• intersubband polaritons
• bosonic lasing
• FTIR
• quantum wells
• strong coupling

The work of this thesis project (Tesi di Laurea Magistrale) is centered on intersubband polaritons, bosonic quasi particles arising from the strong interaction between light and electronic transitions in semiconductor quantum wells. The main experimental result we specifically achieved is the measurement of radiation emitted by an optically pumped polaritonic device, operating in the mid-infrared region of the electromagnetic spectrum. The device was sent to us within the MIR BOSE collaboration, a European research project focused on studying intersubband polariton physics with the perspective to develop a polaritonic device exhibiting bosonic lasing.

We first characterized the device by performing angle-resolved reflectance spectroscopy using a commercial FTIR spectrometer and a reflection unit we built. These results were corroborated and supported by running numerical simulations (a semiclassical RCWA code (S. Zanotto et al, Appl. Phys. Lett. 103, 091110 (2014)) to reconstruct the dispersion relation, which is separated into two distinct branches (the upper polariton and lower polariton) formed by the anticrossing of a cavity mode and an instersubband transition. The intersubband transition occurs between discrete energy levels (called subbands) in the conduction band of AlGaAs-GaAs multi quantum wells exhibiting one dimensional quantum confinement. They constitute the active core region of our device. The cavity mode is instead supported by a double metal resonator with a 1D periodic grating patterning the top surface to enable coupling with external light. When the intersubband transition and the cavity mode are strongly coupled the system is described by new eigenstates: intersubband polaritons. All of these properties were thoroughly discussed. The device is designed take advantage of the interaction between polaritons and longitudinal optical phonons as a resonant scattering mechanism between an initial "pump" injection state and a final state near the k=0 point of the lower polaritonic branch, from which the polaritons decay radiatively emitting light. This is desirable to achieve bosonic final state stimulation, with consequent onset of polariton lasing.

Moving on to the description of the experimental setup, we used as the pump source a high power CO2 laser, previously built in University of Pisa, which was externally triggered and operated in a pulsed regime. The average intensity incident on the polaritonic sample was estimated by measuring the beam waist at the focal point using a knife-edge technique (~ 0.5 mm), reaching 200 W/cm^2. The device was placed in a cryostat to allow cryogenic cooling down to around 78 K, with transparent windows and a rotational degree of freedom to allow alignment at the correct pumping angle. The scattered radiation was collected in the normal direction through a detection unit comprised of lenses, a small interferometer operating in step scan mode and a detector. The alignment of the whole setup posed the first main experimental challenge.

We then performed the actual measurements using both lock-in amplifier and boxcar averaging methods to reduce the background noise, mainly deriving from the CO2 laser, which presented the greatest experimental difficulty. We have verified the presence in the resulting spectrum of a second peak distanced in wavenumber from the pump peak, compatible to emission deriving from the lower polariton after LO phonon scattering. The dependence of the emission on different parameters, which include the temperature, angle and incident power, was tested and interpreted to explain the results. While we have not been able yet to verify the onset of a "bosonic lasing" regime, still to be demonstrated in intersubband polariton systems and the main topic of the research collaboration in which this thesis is inserted, we hope to have provided a relevant step in the direction of achieving this goal.