Search

International audience; Graphene quantum dots, atomically precise nanopieces of graphene, are promising nanoobjects with potential applications in various domains such as photovoltaics, quantum light emitters or bio-imaging. Despite their interesting prospects, precise reports on their photophysical properties remain scarce. Here, we report on a study of the photophysics of C$_{96}$H$_{24}$(C$_{12}$H$_{25}$) graphene quantum dots. A combination of optical studies down to the single molecule level with advanced molecular modeling demonstrates the importance of the coupling to vibrations in the emission process. Optical fingerprints for H-like aggregates are identified. Our combined experimental-theoretical investigations provide a comprehensive description of the light absorption and emission properties of nanographenes, which not only represents an essentiel step towards precise control of sample production, but also paves the way for new exciting physics focused on twisted graphenoid.

Recent years have shown an increasing number of studies dedicated to new light emitters for diverse applications such as optoelectronics, bio-imaging, and quantum technologies. In this context, graphene quantum dots (GQD) have important assets since bottom-up chemistry allows complete control of the structure, opening the way to wide customization of their electronic, optical, and spin properties [1-3]. The full benefit from these opportunities requires addressing GQD’s intrinsic photophysical properties.To do so, single molecule photoluminescence experiment is a powerfull tool [4]. Here, we highlight the influence of vibrations on GQDs’ optical properties, by comparing optical studies to extensive DFT/TDDFT calculations combined with molecular dynamics simulations. Specifically, we discussed their role in the transitions' oscillator strengths [5]. In order to get deeper in the photophysics of GQD, we investigate the spectroscopy of single GQDs at cryogenic temperatures. In particular, we show a narrowing of the emission lines at low temperature, that allows us to characterize and identify vibrational replicas that are characteristic to GQDs [6]. [1] M. G. Debije, J. Am. Chem. Soc. 2004, 126, 4641 [2] X. Yan, X. Cui, and L.-s. Li, J. Am. Chem. Soc. 2010 132, 5944 [3] A. Konishi et al, J. Am. Chem. Soc. 2010, 132, 11021 [4] S. Zhao et al, Nature Communications, 2018, 9, 3470 [5] T. Liu et al, under review [6] T. Liu et al, in preparation

International audience; In the spirit of the core-shell approach that made it possible to greatly enhance the photostability of semiconductor nanoparticles, we study the emission stability of single carbon nanotubes emitting at telecommunication wavelengths protected by a double shell of polymer. Two types of structures with different polymer layers have been studied at cryogenic temperatures, whereas nanotubes wrapped with poly(9,9-di-n-octylfluorenyl-2,7-diyl) and embedded in a bulk polystyrene matrix were used as a reference. A Mandel parameter description applied to the emission intensity and an Allan deviation analysis of the spectral diffusion show quantitatively that the polymer shells stabilize the emission of single carbon nanotubes by more than one order of magnitude in comparison with micelle wrapped nanotubes deposited on a SiO2 surface. This represent an additional step towards the use of single-walled carbon nanotubes as quantum emitters at telecommunication wavelengths.

The development of covalent diazonium functionalization of carbon nanotubes brings unique oportunities in terms of original quantum sources of light. In particular, it should be possible to create a linear ensemble of closely packed single-photon sources along the carbon nanotube backbone. The physical proximity of several dopants would make it possible to couple them simultaneous to a resonant cavity mode and to induce original dynamics and light emission properties. Here, we investigate experimentally the case of a series of four quantum emitters attached to the same carbon nanotube coupled to a high finesse fiber cavity. We explore their properties through PLE, polarization and superlocalization measurements and their light emission properties using time-correlation techniques, photo-luminescence decay measurements.

Atomically precise graphene quantum dots synthesized by bottom-up chemistry are promising versatile single emitters with potential applications for quantum photonic technologies. Toward a better understanding and control of graphene quantum dot (GQD) optical properties, we report on single-molecule spectroscopy at cryogenic temperature. We investigate the effect of temperature on the GQDs’ spectral linewidth and vibronic replica, which we interpret building on density functional theory calculations. Finally, we highlight that the vibronic signatures are specific to the GQD geometry and can be used as a fingerprint for identification purposes.

Progress in quantum computing and quantum cryptography requires efficient, electrically triggered, single-photon sources at room temperature in the telecom wavelengths. It has been long known that semiconducting single-wall carbon nanotubes (SWCNTs) display strong excitonic binding and emit light over a broad range of wavelengths, but their use has been hampered by a low quantum yield and a high sensitivity to spectral diffusion and blinking. In this Perspective, we discuss recent advances in the mastering of SWCNT optical properties by chemistry, electrical contacting and resonator coupling towards advancing their use as quantum light sources. We describe the latest results in terms of single-photon purity, generation efficiency and indistinguishability. Finally, we consider the main fundamental challenges stemming from the unique properties of SWCNTs and the most promising roads for SWCNT-based chip integrated quantum photonic sources. This Perspective describes the recent advances in understanding and controlling the properties of single-wall carbon nanotubes as well as the progress towards the fabrication of new electrically driven single-photon sources.

International audience; At cryogenic temperature and at the single emitter level, the optical properties of single-wall carbon nanotubes depart drastically from that of a one-dimensional (1D) object. In fact, the (usually unintentional) localization of excitons in local potential wells leads to nearly 0D behaviors such as photon antibunching, spectral diffusion, inhomogeneous broadening, etc. Here, we present an hyperspectral imaging of this exciton self-localization effect at the single nanotube level using a super-resolved optical microscopy approach. We report on the statistical distribution of the traps localization, depth and width. We use a quasi-resonant photoluminescence excitation approach to probe the confined quantum states. Numerical simulations of the quantum states and exciton diffusion show that the excitonic states are deeply modified by the interface disorder inducing a remarkable discretization of the excitonic absorption spectrum and a quenching of the free 1D exciton absorption.

The use of carbon nanotubes as single emitters is currently developing. Being made only of surface atoms carbon nanotubes are very sensitive to their environment. A fingerprint of this sensitivity is the variation with time of their emission intensity (blinking) and energy (spectral jitter) in low temperature experiments. To improve the stability of nanotubes, we have synthesized core-double shell structures where two shells of polymers are rolled around the nanotube [1]. Here, we will show the influence of these shells on the stability of both the emission intensity and energy [2]. [1] L. Orcin-Chaix et al, Journal of Material Chemistry C 6, 4786 (2018) [2] L. Orcin-Chaix et al, ACS Appl. Nano Mater. 3, 7291-7296 (2020).

Single photon emission in the near infrared was recently reported in individual carbon nanotubes, either plain or functionalized, with however a moderate quantum yield. Here we use a tunable fiber cavity to enhance light-matter interaction, using the so-called Purcell effect to boost the performances of the single photon source in view of applications in quantum telecommunications. Preferential excitation conditions building on localized excited states, both for plain and functionalized nanotubes, are considered and the performances of the sources in terms of purity, brightness and indistinguishability are discussed.

Because of their emission in the near infrared region, Single-Walled Carbon Nanotubes remain among the most attractive nanomaterials for (opto)electronics, optics and photonics.1 - 3 However, the extreme sensitivity of nanotubes to their environment hinders their applications. Thus, the fabrication of tailor-made functional hybrid materials that preserve the optical properties of SWNTs and facilitate their manipulation is extremely important. One route to take advantage of the protection offered by polymers is to synthesize core/shell nanostructures in which the nanotube is the active core and the polymer, tightly bound to the nanotubes, acts as a protective shell. Here, we describe the synthesis of core-shell nanotube materials made of SWNTs and polystyrene. We developed a two-step strategy that permits to form a stable and homogeneous layer of polymer around the nanotubes by adding first polystyrene via the micelle swelling method and then by locking the structure via radical polymerisation in micelles of styrene and divinylbenzene. After polymerisation and redispersion, the nanotube hybrids can be easily manipulated in solution; they still exhibited photoluminescence properties both in solution and in the solid state demonstrating that the SWNTs embedded in their polystyrene shell are isolated one from each other.4 ,5 References Avouris, P.; Freitag, M.; Perebeinos, V. Nat. Photonics 2008, 2, 341-350. Godin, A. G.; Varela, J. A.; Gao, Z.; Danné, N.; Dupuis, J. P.; Lounis, B.; Groc, L.; Cognet, L. Nat. Nanotechnol. 2017, 12, 238-243. He, X.; Htoon, H.; Doorn, S. K.; Pernice, W. H. P.; Pyatkov, F.; Krupke, R.; Jeantet, A.; Chassagneux, Y.; Voisin, C. Nat. Mater. 2018, 17, 663-670. Orcin-Chaix, L.; Trippé-Allard, G.; Voisin, C.; Okuno, H.; Derycke, V.; Lauret, J.-S.; Campidelli, S. J. Mater. Chem. C 2018, 6, 4786-4792. Orcin-Chaix, L.; Campidelli, S.; Rondin, L.; Fossard, F.; Bretenaker, F.; Chassagneux, Y.; Voisin, C.; Lauret, J.-S. ACS Appl. Nano Mater. 2020, 3, 7291-7296 Figure 1

International audience; We report on the spontaneous noncovalent functionalization of carbon nanotubes with hydropho-bic porphyrin molecules in micellar aqueous solution. By monitoring the species concentrations with optical spectroscopies, we can follow the kinetics of the reaction and study its thermodynamical equilibrium as a function of the reagent concentrations. We show that the reaction is well accounted for by a cooperative Hill equation, reaching a molecular coverage close to a compact monolayer for a porphyrin concentration larger than a diameter-specific threshold concentration. The equilibrium constant is measured for 16 nanotube chiral species. The Gibbs energy of the reaction (of the order of-40 kJ/mol) and its evolution with the nanotube diameter is consistent with theoretical calculations of the binding energy. This thermodynamical study shows a strong preferential binding of TPP molecules to larger diameter nanotubes. This original curvature selectivity can be used to induce diameter selective species enrichment.

Carbon nanotubes have emerged as original single photon source in the near IR and in the telecom bands. To further control their emission rate and directionality, individual carbon nanotube can be coupled to a high Q and small mode volume cavity (Purcell effect). Due to the 1D geometry of carbon nanotube, the coupling between localized exciton and acoustic phonon is particularly strong. This coupling can be used as an additional degree of freedom in cavity coupled single emitters; it brings a very rich physics involving non-markovian decoherence, tripartite coupling, and asymmetric energy exchange... In particular the coupling to phonon can be used to “force” the nanotube to emit preferentially at the cavity frequency rather than at its intrinsic transition frequency.. We implemented this approach to realize a widely tunable single photon source. Jeantet et al, PRL 116, 247402 (2016) Jeantet et al, Nano Lett. 17, 4184 (2017) He et al , Nat. Mat. 17, 663 (2018) Chassagneux et al PRB 97, 205124 (2018)

We use a combination of hyperspectral superlocalization microscopy and excitation photoluminescence spectroscopy techniques to study the localization of one-dimensional excitons in pristine and functionalized single wall carbon nanotubes at low and room temperature. Using numerical simulations, we interpret the nearly resonant excitation spectra either in terms of excited trapped states leading a selective excitation of a trap or in terms of delocalized one dimensional excitonic states. We discuss the implications of this localization for the use of carbon nanotubes as quantum-light emitters. Raynaud et al. Nano Lett. 19, 7210 (2019)

Graphene is a key material for nanoelectronics. Nevertheless, its zero gap makes it unsuitable for applications needing semiconductors with sizeable energy gaps. One way to open a gap in graphene is to use size reduction effects. The reduction of one dimension leads to carbon nanotubes and graphene nanoribbons that are 1D carbon nanostructures. Reducing one more dimension leads to 0D graphene quantum dots. The optical properties of carbon nanotubes have been investigated for approximately 20 years, while the study of graphene quantum dots and nanoribbons is at its infancy. Among potential application of these sp2 carbon nanostructures, the use of graphene nanoribbons and quantum dots as light emitters attracts a lot of attention. Here, I will present our recent results on the optical properties of graphene quantum dots synthesized by bottom-up chemistry [1]. References [1] S. Zhao et al, Nature Communications 9, 3470 (2018)

We develop a theoretical frame to investigate the spectral dependence of the brightness of a single-photon source made of a solid-state nanoemitter embedded in a high-quality factor microcavity. This study encompasses the cases of localized excitons embedded in a one-, two-, or three-dimensional matrix. The population evolution is calculated based on a spin-boson model, using the noninteracting blip approximation. We find that the spectral dependence of the single-photon source brightness (hereafter called spectral efficiency) can be expressed analytically through the free-space emission and absorption spectra of the emitter, the vacuum Rabi splitting, and the loss rates of the system. In other words, the free-space spectrum of the emitter encodes all the relevant information on the interaction between the exciton and the phonon bath to obtain the dynamics of the cavity-coupled system. We compute numerically the spectral efficiency for several types of localized emitters differing by the phonon bath dimensionality. In particular, in low-dimensional systems where this interaction is enhanced, a pronounced asymmetric energy exchange between the emitter and the cavity on the phonon sidebands yields a considerable extension of the tuning range of the source through phonon-assisted cavity feeding, possibly surpassing that of a purely resonant system.

At cryogenic temperatures, the photoluminescence (PL) spectrum of nano-emitters may still be significantly broadened due to interactions with the environment. The interplay of spectral diffusion (SD) and phonon broadening in this context is still a debated issue. Singlewall carbon nanotubes (SWNTs) are a particularly relevant system to address this topic as they show intense spectral diffusion and undergo a high exciton-phonon coupling due to their one-dimensional geometry. Here, we investigate the correlations between the spectral diffusion of the main line and that of the wings in SWNTs quantitatively and demonstrate that the photoluminescence spectrum undergoes spectral jumps as a whole, without distortions. This behavior suggests that the spectral shape of SWNT PL is defined by exciton-phonon interactions and that spectral diffusion results in an additional flat broadening. The methodology developed here can be used to investigate a broad range of non-Lorentzian emitters undergoing spectral diffusion.

A scheme for strong coupling between a single atomic spin and the rotational mode of levitating nanoparticles is proposed. The idea is based on spin readout of nitrogen-vacancy centers embedded in aspherical nanodiamonds levitating in an ion trap. We show that the asymmetry of the diamond induces a rotational confinement in the ion trap. Using a weak homogeneous magnetic field and a strong microwave driving we then demonstrate that the spin of the nitrogen-vacancy center can be strongly coupled to the rotational mode of the diamond.

International audience; Condensed-matter emitters offer enriched cavity quantum electrodynamical effects due to the coupling to external degrees of freedom. In the case of carbon nanotubes, a very peculiar coupling between localized excitons and the one-dimensional acoustic phonon modes can be achieved, which gives rise to pronounced phonon wings in the luminescence spectrum. By coupling an individual nanotube to a tunable optical microcavity, we show that this peculiar exciton–phonon coupling is a valuable resource to enlarge the tuning range of the single-photon source while keeping an excellent exciton-photon coupling efficiency and spectral purity. Using the unique flexibility of our scanning fiber cavity, we are able to measure the efficiency spectrum of the very same nanotube in the Purcell regime for several mode volumes. Whereas this efficiency spectrum looks very much like the free-space luminescence spectrum when the Purcell factor is small (large mode volume), we show that the deformation of this spectrum at lower mode volumes can be traced back to the strength of the exciton-photon coupling. It shows an enhanced efficiency on the red wing that arises from the asymmetry of the incoherent energy exchange processes between the exciton and the cavity. This allows us to obtain a tuning range up to several hundred times the spectral width of the source.

At cryogenic temperature and at the single emitter level, the optical properties of single-wall carbon nanotubes depart drastically from that of a one-dimensional (1D) object. In fact, the (most usually unintentional) localization of the exciton in local potential wells leads to nearly 0D behaviors such as photon antibunching, spectral diffusion, inhomogeneous broadening... Here, we present an hyperspectral imaging of this exciton self-localization effect at the single nanotube level using a super-resolved optical microscopy approach. We report on the statistical distribution of trap depths, size and density. Together with a excitation resolved photoluminescence approach, we show that the excitonic states are deeply modified by the interface disorder inducing a remarkable discretization of the S11 absorption spectrum.

In the last decades, research on graphene has been extensively developed. It is now well established that graphene shows unique physical properties such as extremely high charge-carrier mobility, which promises a wealth of applications in (nano)electronics. However, graphene does not have a bandgap. Therefore, one challenge for scientists is the control and modification of the electronic porperties in order to open a sizable bandgap. In this context nanosized graphene have a lot of assets. The reduction of one dimension of graphene leads to carbon nanotubes (CNT) or graphene nanoribbons (GNRs) while the reduction of the two dimensions leads to graphene quantum dots (GQDs). The size reduction, the topology and the shape of the edges define the electronic and optical properties of this family of objects. In this presentation, I will show our last results on the photophysics of such nanostructures investigated at the single object level.

International audience; We report on the ultraviolet optical response of a color center in hexagonal boron nitride. We demonstrate a mapping between the vibronic spectrum of the color center and the phonon dispersion in hexagonal boron nitride, with a striking suppression of the phonon assisted emission signal at the energy of the phonon gap. By means of nonperturbative calculations of the electron-phonon interaction in a strongly anisotropic phonon dispersion, we reach a quantitative interpretation of the acoustic phonon sidebands from cryogenic temperatures up to room temperature. Our analysis provides an original method for estimating the spatial extension of the electronic wave function in a point defect.

Single-Wall Carbon Nanotubes (SWNTs) are among the very few candidates for single-photon sources operating in the telecom bands since they exhibit large photon antibunching up to room temperature. However, coupling a nanotube to a photonic structure is highly challenging because of the random location and emission wavelength in the growth process. Here, we demonstrate the realization of a widely tunable single-photon source by using a carbon nanotube inserted in an original repositionable fiber micro-cavity : we fully characterize the emitter in the free-space and subsequently form the cavity around the nanotube. This brings an invaluable insight into the emergence of quantum electrodynamical effects. We observe an efficient funneling of the emission into the cavity mode with a strong sub-Poissonian statistics together with an up to 6-fold Purcell enhancement factor. By exploiting the cavity feeding effect on the phonon wings, we locked the single-photon emission at the cavity frequency over a 4~THz-wide band while keeping the mode width below 80~GHz. This paves the way to multiplexing and multiple qubit coupling.

In the version of this Perspective originally published, the x-axis label of Fig. 1d was missing; it should have read ‘Wavelength (nm)’. The units of the y axis of Fig. 3b were incorrect; they should have been meV. And the citation of Fig. 3c in the main text was incorrect; it should have been to Fig. 3b. These issues have now been corrected.

We performed a thorough and quantitative investigation of spectral diffusion at low temperature in carbon nanotubes wrapped in PFO and embedded in a polystyren matrix, a system of interest for applications for quantum light states generation. We show that an intrinsic line shape typical of exciton-acoustic phonon coupling can be traced back down to the µs time-scale, while spectral diffusion yields a feature-less spectral broadening of the zero-phonon line. By tuning physical parameters such as temperature and pump power we investigate the line-shape deformation at several integration time-scales. Finally, we discuss the implications of these optical properties as regards the efficiency of the static and dynamical coupling of carbon nanotube to optical micro-cavities for single photon generation.

Semiconductor lasers based on two-dimensional photonic crystals generally rely on an optically pumped central area, surrounded by un-pumped, and therefore absorbing, regions. This ideal configuration is lost when photonic-crystal lasers are electrically pumped, which is practically more attractive as an external laser source is not required. In this case, in order to avoid lateral spreading of the electrical current, the device active area must be physically defined by appropriate semiconductor processing. This creates an abrupt change in the complex dielectric constant at the device boundaries, especially in the case of lasers operating in the far-infrared, where the large emission wavelengths impose device thicknesses of several micrometres. Here we show that such abrupt boundary conditions can dramatically influence the operation of electrically pumped photonic-crystal lasers. By demonstrating a general technique to implement reflecting or absorbing boundaries, we produce evidence that whispering-gallery-like modes or true photonic-crystal states can be alternatively excited. We illustrate the power of this technique by fabricating photonic-crystal terahertz (THz) semiconductor lasers, where the photonic crystal is implemented via the sole patterning of the device top metallization. Single-mode laser action is obtained in the 2.55-2.88 THz range, and the emission far field exhibits a small angular divergence, thus providing a solution for the quasi-total lack of directionality typical of THz semiconductor lasers based on metal-metal waveguides.

We report on the nonlinear optical response of a monochiral sample of (6,5) single-wall carbon nanotubes by means of broadband two-color pump-probe spectroscopy with selective excitation of the ${S}_{11}$ excitons. By using a moment analysis of the transient spectra, we show that all the nonlinear features can be accurately accounted for by elementary deformations of the linear absorption spectrum. The photogeneration of ${S}_{11}$ excitons induces a broadening and a blueshift of both the ${S}_{11}$ and ${S}_{22}$ excitonic transitions. In contrast, only the ${S}_{11}$ transition shows a reduction of oscillator strength, ruling out population up-conversion. These nonlinear signatures result from many-body effects, including phase-space filling, wave-function renormalization, and exciton collisions. This framework is sufficient to interpret the magnitude of the observed nonlinearities and stress the importance of intersubband exciton interactions. Remarkably, we show that these intersubband interactions have the same magnitude as the intraband ones and bring the major contribution to the photobleaching of the ${S}_{22}$ excitonic transition upon ${S}_{11}$ excitation through energy shift and broadening.

The emission linewidth is a key figure of merit for a quantum emitter. This paper reports on the possibility of reducing the spectral linewidth of single-wall carbon nanotubes. It demonstrates an order of magnitude narrower linewidths compared to the available data due to the enhanced crystalline quality of the carbon nanotubes synthesized by using a laser ablation technique.

We propose the use of a quantum cascade approach to achieve photon emission from intersubband cavity polariton states. Judicious quantum engineering of the electronic band structure and of the microcavity resonator allows one to obtain emission in the strong light–matter coupling regime under electrical excitation. In particular, we show that, using InP-based multiple quantum well structures, large polariton splittings of about 40 meV can be obtained within the mid-infrared range of the electromagnetic spectrum (intersubband transition energy ≈130 meV, corresponding to λ ≈ 9–10 µm). The calculations show that the effect is robust with respect to substantial changes of the electronic doping density or of the linewidth of the intersubband transition.

Carbon nanotubes have recently proven to be promising nano-emitters for future single-photon sources in the telecom wavelengths [1,2,3]. Coupling the nanotube to a micro-cavity brings a invaluable handle to control single-photon emission, regarding the rate, the yield, the directionnality and the extraction. All those features are related to the so-called Purcell effect that results from the interaction of a quasi-two-level system with a high Q and low mode-volume optical cavity. In carbon nanotubes though, the situation is slightly more complex due to the intimate coupling of the excitonic levels with the acoustic phonon bath, resulting in the well-known phonon wings [4]. This additional degree of freedom brings a very rich physics that can be exploited to enlarge the bandwidth of the source in view of multiplexing. A consequence of the non-Markovian decoherence induced by the phonon bath is the asymetry between the phonon-assisted absorption and emission processes. Using those transitions in the Purcell regime can lead to an enhanced single-photon efficiency above the standard limit imposed by the emitter and cavity intrinsic losses. We developped a widely tunable cavity technique, based on laser-engineered optical fibers to investigate this effect. We measure the single-photon emission properties both in the cw and time-resolved modes and explain the strong asymetry of the efficiency with respect to the detuning. [1] A. Jeantet et al., Phys. Rev. Lett. 116, 247402 (2016) [2] S. Khasminskaya et al., Nat. Phot. 10, 727 (2016) [3] X. Ma, et al., Nat. Mat. 10, 671 (2015) [4] F. Vialla et al., Phys. Rev. Lett. 113, 057402 (2013)

International audience; At low temperature the photoluminescence of single-wall carbon nanotubes show a large variety of spectral profiles ranging from ultra narrow lines in suspended nanotubes to broad and asymmetrical line-shapes that puzzle the current interpretation in terms of exciton-phonon coupling. Here, we present a complete set of photoluminescence profiles in matrix embedded nanotubes including unprecedented narrow emission lines. We demonstrate that the diversity of the low-temperature luminescence profiles in nanotubes originates in tiny modifications of their low-energy acoustic phonon modes. When low energy modes are locally suppressed, a sharp photoluminescence line as narrow as 0.7 meV is restored. Furthermore, multi-peak luminescence profiles with specific temperature dependence show the presence of confined phonon modes.

International audience; Photoluminescence excitation measurements in semi-conducting carbon nanotubes show a systematic non-resonant contribution between the well known excitonic resonances. Using a global analysis method, we were able to delineate the contribution of each chiral species including its tiny non-resonant component. By comparison with the recently reported excitonic absorption cross-section on the S22 resonance, we found a universal non-resonant absorbance which turns out to be of the order of one half of that of an equivalent graphene sheet. This value as well as the absorption line-shape in the non-resonant window is in excellent agreement with microscopic calculations based on the density matrix formalism. This non-resonant absorption of semi-conducting nanotubes is essentially frequency independent over 0.5 eV wide windows and reaches approximately the same value betweeen the S11 and S22 resonances or between the S22 and S33 resonances. In addition, the non-resonant absorption cross-section turns out to be the same for all the chiral species we measured in this study. From a practical point of view, this study puts firm basis on the sample content analysis based on photoluminescence studies by targeting specific excitation wavelengths that lead to almost uniform excitation of all the chiral species of a sample within a given diameter range. In contrast to graphene, single-wall carbon nanotubes (SWNTs) show marked resonances in their optical spectrum that primarily reflect the one-dimensional quantum confinement of carriers. These resonances that combine one-dimensional and excitonic characteristics have been extensively investigated and are widely used as finger prints of the (n, m) species [1]. However, spectroscopic studies reveal that the absorption of nanotubes does not vanish between resonances and consists of a wealth of tiny structures, such as phonon side-bands, crossed exci-tons (S ij), or higher excitonic states [2–5]. In ensemble measurements, the non-resonant absorption is even more congested due to the contribution of residual catalyst or amorphous carbon and due to light scattering [6]. In total , a relatively smooth background showing an overall increase with photon energy is observed, from which it is challenging to extract any quantitative information. In this study, we show that thorough photolumines-cence excitation (PLE) measurements yield a much finer insight into the non-resonant absorption of carbon nan-otubes, that reveals the universal features of light-matter interaction in carbon nano-structures [7]. In particular, we show that the non-resonant absorption of SWNTs per unit area well above the S 11 or S 22 resonances reaches an universal value of 0.013±0.003 in good agreement with the value α √ 3 (where α is the fine structure constant) predicted by a simple band-to-band theory. Our study of non-resonant absorption is based on the global analysis of PLE maps of ensembles of carbon nan-otubes that allows us to deconvolute the contribution of each (n, m) species while keeping a high signal to noise

Carbon nanotubes have strong assets for future integrated single-photon sources since their working wavelength can easily be tuned to the telecom C band by choosing an appropriate diameter [1]. In addition, they show a pronounced antibunching both at low and room temperature[2,3]. Nevertheless, the practical implementation of such a single-photon source requires to embed the nanotube into a micro-cavity in order to funnel the emitted photons into a specific mode (and possibly to an optical fiber) and in order to beat the non-radiative recombination by means of Purcell brightening. In order to tackle the stringent spatial and spectral matching conditions, we developed a flexible micro-cavity design where the concave mirror is micro-engineered at the apex of a single mode optical fiber [4] that can be scanned at the surface of the back mirror where nanotubes has been deposited. By doing so, we are able to observe, both by means of photon counting and time-resolved measurements a Purcell effect of the order of 6, which allows us to funnel 90% of the photons into the mode [5]. In addition, by exploiting the cavity feeding effect on the phonon side-bands, we are able to tune the source over a 5 THz spectral window while keeping a high spectral purity (80 GHz) and an anti-bunching rate better than 0.01. [1] V. Ardizzone et al. Phys. Rev. B 91, 121410 (R) (2015) [2] Hogele et al. Phys. Rev. Lett. 100, 217401 (2008) [3] Ma et al. Nature nano. 10, 671 (2015) [4] Hunger et al. N. J. Phys 12, 065038 (2010) [5] Jeantet et al., arXiv1508:06297 (2015)

We analyze the temperature performance of five terahertz (THz)-frequency quantum cascade lasers based on a three-quantum-well resonant-phonon depopulation design as a function of operating frequency in the 2.3-3.8-THz range. We find evidence that the device performance is limited by the interplay between two factors: 1) optical phonon scattering of thermal electrons, which dominates at shorter wavelengths, and 2) parasitic current, which dominates at longer wavelengths. We present a simple model that provides an accurate estimate of the parasitic current in these devices and predicts the dependence of the threshold current density on temperature.

We report single-mode, surface-emitting, mid-IR, photonic-crystal (PhC), quantum-cascade lasers with linearly polarized and highly directional single-lobed emission. A metallic square-lattice photonic crystal with elliptical air holes and pi phase shift was used as the resonator. The 2D feedback coupling--necessary for the operation of the photonic-crystal resonator--is induced by the mismatch between the modes supported by metalized and nonmetalized regions and yields single-mode output with a side-mode suppression ratio30 dB. The elliptical air holes modify the relative intensities of the TM field components (H(x) and H(y)) in the PhC plane, thus yielding linearly polarized emission. The pi phase shift allows the system to produce a single-lobed pattern in the far field with a narrow divergence angle (2.4 degrees x 1.8 degrees). The emission is perfectly orthogonal to the device surface, and the maximum operating temperature--still limited by the metallic ohmic losses--is 240 K.

We report the transfer of a GHz modulation from an optical carrier at telecom wavelengths to a free space THz beam. THz radiation -tunable from 300 GHz to 1.2 THz- is generated by mixing two telecom lasers with an offset frequency in an ultrafast In 0.53 Ga 0.47 As photoconductive device coupled to a broadband antenna. A maximum modulation rate of 20 GHz has been achieved, and the bandwidth is independent of the carrier frequency.

We show how metallic waveguides offer the opportunity of implementing interesting functionalities for semiconductor lasers within a simple technological approach. In the THz, we show that the active region thickness of quantum cascade lasers can be reduced by a factor of 2 without effects on the threshold current density and maximum operating temperature of the laser. Pulsed and continuous-wave operation - with a low threshold Jth= 71 A/cm2 - are obtained for a 5.86-μm-thick THz QC laser. The emission is peaked at λ≈115 μm and the waveguide resonator is based on a metal-metal geometry. In the mid-infrared, we demonstrate surface-plasmon distributed-feedback quantum cascade lasers with a first-order grating realised by the sole patterning the top metallic contact. The devices have a single mode emission with a side-mode suppression ratio greater than 20dB. The emission wavelength at 78K is centred at λ = 7.3 μm and has tuning rate as a function of the temperature of ≈0.4 nm/K.

We demonstrate single-mode, surface-emitting terahertz lasers at λ≈110 µm employing photonic-crystal resonators. The devices employ metal-metal waveguides, and the photonic-crystal lattice is implemented by the sole patterning of the top metal contact, without deep-etching the semiconductor in the photonic crystal holes. The device emission wavelength correctly tunes with the photonic crystal characteristics. In addition, we elucidate the crucial role played by the boundary conditions of the photonic-crystal resonator.

Here, we report the gigahertz-rate modulation of a tunable terahertz carrier. Terahertz radiation, tunable from 300 GHz to 1.2 THz, is generated by mixing two telecom lasers with an offset frequency in an ultrafast In0.53Ga0.47As photoconductive device coupled to a broadband antenna. The microwave modulation applied to the telecom lasers is directly transferred to the terahertz carrier. A maximum modulation rate of 20 GHz has been achieved, and the bandwidth is independent of the carrier frequency.

We demonstrate terahertz microcavity lasers at lambda=112 mum with ultra-low current thresholds of 4 mA and with mode volumes of less than one-cubic-wavelength. Confinement in the longitudinal direction is obtained using almost-circular micro-disk resonators. The guiding properties of surface-plasmons are exploited to guide the mode with the metal contact. The devices laser up to 70 K in pulsed mode, and up to 60 K in continuous-wave.

Absorption spectroscopy of large ensembles of carbon nanotubes show marked resonances, attributed to the different exitonic levels of each chiral species, as well as a relatively smooth background. This signal consists of a large number of different intrinsic and extrinsic contributions, such as phonon sidebands, amorphous carbon absorption, light scattering... It is therefore challenging to extract any quantitative information from these direct absorbance measurements. In contrast, photoluminescence excitation measurements in semi-conducting nanotubes give access to the intrinsic non-resonant absorption contribution between the excitonic resonances. Using a global analysis method, we were able to delineate the contribution of each chiral species including its tiny non-resonant component that shows up as 0.5 eV wide plateaus between the excitonic resonances. By comparison with the recently reported excitonic absorption cross-section on the S22 resonance, we found a universal non-resonant absorption which turns out to be of the order of one half of that of an equivalent graphene sheet. This value as well as the absorption line-shape in the non-resonant window is in excellent agreement with microscopic calculations. This non-resonant absorption cross-section is the same (within our resolution) for all the chiral species we measured in this study.

Photo-excited electron-hole pairs in carbon nanotubes form strongly bound and localized excitons that can recombine radiatively giving rise to near infrared luminescence signals. Due to the coupling to acoustic phonons, this PL line was predicted to be strongly broadened (even at low temperature), leading to asymmetrical meV-broad phonon sidebands. In addition, due to the peculiar one-dimensional geometry, the (possibly narrow) zero phonon line (ZPL) is completely merged into the phonon wings resulting in an overall broad and asymmetric PL line [1]. This picture was recently puzzled by the observation of ultra narrow PL lines in suspended nanotubes [2,3] raising questions about the electron-phonon coupling mechanisms. Here, we show that ultra narrow PL lines can also be observed for regular micelle wrapped nanotubes deposited on a substrate. However, a high resolution measurement allows to identify attenuated sidebands that are reminiscent of the one-dimensional electron-phonon coupling. This observation allows to interpret both the narrow and the broad lines in terms of modified electron-phonon coupling in a unified framework [4]. [1] Galland et al. PRL 101, 067402 (2008) [2] Hofmann et al., Nature nano. 8, 502 (2013) [3] Sarpkaya et al., Nature comm. 4, 2152 (2013) [4] Vialla et al., submitted

not Available.

The use of integrated photonic structures to tailor the behavior of light is extremely promising for optimizing performance and for introducing advanced functionalities into optoelectronic devices. We demonstrate a powerful method based on photonic-band engineering which allows the optimization of the resonator quality factors of devices operating on band-edge photonic-crystal states. We also show that carefully designed π-shifts in two-dimensional photonic-resonators give enhanced beam properties. The application of these general techniques to terahertz quantum cascade lasers yields improved maximum operating temperatures, and angularly narrow, single-lobed surface emission of ≈12°×8°. The devices operate at ≈2.8/2.9 THz, with peak output powers of 5 mW at 78 K.

We regularly return to the wavelength region between 30 µm and 1 mm to exploit this underdeveloped spectral range.

We report distributed-feedback surface-plasmon quantum cascade lasers operating at λ≈7.6μm. The distributed feedback is obtained by the sole patterning of the top metal contact on a surface plasmon waveguide. Single mode operation with more than 30dB side mode suppression ratio is obtained in pulsed mode and at room temperature. A careful experimental study confirms that by varying the grating duty cycle, one can reduce the waveguide losses with respect to standard, unpatterned surface-plasmon devices. This allows one to reduce the laser threshold current of more than a factor of 2 in the 200–300K temperature range. This approach may lead to a fabrication technology for midinfrared distributed-feedback lasers based on a very simple processing.

Surface emitting photonic-crystal quantum cascade lasers operating at λ≈7.3 μm are demonstrated. The photonic crystal resonator is written solely on the top metallization layer. The mismatch between the modes supported by metallized and nonmetallized regions yields enough optical feedback to achieve laser action. The devices exhibit single-mode emission with a side mode suppression ratio of ≈20 dB, the wavelength is lithographically tunable across a range of almost 70 cm−1, and the radiation is emitted from the surface. The maximum operating temperature is 220 K. The divergence of the output beam, which is doughnut-shaped, is approximately 9°.

We demonstrate a framework to understand and predict the far-field emission in terahertz frequency photonic-crystal quantum cascade lasers. The devices, which employ a high-performance three-well active region, are lithographically tunable and emit in the 104-120 microm wavelength range. A peak output power of 7 mW in pulsed mode is obtained at 10 K, and the typical device maximum operating temperature is 136 K. We identify the photonic-crystal band-edge states involved in the lasing process as originating from the hexapole and monopole modes at the G point of the photonic band structure, as designed. The theoretical far-field patterns, obtained via finite-difference time-domain simulations, are in excellent agreement with experiment. Polarization measurements further support the theory, and the role of the bonding wires in the emission process is elucidated.

Surface plasmons are electromagnetic waves originating from electrons and light oscillations at metallic surfaces. Since freely propagating light cannot be coupled directly into surface-plasmon modes, a compact, semiconductor electrical device capable of generating SPs on the device top metallic surface would represent an advantage: not only SP manipulation would become easier, but Au-metalized surfaces can be easily functionalized for applications. Here, we report a demonstration of such a device. The direct proof of surface-plasmon generation is obtained with apertureless near-field scanning optical microscopy, which detects the presence of an intense, evanescent electric field above the device metallic surface upon electrical injection.

We elucidate the effects of the lateral mode structure on the far field pattern of metal-metal ridge-waveguide terahertz quantum cascade lasers. By introducing a 6-μm-wide metal gap on the top metal contact, we suppress odd-parity lateral modes and drastically modify the far field pattern. Measurements are in good qualitative agreement with full three-dimensional finite-difference-time-domain modeling. Experimental evidence of nonuniform current pumping on the intensity distribution of the guided mode (and hence the far field pattern) is also presented and explained in terms of gain guiding.