The amplification of spontaneous emission is used to initiate laser action. As the phase of spontaneous emission is random, the phase of the coherent laser emission (the carrier phase) will also be random each time laser action begins. This prevents phase-resolved detection of the laser field. Here, we demonstrate how the carrier phase can be fixed in a semiconductor laser: a quantum cascade laser (QCL). This is performed by injection seeding a QCL with coherent terahertz pulses, which forces laser action to start on a fixed phase. This permits the emitted laser field to be synchronously sampled with a femtosecond laser beam, and measured in the time domain. We observe the phase-resolved buildup of the laser field, which can give insights into the laser dynamics. In addition, as the electric field oscillations are directly measured in the time domain, QCLs can now be used as sources for time-domain spectroscopy.

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We experimentally demonstrate an integrated semiconductor source of counterpropagating twin photons in the telecom range. A pump beam impinging on top of an AlGaAs waveguide generates parametrically two counterpropagating, orthogonally polarized signal/idler guided modes. A 2 mm long waveguide emits at room temperature one average photon pair per pump pulse, with a spectral linewidth of 0.15 nm. The twin character of the emitted photons is ascertained through a time-correlation measurement. This work opens a route towards new guided-wave semiconductor quantum devices.

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The influence of doping density on the performance of GaAs∕AlGaAs quantum-cascade lasers is presented. A fully self-consistent Schrödinger–Poisson analysis, based on a scattering rate equation approach, was employed to simulate the above threshold electron transport in laser devices. V-shaped local field domain formation was observed, preventing resonant subband level alignment in the high pumping-current regime. The resulting saturation of the maximal current, together with an increase of the threshold current, limits the dynamic working range under higher doping. Experimental measurements are in good agreement with the theoretical predictions.

A high resistance narrow band quantum cascade photodetector (QCD) is presented. Leakage current has been suppressed, increasing the resistivity, thanks to a design in which coupling barriers have been thickened. Useless cross transitions have been eliminated finally leading to a Johnson noise detectivity at 50 K comparable to quantum well infrared photodetectors. Because they work with no dark current, QCDs are very promising for small pixel and large focal plane array applications.

The room temperature operation of InAs∕AlSb quantum cascade lasers is reported. The structure, grown by molecular beam epitaxy on an InAs substrate, is based on a vertical transition design and a low loss n+-InAs plasmon enhanced waveguide. The lasers emitting near 4.5μm operate in pulse regime up to 300K. The threshold current density of 3.18-mm-long lasers is 1.5kA∕cm2 at 83K and 9kA∕cm2 at 300K.

2 AbstractThe spectral gain of bound-to-continuum terahertz quantum cascade lasers (QCLs) is measured as a function of current density using terahertz time-domain spectroscopy.During lasing action the full width at half maximum (FWHM) of the gain is found to monotonically decrease with increasing current density until lasing action stops at which point the FWHM reaches a minimum (0.22THz for a laser operating at 2.1THz).Bandstructure calculations show that the spectral gain narrowing is due to the alignment and misalignment of the injector with the active region as a function of the applied bias field.3 Important progress on terahertz quantum cascade lasers (QCLs) has been achieved in the last few years leading to long wavelength 1 , high power 2 , low current operation 3 , and working temperatures up to 178K 4 . In order to realize further improvements, a better understanding of the gain formation mechanism, and its limiting factors are needed. To this end it is necessary to perform detailed measurements of the gain including its spectral shape. Previous studies have been performed for mid-infraredQCLs. For these measurements, electro-luminescence from a non-lasing cavity is coupledinto an adjacent cavity that shows laser action. 5 However, in the terahertz regime such electro-luminescence based studies are difficult, because of the reduced spontaneous emission at longer wavelengths. Electro-luminescence from the laser cavity has also been used to provide information on upper state lifetimes of terahertz QCLs. 6 In this case to avoid the effect of laser emission, the electro-luminescence from the laser cavity must be collected from a cleaved edge running though the middle of the laser. 7 Multiple probe pulses coupled into the QCL's end facets can also be used to investigate the temporal dynamics of the gain. Coherent population transfer and gain saturation have been observed with this technique at mid-infrared frequencies. 8,9 Recently, terahertz timedomain spectroscopy (TDS) has been shown to be a powerful technique to measure the gain spectra in terahertz QCLs. 10,11,12 Here, a broadband terahertz probe pulse is coupled into the QCL, and the electric field of the transmitted pulses is measured using electrooptic sampling. 13In this letter terahertz TDS is used to investigate the line-width of the spectral gain as a function of current density. Two terahertz QCLs lasers with different bound-tocontinuum designs are studied. One laser emits at 2.1THz 14 and the other emits at 2.9THz. 15 For both devices, as the current density is increased from threshold, we observe a monotonic decrease of the full width at half maximum (FWHM) of the gain. After the 4 laser reaches maximum power, this gain narrowing increases sharply, until laser action ceases. By calculating the band structure for different bias fields, we show the gain narrowing is a consequence of a misalignment of the upper state of the laser transition with the injector miniband.The 2.1THz (2.9THz) sample has an active region thickness of 14µm (12µm), and a...