The spectroscopic detection of complex molecules, such as explosives, requires a much broader spectral tuning range of the employed laser compared to the well-established tunable diode laser spectroscopy of light molecules. In this paper we demonstrate a new type of spectrally-tunable laser modules, which enables an increase of the tuning range by at least one order of magnitude in comparison to the established current-temperature tuning techniques. This was achieved by increasing the operating temperature range through the use of a temperature-variable diamond submount. The module comprises a quantum cascade (QC) laser mounted on the diamond submount with an integrated heater element and temperature sensor, enabling a controlled temperature change of the laser between 77 K and 400 K at a rate of temperature change of up to 2500 K/s. The favorable temperature characteristics allow us to easily tune the laser wavelength on- and off-resonance with the characteristic absorption bands of the material to be detected. With the QC laser module we demonstrate the detection of surface contaminations of around 10 μg/cm2 TNT performing imaging backscattering spectroscopy.

Conventional loudspeaker membranes made of metal or synthetic material such as fabric, ceramics or plastics suffer from nonlinearities and cone breakup modes at fairly low audio frequencies. Due to their mass, inertia and limited mechanical stability the speaker membranes made of conventional materials cannot follow the high frequency excitation of the actuating voice-coil. Low sound velocity causes phase shift and sound pressure losses due to interference of adjacent parts of the membrane at audible frequencies. Therefore, loudspeaker engineers are searching for lightweight but extremely rigid materials to develop speaker membranes whose cone resonances are well above the audible range. With its extreme hardness, paired with low density and high velocity of sound, diamond is a highly promising candidate for such applications. We report on the realization of dome shaped CVD diamond membranes by deposition on curved silicon substrates. Domes with diameters between 20 and 65 mm and with a thickness ranging from 50 to 120 μm were prepared. After deposition, the substrate is dissolved and the rim of the diamond dome is cut by laser scribing. Free standing diamond membranes are mounted onto dynamic voice coils and integrated into tweeter and/or midrange driver chassis. Extended tests and optimisations led to loudspeaker systems that show a second and third harmonic distortion behaviour in the important frequency range between 3 to 10 kHz that is reduced by 40% in comparison to already excellent established values obtained with sapphire membranes. Cone resonance frequencies of CVD diamond membranes are increased by a factor of two, as predicted by simulations.

Diamond lenses are highly desirable for a variety of optical applications including the operation under high-power, high thermal load and under harsh environmental conditions. We report on the realization of CVD-diamond lenses by deposition on structured substrates. Lenses with diameters between 2 and 5 mm and focal lengths between 3.2 and 5.2 mm were prepared. Substrates with a spherical surface or with an array of spherical impressions have been used. CVD-diamond of high phase purity was deposited on these structured substrates using microwave plasma CVD. The growth side of the diamond layer was polished and the substrate removed. Laser cutting separated the lenses. The plano-convex CVD-diamond lenses prepared with this ‘molding’ technique were characterized regarding flatness and spherical aberration of the individual surfaces as well as optical and thermal properties.

Homoepitaxial diamond films grown by chemical vapor deposition from 12C and 13C containing gases have been studied by Fourier transform infrared spectroscopy. A sharp absorption band observed at 3123 cm−1 is attributed to a CH vibrational absorption. It resembles the previously observed 3107 cm−1 absorption band in natural diamond. Isotopic replacement of carbon, hydrogen, and nitrogen by 13C, 2H, and 15N reveals, that a CH center without nitrogen participation is responsible for this new absorption. Furthermore, new hydrogen related electronic transitions have been observed around 7300 cm−1. These lines shift to higher energies in 13C crystals, in accordance with the increase of the electronic band gap.

Diamondlike amorphous hydrogenated carbon (a-.C:H) films have found widespread use as hard coatings for IR optical applications. The refractive index of these films (n=2) allows the deposition of quarter-wave antireflection coatings on germanium. More complex optical designs, however, require the extension of the range of the optical constants. For this reason we studied amorphous hydrogenated carbon-germanium alloys. The corresponding a-C1 xc3ex H films with 0 < X < 1 were grown by ri-plasma deposition using mixtures of a hydrocarbon gas and germane or tetramethylgermanium (TMG) as precursors. Film composition, refractive index and IR absorption spectra were measured. This material system offers the possibility to adjust. the refractive index between 1 .8 and 4.1 and thus allows the deposition of multilayer thin film systems for IR optical applications. As a result of numerical design calculations, a three layer structure with a hard a-C:H top layer is proposed, which acts as a broadband antireflection coating on germanium substrates for 8-1 2 j.m. We report on the growth and performance of these hard multilayer AR coatings.

Using Raman spectroscopy we have studied the film/substrate interface of hard hydrogenated amorphous carbon (a-C:H) films deposited on crystalline silicon and germanium. For film thicknesses below ∼ 100 Å a frequency down shift of the main a-C:H Raman peak is observed. This substrate dependent frequency shift provides evidence for the formation of an amorphous SiC interfacial layer for a-C:H deposited on silicon. This assignment is supported by the comparative study of thick (∼ 1 μm) a-Si1−xCx:H films. The Raman data for a-C:H deposited on germanium and for a-Ge1−xCx:H, respectively, are also compatible with the concept of an amorphous carbide interface layer.

Hard amorphous hydrogenated carbon (a-C:H) films with thicknesses varying from 10 to 2000 Å were plasma deposited on Si or Ge and studied by Raman spectroscopy. Using optical multichannel detection it was possible to record light scattering spectra from even the thinnest films. The main peak in the Raman spectrum, which is at ∼1520 cm−1 for thick (a-C:H) films, shows a frequency down shift for layers thinner than 100 Å. This mode softening, which amounts to 70 cm−1 for 10-Å a-C:H on Si, is substrate dependent. It reflects the free surface of the film as well as the carbon-substrate bonding at the interface. The Raman scattering intensity shows a pronounced enhancement for films with an optical thickness of about 1/4 of the exciting laser wavelength. This behavior is explained in terms of interference-enhanced Raman scattering.

Optical emission analysis and mass spectroscopy was applied to study the rf-glow discharge in hydrocarbons used for the deposition of amorphous hydrogenated carbon (a-C:H). The mass spectra of the positively charged ions in the plasma are characteristic for the hydrocarbon used — e.g. benzene, hexane, or methane. In contrast, the properties of the deposited a-C:H film, such as refractive index, are independent of the precursor gas. Spatially resolved optical measurements show strong emission from C, C2, and CH in the vincinity of the negatively self-biased cathode (substrate) irrespective of the precursor used. Based on these findings we conclude that the fragmentation of the impinging energetic hydrocarbons is one of the key mechanisms for the formation of hard, strongly cross-linked a-C:H films.