No abstract

Optical frequency combs1,2 establish a rigid phase-coherent link between microwave and optical domains and are emerging as high-precision tools in an increasing number of applications 3 . Frequency combs with large intermodal spacing are employed in the field of microwave photonics for radiofrequency arbitrary waveform synthesis 4,5 and for generation of THz tones of high spectral purity in the future wireless communication networks 6,7 . We demonstrate for the first time self-starting harmonic frequency comb generation with a THz repetition rate in a quantum cascade laser. The large intermodal spacing caused by the suppression of tens of adjacent cavity modes originates from a parametric contribution to the gain due to temporal modulations of the population inversion in the laser 8,9 . The mode spacing of the harmonic comb is shown to be uniform to within 5 × 10 −12 parts of the central frequency using multiheterodyne self-detection. This new harmonic comb state extends the range of applications of quantum cascade laser frequency combs 10-13 . Several techniques to generate optical frequency combs (OFCs) have been demonstratedin the last decades based on different nonlinear mechanisms that fulfill the modelocking condition. Originally, passively modelocked lasers based on saturable absorption and Kerr lensing were used to create short light pulses, and were subsequently shown to also constitute frequency combs. This type of modelocking is an example of amplitude-modulated modelocking, so-named for the temporal behavior of the electric field of the emitted light. However, these techniques usually result in elaborate optical systems. More recently, new routes promising chip-scale comb generators have been investigated based on optically-pumped ultra-high-quality-factor crystalline microresonators 14-16 and on broadband quantum cascade lasers (QCLs) with specially designed multistage active regions 10,17 . In both cases the essential underlying mechanism responsible for the generation of OFCs is cascaded four-wave mixing (FWM) enabled by a third-order χ (3) Kerr nonlinearity. The temporal behavior of these OFCs is not restricted to ultrashort pulses but can represent rather sophisticated waveforms due to a non-trivial relationship among the spectral phases of the comb teeth. In fact, the output of a QCL-based frequency comb resembles that of a frequency-modulated laser with nearly constant output intensity 10,18 .A novel mechanism of OFC generation in QCLs was suggested by the recent discovery of a new laser state 19 , which comprises many modes separated by higher harmonics of the 2 cavity free spectral range (FSR) (Figure 1a). This spectrum radically differs from that of fundamentally modelocked QCL combs where adjacent cavity modes are populated ( Figure 1b THz repetition rates in other semiconductor lasers 20 . Rather, the modes are locked passively due to the behavior of the QCL gain medium itself.In this work we employed two Fabry-Perot (FP) QCLs fabricated from the same growth process with 6 mm-long cavit...

Optical frequency combs have revolutionized the fields of high-resolution and precision atomic spectroscopy due to their high coherence, wide spectral bandwidth and absolute traceability [1,2]. Initially developed in the nearinfrared (NIR) spectral region, frequency combs are now being extended to other parts of the spectrum. In particular, extending the spectral range of frequency combs into the mid-infrared (MIR) and terahertz (THz) regions will open new possibilities in the fields of frequency metrology, molecular spectroscopy, chemical analysis and medical diagnosis [3], as the fundamental roto-vibrational absorption lines of a variety of molecules lie in this spectral region.Different schemes have been investigated for generating MIR frequency combs. A well-established approach consists of transferring frequency combs from the near-infrared region into the MIR region through nonlinear processes using, for example, optical parametric oscillators [4,5] or difference frequency generation in fiber-based NIR combs [6][7][8].Other examples include MIR combs generated by transition metals incorporated into chalcogenide hosts [9,10] or Thulium-doped silica fiber lasers [11]. These sources are now well-established and applications such as MIR highresolution spectroscopy are possible. These methods guarantee good spectral coverage and coherence, but usually require delicate experimental set-ups with large footprints.Significant effort has been recently made for achieving chip-based MIR frequency combs. Microresonator frequency combs (Kerr-combs) have been significantly improved [12][13][14][15] and have been extended to the MIR region [16][17][18]. Although Kerr-combs can be produced in different material platforms, they still require a high-power continuous wave (CW) laser as well as an evanescent coupling system, especially difficult to achieve in the MIR and THz regions.Quantum cascade lasers (QCL) have proven to be semiconductor lasers capable of generating comb radiation in the MIR and THz regions [19][20][21]. As the comb formation takes place directly in the QCL active region, QCL frequency combs (QCL-combs) offer the unique possibility of a completely integrated chip-based system capable of performing broadband high-resolution spectroscopy. Such a compact system is ideal for applications requiring the detection of several different molecules masked by a complex background matrix.Meanwhile, dual-comb spectroscopy using QCL-combs has been demonstrated [22] and a theoretical description of the comb formation has recently been developed [23,24]. However, key characteristics of QCL-combs -such as optical bandwidth and power-per-mode distribution -still need to be improved in order to better address spectroscopy applications.Group delay dispersion (GDD) plays an important role in the formation of 20,24]. In this work, we investigate a scheme for controlling the dispersion in MIR QCL-combs. We demonstrate that a dispersion compensation scheme based on a Gires-Tournois Interferometer [25] (GTI) directly integrated int...

Electro-optic modulators are essential for sensing, metrology and telecommunications. Most target fiber applications. Instead, metasurface-based architectures that modulate free-space light at gigahertz (GHz) speeds can boost flat optics technology by microwave electronics for active optics, diffractive computing or optoelectronic control. Current realizations are bulky or have low modulation efficiencies. Here, we demonstrate a hybrid silicon-organic metasurface platform that leverages Mie resonances for efficient electro-optic modulation at GHz speeds. We exploit quasi bound states in the continuum (BIC) that provide narrow linewidth (Q = 550 at $${\lambda }_{{{{{{{{\rm{res}}}}}}}}}=1594$$ λ res = 1594  nm), light confinement to the non-linear material, tunability by design and voltage and GHz-speed electrodes. Key to the achieved modulation of $$\frac{{{\Delta }}T}{{T}_{\max }}=67 \%$$ Δ T T max = 67 % are molecules with r33 = 100 pm/V and optical field optimization for low-loss. We demonstrate DC tuning of the resonant frequency of quasi-BIC by $${{\Delta }}{\lambda }_{{{{{{{{\rm{res}}}}}}}}}=$$ Δ λ res =  11 nm, surpassing its linewidth, and modulation up to 5 GHz (fEO,−3dB = 3 GHz). Guided mode resonances tune by $${{\Delta }}{\lambda }_{{{{{{{{\rm{res}}}}}}}}}=$$ Δ λ res =  20 nm. Our hybrid platform may incorporate free-space nanostructures of any geometry or material, by application of the active layer post-fabrication.