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Perfluorinated sulfonic acid ionomers are well known for their unique water uptake properties and chemical/mechanical stability. Understanding their performance–stability trade-offs is key to realizing membranes with optimal properties. Terahertz time-domain spectroscopy has been demonstrated to resolve water states inside industrially relevant membranes, producing qualitatively agreeable results to conventional gravimetric analysis and prior demonstrations. Using the proposed humidity-controlled terahertz time-domain spectroscopy, here we quantify this detailed water information inside commercially available Nafion membranes at various humidities for direct comparison against literature values from dynamic vapor sorption, differential scanning calorimetry, and Fourier transform infrared spectroscopy on selected samples. Using this technique therefore opens up opportunities for rapid future parameter space investigation for membrane optimization.

Recent advancements in terahertz (THz) technology have heralded the development of cutting-edge sources, detectors, and modulators, catalyzing progress in fields such as molecular spectroscopy, 6G wireless communication, and biomedical imaging. Central to these innovations is terahertz time-domain spectroscopy (THz-TDS), which has garnered significant attention in both foundational and applied research. THz-TDS's unique capability to concurrently obtain time-domain and frequency-domain spectral data, along with its non-destructive nature, positions it as an indispensable tool for elucidating the physical and chemical properties of various materials without causing damage. This technique provides profound insights into diverse fields, such as molecular dynamics and vibrational modes of crystal lattices. Leveraging the state-of-the-art measurements enabled by THz-TDS, this thesis delves into the characterization of advanced materials and the examination of light interactions within artificial photonic structures. It showcases a variety of innovative THz-TDS applications aimed at propelling research in diverse areas: Initially, the thesis presents material characterization applications utilizing a standard THz-TDS system configured for both transmission and reflection measurements. Through the analysis of different transmission characteristics of samples, it highlights two exemplary methods: evaluating the surface roughness of 3D-printed metals to within micrometer accuracy, and determining the shielding effectiveness of bio-degradable THz absorbers up to 99.99999%. These methodologies underscore the pivotal role of THz-TDS in advancing next-generation manufacturing technologies. Furthermore, the thesis explores the investigation of artificial photonic structures using a general THz-TDS system. It details the initial measurements of two types of metamaterials—each designed for high Q-factor enhancement and increased light absorption—using both standard and cryogenic TH

Active modification of the polarization state is a key feature for the next-generation of wireless communications, sensing, and imaging in the THz band. The polarization modulation performance of an integrated metamaterial/graphene device is investigated via a modified THz time domain spectroscopic system. Graphene’s Fermi level is modified through electrostatic gating, thus modifying the device’s overall optical response. Active tuning of ellipticity by >0.3 is reported at the resonant frequency of 0.80 THz. The optical activity of transmitted THz radiations is continuously modified by > 21.5deg at 0.71 THz. By carefully selecting the transmitted frequency with the relative angle between the incoming linear polarization and the device’s symmetry axis, active circular dichroism and optical activity are almost independently exploited. Finally, this all-electronically tuneable versatile polarization device can be used in all applications requiring an ultrafast modulation such as polarization spectroscopy, imaging, and THz wireless generation.

Metamaterial resonators have become an efficient and versatile platform in the terahertz frequency range, finding applications in integrated optical devices, such as active modulators and detectors, and in fundamental research, e.g., ultrastrong light–matter investigations. Despite their growing use, characterization of modes supported by these subwavelength elements has proven to be challenging and it still relies on indirect observation of the collective far-field transmission/reflection properties of resonator arrays. Here, we present a broadband time-domain spectroscopic investigation of individual metamaterial resonators via a THz aperture scanning near-field microscope (a-SNOM). The time-domain a-SNOM allows the mapping and quantitative analysis of strongly confined modes supported by the resonators. In particular, a cross-polarized configuration presented here allows an investigation of weakly radiative modes. These results hold great potential to advance future metamaterial-based optoelectronic platforms for fundamental research in THz photonics.

Terahertz time-domain spectroscopy has experienced significant progress in imaging, spectroscopy, and quality inspection, e.g., for semiconductor packaging or the automotive industry. Additive manufacturing alloys (also known as alloys for use in 3D printing) have risen in popularity in aerospace and biomedical industries due to the ability to fabricate intricate designs and shapes with high precision using materials with customized mechanical properties. However, these 3D-printed elements need to be polished thereafter, where the surface roughness is inspected using techniques such as the laser scanning microscope. In this study, we demonstrate the use of terahertz time-domain spectroscopy to assess the average roughness profile and height leveling of stainless steel for comparisons against the same parameters acquired using laser scanning microscopy. Our results highlight the potential of the proposed technique to rapidly inspect 3D-printed alloys over large areas, thus providing an attractive modality for assessing surface profiles of AM-manufactured terahertz components in the future.

Dynamic control of terahertz (THz) waves is a significant area of intense research. For modulating THz waves, metamaterials have emerged as a promising solution. This article comments on an innovative approach that was recently published.

Polyethylene glycol (PEG) hydrogel are commonly used as both scaffolds and drug delivery systems to aid in cellular differentiation, thus developing a good understanding of their water properties is key to PEG optimisation. Terahertz (THz) imaging has previously been used to analyse the water content inside polymer films in ambient environment. Here, we apply this approach to extract the desorption profiles of PEG hydrogels of four concentrations producing results that indicate confinement effects due to porosity on the diffusional properties of water. These preliminary data therefore highlight the broad applicability of THz imaging for the analysis of water properties of PEG hydrogels.

Research in the terahertz (THz) band, which is broadly defined as 0.1-10 THz, is an active area of research driven by applications in sixth generation (6G) and beyond for communications, spectroscopy, imaging, and sensing. In order to exploit the full potential of all these applications, fast integrated circuitry is required. This work revolves around removing this bottleneck. Achievement of efficient dynamic modulation requires the implementation of active material. Amongst many different approaches to achieve active modulation, metamaterials/graphene-based technology is establishing itself as a benchmark for THz operation due to its versatility, power efficiency, small footprint, and integration capabilities. Our devices have been modulated all-electronically, as described in Chapters 4 and 6, and all-optically as reported in Chapter 5. The fabrication of the novel design based on metamaterial (MM) and graphene for amplitude, phase, and polarization modulations is reported in Chapter 3. The optoelectronic behaviour of this modulator is tested in a THz time-domain spectroscopy (THz-TDS) setup as demonstrated in Chapter 4. By choosing the appropriate THz-TDS setup configuration, a spectral amplitude extinction ratio of >10 dB (>93%) at the resonant frequency of 0.8 THz is demonstrated. The spectral phase of THz radiations is actively tuned by >27o at 0.62 THz frequency. Linear to circular polarization conversion with nearly 100% of conversion efficiency is reported demonstrating almost an independent control of circular dichroism (CD) and optical activity (OA) as mentioned explicitly in Chapter 6. Dynamic changes of ellipticity are reported to exceed 0.3 in ratio at resonance. The OA of transmitted THz radiations is continuously rotated by >21.5o at 0.71 THz by varying the gate. These values are in line with acquainted literature with graphene-based or 2-dimensional electron gas modulators but with higher reconfiguration speed. The helicity, either right or left cir

We report on the ultrafast modulation of a graphene loaded artificial metasurface realized on a SiO2/Si substrate by near-IR laser pump, detected via terahertz probe at the resonant frequency of ∼0.8 THz. The results have been acquired by setting the Fermi energy of graphene at the Dirac point via electrostatic gating and illuminating the sample with 40 fs pump pulses at different fluences, ranging from 0.9 to 0.018 mJ/cm2. The sub-ps conductivity rising time was attributed to the combined effect of the ultrafast generation of hot carriers in graphene and electron–hole generation in silicon. In correspondence of the resonance, it was possible to clearly distinguish a partial recovery time of ∼2 ps mainly due to carrier-phonon relaxation in graphene, superimposed to the > 1 ns recovery time of silicon. The resonant metasurface yielded ∼6 dB modulation depth in E-field amplitude at 0.8 THz for the range of fluences considered. These measurements set an upper limit for the reconfiguration speed achievable by graphene-based terahertz devices. At the same time, this work represents a great progress toward the realization of an ultrafast THz optoelectronic platform for a plethora of applications, ranging from the investigation of the ultrastrong light-matter regime to the next generation wireless communications.

Many mid- and far-infrared semiconductor photodetectors rely on a photonic response, when the photon energy is large enough to excite and extract electrons due to optical transitions. Toward the terahertz range with photon energies of a few milli–electron volts, classical mechanisms are used instead. This is the case in two-dimensional electron systems, where terahertz detection is dominated by plasmonic mixing and by scattering-based thermal phenomena. Here, we report on the observation of a quantum, collision-free phenomenon that yields a giant photoresponse at terahertz frequencies (1.9 THz), more than 10-fold as large as expected from plasmonic mixing. We artificially create an electrically tunable potential step within a degenerate two-dimensional electron gas. When exposed to terahertz radiation, electrons absorb photons and generate a large photocurrent under zero source-drain bias. The observed phenomenon, which we call the “in-plane photoelectric effect,” provides an opportunity for efficient direct detection across the entire terahertz range.

The terahertz (0.1 THz-10 THz) range represents a fast-evolving research and industrial field. The great interest for this portion of the electromagnetic spectrum, which lies between the photonics and the electronics ranges, stems from the unique and disruptive sectors where this radiation finds applications in, such as spectroscopy, quantum electronics, sensing and wireless communications beyond 5G. Engineering the propagation of terahertz light has always proved to be an intrinsically difficult task and for a long time it has been the bottleneck hindering the full exploitation of the terahertz spectrum. Amongst the different approaches that have been proposed so far for terahertz signal manipulation, the implementation of metamaterials has proved to be the most successful one, owing to the relative ease of realisation, high efficiency and spectral versatility. In this review, we present the latest developments in terahertz modulators based on metamaterials, while highlighting a few selected key applications in sensing, wireless communications and quantum electronics, which has particularly benefitted from these developments

We investigate the modulation performance of a metamaterial/graphene optoelectronic device in the THz range. Operating characteristics such as amplitude, phase and polarization modulations of broadband THz radiation are reported. The accomplishments of modulation depth include >75% in spectral amplitude, >15∘ in spectral phase, >0.2 active modulation of ellipticity ratio, and active rotational angle changes of 20°. These achievements are the key elements towards efficient manipulations of THz radiation for applications such as next-generation wireless communications, spectroscopy, and imaging.

Terahertz time domain spectroscopy (THz-TDS) has been demonstrated to be capable of quantifying water uptake and retention properties of Nafion proton exchange membranes (PEMs). With a growing interest in thinner membranes, we analysis thin membranes to reveal its water uptake and retention properties

The realization of terahertz external amplitude modulators with a carrier frequency of 0.8 THz is presented for application in the next generation near-field wireless communications.

An antenna-coupled dual-gated two-dimensional electron gas (2DEG) based on a GaAs-AlGaAs heterostructure shows a pronounced response to 2 THz radiation. The device is shown to be a direct detector, and its photoresponse arises without any source-drain bias. The detection is based on a novel mechanism that yields a substantially stronger photoresponse than predicted by the classical plasma-wave self-mixing and other mechanisms.

This work presents an application of the TRIZ methodology to an industrial project dealing with laser pyrolysis of tires, bitumen, and other wasting materials. Usually in the literature you can find works that describe how to apply TRIZ to single cases of problem solving, to conceive new products or to strategic planning activities. In this paper instead we present a methodological approach that combines the use of different TRIZ tools to support an entire industrial project. Triz tools will be briefly introduced to show how they can be used to decide between different industrial pyrolysis technologies, identifying strengths and weaknesses of existing laser applications, suggesting patents around the existing idea, designing the innovative idea of a laser chamber to test the effectiveness of the process. More in details, the evolution trees and the macro-micro law of technical evolution were revisited and suggested for analyzing and organizing results of a big search about the state of art. Contradictions and Ideal Final Result were used to analyze existent products and design the new ones. TRIZ methodology is also useful for building a storyboard able to convince other stakeholders of the credibility of the results and to create a team of companies and academics willing to invest in such a pioneering idea.

Active control of chirality in artificial media such as metamaterials is fundamental in many scientific areas, ranging from research into fundamental optical phenomena to the investigation of novel materials, spectroscopy, and imaging. Precise control of the light polarization states has great importance for light‐matter interaction in chemistry and biology, as media with diverse chiral properties react differently to the incoming polarization of light. In this work an active double layer metamaterial device based on vertically stacked ring resonators is realized by integrating electrostatically tunable graphene as an active element. The device is characterized with a THz time domain spectroscopic system demonstrating an all‐electrical control of circular dichroism and optical activity at ≈2 THz, reporting a tunable ellipticity of 0.55–0.98 and >20° rotation of the plane polarization, respectively, by modifying the conductivity of graphene. Further integration with a narrow frequency quantum cascade laser emitting at ≈1.9 THz, in a crossed polarizer experimental arrangement, realizes an active amplitude modulator, hence highlighting the versatility of this approach. These results represent an important milestone for the investigation of novel concepts in optics and in several applications in the THz range, such as wireless communications and spectroscopy.

Photonic engineering of the terahertz emission from a quantum cascade laser (QCL) is fundamental for the exploitation of this unique source in a myriad of applications where it can be implemented, such as spectroscopy, imaging and sensing. Active control of the frequency, power, polarization and beam profile has been achieved through a variety of approaches. In particular, the active control of the emitted frequency, which is difficult to determine a priori, has been achieved through the integration of a photonic structure, and/or by using external cavity arrangements. In this work an external cavity arrangement which implements a metamaterial/graphene optoelectronic mirror as external feedback element is proposed and demonstrated. The reflectivity and dispersion properties of the external active mirror were tuned via electrostatically gating graphene. It was possible to electronically reproduce the mode-switch occurring in a QCL emitting ~ 2.8 THz by mechanically changing the external cavity length formed by an Au mirror. The external cavity arrangement was investigated and described in the framework of self-mixing theory. These results open a way for all-electronic engineering of the QCL emission by the use of a fast reconfigurable external mirror. This approach can uniquely address both power and frequency control, with ~ 100 MHz reconfiguration speeds, using an integrated external element. Furthermore, the metamaterial/graphene mirror strong dispersive properties might be implemented for active mode locking of THz QCLs. Finally, this approach offers a unique opportunity to study the laser dynamics and mode competition in THz QCLs in the self-mixing feedback regime.

Active control of chirality in the terahertz frequency range is of great importance in many scientific areas, which include research into fundamental optical phenomena, investigation of novel materials, spectroscopy, imaging, wireless communications and chemistry. The lack of efficient, integrated and fast-reconfigurable polarization modulators has hindered, so far, the full exploitation of applications in all the aforementioned fields. Metamaterials are artificial resonant elements possessing unique remarkable properties such as high efficiency and miniaturization capability. The interplay of metallic metamaterial arrays with electrostatically tunable monolayer graphene has been demonstrated to be a valid approach for the realization of a novel class of THz devices. In this work, the realization of active chiral graphene/metamaterial modulator is presented. The versatility of this experimental approach allowed the device integration with broadband sources such as terahertz time domain spectrometers as well as with quantum cascade lasers. A continuous rotation of the polarization plane > 30° has been reported with a reconfiguration speed > 5 MHz. These results pave the way to the integration of fast terahertz polarization modulators in all the applications where these devices are in great demand.

The terahertz (THz) quantum cascade laser (QCL) provides a versatile tool in a plethora of applications ranging from spectroscopy to astronomy and communications. In many of these fields, compactness, single mode frequency emission, and low threshold are highly desirable. The proposed approach, based on line defects in a photonic crystal (PhC) matrix, addresses all these features while offering unprecedented capabilities in terms of flexibility, light waveguiding, and emission directionality. Nine line-defect QCLs were realized in a triangular lattice of pillars fabricated in the laser active region (AR), centered around ∼2 THz by tuning the photonic design. A maximal 36% threshold reduction was recorded for these ultraflat dispersion line-defect QCLs in comparison to standard metal-metal QCL. The mode selectivity is an intrinsic property of the chosen fabrication design and has been achieved by lithographically scaling the dimension of the defect pillars and by acting on the PhC parameters in order to match the AR emission bandwidth. The measured line-defect QCLs emitted preferentially in the single frequency mode in the propagation direction throughout the entire dynamic range. An integrated active platform with multiple directional outputs was also fabricated as proof-of-principle to demonstrate the potential of this approach. The presented results pave the way for integrated circuitry operating in the THz regime and for fundamental studies on microcavity lasers.

Within the last years there has been a tremendous thrust into research and technology in the THz spectral region (broadly defined as 0.1-10 THz) mainly driven by the unique potential where this radiation finds applications in, such as imaging, spectroscopy and communication. In all these fields a fast, integrated and versatile platform for modulating light is required. Metamaterial/graphene devices fulfill all these requirements as their subwavelength nature lends itself naturally to strong light-matter interaction, and therefore highly efficient and miniaturized devices. Graphene's unique properties, e.g. the large carrier concentration modulation, provide a large degree of compatibility with several architectures which can be exploited in a range of modulation or detection schemes. Finally, metamaterial/graphene devices realize a fast, versatile platform, which can be easily scaled to other frequencies, and adapted into amplitude, frequency, polarization and phase modulators, as well as integrated detectors, for the next generation of wireless-communication.

Optoelectronic modulators that operate by the electrical tuning of plasmonic resonator structures have demonstrated fast (>MHz) manipulation of terahertz (THz) radiation for communications, imaging, and spectroscopy applications. Among this class of THz device, chiral metamaterial-based polarization modulators have attracted increasing attention due to the importance of THz polarization control for chemistry, biology, and spectroscopy applications, as well as for THz communication protocols. In this paper, active polarization modulation of a THz quantum cascade laser is demonstrated by the electrical tuning of a 2D chiral metamaterial array. The operating principle of this device is based on an electromagnetically induced transparency analogue, produced by the coupling between a bright resonator and two dark resonators. The orientation of these resonators is such that a radiating electric dipole orthogonal to the incident electric field polarization is induced, causing a rotation of the polarization angle of the transmitted radiation. By variably dampening the dark resonators using graphene, the coupling condition is electrically modulated such that continuous tuning of the transmitted polarization angle is achieved. This device, operating at room temperature, can be readily implemented as a fast, optoelectronic, polarization modulator with a maximum tuning range of 20 degrees at 1.75 THz, with demonstrated reconfiguration speeds of >5 MHz.

Terahertz (THz) science and technology has experienced tremendous progress in recent years, such as in spectroscopy, imaging, pharmaceutical research [1] and wireless communications. These applications require electrically tuneable devices to modulate the THz properties, including the amplitude, frequency and polarization. The integration of resonant plasmonic/metamaterial devices with graphene, has proved a successful route for the realisation of fast reconfigurable, efficient THz optoelectronic devices [2], via electrical tuning of graphene integrated with plasmonic resonant structures. An active THz modulator is presented based on a chiral metamaterial array containing metallic features, loaded with graphene. The device makes use of an electromagnetically induced transparency analogue produced via the capacitive coupling of bright and dark resonators, the latter actively damped with graphene, exploited for frequency modulation in Ref. [2]. The active area is 1.2 x 1.2 mm, consisting of a 2D chiral metamaterial array comprising 27 x 27 unit cells, shown in Fig. 1a. The resonators were defined using electron-beam lithography, and thermal evaporation of Ti/Au (10/70nm). These features were deposited on top of a 300 nm insulating layer of SiO2 on a boron p-doped silicon substrate. Chemical vapour deposition grown graphene was defined into 3.25 x 3.25 µm2 patches through e-beam lithography.

We employed a metallic wire grating loaded with graphene and operating in total internal reflection (TIR) geometry to realize deep and broadband THz modulation. The non-resonant field enhancement effect of the evanescent wave in TIR geometry and in the subwavelength wire grating was combined to demonstrate a∼ 77% modulation depth (MD) in the frequency range of 0.2 –1.4 THz. This MD, achieved electrically with a SiO2∕Si gated graphene device, was 4.5 times higher than that of the device without a metal grating in transmission geometry. By optimizing the parameters of the metallic wire grating, the required sheet conductivity of graphene for deep modulation was lowered to 0.87 mS. This work has potential applications in THz communication and real-time THz imaging

Optoelectronic terahertz modulators, operated by actively tuning metamaterial, plasmonic resonator structures, have helped to unlock a myriad of terahertz applications, ranging from spectroscopy and imaging to communications. At the same time, due to the inherently versatile dispersion properties of metamaterials, they offer unique platforms for studying intriguing phenomena such as negative refractive index and slow light. Active resonance frequency tuning of a metamaterial working in the terahertz regime is achieved by integrating metal-coupled resonator arrays with electrically tunable graphene. This metamaterial device exploits coupled plasmonic resonators to exhibit an electromagnetically induced transparency analog, resulting in the splitting of the resonance into coupled hybrid optical modes. By variably dampening one of the resonators using graphene, the coupling condition is electrically modulated and continuous tuning of the metamaterial resonance frequency is achieved. This device, operating at room temperature, can readily be implemented as a fast, optoelectronic, tunable band pass/reject filter with a tuning range of ≈100 GHz operating at 1.5 THz. The reconfigurable dispersion properties of this device can also be implemented for modulation of the group delay for slow light applications.

Terahertz laser sources with stable power levels are requested for astronomical, communication and spectroscopic applications. Here we demonstrate the amplitude stabilization of a terahertz quantum cascade laser with a graphene loaded split-ring-resonator array. This integrated amplitude modulator operates at room temperature and is capable of actively modulating the quantum cascade laser power level and stabilizing the power output via a PID loop. The laser power fluctuation was reduced from 1.82% to 0.19% of the total power

We demonstrate the amplitude stabilization of a 2.85 THz quantum cascade laser with a graphene loaded split-ring-resonator array, acting as an external amplitude modulator. The transmittance of the modulator can be actively changed by modifying the graphene conductivity via electrostatic back-gating. The modulator operates at room temperature and is capable of actively modulating the quantum cascade laser power level and thus stabilizing the power output via a proportional-integral-derivative feedback control loop. The stability was enhanced by more than 10 times through actively tuning the modulation. Furthermore, this approach can be used to externally control the laser power with a high level of stability

We present the continuous frequency modulation of a metamaterial resonance using selective damping of coupled plasmonic resonators with electrostatically gated graphene. A resonance frequency tuning range >150 GHz is achieved at 1.5 THz making this device suitable for use as an optoelectronic, tunable frequency modulator for THz frequencies.