Below Optical Breakdown Threshold Photoionization Mode (B/OB)
| This section may have been copied and pasted from https://sites.google.com/site/tiberiusbrastaviceanu/home/laser-applications/photoionization/single-photonmodesp2, possibly in violation of Wikipedia's copyright policy. Please remedy this by editing this article to remove any non-free copyrighted content and attributing free content correctly, or flagging the content for deletion. Please be sure that the source of the copyright violation is not itself a Wikipedia mirror. |
B/OB mode is an intermediary between the optical breakdown mode (OB mode) and the filamentary mode (F mode). The plasma density generated in this mode can go from 0 to the critical value i.e. optical breakdown threshold. Intensities reached inside the B/OB zone can range from multi-photon ionization threshold to the optical breakdown threshold. In the visible-IR domain, B/OB mode is obtained under very tight external focusing (high numerical aperture), to avoid self-focusing, and for intensities below optical breakdown threshold. In the UV regime, where optical breakdown 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 (only a few micrometres). 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 optical breakdown threshold. Optical breakdown intensity threshold is strongly correlated to the input intensity only at short pulse durations. Therefore, one important requirement, in order to systematically avoid the optical breakdown, is to operate at short pulse durations. In order for the ionization to take place, multi-photon ionization intensity threshold must be reached. The idea is to adjust the duration of the laser pulse so that multi-photon ionization, and perhaps to a lesser extent avalanche ionization, 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 multi-photon ionization, single-photon ionization, and perhaps to a lesser extent avalanche ionization, are operating, whereas for the former, only single-photon ionization is operating.
B/OB relies mostly on MPI processes. Therefore, it is more selective than 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 avalanche ionization is greatly reduced, and the effects are dominated by multi-photon ionization 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 numerical aperture, and to account for detailed features of plasma spatial distribution.
The B/OB mode was described by A. Vogel et al. .
Read more about this topic: Photoionization Mode
Famous quotes containing the words optical, breakdown, threshold and/or mode:
“It is said that a carpenter building a summer hotel here ... declared that one very clear day he picked out a ship coming into Portland Harbor and could distinctly see that its cargo was West Indian rum. A county historian avers that it was probably an optical delusion, the result of looking so often through a glass in common use in those days.”
—For the State of New Hampshire, U.S. public relief program (1935-1943)
“With the breakdown of the traditional institutions which convey values, more of the burdens and responsibility for transmitting values fall upon parental shoulders, and it is getting harder all the time both to embody the virtues we hope to teach our children and to find for ourselves the ideals and values that will give our own lives purpose and direction.”
—Neil Kurshan (20th century)
“I am admonished in many ways that time is pushing me inexorably along. I am approaching the threshold of age; in 1977 I shall be 142. This is no time to be flitting about the earth. I must cease from the activities proper to youth and begin to take on the dignities and gravities and inertia proper to that season of honorable senility which is on its way.”
—Mark Twain [Samuel Langhorne Clemens] (1835–1910)
“Yours of the 24th, asking “the best mode of obtaining a thorough knowledge of the law” is received. The mode is very simple, though laborious, and tedious. It is only to get the books, and read, and study them carefully.... Work, work, work, is the main thing.”
—Abraham Lincoln (1809–1865)