Frequency-modulated combs via on-chip field enhancement

Frequency-modulated (FM) combs feature flat intensity spectra with a linear frequency chirp, useful for metrology and sensing applications. Generating FM combs in semiconductor lasers generally requires a fast saturable gain, usually limited by the intrinsic gain medium properties. Here, we show how a spatial modulation of the laser gain medium can enhance the gain saturation dynamics and nonlinearities to generate self-starting FM combs. We demonstrate this with tapered planarized THz quantum cascade lasers (QCLs). While simple ridge THz QCLs typically generate combs which are a mixture of amplitude and frequency modulation, the on-chip field enhancement resulting from extreme spatial confinement leads to an ultrafast saturable gain regime, generating a pure FM comb with a flatter intensity spectrum, a clear linear frequency chirp and very intense beatnotes up to -30 dBm. The observed linear frequency chirp is reproduced using a spatially inhomogeneous mean-field theory model which confirms the crucial role of field enhancement. In addition, the modified spatial temperature distribution within the waveguide results in an improved hightemperature comb operation, up to a heat sink temperature of 115 K, with comb bandwidths of 600 GHz at 90 K. The spatial inhomogeneity also leads to dynamic switching between various harmonic states in the same device.Comment: 9 pages, 6 figure

Planarized THz quantum cascade lasers for broadband coherent photonics

Recently, there has been a growing interest in integrated THz photonics for various applications in communications, spectroscopy and sensing. We present a new integrated photonic platform based on active and passive elements integrated in a double-metal, high confinement waveguide layout planarized with a low-loss polymer. An extended top metallization results in low waveguide losses and improved dispersion, thermal and RF properties, as it enables to decouple the design of THz and microwave cavities. Free-running on-chip quantum cascade laser combs spanning 800 GHz, harmonic states over 1.1 THz and RF-injected broadband incoherent states spanning over nearly 1.6 THz are observed. With a strong external RF drive, actively mode-locked pulses as short as 4.4 ps can be produced, as measured by SWIFTS. We demonstrate as well passive waveguides with low insertion loss, enabling the tuning of the laser cavity boundary conditions and the co-integration of active and passive components on the same THz photonic chip.Comment: 27 pages, 5 figure

Proximity array device: a novel photon detector working in long wavelengths

We present here an innovative photon detector based on the proximity junction array device (PAD) working at long wavelengths. We show that the vortex dynamics in PAD undergoes a transition from a Mott insulator to a vortex metal state by application of an external magnetic field. The PAD also evidences a Josephson I-V characteristic with the external field dependent tunneling current. At high applied currents, we observe a dissipative regime in which the vortex dynamics is dominated by the quasi-particle contribution from the normal metal. The PAD has a relatively high photo-response even at frequencies below the expected characteristic frequency while, its superconducting properties such as the order parameter and the Josephson characteristic frequency can be modulated via external fields to widen the detection band. This device represents a promising and reliable candidate for new high-sensitivity long-wavelength detectors. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Low-temperature stability and sensing performance of mid-infrared bloch surface waves on a one-dimensional photonic crystal

The growing need for new and reliable surface sensing methods is arousing interest in the electromagnetic excitations of ultrathin films, i.e., to generate electromagnetic field distributions that resonantly interact with the most significant quasi-particles of condensed matter. In such a context, Bloch surface waves turned out to be a valid alternative to surface plasmon polaritons to implement high-sensitivity sensors in the visible spectral range. Only in the last few years, however, has their use been extended to infrared wavelengths, which represent a powerful tool for detecting and recognizing molecular species and crystalline structures. In this work, we demonstrate, by means of high-resolution reflectivity measurements, that a one-dimensional photonic crystal can sustain Bloch surface waves in the infrared spectral range from room temperature down to 10 K. To the best of our knowledge, this is the first demonstration of infrared Bloch surface waves at cryogenic temperatures. Furthermore, by exploiting the enhancement of the surface state and the high brilliance of infrared synchrotron radiation, we demonstrate that the proposed BSW-based sensor has a sensitivity on the order of 2.9 cm–1 for each nanometer-thick ice layer grown on its surface below 150 K. In conclusion, we believe that Bloch surface wave-based sensors are a valid new class of surface mode-based sensors for applications in materials science

Studio di bolometri per il lontano infrarosso

La regione dello spettro elettromagnetico che si riconduce al terahertz è considerata un'autentica zona di frontiera per differenti ambiti tra i quali, per citarne solo alcuni, la fisica, la chimica, la biologia, la scienza dei materiali, la medicina e l'astrofisica. La tecnologia nella "terahertz gap" è ancora agli inizi, scarse sono le sorgenti in questa regione e per quanto riguarda i rivelatori ancora grossi miglioramenti vanno fatti nell'ottica di ottenere un insieme integrato di rivelatori terahertz sensibili e veloci ma non limitati dal rumore termico di background. E' proprio in questo vivace panorama che si può collocare questo lavoro di tesi. Partendo infatti dal lavoro del gruppo di Pekola durante i tre anni di dottorato è stato messo a punto il processo di fabbricazione dello SHAB (Superconducting Hotspot Airbridge Bolometer), microbolometro superconduttore sensibile alla radiazione elettromagnetica THz. E' stata ottimizzata la sua caratterizzazione elettrica a 4.2 K in elio liquido ed in vuoto ed effettuata una completa caratterizzazione ottica mediante l'utilizzo di un criostato a finestra ottica. L'obiettivo prossimo sarà quello di utilizzare il nostro SHAB per la realizzazione di una fotocamera al terahertz che possa essere inserita in un sistema ottico pensato per applicazioni medicali in cui la sorgente sarà un laser a cascata quantica (QCL). Lo SHAB infatti, caratterizzato dall'estrema flessibilità ad integrarsi in configurazioni costituite da più rivelatori vicini, risulta essere uno dei migliori candidati per tali applicazioni nel panorama dei moderni rivelatori al THz. Altro lavoro svolto in questa tesi è lo studio di bolometri di tipo spiderweb per applicazioni astrofisiche la cui fabbricazione è stata realizzata in collaborazione con l'Univeristy of California Santa Barbara. Particolare interesse in questo contesto è stato rivolto alla criogenia necessaria per la caratterizzazione a 320 mK di tali rivelatori

THz frequency combs form dispersion-compensated antenna-coupled ring quantum cascade lasers

We report comb operation of RF injected ring Quantum Cascade Lasers. A coupled waveguide approach is implemented for dispersion compensation while passive bullseye antenna improves the device power extraction and far field. Phase sensitive measurements are presented which hints at the presence of soliton state

Superconducting complementary metasurfaces for THz ultrastrong light-matter coupling

A superconducting metasurface operating in the THz range and based on the complementary metamaterial approach is discussed. Experimental measurements as a function of temperature and magnetic field display a modulation of the metasurface with a change in transmission amplitude and frequency of the resonant features. Such a metasurface is successively used in a cavity quantum electrodynamic experiment displaying ultrastrong coupling to the cyclotron transition of two-dimensional electron gas. A finite element modeling is developed and its results are in good agreement with the experimental data. In this system a normalized coupling ratio of $\frac{\Omega }{{{\omega }_{c}}}=0.27$ is measured and a clear modulation of the polaritonic states as a function of the temperature is observed.ISSN:1367-263