Single walled carbon nanotube (SWCNT) networks present outstanding potential for the development of SWCNT-based gas sensors. Due to the complexity of the transport properties of this material, the physical mechanisms at stake during exposure to gas are still under debate. Previously suggested mechanisms are charge transfer between gas molecules and SWCNT and Schottky barrier modulation. By comparing electrical measurements with an analytical model based on Schottky barrier modulation, we demonstrate that one mechanism or the other is predominant depending on the percolation of metallic carbon nanotubes. Below the metallic SWCNT percolation threshold, sensing is dominated by the modulation of the Schottky barrier, while above this threshold, it is only attributed to a charge transfer between SWCNT and gas molecules. Both mechanisms are discussed in terms of sensitivity and resolution leading to routes for the optimization of a gas sensor architecture based on highly enriched semiconducting carbon nanotube films.

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A theoretical and experimental investigation of the effects of mode coupling in a resonant macroscopic quantum device is achieved in the case of a ring laser. In particular, we show both analytically and experimentally that such a device can be used as a rotation sensor provided the effects of mode coupling are controlled, for example through the use of an additional coupling. A possible generalization of this example to the case of another resonant macroscopic quantum device is discussed.PACS numbers: 42.65. Sf, 42.62.Eh, 06.30.Gv, 42.55.Rz Devices using macroscopic quantum effects [1] and their associated phenomenon of interference to detect rotations can be arbitrarily divided into two classes [2], namely non-resonant and resonant devices. In devices of the first class, the rotation-induced phase shift is detected by looking at the displacement of an interference pattern. Among such devices are the fiber optic gyroscope [3], the gyromagnetic gyroscope [4], the superfluid gyrometer [5] and the atomic interferometer [6]. In devices of the second class, the rotation is detected through a beat signal. Two examples of resonant and potentially rotation-sensitive devices are the ring laser [7] and the superfluid (e.g. liquid helium or Bose-Einstein condensed gas) in a ring container [8]. In these two examples, it has been shown [9, 10] that a non-linear mode coupling effect plays a crucial role in the system dynamics, usually preventing it from operating in a rotation sensitive regime, unless an additional adequate coupling is set.For example, in the case of the ring laser, the bidirectional emission regime, required for rotation sensing, can be inhibited because the counter-propagating modes share the same gain medium and can be subject to mode competition. This problem is usually circumvented by choosing for the gain medium a two isotopes mixture of helium and neon and by tuning the laser emission frequency out of resonance with the atoms at rest. Provided the detuning value is bigger than the atomic natural linewidth, the gain medium can then be considered as being inhomogeneously broadened and coexistence of the counter-propagating modes occurs [11]. In the case of solid-state ring lasers, the gain medium is homogeneously broadened, leading to a strongly coupled situation resulting in laser emission in only one direction [10].We report in this Letter theoretical and experimental investigation of mode coupling control in a solid-state (Nd:YAG) ring laser. The main natural sources of coupling between the counter-propagating modes are identified, and their role in the laser dynamics is discussed. An additional coupling source is introduced in order to ensure the coexistence of the counter-propagating modes.A condition for rotation sensing is then analytically derived, and an experimental confirmation of this theoretical investigation is reported. It is eventually pointed out that the two-level toy model developed in [9] to describe a superfluid placed in a rotating ring container leads to a similar rotation sens...

We report an experimental study of the 1.064 µm transition dipoles in neodymium doped yttrium aluminium garnet (Nd-YAG) by measuring the coupling constant between two orthogonal modes of a laser cavity for different cuts of the YAG gain crystal. We propose a theoretical model in which the transition dipoles, slightly elliptic, are oriented along the crystallographic axes. Our experimental measurements show a very good quantitative agreement with this model, and predict a dipole ellipticity between 2% and 3%. This work provides an experimental evidence for the simple description in which transition dipoles and crystallographic axes are collinear in Nd-YAG (with an accuracy better than 1 deg), a point that has been discussed for years. PACS numbers: 42.70.Hj, 42.55.Rz While Nd-YAG is one of the most (if not the most) commonly used solid-state laser crystals, the exact orientation of transition dipoles within it is still, paradoxically, an unresolved problem. This is probably owing to the fact that for most applications it is sufficient to consider the Nd-YAG crystal (usually grown along the 111 crystallographic axis) as isotropic, although it has been known for long [1,2] that Nd 3+ ions in this configuration rather see a D 2 symmetry, with six possible dodecahedral orientations. The influence of crystal symmetry on dipole orientations has been previously studied in saturable absorbers such as Cr-YAG [3,4,5] and Tm-YAG [6,7]. In the first case, it has been clearly established that transition dipoles were aligned with the crystal axes (labeled 100 , 010 and 001 ) [4,5] while in the second case it has been found that they were rather collinear with the 110 , 011 and 101 directions [7]. In the case of Nd-YAG, the answer to this question is still unclear in spite of several previous studies involving in particular dynamical polarization effects in Nd-YAG lasers [8,9,10,11].In this paper, we propose a new approach to probe the orientation of transition dipoles in Nd-YAG, by measuring the coupling constant between two linearly-polarized orthogonal modes of a laser cavity for different cuts of the gain crystal, using a steady-state method similar to the one described in [12]. The measured coupling constant is a dimensionless ratio between cross-saturation and selfsaturation coefficients, which is relatively independent of most laser parameters (pumping rate, birefringence,. . .), hence a good indicator for testing the validity of theoretical models. Our study deals for the most part with the 1064.15 nm emission line, sometimes referred to as R2, between the upper doublet of 4 F 3/2 and the Y3 level of 4 I 11/2 . As a matter of fact, it is known from previous studies [13,14] that the R1 line at 1064.4 nm (between the lower doublet of 4 F 3/2 and the Y2 level of 4 I 11/2 ) has a very small contribution to the overall gain, especially at low pumping rates.The paper is organized as follows. We first propose a theoretical model for calculating the coupling constant between two orthogonal modes of a Nd-YAG laser cavity, on t...