The authors of a paper in Nature Photonics have been forced to walk back their article after learning from another group of researchers that their conclusions likely were an, ahem, optical illusion.
The paper, “Greatly enhanced continuous-wave terahertz emission by nano-electrodes in a photoconductive photomixer,” appeared in January 2012 and came from a team led by that included Aaron Danner, an optics expert at the National University of Singapore. As the abstract of the paper explains (to physicists, anyway):
An efficient, room-temperature-operation continuous-wave terahertz source will greatly benefit compact terahertz system development for high-resolution terahertz spectroscopy and imaging applications. Here, we report highly efficient continuous-wave terahertz emission using nanogap electrodes in a photoconductive antenna-based photomixer. The tip-to-tip nanogap electrode structure provides strong terahertz field enhancement and acts as a nano-antenna to radiate the terahertz wave generated in the active region of the photomixer. In addition, it provides good impedance-matching to the terahertz planar antenna and exhibits a lower RC time constant, allowing more efficient radiation, especially at the higher part of the terahertz spectrum. As a result, the output power of the photomixer with the new nanogap electrode structure in the active region is two orders of magnitude higher than for a photomixer with typical interdigitated electrodes. The terahertz emission bandwidth also increases by a factor of more than two.
Turns out that, despite undergoing conventional peer review — so we assume — the conclusions were based on a mirage: namely, that the equipment Danner’s the group had used in their experiment functioned as they’d believed. Evidently, it didn’t.
According to the retraction notice:
In this Article, we presented a nanogap continuous-wave terahertz photomixer that exhibited a two orders of magnitude intensity enhancement compared with a photomixer with conventional interdigitated electrodes, which we used as a reference device. We estimated the powers of the devices based on the blackbody power of a Hg lamp, as described in the Methods section and shown in Fig. 3b of this Article. After receiving a communication from J. Mangeney, R. Colombelli, E. Peytavit, J. F. Lampin, M. Jarrahi, S. Barbieri, M. Wanke and M. A. Belkin, we measured the absolute output of our device using a loaned pyrodetector (Gentec-EO, THZ1.5B-BL-USB). We measured the absolute output power to be about 1 μW, instead of 100 μW as initially estimated and shown in Fig. 3b. Due to this error, we wish to retract the Article, even though the reported relative enhancement and the idea of using nanogap electrodes are still valid. We apologize to the readers for any adverse consequences that may have resulted from the paper’s publication.
The paper has been cited eight times, according to Thomson Scientific’s Web of Knowledge.
Danner, in an email, said:
Obviously we are all disappointed to retract a paper from Nature Photonics.
Just a guess by an outsider: the error in absolute output power could not have been caught in peer review by its very nature. The abstract states a two order of magnitude increase in output power as compared to a conventional photomixer, apparently of the same type. If you believe the retraction, the only problem is that they were having the same increase from a much lower base. It seems to me that the paper should be resubmitted with the new, apparently more accurate, absolute output figures. This reaction should be understood to be by a complete outsider to the field.
Yeah.. this seems like a legit mistake that will be corrected and republished.
I honestly can’t understand why a retraction is necessary. Whatever happened to errata? Either scientists’ and publishers’ Latin is getting worse, or they are turning to absolute extremes to deal with pesky problems and truly minor errors that can be resolved through a simple declaration and statement of where the error lies. This sounds like a valid error (minor one at that) and deserves nothing more than an online erratum. Why are publishers jumping from error to retraction without the possibility of an erratum. The oddest thing, truly, is that the authors are requesting a retraction.
My guess on the reason for the retraction is the corrected article might not meet the level of novelty for Nature Photonics. If that was the case, the authors and editors may have decided that a retraction by the authors has a better appearance than from the editors. The authors could still submit the corrected article to another journal.
The only reason of publishing the original version of that paper is “the enhancement” is two-order higher than usual photomixer. According to their second statement, such great enhancement is no longer there, and then there is no point of publication at Nature Photonics.