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(a) Schematic representation of a generic dual-comb spectroscopy setup. A sample frequency comb (FC) with repetition rate frep interrogates a molecular sample inside a spectroscopy cell. The molecular information is encoded in the sample FC and is downconverted to radio frequencies, thanks to multiheterodyne mixing with a reference FC with slightly different repetition rate frep + Δfrep.
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Broadband, quantum-engineered, quantum cascade lasers (QCLs) are the most powerful chip-scale sources of optical frequency combs (FCs) across the mid-infrared and the terahertz (THz) frequency range. The inherently short intersubband upper state lifetime spontaneously allows mode proliferation, with large quantum efficiencies, as a result of the in...
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In recent years, terahertz waves have attracted significant attention for their promising applications. Due to a broadband optical response, an ultra-fast relaxation time, a high nonlinear coefficient of graphene, and the flexible and controllable physical characteristics of its meta-structure, graphene metamaterial has been widely explored in inte...
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... These features, combined with their small footprint, intrinsic spectral purity [7], continuous wave (CW) operation, and relatively high output powers (>2 W) [8], make these sources an ideal platform for the observation of mode-locked multimode regimes [9] and quantum correlation effects [7]. The experimental discovery that QCLs can spontaneously emit optical frequency combs (OFCs) [9][10][11] has dramatically increased their technological potential in metrology [12], communication [13], sensing [14], dual-comb imaging and spectroscopy [15,16], and quantum technologies [14,17,18]. ...
... These features, combined with their small footprint, intrinsic spectral purity [7], continuous wave (CW) operation, and relatively high output powers (>2 W) [8], make these sources an ideal platform for the observation of mode-locked multimode regimes [9] and quantum correlation effects [7]. The experimental discovery that QCLs can spontaneously emit optical frequency combs (OFCs) [9][10][11] has dramatically increased their technological potential in metrology [12], communication [13], sensing [14], dual-comb imaging and spectroscopy [15,16], and quantum technologies [14,17,18]. ...
... In contrast to the standard, evenly spaced comb operation, where the mode separation matches the cavity round trip frequency, in the harmonic state, optical power is concentrated in a few modes spaced by several multiples of the cavity free spectral range (FSR) [21]. For both QCL FCs and HFCs, quantum correlations are expected to be established by resonant four-wave-mixing (FWM) [9,14,22,23] enabled by the third-order χ (3) Kerr nonlinearity, which is the mechanism responsible for QCL comb mode proliferation and for frequency-and phase-locking the side modes [5,6]. FWM has also been identified as the core mechanism of entanglement among laser modes in passive micro-resonator-based comb emitters [24], triggering an increased interest for the exploitation of the same effect in standard Fabry-Perot (FP) QCL FCs. ...
Optical frequency combs (OFCs), which establish a rigid phase-coherent link between the microwave and optical domains of the electromagnetic spectrum, are emerging as key high-precision tools for the development of quantum technology platforms. These include potential applications for communication, computation, information, sensing, and metrology and can extend from the near-infrared with micro-resonator combs, up to the technologically attractive terahertz (THz) frequency range, with powerful and miniaturized quantum cascade laser (QCL) FCs. The recently discovered ability of the QCLs to produce a harmonic frequency comb (HFC)—a FC with large intermodal spacings—has attracted new interest in these devices for both applications and fundamental physics, particularly for the generation of THz tones of high spectral purity for high data rate wireless communication networks, for radio frequency arbitrary waveform synthesis, and for the development of quantum key distributions. The controlled generation of harmonic states of a specific order remains, however, elusive in THz QCLs. Here, and by design, we devise a strategy to obtain broadband HFC emission of a pre-defined order in a QCL. By patterning $n$ n regularly spaced defects on the top surface of a double-metal Fabry–Perot QCL, we demonstrate harmonic comb emission with modes spaced by an ( $n + 1$ n + 1 ) free spectral range and with an optical power/mode of ${\sim}{270}\;\unicode{x00B5} {\rm W}$ ∼ 270 µ W .
... In the rapidly advancing landscape of quantum science and technology, the development of quantum devices operating at terahertz (THz) frequencies has emerged as a frontier that prospects an immense potential for a variety of applications, including communications and imaging [1], sensing [2], materials science, and metrology [3]- [6]. Among the most recent developments of THz technologies, the experimental discovery that THz frequency quantum cascade lasers (QCLs), miniaturized quantum sources, can emit optical frequency combs spontaneously [7] has opened new areas for fundamental study and application-oriented developments [8]. THz frequency combs (FCs), characterized by a series of evenly spaced and coherent spectral lines, may indeed lead to a paradigm shift in the spontaneous generation of quantum states of light [9], providing a fundamental metrological tool to generate in phase optical modes with tailored frequencies of pre-defined spectral spacing. ...
Optical frequency comb synthesizers, operating in the harmonic regime, are metrological sources in which the emitted optical power is concentrated in a few modes, spaced by several multiples of the cavity free spectral range (FSR). This behavior reflects in a large correlation degree and, in principle, in an increased optical power per mode. In miniaturized quantum cascade lasers (QCLs), harmonic frequency combs (HFCs) are hence particularly attracting to explore quantum correlation effects between adjacent harmonic modes, enabled by the inherently large gain media third-order χ ⁽³⁾ Kerr nonlinearity, even if controlled generation of stable HFCs of predefined order, is typically demanding in such electrically pumped sources. Here, we demonstrate stable 2nd order and 3rd order HFC emission in terahertz frequency QCLs by respectively patterning an individual or a couple of equally spaced distributed multilayer graphene absorbers on the top metallic waveguides.
... Recent developments in on-chip terahertz sources without external antennas include Quantum Cascade Lasers (QCLs), High Electron Mobility Transistors (HEMTs), and Resonant Tunneling Diodes (RTDs) [46]. QCLs, relying on optical transitions in repeated stacks of semiconductor nanostructures [47][48][49], have seen substantial progress, achieving tunability within 1-5 THz and emitting over 1 W in pulsed mode at 10 K [50]. However, challenges persist in reaching room temperature operation conditions while generating signals in the continuous wave regime. ...
Graphene-based Field-Effect Transistors (FETs) integrated with microstrip patch antennas offer a promising approach for terahertz signal radiation. In this study, a dual-stage simulation methodology is employed to comprehensively investigate the device’s performance. The initial stage, executed in MATLAB, delves into charge transport dynamics within a FET under asymmetric boundary conditions, employing hydrodynamic equations for electron transport in the graphene channel. Electromagnetic field interactions are modeled via Finite-Difference Time-Domain (FDTD) techniques. The second stage, conducted in COMSOL Multiphysics, focuses on the microstrip patch antenna’s radiative characteristics. Notably, analysis of the S11 curve reveals minimal reflections at the FET’s resonant frequency of 1.34672 THz, indicating efficient impedance matching. Examination of the radiation pattern demonstrates the antenna’s favorable directional properties. This research underscores the potential of graphene-based FETs for terahertz applications, offering tunable impedance matching and high radiation efficiency for future terahertz devices.
... Frequency combs based on THz QCLs have been demonstrated with heterogeneous [81][82][83]135] or homogeneous [84][85][86] active region designs. THz QCL comb formation is due to the large third-order χ 3 ( ) Kerr nonlinearity of the QCL gain medium, which, in turn, determines a robust interaction between adjacent modes via the four-wave mixing (FWM) process [136][137][138]. However, in a free-running THz QCL, the modes are not uniformly spaced, due to chromatic dispersion [84,139]. ...
Terahertz (THz) quantum cascade laser (QCL) is an electrically pumped unipolar photonic device in which light emission takes place due to electronic transitions between subbands formed by multiple strongly coupled quantum wells. THz QCL is arguably the most promising solid-state source to realize various THz applications, such as high-resolution spectroscopy, real-time imaging, chemical and biological sensing, and high-speed wireless communication. To date, THz QCLs have covered emitting frequency from 1.2 to 5.4 THz when operating without the assistance of an external magnetic field. The highest output power is in hundreds milliwatt and watt levels continuous-mode and pulsed-mode operations, respectively. THz QCL-based local oscillators have been implemented in astronomy for the identification of atoms and ions. However, there are also limitations, including under room-temperature operation, large divergent beam, narrow single-mode frequency tuning range, incomplete polarization control, and narrow-range frequency comb operation that hinder the widespread applications of THz QCLs. Continuous efforts have been made to improve those THz QCL properties in order to satisfy the requirements of different THz applications. This report will review the key output characteristic developments of THz QCLs in the past few years, which aim to speed up THz QCLs toward practical applications.
... Among different THz radiation sources, the electrically pumped THz quantum cascade laser (QCL), showing high output power, 19 wide emission frequency range, 20,21 high operation temperature, [22][23][24] high-quality far-field beam, 25,26 and narrow intrinsic linewidth, 27 is an ideal candidate for high-precision frequency comb operation. 28,29 Although the intrinsic linewidth of THz QCLs is narrow, the practical devices normally show broad linewidths due to the disturbances such as temperature drift, current variation, optical feedback and other environmental noises. So far, many efforts have been devoted to improving the stability of THz QCLs. ...
... For this reason QCLs can emit in frequency ranges that normally are not achievable with other types of semiconductor lasers, as the mid-IR and the THz ranges. Furthermore, they can be exploited in several types of different applications, such as free space communication [67], [66], [41], [15], material and molecular analysis and spectroscopy [19], [14], interferometry [22], metrology [58] and nonlinear optics [75], in both mid-IR and THz regions. In the following part of this chapter we will review in more detail some peculiar characteristics of QCLs that we mentioned in this introductory part about this class of semiconductor lasers. ...
This work concerns about the study of the Quantum Cascade Lasers' dynamics from theoretical and numerical point of view, by exploring different aspects of this vast field of study.
Firstly, the self-generation of optical frequency combs by exploiting these devices in the Fabry-Perot (FP) configuration has been studied by introducing the Effective Semiconductor Maxwell-Bloch Equations (ESMBEs), a model which encompasses the main features of semiconductors materials and includes in the case of FP cavity the Spatial-Hole Burning (SHB). The simulation study based on the integration of the ESMBEs has allowed the reproduction of relevant experimental achievements and the highlight of the role of some parameters such as the linewidth enhancement factor. Furthermore, this model has been retrieved for the case of ring Quantum Cascade Laser, and also in this case a simulation study has been performed.
The case of ring QCL have also been studied by introducing a reduced model based on a single master equation. This model, which is valid in the hypotheses of fast carriers and near threshold operation, presents a shorter simulation time, and showed an agreement with the ESMBEs results also for values of the current relevantly above the laser threshold.
The ring QCL with an optical injected field has also been considered and numerically studied, achieving the reproduction of experimentally found regimes such as temporal solitons and Turing rolls.
The last part of the work is dedicated to the study of a Terahertz (THZ)- QCL in presence of optical feedback, in a self-mixing interferometry setup which is combined with a probing-type microscopy technique, in order to produce a system for the nano-imaging of resonant materials, called Self-Detection scattering type near field optical microscopy (SD s-SNOM). A theoretical study based on the Lang-Kobayashi Equations and a simulation analysis of this setup have been performed in order to explore the possibility to retrieve the dielectric properties of materials with phonon resonances in the THZ region with a resolution far beyond the diffraction limit.
... QCLs are semiconductor lasers relying on inter-conduction-band quantum well transitions [1] and their emission domain ranges from the mid-infrared (mid-IR) to the terahertz (THz) region [2]. The topic of QCL has undergone fast development, with important breakthroughs still to be expected in the coming years [3][4][5]. The first operational device, working at cryogenic temperature and in pulsed mode, was produced at Bell Labs in 1994 after years of theoretical and technological improvements [6]. ...
... Indeed, both laser subbands of a QCL are within the conduction band and exhibit the same reciprocal space curvature. Equation (5) shows that α vanishes at the transition frequency, independently of n, hence characterizing detuned oscillators with ν ̸ = ν 0 . It has to be noted that equation (5) can be applied to any laser that can be described by a two-level system (e.g. ...
The purpose of this article is to gather recent findings about the non-linear dynamics of distributed feedback quantum cascade lasers, with a view on practical applications in a near future. As opposed to other semiconductor lasers, usually emitting in the visible or the near-infrared region, quantum cascade laser technology takes advantage of intersubband transitions and quantum engineering to emit in the mid-infrared and far-infrared domain. This peculiarity and its physical consequences were long considered as a detrimental characteristic to generate non-linear dynamics under external optical control. However, we show that a wide diversity of phenomena, from high-dimensional chaos to giant pulses can be observed when the quantum cascade laser is under external optical feedback or under optical injection and with a continuous current bias. Most of these phenomena have already been observed in other semiconductor lasers under optical feedback or under optical injection, which allows us comparing quantum cascade lasers with their interband counterparts.
... Alternatively, the photonic approach utilizes optical and optoelectronic devices to step down to THz frequencies, with a large variety of designs having a wide range of operating principles [10], [11]. Some examples are Quantum Cascade Lasers (QCLs) [12], [13], heterodyne/photomixing structures [14], and laser-driven sources [15]. However, these designs are often bulky, possess low efficiency and output power, require cryogenic temperatures, and offer minimal tunability. ...
Terahertz communication is envisioned as a key technology not only for the next generation of macro-scale networks (e.g., 6 G), but also for transformative networking applications at the nanoscale (e.g., wireless nanosensor networks and wireless networks on chip). In this paper, an on-chip THz nano-generator with amplitude and frequency modulation capabilities is presented. The proposed device uses and leverages the tunability of the Dyakonov-Shur instability for the growth and modulation of plasmonic oscillations in the two-dimensional electron gas channel of a graphene-based high-electron-mobility transistor. An in-house developed finite-element-methods platform, which self-consistently solves the hydrodynamic model and Maxwell's equations, is utilized to provide extensive numerical results demonstrating the device's functionality along with ultra-wide bandwidth and high modulation index capabilities.
... Thanks to their intrinsic coherence in the frequency and time domains, frequency combs are widely used for ultrahigh-resolution spectroscopy and hyperspectral imaging, 215 time-domain nanoimaging, quantum science and technology, metrology, nonlinear optics, and frequency multiplexing for the generation of short optical pulses. 216 All these applications are typically addressed by using different architectures and different QCL designs. Programmable metasurfaces could impact this field by introducing a reconfigurable component that can be customtailored according to the specific task. ...
The discovery of graphene and its fascinating capabilities has triggered an unprecedented interest in inorganic two-dimensional (2D) materials. van der Waals layered materials such as graphene, hexagonal boron nitride, transition metal dichalcogenides, and the more recently re-discovered black phosphorus (BP) indeed display an exceptional technological potential for engineering nano-electronic and nano-photonic devices and components “by design,” offering a unique platform for developing new devices with a variety of “ad hoc” properties. In this Perspective article, we provide a vision on the key transformative applications of 2D nanomaterials for the development of nanoelectronic, nanophotonic, optical, and plasmonic devices at terahertz frequencies, highlighting how the rich physical phenomena enabled by their unique band structure engineering can allow them to boost the vibrant field of quantum science and quantum technologies.
... The advent of grain-of-rice-size THz lasers on a chip, operating at 250 K, 8 a temperature reachable with a plug-in cooler, and chip-scale THz frequency combs 9 will trigger the development of compact and technologically relevant THz systems. However, producing low-cost (<10k $) and scalable multipixel THz detectors operating at room temperature (RT), with noise equivalent powers (NEP) < 1 nWHz −1/2 , suitable for real-time detection or quantum applications, is still elusive, in particular for operating frequencies >2THz, appealing for broadband integrated systems comprising miniaturized quantum cascade laser (QCL) combs. ...
... However, producing low-cost (<10k $) and scalable multipixel THz detectors operating at room temperature (RT), with noise equivalent powers (NEP) < 1 nWHz −1/2 , suitable for real-time detection or quantum applications, is still elusive, in particular for operating frequencies >2THz, appealing for broadband integrated systems comprising miniaturized quantum cascade laser (QCL) combs. 9 Focal plane arrays, the most commonly employed architectures for multipixel imaging, are currently based on either microbolometers 10 or complementary metal-oxidesemiconductor (CMOS) image sensors. Commercially available microbolometers have a low NEP ∼ 30 pWHz −1/2 , 10 with a slow response time (τ) in the range ∼10−1000 μs. ...
The scalable synthesis and transfer of large-area graphene underpins the development of nanoscale photonic devices ideal for new applications in a variety of fields, ranging from biotechnology, to wearable sensors for healthcare and motion detection, to quantum transport, communications, and metrology. We report room-temperature zero-bias thermoelectric photodetectors, based on single- and polycrystal graphene grown by chemical vapor deposition (CVD), tunable over the whole terahertz range (0.1–10 THz) by selecting the resonance of an on-chip patterned nanoantenna. Efficient light detection with noise equivalent powers <1 nWHz–1/2 and response time ∼5 ns at room temperature are demonstrated. This combination of specifications is orders of magnitude better than any previous CVD graphene photoreceiver operating in the sub-THz and THz range. These state-of-the-art performances and the possibility of upscaling to multipixel architectures on complementary metal-oxide-semiconductor platforms are the starting points for the realization of cost-effective THz cameras in a frequency range still not covered by commercially available microbolometer arrays.
