Nanodevices for mid-infrared single molecule spectroscopy

Single molecule spectroscopy is a fascinating technique with numerous applications for fundamental research in many areas from physics to life sciences. Currently, detection of single molecules is obtained by experimental techniques employing scanning tunneling and atomic force microscopy and by optical spectroscopy in the visible spectral range. Vibrating and rotational modes of many organic molecules are found, however, in the spectral range of the mid-infrared at wavelengths between 3 and 30 microns (in the so called molecular finger print range). Although, this spectral range is therefore of high technical interest, all spectroscopic tools currently available for the mid-infrared are by far insufficient for detection of single molecules. Hence, in this project we develop new concepts and new devices for spectroscopy in the mid-infrared with high spectral resolution and high sensitivity with the aim to allow the investigation of single organic molecules in gaseous atmosphere. In particular we develop lead-salt based microcavity devices with high Finesse, which benefit from the high dielectric constant not found in other semiconductors. These devices operate either as vertical-cavity surface-emitting lasers, or as highly sensitive detectors exhibiting an optical response at a certain single spectral line, or as miniature gas absorption cells. To achieve high sensitivity, the molecules under investigation are placed into the laser resonators, whereas lateral patterning is used to obtain the spatial resolution required for single molecule detection in the order of 150 nm. We will fabricate and test devices based on vertically emitting as well as laterally emitting lasers. In both cases the detectors will be integrated on the same chip. In addition, we plan to assemble a highly compact mid-infrared spectrometer of matchbox size for spectroscopy on molecule ensembles. This will be done by the use of a separate laser and detector in combination with a high Q Fabry-Perot resonator used as miniaturized gas absorption cell. This spectrometer will facilitate important applications such as trace gas detection and pollution monitoring.

The aim of this project has been the development of novel nano-materials and novel opto-ectronic devices based on these materials, which are active in the infrared spectral region. In particular novel light sources and detectors should have been developed to assemble them to sensitive detection systems. Such integrated detection systems are desired to reveal specific molecules in gas mixtures, which is important for environmental analysis, climate research, clinical diagnostics, exhaust gas analysis and process control. As an example, within this project, within a collaboration with the research department of Siemens in Erlangen, a infrared-camera has been developed and introduced, which has a photosensitive layer based on various organic and inorganic solution processed semiconductors. Thus, the production costs for this camera is by far reduced in respect to conventional infrared cameras. The material which makes this infrared camera sensitive to the infrared, are colloidal lead-sulfide nanocrystals produced by us. The photodiodes, forming the individual pixels of the camera are not only simple to produce, but they have also outstanding characteristics in respect to rectification, quantum yield and lifetime. Furthermore, their operation principle is new, because the detection principle which is based on charge separation is obtained in a blend containing three different components. Such cameras could be applied in the near future also in cars to assist drivers at night or at bad weather by improving the visibility. Besides the cameras, also semiconductor disc lasers from lead-salt materials were developed, which operate at very long wavelength of 4.3 micrometers, while so far the emission of conventional semiconductor lasers operating at room temperature has been restricted to wavelength small then 3.3 micrometers. At longer wavelength usually strong nonradiative losses prohibit laser radiation. For the new lead-salt lasers these losses are strongly reduced, thus the laser operates at relatively small pump powers and it radiates at a single very well defined wavelength. The laser active material is deposited in ultrahigh vacuum, in particular it is a lead-selenide "quantum" film with a thickness corresponding to only 30 atomic layers. As a special result we have also obtained photoconductive detectors simply prepared by inkjet-printing. These detectors exhibit a high sensitivity which is at room temperature about 100 to 1000 times higher than that of epitaxially grown quantum dot photodetectors. These detectors operate not only in the visible spectral region but also in the infrared up to wavelength of 3 micrometers. This is the longest wavelength for which operation of an electro-optical devices, fabricated from solution processed materials, has been obtained.