Conference has limited amount of time for each call, so most of the technical talks are highly specialized. As a result, you need to be familiar with the topic or extremely sharp in picking up the ideas in order to grasp the content immediately. This is definitely not the case in Professor Murnane’s presentation. She started by carefully turning the clock back to the beginning for the century, revisited the idea behind the lasers proposed by Einstein (such as stimulated emission, population inversion), and naturally explained why table top X-ray laser is a challenging topic to tackle even today. For a laser to function (lase), you need to pump the energy to the gain medium to induce population inversion. However, population inversion is very difficult to achieve when the wavelength becomes shorter and shorter. And as we all know, X-ray has very short wavelength.
She gave us an interesting table: To generate 1 um/1 nm/1 angstrom wavelength laser, you need to pump in about 1 mW/1 TW/1 PW energy. This is clearly not very feasible. To squeeze 1 PW from a table top laser does not seem to happen in the near future. This is also why we only saw coherent X-ray by means of powerful cyclotron before 80s.
However, with the discovery of the super-continuum and high harmonic generation (HHG), the dream of tabletop x-ray laser has re-emerged in the 80s. With the pioneering work put down by Franken, Alfano, Bloembergen, and etc., scientists realize that they can use ultrafast pico/femto second laser to induce the nonlinear effect in the gain media to generate coherent radiation with broader and broader spectra even covering the X-ray region. It is also has extremely short pulse duration due to its width of the spectra. Up to now, > 5000 orders of HHG have been observed, and laser pulse is pushing to the Zepto-second regime. The recipe to create this phenomenon is straightforward: Using a high peak power femto-second laser to pump the gas in the chamber (He gas for example), and then let the nature takes care of the business. A very illustrative graph description can be found here.
As you can see from any story, things are not that simple…
What are the challenges we face? Professor Murnane explained clearly: cut-off energy barrier, and phase mis-matching. Due to the mechanism of HHG, the energy of the generated photon is limited by cut-off energy, which is proportional lamba^2 of the input laser wavelength. This seems to be solvable, since we now have mid-IR femtosecond laser. However, when the gain media (gas in most cases) is pumped with longer wavelength laser, the electrons tend to diffuse more. This diffusion will cause the conversion efficiency (converting the energy from mid-IR to X-ray) to drop and is proportional to lambda ^-5 of the input laser. This is a dilemma – you need longer wavelength input laser, but then your conversion efficiency drops significantly.
On the hand, in order to add the X-ray radiation coherently, you need to create a condition called phase matching. This condition allows the X-ray radiation adds coherently and amplifies as it travels though the gain media. Fortunately, nature is on our side on this one. The overlap of the ionized gas (gas is ionized after high power laser travels through) and neutral gas naturally create a short window where phase matching condition can exist. This condition makes the intensity of the X-ray radiation amplifies exponentially and is proportional to the number^2 of the gas. This phase matching condition also solves the first dilemma. Since the low conversion efficiency can be overcome by having more gas in the chamber as the gain media. These difficult and complicated ideas were laid out nicely and conveyed to the audience by Professor Mrunane. !Bravo!
Using this method, the conversion efficiency is ~ 10^-6 with the energy of the photon in the 300--1keV range, with the input laser being the 3.9 um mid-IR femtosecond laser. By using even longer wavelength laser, we may be able to push into the hard X-ray regime.
The application of the X-ray tabletop laser is essentially everywhere. It allows us to capture the fast events in the nature. All of these used to belong to theoretical pondering since the time scale of these events are too fast to observe. In addition, due to its short wavelength, we can now study nanoscale imaging and material characterization near the wavelength limit. Examples such as nanoscale spin dynamics, electron dynamics in quantum dots, correlated materials, electrons moving around in the nanosystems, acoustic nano-metrology, nanoscale heat flow, nanoscale coherent imaging are just some fascinating developments presented by professor Murnane.
She ended the presentation with a timeline chart, which I find highly philosophic and encouraging to the young scientists:
1937 first NMR observed à1980 first commercial MRI. Now MRI is vital in many medical applications.
1977 first HHG observed à 20xx, first commercial tabletop X-ray laser? And what will the impact be?
It took decades of development to shine in this line of work! Science is accumulation of knowledge, with endeavor of much wisdom. It can be slow sometimes, but it is always fun, intellectually challenging, and rewarded.
Posted: 10/8/2013 8:35:31 AM by
By Frank Kuo
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