Gaetano Scamarcio

Quantum Cascade Laser

A semiconductor injection laser that differs in a fundamental way from diode lasers has been demonstrated. It is built out of quantum semiconductor structures that were grown by molecular beam epitaxy and designed by band structure engineering. Electrons streaming down a potential staircase sequentially emit photons at the steps. The steps consist of coupled quantum wells in which population inversion between discrete conduction band excited states is achieved by control of tunneling. A strong narrowing of the emission spectrum, above threshold, provides direct evidence of laser action at a wavelength of 4.2 micrometers with peak powers in excess of 8 milliwatts in pulsed operation. In quantum cascade lasers, the wavelength, entirely determined by quantum confinement, can be tailored from the mid-infrared to the submillimeter wave region in the same heterostructure material.

Quartz-Enhanced Photoacoustic Spectroscopy: A Review

A detailed review on the development of quartz-enhanced photoacoustic sensors (QEPAS) for the sensitive and selective quantification of molecular trace gas species with resolved spectroscopic features is reported. The basis of the QEPAS technique, the technology available to support this field in terms of key components, such as light sources and quartz-tuning forks and the recent developments in detection methods and performance limitations will be discussed. Furthermore, different experimental QEPAS methods such as: on-beam and off-beam QEPAS, quartz-enhanced evanescent wave photoacoustic detection, modulation-cancellation approach and mid-IR single mode fiber-coupled sensor systems will be reviewed and analysed. A QEPAS sensor operating in the THz range, employing a custom-made quartz-tuning fork and a THz quantum cascade laser will be also described. Finally, we evaluated data reported during the past decade and draw relevant and useful conclusions from this analysis.

Measurement of subband electronic temperatures and population inversion in THz quantum-cascade lasers

We compare the electronic temperatures and the population inversion both below and above the lasing threshold in three quantum-cascade lasers (QCLs) operating at 2.8THz, 3.2THz, and 3.8THz using microprobe band-to-band photoluminescence. In the lasing range, while the ground-state temperature remains close to the lattice one (90K–100K), the upper radiative state heats up to ∼200K. From the measured thermal resistance and the power dependence of the ground-state electronic temperature, we get a value of the electron-lattice energy relaxation rate comparable with that typical of midinfrared QCLs.

Interfacial electronic effects in functional biolayers integrated into organic field-effect transistors

Biosystems integration into an organic field-effect transistor (OFET) structure is achieved by spin coating phospholipid or protein layers between the gate dielectric and the organic semiconductor. An architecture directly interfacing supported biological layers to the OFET channel is proposed and, strikingly, both the electronic properties and the biointerlayer functionality are fully retained. The platform bench tests involved OFETs integrating phospholipids and bacteriorhodopsin exposed to 1–5% anesthetic doses that reveal drug-induced changes in the lipid membrane. This result challenges the current anesthetic action model relying on the so far provided evidence that doses much higher than clinically relevant ones (2.4%) do not alter lipid bilayers’ structure significantly. Furthermore, a streptavidin embedding OFET shows label-free biotin electronic detection at 10 parts-per-trillion concentration level, reaching state-of-the-art fluorescent assay performances. These examples show how the proposed bioelectronic platform, besides resulting in extremely performing biosensors, can open insights into biologically relevant phenomena involving membrane weak interfacial modifications.

Part-per-trillion level SF6 detection using a quartz enhanced photoacoustic spectroscopy-based sensor with single-mode fiber-coupled quantum cascade laser excitation.

A sensitive spectroscopic sensor based on a hollow-core fiber-coupled quantum cascade laser (QCL) emitting at 10.54 μm and quartz enhanced photoacoustic spectroscopy (QEPAS) technique is reported. The design and realization of mid-IR fiber and coupler optics has ensured single-mode QCL beam delivery to the QEPAS sensor. The collimation optics was designed to produce a laser beam of significantly reduced beam size and waist so as to prevent illumination of the quartz tuning fork and microresonator tubes. SF(6) was selected as the target gas. A minimum detection sensitivity of 50 parts per trillion in 1 s was achieved with a QCL power of 18 mW, corresponding to a normalized noise-equivalent absorption of 2.7×10(-10) W·cm(-1)/Hz(1/2).

High-power inter-miniband lasing in intrinsic superlattices

We report the realization of a mid-infrared (λ≃7 μm) quantum-cascade laser, in which the emission process takes place between the two lowest minibands of an intrinsic superlattice. Contrary to previous lasers based on doped superlattices, here the dopants are located only inside suitably designed injector regions, where positive ionized donors and negative electrons are arranged to compensate the applied external field across the superlattices. This reduces impurity scattering and translates into low threshold currents (4.2 kA/cm2 at 10 K) and into room temperature operation, without compromising the large current-carrying capabilities of the minibands. Peak powers of ∼1.3 W per facet have been obtained from broad-area devices at 10 K, with still more than 1 W at 120 K and 400 mW at 200 K. Effects related to the finite size of the superlattices become visible in the spectral properties, owing to the reduced broadening, and have to be taken into account to accurately describe the laser’s behavior.

High-performance superlattice quantum cascade lasers

Superlattice quantum cascade (QC) lasers based on optical transitions between conduction minibands are unipolar semiconductor lasers with high-current-carrying capability and attendant high optical power due to miniband transport in the active and in the injector regions. Other advantages include the intrinsic population inversion associated with the large interminiband-to-intraminiband relaxation time ratio and the high oscillator strength of the laser transition at the superlattice Brillouin zone boundary. This oscillator strength is significantly larger than that of intersubband transitions in double-quantum-well active regions of conventional cascade lasers, particularly at long infrared wavelengths (/spl ges/10 /spl mu/m). Following a brief review of results on conventional QC lasers, design considerations for superlattice cascade lasers are discussed, along with recent advances in long-wavelength (11 /spl mu/m) structures with doped active regions. Dopants broaden the gain spectrum and increase the laser threshold. Two laser designs that avoid doping of the active regions without causing electric-field-induced localization of the superlattice states are then presented, along with experimental results. In the first one, modulation doping creates a space-charge electric field that compensates the voltage drop across the undoped superlattice active regions. In the second scheme, the latter are designed with quantum wells of varying thickness (chirped superlattice), so that under application of the external field, the localized quantum well states overlap, forming minibands. Both schemes lend to considerably lower threshold current densities than devices with doped active regions, as well as to much higher peak optical power and to room-temperature operation. A record peak power of 500 mW at /spl lambda/=7.6 /spl mu/m at room temperature is obtained with the chirped design. The latter also leads to the longest operating wavelength of any other QC laser (17 /spl mu/m). The last section of the paper describes superlattice cascade lasers operating simultaneously at two or more widely different wavelengths.

Terahertz quartz enhanced photo-acoustic sensor

A quartz enhanced photo-acoustic sensor employing a single-mode quantum cascade laser emitting at 3.93 Terahertz (THz) is reported. A custom tuning fork with a 1 mm spatial separation between the prongs allows the focusing of the THz laser beam between them, while preventing the prongs illumination. A methanol transition with line-strength of 4.28 × 10−21 cm has been selected as target spectroscopic line. At a laser optical power of ∼ 40 μW, we reach a sensitivity of 7 parts per million in 4s integration time, corresponding to a 1σ normalized noise-equivalent absorption of 2 × 10−10 cm−1W/Hz½.

Thermal modeling of GaInAs∕AlInAs quantum cascade lasers

We measured the facet temperature profiles of GaInAs∕AlInAs quantum cascade lasers (QCLs) operating in continuous wave mode by means of microprobe photoluminescence. These results were used to evaluate the in-plane (k‖) and the cross-plane (k⊥) thermal conductivities of the active region and to validate a two-dimensional model for the anisotropic heat diffusion in QCLs. In the temperature range of 80–250K, k⊥ monotonically increases with temperature and remains one order of magnitude smaller than the thermal conductivities of bulk constituent materials. We found an excellent agreement between the calculated and experimental values of the thermal resistance of GaInAs∕AlInAs QCLs operating in continuous wave up to 400K. Comparison between the calculated thermal performances of QCLs sharing the same active region structure, but having either a buried or a ridge waveguide, shows that devices with Au contact layers thicker than 4μm have better thermal properties than the buried structures.

Simultaneous measurement of the electronic and lattice temperatures in GaAs/Al0.45Ga0.55As quantum-cascade lasers: Influence on the optical performance

We measured the electronic and lattice temperatures in steady-state operating GaAs/AlGaAs quantum-cascade lasers, by means of microprobe band-to-band photoluminescence. Thermalized hot-electron distributions with temperatures up to 800 K are established. The comparison of our data with the analysis of the temperature dependence of device optical performances shows that the threshold current is determined by the lattice temperature.