Marcella Giovannini

External cavity quantum-cascade laser tunable from 8.2to10.4μm using a gain element with a heterogeneous cascade

A heterogeneous quantum-cascade structure based on two bound-to-continuum designs emitting at 9.6 and 8.4μm is presented. Its spontaneous emission spectrum at room temperature has a full width at half maximum of 350cm−1 and shows a variation of intensity of less than 20% over more than 200cm−1. External cavity lasers using a grating in Littrow configuration and antireflection coated chips with this active region could be tuned over 265cm−1 from 8.2to10.4μm, that is, over 24% of the center wavelength.

Room-temperature, continuous-wave, single-mode quantum-cascade lasers at λ≃5.4μm

We demonstrate room-temperature, single-mode, continuous-wave operation of a λ≃5.4μm quantum-cascade laser up to the temperature of 30°C. Processing is done using standard lithography in a ridge waveguide mounted junction-up. The active region is based on a bound-to-continuum transition. The high performances were achieved with a low active region doping and a thick electroplated gold deposition, resulting in a characteristic temperature of T0=155K in continuous-wave with a threshold current density of jth=2.05kA∕cm2 at 300K.

Biomedical terahertz imaging with a quantum cascade laser

We present biomedical imaging using a single frequency terahertz imaging system based on a low threshold quantum cascade laser emitting at 3.7THz (λ=81μm). With a peak output power of 4mW, coherent terahertz radiation and detection provide a relatively large dynamic range and high spatial resolution. We study image contrast based on water/fat content ratios in different tissues. Terahertz transmission imaging demonstrates a distinct anatomy in a rat brain slice. We also demonstrate malignant tissue contrast in an image of a mouse liver with developed tumors, indicating potential use of terahertz imaging for probing cancerous tissues.

Imaging with a Terahertz quantum cascade laser.

We demonstrate bio-medical imaging using a Terahertz quantum cascade laser. This new optoelectronic source of coherent Terahertz radiation allows building a compact imaging system with a large dynamic range and high spatial resolution. We obtain images of a rat brain section at 3.4 THz. Distinct regions of brain tissue rich in fat, proteins, and fluid-filled cavities are resolved showing the high contrast of Terahertz radiation for biological tissue. These results suggest that continuous-wave Terahertz imaging with a carefully chosen wavelength can provide valuable data on samples of biological origin; these data appear complementary to those obtained from white-light images.

Intersubband linewidths in quantum cascade laser designs

We present a model to a priori calculate the temperature and field dependent intersubband linewidth of the optical transition in quantum cascade laser designs. Besides intra- and intersubband lifetime broadening, it comprises interface roughness scattering based on the approach of Tsujino et al. [Appl. Phys. Lett. 86, 062113 (2005)]. We verified our model with experimental data of quantum cascade lasers having different linewidths. Excellent agreement with the experiment was found for the two-phonon resonance design. Linewidths are slightly overestimated in the bound-to-continuum design. Differential gain and threshold current density are in excellent agreement for the two-phonon resonance design. Although the slope efficiency is somewhat underestimated at low temperatures, there is still reasonable agreement with the experiment.

Interface-roughness-induced broadening of intersubband electroluminescence in p-SiGe and n-GaInAs∕AlInAs quantum-cascade structures

The effect of intrasubband interface roughness scattering on intersubband transition linewidths in double-quantum-well and quantum-cascade (QC) structures is studied. In n-GaInAs∕AlInAs structures, the calculated ratios between the linewidths of the spatially vertical and diagonal transitions agree with the experimental values. In p-Si∕Si0.2Ge0.8 QC structures, the experimentally observed linewidth is a factor of 4–7 smaller than the predicted value. However, by assuming a vertical interface correlation between adjacent interfaces separated by less than ∼1.5nm, the theory reproduces the experiment. Transmission electron microscopy of the SiGe QC sample reveals this vertical correlation, supporting the model.

Continuous-wave operation of a broadly tunable thermoelectrically cooled external cavity quantum-cascade laser

Continuous-wave operation of an external cavity quantum-cascade laser on a thermoelectric cooler is reported. The active region of the gain element was based on a bound-to-continuum design emitting near 5.15 microm. The external cavity setup was arranged in a Littrow configuration. The front facet of the gain chip was antireflection coated. The laser could be tuned over more than 170 cm(-1) from 4.94 to 5.4 microm and was single mode over more than 140 cm(-1). The output power was in excess of 10 mW over approximately 100 cm(-1) and in excess of 5 mW over approximately 130 cm(-1) at -30 degrees C.

High-Performance Bound-to-Continuum Quantum-Cascade Lasers for Broad-Gain Applications

Based on the bound-to-continuum active region design, we shall present a high performance continuous-wave (CW) quantum-cascade laser. In contrast to high performance lasers based on a two-phonon resonance transition and a narrow linewidth (< 165 cm-1), the device presented here exhibits a spontaneous emission full-width at half-maximum as large as 295 cm-1. Thus, such devices are very suitable for broadband tuning. At 30degC, it shows a maximum output power and slope efficiency of 188 mW and 500 mW/A, as well as a threshold current density of only 1.79 kA/cm2. Furthermore, at this temperature, the device demonstrates an internal differential quantum efficiency of 71% and a wall plug efficiency of 2.0%. The maximum CW operation temperature reached is 110degC. A thermal resistance of 4.3 K/W was attained by epi-down mounting on diamond submounts. The waveguide losses of 14 cm-1 are explained by intersubband absorption in addition to free-carrier absorption.

Direct measurement of the linewidth enhancement factor by optical heterodyning of an amplitude-modulated quantum cascade laser

A measurement of the linewidth enhancement factor α of a distributed feedback quantum cascade laser is presented. The measurement is based on a heterodyning experiment, in which one of the lasers is modulated at radio frequency. A value of α=0.02±0.20 is obtained for a modulation frequency of 500MHz. As the frequency is decreased, α increases and is consistent with a thermal chirp effect.

InP-based quantum cascade detectors in the mid-infrared

We present two InP-based quantum cascade detectors (QCDs) in the mid-infrared wavelength range. Their narrow band detection spectra are centered at 5.3 and 9μm. A vertical intersubband transition followed by a carefully designed extraction cascade, which is adapted to the LO-phonon energy, leads to 10K responsivities R of 3.2 and 9.0mA∕W and background limited detectivities DBLIP* of 2×108 and 3×109 Jones, for the 5.3 and the 9μm devices, respectively. Detection has been observed up to device temperatures of 300K (RT), albeit reasonable performance is restricted to temperatures below 150K (5.3μm) and 70K (9μm). Designed for zero bias operation, QCDs do not produce any dark current and therefore do not suffer from dark current noise and capacitance saturation at long integration times, making them ideal devices for large focal plane arrays.