Abstract:We present the first optical sensor based on Surface Plasmon Resonance (SPR) operating in the mid-infrared range. The experimental setup is based on a Kretschmann geometry with Ti/Au layers deposited on a CaF 2 prism where light excitation is provided by a Quantum Cascade Laser (QCL) source. Evidence of SPR is presented and the sensing capability of the system is demonstrated by using CO 2 and N 2 mixtures as test samples. Due to the absorption of CO 2 at this wavelength, it is shown that the sensitivity of this configuration is five times higher than a similar SPR sensor operating in the visible range of the spectrum.

Esaki and Tsu's superlattice 1 , made by alternating two different semiconductor materials, was the first one-dimensional artificial crystal that demonstrated the ability to tailor semiconductor properties. One motivation of this work was the realization of the Bloch oscillator 2,3 and the use of its particular dispersive optical gain 4,5 to achieve a tuneable source of electromagnetic radiation. However, these superlattices were electrically unstable in the steady state 6 . Fortunately, because it is based on scatteringassisted transitions, this particular gain does not arise only in superlattices, but also more generally in semiconductor heterostructures 7,8 such as quantum cascade lasers 9 (QCLs), where the electrical stability can be controlled 10 . Here, we show the unambiguous spectral signature of Bloch gain in a special QCL designed to enhance the latter by exhibiting laser action in the condition of weak to vanishing population inversion.In solids, electrons have a fixed relation between momentum and energy: they move along energy bands, as known from condensed-matter theory. When an electric field is applied they are accelerated but the lattice forces a periodic motion at a definite Bloch frequency. This phenomenon is known as Bloch oscillations, and the idea was successfully used by Zener to explain the dielectric breakdown 3 . However, in usual solids the strong scattering due to impurities and carrier-carrier interaction prevents the observation of such oscillations, as the lattice constant is too short to allow the electrons to complete even one oscillation cycle. In superlattices, the lattice constant can be chosen and a subtle engineering may allow electrons to achieve a few oscillations before scattering. As this phenomenon is fascinating from a condensed-matter point of view, it also opens new perspectives for optics because charge oscillations naturally couple to radiation and offer a way to emit coherent radiation.Therefore, the important question is whether these oscillations can be self-sustained and provide optical gain. First, Ktitorov 4 and then Ignatov and Romanov 5 addressed the problem theoretically with Boltzmann equations and succeeded in providing a definitive signature for Bloch oscillations in superlattices in terms of a particular spectral response: the Bloch oscillations are found to amplify the electromagnetic field (optical gain) on the low-energy side of the oscillation frequency, whereas they absorb photons on the high-energy side. This particular shaped gain-Bloch gain-is the main feature of the Bloch oscillator. A series of experiments 11-13 using pulsed ultrafast techniques have successfully shown the existence of Bloch oscillations as electrons are pumped in a higher energy band and collectively oscillate over their dephasing time. However, the Bloch gain extends to zero frequencies and the structure becomes unstable in the steady state, so far preventing the observation of net gain in superlattices, although some evidence in photocurrent 14 and more recently in absorpt...

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In this work, we demonstrate broad electrical tuning of quantum cascade lasers at 9.25 μm, 8.5 μm, and 4.4 μm in continuous wave operation using Vernier-effect distributed Bragg reflectors based on superstructure gratings. Integrated micro-heaters allow to switch from one Vernier channel to the other, while predictable and mode-hop free tuning can be obtained in each channel modulating the laser current with a side mode suppression ratio as high as 30 dB. The resulting device behaves effectively as a switchable multicolour tunable source. Tuning up to 6.5% of the central wavelength is observed. To prove the importance of the developed devices for high resolution molecular spectroscopy, a N2O absorption spectrum has been measured.

We present single mode quantum cascade lasers including a microscopic heater for spectral emission tuning. Through the use of a buried heater element, the active region temperature can be modified without changing the submount one. Emission frequency tuning in continuous-wave as large as 9 cm(-1) at 1270 cm(-1) and 14 cm(-1) at 2040 cm(-1) are observed, corresponding to an increase of the active region temperatures of ∼ 90 K. Due to the proximity of the heaters to the active region, emission can be modulated at several kHz range and the absence of moving parts guarantees the mechanical stability of the system. This method can be successfully applied to all buried heterostructure lasers, becoming an attractive solution for molecular spectroscopy in the IR. Using the presented devices, molecular absorptions of N(2)O have been measured between 1270 cm(-1) and 1280 cm(-1) and are in agreement with data from the HITRAN database.