The spectral imaging and detection of mid-infrared wavelengths is emerging as an enabling technology of great technical and scientific interest, primarily because important chemical compounds display unique and strong mid-infrared spectral fingerprints that reveal valuable chemical information. Modern quantum cascade lasers have evolved as ideal coherent mid-infrared excitation sources, but simple, low-noise, room-temperature detectors and imaging systems lag behind. We address this need by presenting a novel, field-deployable, upconversion system for sensitive, two-dimensional, midinfrared spectral imaging. A room-temperature dark noise of 0.2 photons/spatial element/second is measured, which is a billion times below the dark noise level of cryogenically cooled InSb cameras. Single-photon imaging and a resolution of up to 200 3 100 spatial elements are obtained with a record-high continuous-wave quantum efficiency of ∼20% for polarized incoherent light at 3 mm. The proposed method is relevant for existing and new mid-infrared applications such as gas analysis and medical diagnostics.O ptical spectroscopy within the ultraviolet, visible and nearinfrared regimes has for decades been an indispensable method for the identification and quantization of chemical analytes. However, emerging mid-infrared applications in environmental gas monitoring or the life sciences call for improved detection systems that challenge today's capabilities in terms of sensitivity and/or imaging functionality. For example, in the face of global warming, mid-infrared detectors capable of measuring minute gas concentrations are required, because important greenhouse gases such as CO 2 , CO, CH 4 and N 2 O have fundamental absorption bands located in the mid-infrared 1 . For example, CO requires a detection sensitivity on the order of 100 ppb (parts per billion) 2 . Monitoring atmospheric trace molecules at these levels provides important inputs for the climate models used for studying global warming and its consequences for life on Earth 3 . In life science, the spectral regime from 0.3 to 2 mm has already been utilized for fundamental studies of breath analysis. However, significant improvements can be expected from using mid-infrared spectroscopy 4,5 . The on-line detection of the numerous different molecules (.1,000) in exhaled human breath may lead to new non-invasive diagnostics tool for doctors. However, such biomarkers are frequently below ppb levels. Indeed, the exhaled concentration of ethane (at 3.3 mm), which is used as a marker for asthma and chronic obstructive pulmonary disease, is found at 100 ppt (parts per trillion) levels, clearly demonstrating the requirement for highly sensitive methods 5 . Similarly, 1-butanol and 3-hydroxy-2-butanone in breath could be useful biomarkers for lung cancer 6 .In the 3-15 mm wavelength regions, two-dimensional mid-infrared spectral imaging demonstrates potential for identifying cancerous tissue, providing a new tool for cancer diagnostics. In this wavelength region, each organic compound an...

Based on enhanced upconversion, we demonstrate a highly efficient method for converting a full image from one part of the electromagnetic spectrum into a new desired wavelength region. By illuminating a metal transmission mask with a 765 nm Gaussian beam to create an image and subsequently focusing the image inside a nonlinear PPKTP crystal located in the high intra-cavity field of a 1342 nm solid-state Nd:YVO 4 laser, an upconverted image at 488 nm is generated. We have experimentally achieved an upconversion efficiency of 40% under CW conditions. The proposed technique can be further adapted for high efficiency mid-infrared image upconversion where direct and fast detection is difficult or impossible to perform with existing detector technologies.

Optical trapping has enabled a multitude of applications focusing, in particular, on non-invasive studies of cellular material. The full potential of optical trapping has, however, not yet been exploited due to restricted access to the trapped samples, caused by high numerical aperture objectives needed to focus the trapping laser beams. Here, we use our recently developed biophotonics workstation to overcome this limitation by introducing probing and spectroscopic characterization of optically trapped particles in a side-view geometry perpendicular to the trapping beams rather than in the traditional top-view geometry parallel to the trapping beams. Our method is illustrated by CARS and fluorescence spectroscopy of trapped polystyrene beads. The side-view geometry opens intriguing possibilities for accessing trapped particles with optical as well as other types of probe methods independent from the trapping process.

Quantum measurements using single-photon detectors are opening interesting new perspectives in diverse fields such as remote sensing, quantum cryptography and quantum computing. A particularly demanding class of applications relies on the simultaneous detection of correlated single photons. In the visible and near infrared wavelength ranges suitable single-photon detectors do exist. However, low detector quantum efficiency or excessive noise has hampered their mid-infrared (MIR) counterpart. Fast and highly efficient single-photon detectors are thus highly sought after for MIR applications. Here we pave the way to quantum measurements in the MIR by the demonstration of a room temperature coincidence measurement with non-degenerate twin photons at about 3.1 μm. The experiment is based on the spectral translation of MIR radiation into the visible region, by means of efficient up-converter modules. The up-converted pairs are then detected with low-noise silicon avalanche photodiodes without the need for cryogenic cooling.