Fabrizio Castellano

Ultrastrong coupling regime and plasmon polaritons in parabolic semiconductor quantum wells.

Ultrastrong coupling is studied in a modulation-doped parabolic potential well coupled to an inductance-capacitance resonant circuit. In this system, in accordance to Kohns theorem, strong reduction of the energy level separation caused by the electron-electron interaction compensates the depolarization shift. As a result, a very large ratio of 27% of the Rabi frequency to the center resonance frequency as well as a polariton gap of width 2π × 670  GHz are observed, suggesting parabolic quantum wells as the system of choice in order to explore the ultrastrong coupling regime.

Low divergence Terahertz photonic-wire laser

Edge emitting, terahertz quantum cascade photonic-wire lasers, based on a third order Bragg grating are presented. Devices with a power consumption as low as 300mW, with a single frequency output power of more than 1.5mW are demonstrated. Their maximum operating temperature in continuous-wave mode operation is 110K and the emission is concentrated in a narrow beam (~30 degrees divergence). Larger structure based on the same design show more than 10mW output power and more than 200mW/A slope efficiency at 10K continuous-wave operation.

Ultra-broadband heterogeneous quantum cascade laser emitting from 2.2 to 3.2 THz

We present a heterogeneous terahertz quantum cascade laser that emits continuously between 2.2 and 3.2 THz, covering an emission range of over 40% around the central frequency. Devices were realized by stacking different active region designs into a double-metal waveguide. They operate up to 125 K with 15 mW peak power at 10 K in pulsed mode. Smaller devices show broadband emission also in continuous wave. Time-resolved measurements of the emission spectra were realized, confirming the broadband emission within a 5 ns time window.

Room temperature terahertz polariton emitter

Terahertz (THz) range electroluminescence from intersubband polariton states is observed in the ultra strong coupling regime, where the interaction energy between the collective excitation of a dense electron gas and a photonic mode is a significant portion of the uncoupled excitation energy. The polaritons increased emission efficiency along with a parabolic electron confinement potential allows operation up to room temperature in a nonresonant pumping scheme. This observation of room temperature electroluminescence of an intersubband device in the THz range is a promising proof of concept for more powerful THz sources.

Asymmetric heterostructure for photovoltaic InAs quantum dot infrared photodetector

A photovoltaic InAs quantum dot-under-a-well photodetector is reported with a peak responsivity at 7 μm wavelength. In this structure, we implement an improved injection scheme, which allows a controlled feeding of the quantum dots through a modulation-doped InGaAs quantum well injector. A thin Al0.3Ga0.7As barrier significantly reduces the dark current and, at the same time, is responsible for the photovoltaic behavior. At 4 K and no applied bias, a responsivity of 2.5 mA/W and a detectivity of D∗=2.3×1010 cm Hz1/2/W in the dark is measured. The TBLIP of the device is 60 K and the D∗ at this temperature is 2×109 cm Hz1/2/W.

Continuous tuning of terahertz distributed feedback quantum cascade laser by gas condensation and dielectric deposition

We present the continuous tuning of a terahertz quantum cascade laser based on 3rd order distributed feedback. This source provides a single mode emission with low dissipation power (∼400 mW) and milliwatt-level output power. The laser is tuned by nitrogen-gas condensation or by dielectric deposition. The former method offers a reversible, reproducible, and continuous mode-hop free tuning over 25 GHz at a frequency of ∼3.3 THz (Δf/f ∼ 0.75%). The latter is convenient for precise post-process targeting of a desired frequency.

Tuning the dynamic properties of electrons between a quantum well and quantum dots

We present a study of the dynamic properties of electrons tunneling from an InGaAs quantum well to self assembled InAs quantum dots. The experiments were conducted on three highly asymmetric quantum dot infrared photodetectors, where the quantum well and quantum dots were separated by a composite GaAs/AlGaAs/GaAs barrier, which varied from 3.5 nm to 7.0 nm. We performed interband (photoluminescence) and intraband (photocurrent) measurements to characterize the spectral properties of the well and the dots. The photoluminescence measurements revealed that the two nanostructures are decoupled when the device is at zero bias. By intraband pump-probe experiments and photocurrent saturation experiments, we were able to extrapolate a refilling time τ from the well to the dots, which varied from a few μs for the thinnest barrier and hundreds of μs for the thickest one. The extracted values are in good agreement with characteristic tunneling times computed by using a model based on the theoretically predicted tran...

Room-temperature transverse-electric polarized intersubband electroluminescence from InAs/AlInAs quantum dashes

We report the observation of transverse electric polarized electroluminescence from InAs/AlInAs quantum dash quantum cascade structures up to room temperature. The emission is attributed to the electric field confined along the shortest lateral dimension of the dashes, as confirmed by its dependence on crystallographic orientation both in absorption measurements on a dedicated sample and from electroluminescence itself. From the absorption, we estimate a dipole moment for the observed transition of 〈x〉 = 1.7 nm. The electroluminescence is peaked at around 110 meV and increases with applied bias. Its temperature dependence shows a decrease at higher temperatures limited by optical phonon emission.

Loss mechanisms of quantum cascade lasers operating close to optical phonon frequencies

The extension of the operating frequency of Quantum Cascade Lasers (QCL) into the 20−50 μm regime is a desirable goal as it would bridge the gap between mid-infrared and THz devices. Coherent light emitters in this spectral range are also needed for spectroscopy and radio astronomy applications. Since little attention has been devoted to the subject in the past, we investigate the dominant loss mechanisms of QCLs in this spectral range. We report on an InGaAs/InAlAs QCL in an InP dielectric waveguide emitting at 23 μm wavelength whose electroluminescence spectrum shows an anomalous low-frequency cut which prevents laser action at low electric field. We also observe similar line shape in other GaAs/AlGaAs devices. The spectral features are analyzed and explained in terms of refractive index anomalies induced by phonon resonances.

Sequential multiphoton strategy for semiconductor-based terahertz detectors

A semiconductor-based terahertz-detector strategy, exploiting a bound-to-bound-to-continuum architecture, is presented and investigated. In particular, a ladder of equidistant energy levels is employed, whose step is tuned to the desired detection frequency and allows for sequential multiphoton absorption. Our theoretical analysis demonstrates that the proposed multisubband scheme could represent a promising alternative to conventional quantum-well infrared photodetectors in the terahertz spectral region.