition and its structure), as well as on the dissociation channels (ionization, molecular dissociation) that are possible during the interaction. For sub-ps pulse durations, the primary events are ionization and excitation of atomic systems. Bound breaks occur only after the passage of the laser pulse, as the excited molecules and the plasma relax. It is important to note here that the laser pulse doesn't interfere with the relaxation processes, it just creates the initial conditions. For ns pulse durations, relaxation processes occur during the laser pulse, and here we would have to take into account the direct influence of the optoelectric field, but B/OB is not possible in this time regime.

Average local intensity

This parameter affects the ionization and excitation channels, and subsequent relaxation mechanisms (on a fs-ps time scale), and consequently it affects the nature of primary photolytic species. The local intensity also has an impact on the energy spectrum of free electrons, and on the plasma density, which in turn affects the relative yield of secondary photolytic species created during the physico-chemical stage, during plasma relaxation (on a ps time scale); and ultimately it affects the nature and the relative yield of long-term (stable) photolytic species.

Although the heat generated during B/OB is small compared to the OB mode, whatever amount of energy is transfered into heat, depends on the average input intensity.

Wavelength

The wavelength has a selective effect on ionization, dissociation, and excitation channels, and directly controls the nature of primary photolytic species. It also affects the free electron energy spectrum, as well as on the distance between free electrons and their geminate ions, which in turn affects the yields of secondary photolytic species, and ultimately the long-term photolytic effects.

Pulse duration

As mentioned earlier, the pulse duration has a direct effect on the final plasma density, as more time is available to the MPI processes to generate free charges. This alone can have an important effect on the recombination chemistry, and ultimately on the yield of long-term (stable) photolytic species.

Furthermore, the energy spectrum of the (quasi-)free electrons is determined by their history during the laser pulse. At birth, the free electrons have specific energies, corresponding to the electronic structure on the material and to the wavelength of the ionizing laser pulse. Only a couple of fs later, electron-electron and the electron-phonon collisions work toward homogenization of the energy spectrum of the electron gas. For very short pulse durations (<10fs) it becomes important to take into account the dynamics of the electron gas energy distribution [5], in order to understand the formation of secondary photolytic species (created after the passage of the laser pulse, during plasma relaxation).

Chirp

The temporal spectral distribution can influence several ultra-fast processes (select ionization and excitation channels). By controlling this aspect of the laser pulse we can influence the outcome of the photolysis process. A. Lindinger et al. demonstrated how the wavelength time distribution affects ionization and fragmentation of Na₃ clusters [7].

Temporal intensity distribution

Some ultra-fast processes are very sensitive to the temporal intensity distribution, as well as to the intensity change rate. Understanding temporal pulse breaking and steepening becomes important. By controlling these aspects of the laser pulse we control these ultra-fast processes. That can have important consequences on the yields of primary photolytic species.

Pulse train

By shaping the temporal intensity distribution ultra-fast processes can be controlled. Sinusoidal spectral phase modulations create pulse trains of several pulses with controllable pulse separations. A. Lindinger et al. used this technique to demonstrate how the intensity time distribution affects ionization and fragmentation of Na₂ and NaK clusters [7]. The same principles apply to other molecular systems.

A temporal pulse-break can also occur during a non-linear propagation of a powerful laser pulse in a dielectric medium.

Steepning

The temporal intensity distribution can be dramatically modified during propagation of a powerful laser pulse in a dielectric medium. Some ultra-fast processes are sensitive to the rate of change in intensity. Using non-linear propagation models as a guide, one can modify the temporal pulse shape according to specific needs.

Polarization

In the case of crystals the net effect is clear, OB intensity threshold, and the plasma density reached at the end of the laser pulse depend on polarization. This is because MPI processes are sensitive to the relative orientation between the polarization of light (for a linearly polarized laser beam) and the molecular or lattice axes. In conclusion, all other pulse parameters being kept the same, polarization alone can have an impact on plasma density, free electron energy spectrum, and on the type of primary photolytic species created. As a consequence, the long-term photolytic effects can also be affected.

In an amorphous material, molecules are arranged in an arbitrary manner, so the polarization doesn’t change the overall distribution of photolytic species. Although, we have to keep in mind that MPI and SPI depend on molecular orientation. Consequently, the polarization operates a selection on the molecules that are ionized. It induces an anisotropy within the affected region, i.e. molecules that undergo certain processes under the influence of the laser light tend to be oriented in given directions.

The transition from linear to circular polarization can also affect the plasma density in the B/OB mode, as for circularly polarized light higher-order multiphoton processes are significantly inhibited.

Material impurities

In the B/OB mode MPI processes are mainly responsible for ionization. Consequently, this photoionization mode can be very specific: the wavelength can be tuned to affect (ionize, dissociate, or excite) only a given species.

Control of dose spatial distribution

In the B/OB ionization mode, the affected area is actually the area where the intensity is above MPI threshold; and it is always smaller then the area covered by the entire beam (or then the focus area). In the visible-IR wavelength domain, tight focusing is necessary in order to avoid self-focusing. The dose distribution is in this case very localized. However, in the UV domain, where OB intensity threshold is below self-focusing threshold, tight focusing is not necessary. The affected area is the space where the local intensity is above MPI threshold, and below OB threshold. This area can be shaped at will, using different external optical devices.

Use of lenses

In the visible-IR domain tight focusing (or high NA values) is necessary for the B/OB modes in order to avoid self-focusing. Lens also help to control the local intensity, the dimensions, the shape, and the position within the sample, of the affected area. In the UV regime, where OB's intensity threshold is below self-focusing threshold, tight focusing is not necessary.

Material impurities

In the B/OB mode, MPI processes are mainly responsible for ionization. Consequently, this photoionization mode can be very specific: the wavelength can be tuned to affect (ionize, dissociate, or excite) only a given entity. The dose is then spatially distributed according to the global spatial intensity distribution, and at the microscopic level, it is centered on every atoms or molecules affected by the laser light. In the figure below we can see pinpoint bulk damages (with a diameter that rages from a few to a few hundreds of micrometers) in deuterated potassium dihydrogen phosphate (DKDP), along the trajectory of a 351nm, 1-10ns, laser pulse [8]. The fluence was kept practically constant, and the pulse duration was varied: 1, 3 and 10ns (from left to right on the picture). It is believed that these point-damages are OB centers, initiated by nano-particles, or cluster defects. I assume that in the fs time regime, and for intensities below OB threshold, you would still have plasma generation around these impurities.

Other effects

Heat deposition

To be continued ...

References

[1] Nanodissection of human chromosomes with near-infrared femtosecond laser pulses; K. König, I. Riemann, W. Fritzsche; Optics Letters, Vol. 26, No. 11, June 1, 2001

[2] Mechanisms of femtosecond laser nanosurgery; of cells and tissues; A. Vogel, J. Noack, G. Huttman, G. Paltauf; Appl. Phys. B 81, 1015–1047 (2005)

[3] Femtosecond laser disruption of subcellular organelles in a living cell; Wataru Watanabe, Naomi Arakawa, Sachihiro Matsunaga, Tsunehito Higashi, Kiichi Fukui, Keisuke Isobe, Kazuyoshi Itoh; Optics Express, Vol. 12, No. 18, 4203; 6 September 2004

[4] Intracellular disruption of mitochondria in a living HeLa cell with a 76-MHz femtosecond laser oscillator; Tomoko Shimada, Wataru Watanabe, Sachihiro Matsunaga, Tsunehito Higashi, Hiroshi Ishii, Kiichi Fukui, Keisuke Isobe, Kazuyoshi Itoh; Optics Express, Vol. 13, No. 24, 9869, 28 November 2005

[5] Microscopic processes in dielectrics under irradiation by subpicosecond laser pulses; A. Kaiser, B. Rethfeld, M. Vicanek and G. Simon, Physical Review B, 1 May 2000-I, Vol. 61, No. 17

[6] Femtosecond-laser-produced low-density plasmas in transparent biological media: A tool for the creation of chemical, thermal and thermomechanical effects below the optical breakdown threshold; AlfredVogel , Joachim Noack , Gereon Huttmann , and Gunther Paltauf; Proceedings of SPIE Vol. 4633 (2002) © 2002 SPIE

[7] Optimal control on multi-photon ionization dynamics of small alkali aggregates; A. Lindinger, A. Bartelt, C. Lupulescu, Vajda, L. Wöste; Proceedings of SPIE Vol. 5258 IV Workshop on Atomic and Molecular Physics

[8] Laser-induced damage in deuterated potassium dihydrogen phosphate; Alan K. Burnham, Michael Runkel, Michael D. Feit, Alexander M. Rubenchik, Randy L. Floyd, Teresa A. Land, Wigbert J. Siekhaus, and Ruth A. Hawley-Fedder; Applied Optics, 20 September 2003, Vol. 42, No. 27

[9] Laser Ablation and Micromachining with Ultrashort Laser Pulses; X. Liu, D. Du, and G. Mourou; 1706 IEEE Journal of Quantum Electronics, Vol. 33, No. 10, October 1997