As a pioneer in organic semiconductors (for example, intrinsic properties of conjugated polymers, first blue polymer LED, organic photodiode on newspaper) who has been in the field for 28 years, I highly appreciated reading your article “Organic Semiconductor Lasers — The pump is the challenge” (Photonic Frontiers, June 2008; www.laserfocusworld.com/articles/330755), and I would like to make some comments.
High current densities are possible in organic materials if they are:
- Highly pure (less than 1015 defects per cm3)
- In the form of single crystals or high-quality thin films
- Pumped with short current pulses (less than 1 µs)
- Excellently anchored thermally
- Handled and driven in inert atmosphere or perfectly packaged (concerning oxygen and water)
I am 100% positive that it will be possible to realize an electrically pumped organic laser because the creation of high current densities is not an issue (see above), there are a number of materials in which the carrier absorption is sufficiently away from the spectral position of the expected laser action, and modern structuring techniques allow the devicing of organic layers and multilayers without affecting purity.
We have published an interesting review concerning organic lasers (G. Kranzelbinder and G. Leising, Rep. Prog. Phys. 63, 729, May 2000). It is a very stimulating competition and race for achieving the first realization of such an “OLASER.”
Günther Leising
Institute of Solid State Physics
Graz University of Technology, Austria
[email protected]
www.leising.at
Contact-lens virtual display would need optics
With regard to “Contact lens could create virtual display” (April 2008, p. 19; www.laserfocusworld.com/articles/325415.
This very clever fabrication technique is erroneously titled. No display can be placed so close to the iris of the eye and hope to be imaged on to the retina without the presence of reimaging optics. The situation is analogous to placing a display on the front surface of a camera lens that is focused at a long distance, and expecting to image it.
An object at distance u from a lens of focal length f forms an image at a distance v from the lens, according to the relationship: 1/u + 1/v = 1/f, so v = u.f/(u – f).
It is immediately obvious that if u is less than f, then v is negative (in other words, not a real image). In fact, for the normal human eye, u would have to be greater than the distance to the “near point” of focus, which is about 200 mm, if a sharp image is to be perceived.
However the other putative uses are interesting and probably valid.
David Beach
Prebbleton, New Zealand
[email protected]
The researcher replies:
Your calculation is correct and your point is well taken. An object placed on the surface of the eye cannot be in focus. We plan to use two techniques for generating an in-focus image, but are not prepared to discuss the technical details at this time. Thank you for the question.
Babak Parviz
University of Washington
[email protected]