Quasi-periodic and random THz photonic resonators

In the last decades the fields of photonics and nanotechnology have led to some impressive scientific and technological achievements. Among them, the exploration of yet unexploited spectral regions, such as the Terahertz (THz) range, i.e. wavelengths of 60micron-300 micron, has been a major breakthrough. On the one hand, this was possible after the novel theoretical concept of light amplification in multiple quantum wells and superlattices was proposed in the Seventies, introducing the groundbreaking quantum cascade laser (QCL) idea. On the other hand, the development of new nanofabrication technologies and crystal growth techniques, such as the molecular beam epitaxy (MBE), allowed an unprecedented control over the material structure, down to the deposition of nanometer-thick semiconductor layers. This paved the way to the practical realization of electrically pumped multi-stage gain media, the QCL, and to the successful demonstrations of their operation in a broad frequency range, from the mid-IR to the far-infrared. Apart from the purely scientific interest, Terahertz photonics has now a fundamental role in many applications, like metrology, spectroscopy, biomedical and pharmaceutical imaging, quality and process control, communications and security. Nowadays, a lot of effort is made to improve the performance of Terahertz QCL in terms of optical power, efficiency, beam pattern, frequency control and thermal management. Some of these crucial issues can be addressed by the use of photonic structures, i.e. specially designed patterns of dielectric scatterers superimposed to the active region. Such structures can be implemented in one- (1D), two (2D)- or three (3D)-dimensional architectures, to provide a tight control of the frequencies and far-field emission pattern of the laser. Periodic photonic crystals have been studied for long time, providing intriguing insights. More recently, aperiodic patterns have attracted increasing attention due to their greater flexibility and the possibility to study and explore novel physical phenomena. The aim of the present thesis is to design, fabricate and investigate the transport and optical behavior of THz QCLs exploiting distributed feedback, achieved through the use of 2D quasi-periodic and random resonators. The main goal is to demonstrate multimode emission over a broad frequency bandwidth, centered around 3.1 THz. Unlike perfect photonic crystals, quasi-crystal geometries do not possess discrete translational invariance, yet they do possess long-range order which gives rise to a rich spectrum. After developing a simulation code based on the generation algorithm called "Generalized Dual Method", we designed the following quasi-crystal geometries: i) a 7-fold pattern with a perfect symmetry under 2π/7 rotations around a central axis, ii) an imperfect 7-fold geometry where small defect points are introduced. This allowed to compare the effects of introducing a small amount of disorder in the design of the photonic structures. In order to understand the effect of a further increase of disorder, a third type of random structures was also studied, whose scatterers positions were extracted from a uniform pseudo-random distribution. We then simulated these photonic structures using the numerical approach of finite elements analysis, to understand how light propagation is affected by the size, the number and the arrangement of the scatterrers. A set of devices for each geometry was selected among those with the largest number of electromagnetic modes with predicted high quality factors Q. They were then nano-fabricated with the same QCL active region in a cleanroom facility, using a combination of UV optical lithography, plasma-assisted etching, metal deposition, chemical processes and ultrasonic wedge bonding. Finally, all lasers were characterized electrically and optically to study how the different physical and geometrical parameters affect the lasing threshold, the slope efficiency, the emitted power and the far-field intensity profile. The emission spectra were probed via Fourier Transform Infrared Spectroscopy (FT-IR), demonstrating the predicted multimode emission in most devices. In a future perspective, such multimode emission could be used to mode-lock radiation in a THz QCL, for example using passive optical components. An interesting possibility is the future integration of graphene in QCL to exploit its saturable absorption in the THz region. To this end, the transmission of THz radiation through a few layers of graphene transferred on an intrinsic silicon substrate was measured, reporting saturable absorption in the THz

Continuous-wave highly-efficient low-divergence terahertz wire lasers.

Terahertz (THz) quantum cascade lasers (QCLs) have undergone rapid development since their demonstration, showing high power, broad-tunability, quantum-limited linewidth, and ultra-broadband gain. Typically, to address applications needs, continuous-wave (CW) operation, low-divergent beam profiles and fine spectral control of the emitted radiation, are required. This, however, is very difficult to achieve in practice. Lithographic patterning has been extensively used to this purpose (via distributed feedback (DFB), photonic crystals or microcavities), to optimize either the beam divergence or the emission frequency, or, both of them simultaneously, in third-order DFBs, via a demanding fabrication procedure that precisely constrains the mode index to 3. Here, we demonstrate wire DFB THz QCLs, in which feedback is provided by a sinusoidal corrugation of the cavity, defining the frequency, while light extraction is ensured by an array of surface holes. This new architecture, extendable to a broad range of far-infrared frequencies, has led to the achievement of low-divergent beams (10°), single-mode emission, high slope efficiencies (250 mW/A), and stable CW operation

Highly efficient surface-emitting semiconductor lasers exploiting quasi-crystalline distributed feedback photonic patterns

Abstract: Quasi-crystal distributed feedback lasers do not require any form of mirror cavity to amplify and extract radiation. Once implemented on the top surface of a semiconductor laser, a quasi-crystal pattern can be used to tune both the radiation feedback and the extraction of highly radiative and high-quality-factor optical modes that do not have a defined symmetric or anti-symmetric nature. Therefore, this methodology offers the possibility to achieve efficient emission, combined with tailored spectra and controlled beam divergence. Here, we apply this concept to a one-dimensional quantum cascade wire laser. By lithographically patterning a series of air slits with different widths, following the Octonacci sequence, on the top metal layer of a double-metal quantum cascade laser operating at THz frequencies, we can vary the emission from single-frequency-mode to multimode over a 530-GHz bandwidth, achieving a maximum peak optical power of 240 mW (190 mW) in multimode (single-frequency-mode) lasers, with record slope efficiencies for multimode surface-emitting disordered THz lasers up to ≈570 mW/A at 78 K and ≈720 mW/A at 20 K and wall-plug efficiencies of η ≈ 1%

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Photonic engineering of CW, ultrabroad gain, aperiodic quantum cascade lasers at terahertz frequencies integrations with 2D materials and study of the optical mode dynamics

The terahertz (THz) frequency range of the electromagnetic spectrum is usually defined in the range between 0.1 THz and 10 THz, corresponding to wavelengths in the interval from 3 mm to 30 \ub5m, lying in-between the infrared and the microwave spectral regimes. In recent years, the progress of THz technology has fostered interdisciplinary research in spectroscopy and tomography to map macroscopic systems, (chemical detection and imaging, amongst others) or microscopic ones, such as nanoparticles and nanowires on either static or dynamic timescales. THz radiation is commonly generated with photoconductive emitters, semiconductor diodes, free-electron lasers, photomixing, and beating of a pump and idler signal from non-linear crystals. These approaches are often bulky, expensive or with limited optical powers. The breakthrough demonstration of quantum cascade lasers operating in the far-infrared, and based on quantum engineered heterostructures, paved the way to the development of much more compact, efficient and powerful semiconductor THz sources. Thanks to the atomic-layer resolution ensured in the heterostructure growth by molecular beam epitaxy (MBE), very accurate designs can be implemented via a proper sequence of quantum barriers and quantum wells. In this way, sharp discontinuities in the conduction and valence bands edges are created, in order to manipulate the electron energy levels and wavefunction localization, and to provide optical intersubband transitions at the desired frequencies. [...

Impact ionization in low-band-gap semiconductors driven by ultrafast terahertz excitation: Beyond the ballistic regime

Using two-dimensional THz spectroscopy in combination with numerical models, we investigate the dynamics linked to carrier multiplication caused by high-field THz excitation of the low-gap semiconductor InSb. In addition to previously reported dynamics connected with quasiballistic carrier dynamics, we observe other spectral and temporal features that we attribute to impact ionization for peak fields above 60 kV/cm, which continue up to the maximum investigated peak field of 430 kV/cm. At the highest fields we estimate a carrier multiplication factor greater than 10 due to impact ionization, which is well-reproduced by a numerical simulation of the impact ionization process which we have developed.ISSN:1098-0121ISSN:0163-1829ISSN:1550-235XISSN:0556-2805ISSN:2469-9969ISSN:1095-3795ISSN:2469-995

Highly efficient surface-emitting semiconductor lasers exploiting quasi-crystalline distributed feedback photonic patterns

Abstract: Quasi-crystal distributed feedback lasers do not require any form of mirror cavity to amplify and extract radiation. Once implemented on the top surface of a semiconductor laser, a quasi-crystal pattern can be used to tune both the radiation feedback and the extraction of highly radiative and high-quality-factor optical modes that do not have a defined symmetric or anti-symmetric nature. Therefore, this methodology offers the possibility to achieve efficient emission, combined with tailored spectra and controlled beam divergence. Here, we apply this concept to a one-dimensional quantum cascade wire laser. By lithographically patterning a series of air slits with different widths, following the Octonacci sequence, on the top metal layer of a double-metal quantum cascade laser operating at THz frequencies, we can vary the emission from single-frequency-mode to multimode over a 530-GHz bandwidth, achieving a maximum peak optical power of 240 mW (190 mW) in multimode (single-frequency-mode) lasers, with record slope efficiencies for multimode surface-emitting disordered THz lasers up to ≈570 mW/A at 78 K and ≈720 mW/A at 20 K and wall-plug efficiencies of η ≈ 1%

Self-mixing interferometry and near-field nanoscopy in quantum cascade random lasers at terahertz frequencies

We demonstrate that electrically pumped random laser resonators, operating at terahertz (THz) frequencies, and comprising a quantum cascade laser heterostructure, can operate as sensitive photodetectors through the self-mixing effect. We devise two-dimensional cavities exploiting a disordered arrangement of surface holes that simultaneously provide optical feedback and allow light out-coupling. By reflecting the emitted light back onto the surface with random holes pattern, and by varying the external cavity length, we capture the temporal dependence of the laser voltage, collecting a rich sequence of interference fringes that follow the bias-dependent spectral emission of the laser structure. This provides a visible signature of the random laser sensitivity to the self-mixing effect, under different feedback regimes. The latter effect is then exploited, in the near-field, to demonstrate detectorless scattering near-field optical microscopy with nanoscale (120 nm) spatial resolution. The achieved results open up possibilities of detectorless speckle-free nano-imaging and quantum sensing applications across the far-infrared