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We perform Ramsey spectroscopy on the ground state of ultracold 87Rb atoms magnetically trapped on a chip in the Knudsen regime. Field inhomogeneities over the sample should reduce the contrast of the Ramsey frindges by a factor of 5 for 5 s of interrogation. We measure hardly any decay, only a factor 1.08 corresponding to a coherence time of the order of one minute. This new effect allows us to increase the quality factor of the microwave atomic resonance. We explain this surprising result by a spin self-rephasing mechanism induced by the identical spin rotation effect. The study of decoherence under metrological condition is the key to this synchronisation regime where non-linear effects dominate the evolution of the cloud. We propose a theory of this synchronization mechanism and obtain good agreement with the experimental observations. The effect is general and may appear in other physical systems with trapped atoms. In particular, this effect will exist for fermions and could be exploited to enhance the stability of optical lattice clocks.

Semiconductor lasers based on two-dimensional photonic crystals generally rely on an optically pumped central area, surrounded by un-pumped, and therefore absorbing, regions. This ideal configuration is lost when photonic-crystal lasers are electrically pumped, which is practically more attractive as an external laser source is not required. In this case, in order to avoid lateral spreading of the electrical current, the device active area must be physically defined by appropriate semiconductor processing. This creates an abrupt change in the complex dielectric constant at the device boundaries, especially in the case of lasers operating in the far-infrared, where the large emission wavelengths impose device thicknesses of several micrometres. Here we show that such abrupt boundary conditions can dramatically influence the operation of electrically pumped photonic-crystal lasers. By demonstrating a general technique to implement reflecting or absorbing boundaries, we produce evidence that whispering-gallery-like modes or true photonic-crystal states can be alternatively excited. We illustrate the power of this technique by fabricating photonic-crystal terahertz (THz) semiconductor lasers, where the photonic crystal is implemented via the sole patterning of the device top metallization. Single-mode laser action is obtained in the 2.55-2.88 THz range, and the emission far field exhibits a small angular divergence, thus providing a solution for the quasi-total lack of directionality typical of THz semiconductor lasers based on metal-metal waveguides.

We present a compact atomic clock interrogating ultracold 87Rb magnetically trapped on an atom chip. Very long coherence times sustained by spin self-rephasing allow us to interrogate the atomic transition with 85% contrast at 5 s Ramsey time. The clock exhibits a fractional frequency stability of $5.8\times 10^{-13}$ at 1 s and is likely to integrate into the $1\times10^{-15}$ range in less than a day. A detailed analysis of 7 noise sources explains the measured frequency stability. Fluctuations in the atom temperature (0.4 nK shot-to-shot) and in the offset magnetic field ($5\times10^{-6}$ relative fluctuations shot-to-shot) are the main noise sources together with the local oscillator, which is degraded by the 30% duty cycle. The analysis suggests technical improvements to be implemented in a future second generation set-up. The results demonstrate the remarkable degree of technical control that can be reached in an atom chip experiment., 12 pages, 11 figures

We have used the Pockels effect in ZnTe to demonstrate that the frequency and the phase of a 2.7Terahertz Quantum Cascade Laser can be actively stabilized to the n-th harmonic of the 90MHz repetition rate of a commercial, mode-locked Er-doped fiber-laser. The beating between the stabilized QCL frequency and the harmonic of the repetition rate yielded a signal-to-noise ratio of 80dB in 1Hz bandwidth (limited by the resolution of our spectrum analyzer). The technique demonstrated is inherently broadband, in the sense that it allows to directly stabilize any QCL source provided that its frequency is within the spectral bandwidth of the fs-laser (∼5THz).

At Terahertz (THz) frequencies metals are still excellent materials to guide and confine electromagnetic radiation with relatively low losses. Therefore the concepts developed in the microwave range to design efficient waveguides and resonators can be successfully transferred up to this frequency region. A successful example of such "technology transfer" is the so-called metal-metal resonator, effectively used as a waveguide for THz Quantum Cascade Lasers (QCLs). This type of resonator is essentially a downscaled version of a microstrip waveguide, widely used at microwave frequencies. In this work we report on microwave impedance measurements of metal-metal ridge-waveguide THz QCLs. Experimental data, recorded at 4K in the 100MHz-55GHz range, are well reproduced by distributed-parameter transmission-line simulations, showing that the modulation cutoff is limited by the propagation losses that increase for higher microwave frequencies, yielding a 3dB modulation bandwidth of ~70GHz for a 1mm-long ridge. By using a shunt-stub matching we demonstrate amplitude modulation of a 2.3THz QCL up to 24GHz. In the last part of this work we discuss the experimental evidence of a feedback-coupling between the intracavity THz field and the microwave field generated by the beating of the Fabry-Perot longitudinal modes above the lasing threshold.

International audience; Mode-locked femtosecond lasers have revolutionized the field of optical metrology by allowing the realization of ultra-stable phase-coherent links between the optical-frequency domain and the radiofrequency range. In this work we have used the electro-optic effect in ZnTe (ref. 5) to demonstrate that the frequency and the phase of a 2.7 THz quantum cascade laser can be actively stabilized to the nth harmonic of the 90 MHz repetition rate (frep) of a commercial, mode-locked erbium-doped fibre laser. The beating between the stabilized quantum cascade laser frequency and the harmonic of frep yield a signal-to-noise ratio of 80 dB in a bandwidth of 1 Hz. The technique is inherently broadband, that is, it is applicable to any quantum cascade laser source provided that its frequency falls within the spectral bandwidth of the femtosecond laser (~5 THz). Furthermore, it is an ideal tool with which to control the phase of different quantum cascade lasers using light and compact fibre technology rather than superconducting bolometer mixers.

International audience; We report on microwave impedance measurements of metal-metal ridge-waveguide terahertz quantum cascade lasers. Experimental data, recorded at 4 K in the 100 MHz–55 GHz range, are well reproduced by distributed-parameter transmission-line simulations, showing that the modulation cutoff is limited by the propagation losses that increase for higher microwave frequencies, yielding a 3 dB modulation bandwidth of ∼70 GHz for a 1 mm-long ridge. By using a shunt-stub matching we demonstrate amplitude modulation of a 2.3 THz QCL up to 24 GHz.

International audience; Quantum Cascade Lasers (QCL) are probably the most promising sources for Terahertz wave generation. The key issue especially for double metal QCLs is the extraction and the beam shaping of the emitted power. We propose a solution with a TEM-Horn Antenna and we compare it to the most recently proposed solutions: plasmonic array, Silicon-lens etc. New Terahertz antenna designs will also be presented.

Portability, low cost and fast acquisition rates are key features that a THz imaging system should satisfy for extended commercialized applications. With regards to these features, the source - detector association of a THz Quantum Cascade Laser (QCL) with an un-cooled micro-bolometer two-dimensional array looks promising for THz active imaging. QCLs performance is rapidly improving, with higher operating temperatures and output powers recently demonstrated. On the detector side, un-cooled micro-bolometer array opens the way to real-time video rate, with no raster scanning and potential low cost. In parallel to the development of room temperature micro-bolometer sensors specifically designed for the THz range, the authors have characterized experimentally the sensitivity of CEA-LETI standard amorphous Silicon infrared microbolometers illuminated by a 3THz QCL. The sensitivity of these existing sensors is then compared to the expected sensitivity of the CEA-LETI upcoming THz sensors.

We demonstrate single-mode, surface-emitting terahertz lasers at λ≈110 µm employing photonic-crystal resonators. The devices employ metal-metal waveguides, and the photonic-crystal lattice is implemented by the sole patterning of the top metal contact, without deep-etching the semiconductor in the photonic crystal holes. The device emission wavelength correctly tunes with the photonic crystal characteristics. In addition, we elucidate the crucial role played by the boundary conditions of the photonic-crystal resonator.

International audience; We present the operation and the far field radiation pattern of a 2.9 THz double metal quantum cascade laser (QCL) with a micro TEM-horn antenna. An improvement of both the radiation pattern and the collected power level (typically a factor of 2 with f/1 optics) are observed in comparison with the QCL without antenna.

International audience; A 2.8 THz metal-metal quantum cascade laser operating with a microtransverse-electromagnetic-horn antenna has been demonstrated. By comparing the far-field radiation pattern to a standard ridge cavity with a cleaved facet, a striking improvement of the directionality is observed. The effectiveness of the horn antenna is critically dependent on the control of the lateral modes in the laser cavity.

International audience; Quantum Cascade Lasers are one the most promising approach for the generation of THz waves and particularly in a double plasmon waveguide topology. Stripline resonators with a quasi-TEM mode are indeed interesting for THz-QCLs thanks to a high confinement factor combined with low losses. However they present two major drawbacks: the first is a poor outcoupling of the radiation, and the second is a highly divergent beam. In this communication we propose an original approach to solve simultaneously these two identified problems. It consists on the use of a metallic triangle connected at one end of the top metallic strip in order to realize with the ground plane a TEM- Horn Antenna (TEM-HA). We have first validated this approach with electromagnetic simulations based on a finite integration technique (FIT). The use of the TEM-HA improves the far-field radiation pattern, which presents a higher directivity in the direction of the antenna. We have also validated experimentally this concept on a QCL designed to emit at f=2 THz (lambda = 150 mum). The use of the antenna increases the collected power by a factor 2-to-3 compared to the case without antenna. We expect an even higher outcoupling by optimising the connection of the antenna to the QCL (electrical connection and position). Finally, as we believe that the realisation of a full monolithic device is the best approach to integrate the TEM-HA on the QCL, we will discuss on the technological issues raised by such a device.

The use of integrated photonic structures to tailor the behavior of light is extremely promising for optimizing performance and for introducing advanced functionalities into optoelectronic devices. We demonstrate a powerful method based on photonic-band engineering which allows the optimization of the resonator quality factors of devices operating on band-edge photonic-crystal states. We also show that carefully designed π-shifts in two-dimensional photonic-resonators give enhanced beam properties. The application of these general techniques to terahertz quantum cascade lasers yields improved maximum operating temperatures, and angularly narrow, single-lobed surface emission of ≈12°×8°. The devices operate at ≈2.8/2.9 THz, with peak output powers of 5 mW at 78 K.

We regularly return to the wavelength region between 30 µm and 1 mm to exploit this underdeveloped spectral range.

We elucidate the effects of the lateral mode structure on the far field pattern of metal-metal ridge-waveguide terahertz quantum cascade lasers. By introducing a 6-μm-wide metal gap on the top metal contact, we suppress odd-parity lateral modes and drastically modify the far field pattern. Measurements are in good qualitative agreement with full three-dimensional finite-difference-time-domain modeling. Experimental evidence of nonuniform current pumping on the intensity distribution of the guided mode (and hence the far field pattern) is also presented and explained in terms of gain guiding.

By directly modulating the bias voltage of a double-metal waveguide, 2.8THz quantum cascade laser, we observe the appearance of multiple gigahertz sidebands in the emission spectrum, with a spacing that can be continuously tuned up to 13GHz. By using an upconversion technique, the terahertz spectrum is shifted at 1.57μm, and displayed on an optical spectrum analyzer. A marked increase in the number of sidebands is observed when the modulation frequency approaches the round-trip frequency (∼12.3GHz). The laser packaging high frequency response has been measured using a microwave rectification technique, and is limited by the bond-wire parasitic inductance.

We demonstrate full control of the spectral and spatial emission properties of terahertz quantum cascade lasers using metallic photonic-crystals for lateral confinement. Single-mode, surface-emission is obtained at a frequency of 2.8–2.9 THz using a photonic-crystal design with a square lattice geometry. The resonator quality factor is then optimised, using a graded photonic-crystal architecture, and this leads to an increased maximum operating temperature of 151K. Furthermore, the introduction of a carefully designed pi-shift yields single-lobed surface emission (∼ 12° × 8°), with a peak output power of 5 mW at 78K.