Excitons In Carbon Nanotubes Research Articles

Optical coupling of individual air-suspended carbon nanotubes to silicon microcavities.

Carbon nanotubes are a telecom band emitter compatible with silicon photonics, and when coupled to microcavities, they present opportunities for exploiting quantum electrodynamical effects. Microdisk resonators demonstrate the feasibility of integration into the silicon platform. Efficient coupling is achieved using photonic crystal air-mode nanobeam cavities. The molecular screening effect on nanotube emission allows for spectral tuning of the coupling. The Purcell effect of the coupled cavity-exciton system reveals near-unity radiative quantum efficiencies of the excitons in carbon nanotubes.

Evidence for near-unity radiative quantum efficiency of bright excitons in carbon nanotubes from the Purcell effect

The efficiencies of photonic devices are primarily governed by radiative quantum efficiency, which is a property given by the light-emitting material. Quantitative characterization for carbon nanotubes, however, has been difficult despite being a prominent material for nanoscale photonics. Here we estimate the radiative quantum efficiency of bright excitons in carbon nanotubes by modifying the exciton dynamics through cavity quantum electrodynamical effects. Silicon photonic crystal nanobeam cavities are used to induce the Purcell effect on individual carbon nanotubes. Spectral and temporal behavior of the cavity enhancement is characterized by photoluminescence microscopy and the fraction of the radiative decay process is evaluated. We find that the radiative quantum efficiency can reach near unity for bright excitons in long air-suspended carbon nanotubes at room temperature.Received 19 March 2021Revised 27 February 2022Accepted 23 March 2022DOI:https://doi.org/10.1103/PhysRevResearch.4.L022011Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCavity quantum electrodynamicsExcitonsPhysical SystemsNanotubesOptical microcavitiesTechniquesPhotoluminescenceCondensed Matter, Materials & Applied Physics

(Invited) Near-Unity Radiative Quantum Efficiency of Excitons in Carbon Nanotubes

We experimentally determine the radiative quantum efficiency of bright excitons in carbon nanotubes by modifying the exciton dynamics through cavity quantum electrodynamical effects [1]. Specially designed air-mode silicon nanobeam cavities [2,3] are used to obtain efficient coupling to individual air-suspended carbon nanotubes. Spectral and temporal behavior of the cavity enhancement is characterized by photoluminescence spectroscopy at room temperature, and the fraction of the radiative decay process is evaluated. We find that the radiative quantum efficiency is near unity for the bright excitons in suspended nanotubes.Work supported in part by MIC (SCOPE 191503001), JSPS (KAKENHI JP20H02558, JP20J00817, JP20K15199), MEXT (Nanotechnology Platform JPMX09F19UT0072), and RIKEN (Incentive Research Project). H.M. acknowledges support from RIKEN (Junior Research Associate Program). Finite-difference time-domain calculations are performed using HOKUSAI BigWaterfall supercomputer at RIKEN. We thank the Advanced Manufacturing Support Team at RIKEN for technical assistance.[1] H. Machiya, D. Yamashita, A. Ishii, Y. K. Kato, in preparation.[2] R. Miura, S. Imamura, R. Ohta, A. Ishii, X. Liu, T. Shimada, S. Iwamoto, Y. Arakawa, Y. K. Kato, Nat. Commun. 5, 5580 (2014).[3] H. Machiya, T. Uda, A. Ishii, Y. K. Kato, Appl. Phys. Lett. 112, 021101 (2018).

DECaNT: Simulation tool for diffusion of excitons in carbon nanotube films

We present the numerical tool DECaNT (Diffusion of Excitons in Carbon NanoTubes) that simulates exciton transport in thin films of carbon nanotubes. Through a mesh of nanotubes generated using the Bullet Physics C++ library, excitons move according to an ensemble Monte Carlo algorithm, with the scattering rates that account for tube chirality, orientation, and distance. We calculate the diffusion tensor from the position–position correlation functions and analyze its anisotropy and dependence on the film composition, morphology, and defect density.

Finite‐Difference Time‐Domain Simulation of Strong‐Field Ionization: A Perfectly Matched Layer Approach

A finite‐difference time‐domain (FDTD) scheme with perfectly matched layers (PMLs) is considered for solving the time‐dependent Schrödinger equation and simulating the laser‐induced ionization of two systems: 1) an electron initially bound to a 1D δ potential and 2) excitons in carbon nanotubes (CNTs). The performance of PMLs based on different absorption functions is compared, where slowly growing functions are found to be preferable. PMLs are shown to be able to reduce the computational domain, and thus the required numerical resources, by several orders of magnitude. This is demonstrated by testing the proposed method against an FDTD approach without PMLs and a very large computational domain. It is shown that PMLs outperform the well‐known exterior complex scaling (ECS) technique for short‐range potentials when implemented in FDTD. For long‐range potentials such as in CNTs, however, ECS performs better than PMLs when propagating over many periods.

(Invited) Super-Localization of Excitons in Carbon Nanotubes at Cryogenic Temperatures

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.

Dipolar and charged localized excitons in carbon nanotubes

We study both experimentally and theoretically the fundamental interplay of exciton localization and polarization in semiconducting single-walled carbon nanotubes. From Stark spectroscopy of individual carbon nanotubes at cryogenic temperatures, we identify localized excitons as permanent electric dipoles with dipole moments of up to $1\phantom{\rule{3.33333pt}{0ex}}e\AA{}$. Moreover, we demonstrate field-effect doping of localized excitons with an additional charge which results in defect-localized trions. Our findings, in qualitative agreement with theoretical calculations, not only provide fundamental insight into the microscopic nature of localized excitons in carbon nanotubes, they also signify their potential for sensing applications and may serve as guidelines for molecular engineering of exciton-localizing quantum dots in other atomically thin semiconductors including transition metal dichalcogenides.

Radiative Pumping and Propagation of Plexcitons in Diffractive Plasmonic Crystals.

Strong coupling between plasmons and excitons leads to the formation of plexcitons: quasiparticles that combine nanoscale energy confinement and pronounced optical nonlinearities. In addition to these localized modes, the enhanced control over the dispersion relation of propagating plexcitons may enable coherent and collective coupling of distant emitters. Here, we experimentally demonstrate strong coupling between carbon nanotube excitons and spatially extended plasmonic modes formed via diffractive coupling of periodically arranged gold nanoparticles (nanodisks, nanorods). Depending on the light-matter composition, the rather long-lived plexcitons (>100 fs) undergo highly directional propagation over 20 μm. Near-field energy distributions calculated with the finite-difference time-domain method fully corroborate our experimental results. The previously demonstrated compatibility of this plexcitonic system with electrical excitation opens the path to the realization of a variety of ultrafast active plasmonic devices, cavity-assisted energy transport and low-power optoelectronic components.

Purcell-enhanced quantum yield from carbon nanotube excitons coupled to plasmonic nanocavities

Single-walled carbon nanotubes (SWCNTs) are promising absorbers and emitters to enable novel photonic applications and devices but are also known to suffer from low optical quantum yields. Here we demonstrate SWCNT excitons coupled to plasmonic nanocavity arrays reaching deeply into the Purcell regime with Purcell factors (FP) up to FP = 180 (average FP = 57), Purcell-enhanced quantum yields of 62% (average 42%), and a photon emission rate of 15 MHz into the first lens. The cavity coupling is quasi-deterministic since the photophysical properties of every SWCNT are enhanced by at least one order of magnitude. Furthermore, the measured ultra-narrow exciton linewidth (18 μeV) reaches the radiative lifetime limit, which is promising towards generation of transform-limited single photons. To demonstrate utility beyond quantum light sources we show that nanocavity-coupled SWCNTs perform as single-molecule thermometers detecting plasmonically induced heat at cryogenic temperatures in a unique interplay of excitons, phonons, and plasmons at the nanoscale.

Carbon nanotubes: Wiry matter-light coupling.

Electrical injection into polaritons, built from admixing excitons in carbon nanotubes with light in a surrounding microcavity, has been achieved.

(Invited) Environmental Effects in Cryogenic Carbon Nanotube Spectroscopy

The photophysical properties of cryogenic carbon nanotubes are dominated by exciton localization. As exciton localization is commonly accompanied by exciton polarization, localized carbon nanotube excitons are particularly sensitive to electrostatic fluctuations in their immediate environment. We will discuss the realm of environmental effects observed in cryogenic spectroscopy of individual carbon nanotubes in different dielectric environments.

(Invited) Optical Signatures of Carbon Nanotube Excitons Coupled to Plasmonic Nanocavities and Metallo-Dielectric Antennas

Single-walled carbon nanotubes (SWCNTs) have recently gained tremendous interest as a nanomaterial for next generation optoelectronics and quantum photonic devices, in particular through the demonstration of quantum light emission at room temperature in doped SWCNTs [1]. For further advances and to enable practical applications in quantum information processing light enhancement strategies are required. To this end plasmonic nanocavities such as bowtie antennas feature nanometer gaps that can support ultra-small mode volumes which are particularly attractive to enhance the exciton emission of single-walled carbon nanotubes (SWCNTs). In this talk we will discuss several technique to achieve the required spectral, spatial, and orientational resonance between SWCNT excitons and nanoplasmonic modes. Both time integrated and time-resolved measurements reveal characteristic signatures demonstrating pronounced coupling in the Purcell regime. As a test system we employ co-polymer wrapped carbon nanotubes for which we have recently shown that the polymer backbone can lead to efficient acoustic-phonon localization implying long exciton coherence times and ultra-narrow linewidth [2]. In addition, we will discuss SWCNT excitons coupled to metallo-dielectric antennas and evaluate the light collection efficiency enhancements achievable with this approach. Finally, we will discuss the prospects of achieving transform limited single photons in these systems. [1] X. Ma, N.F. Hartmann, J.K.S. Baldwin, S.K. Doorn, and H. Htoon, “Room-temperature single-photon generation from solitary dopants of carbon nanotubes”, Nature Nanotech 10, 671–675 (2015). [2] I. Sarpkaya, Ehsaneh D. Ahmadi, Gabriella D. Shepard, Kevin S. Mistry, Jeffrey L. Blackburn, and S. Strauf, “Strong Acoustic Phonon Localization in Copolymer-Wrapped Carbon Nanotubes”, ACS Nano 9, 6383 (2015).

Controlling signal transport in a carbon nanotube opto-transistor.

With the highly competitive development of communication technologies, modern information manufactures place high importance on the ability to control the transmitted signal using easy miniaturization materials. A controlled and miniaturized optical information device is, therefore, vital for researchers in information and communication fields. Here we propose a controlled signal transport in a doubly clamped carbon nanotube system, where the transmitted signal can be controlled by another pump beam. Pump off results in the transmitted signal off, while pump on results in the transmitted signal on. The more pump, the more amplified output signal transmission. Analogous with traditional cavity optomechanical system, the role of optical cavity is played by a localized exciton in carbon nanotube while the role of the mechanical element is played by the nanotube vibrations, which enables the realization of an opto-transistor based on carbon nanotube. Since the signal amplification and attenuation have been observed in traditional optomechanical system, and the nanotube optomechanical system has been realized in laboratory, the proposed carbon nanotube opto-transistor could be implemented in current experiments and open the door to potential applications in modern optical networks and future quantum networks.

High energetic excitons in carbon nanotubes directly probe charge-carriers.

Theory predicts peculiar features for excited-state dynamics in one dimension (1D) that are difficult to be observed experimentally. Single-walled carbon nanotubes (SWNTs) are an excellent approximation to 1D quantum confinement, due to their very high aspect ratio and low density of defects. Here we use ultrafast optical spectroscopy to probe photogenerated charge-carriers in (6,5) semiconducting SWNTs. We identify the transient energy shift of the highly polarizable S33 transition as a sensitive fingerprint of charge-carriers in SWNTs. By measuring the coherent phonon amplitude profile we obtain a precise estimate of the Stark-shift and discuss the binding energy of the S33 excitonic transition. From this, we infer that charge-carriers are formed instantaneously (<50 fs) even upon pumping the first exciton, S11. The decay of the photogenerated charge-carrier population is well described by a model for geminate recombination in 1D.

(Invited) Photophysics of Localized Excitons in Carbon Nanotubes

Modification of carbon nanotube photophysical properties by charge-carrier doping, chemical side-wall functionalization or temperature-driven localization has emerged as an increasingly studied research topic over the past years. In this context, we will discuss signatures of cryogenic exciton localization in different carbon nanotube materials as well as experimental and theoretical aspects of Stark spectroscopy of individual carbon nanotubes in transverse electric field.

Spaser Made of Graphene and Carbon Nanotubes

Spaser is a nanoscale source of surface plasmons comprising a plasmonic resonator and gain medium to replenish energy losses. Here we propose a carbon-based spaser design in which a graphene nanoflake (GNF) resonator is coupled to a carbon nanotube (CNT) gain element. We theoretically demonstrate that the optically excited CNT can nonradiatively transfer its energy to the localized plasmon modes of the GNF because of the near-field interaction between the modes and the CNT excitons. By calculating the localized fields of the plasmon modes and the matrix elements of the plasmon-exciton interaction, we find the optimal geometric and material parameters of the spaser that yield the highest plasmon generation rate. The results obtained may prove useful in designing robust and ultracompact coherent sources of surface plasmons for plasmonic nanocircuits.

Direct observation of substrate induced exciton in carbon nanotube

We have successfully measured the electroluminescence spectra of a single-walled carbon nanotube (CNT) grown to serpentine shape on quartz substrate. We observe two emission peaks: One locates at 0.85 eV and is identified as the usual E11 exciton peak, and the other locates at slightly higher energy of 0.94 eV with similar symmetrical line shape and comparable intensity. However, the extra peak is substantially wider and it broadens with increasing current at unusually faster speed. We show that the extra peak is not from interband transitions, and ascribe it to a type of exciton induced by the formation of substrate-CNT superlattice. The periodic surface potential of the substrate modulates the CNT band structure, causes degeneracy lifting and band flattering at the Brillouin zone, and generates the higher energy exciton. For confirmation, a similar device is fabricated using amorphous SiO2 substrate to avoid the formation of the superlattice. Indeed, the extra emission peak disappears.

Electronic and optical gap renormalization in carbon nanotubes near a metallic surface

Renormalization of quasiparticles and excitons in carbon nanotubes (CNTs) near a metallic surface has been studied within a many-body formalism using an embedding approach newly implemented in the GW and Bethe-Salpeter methods. The quasiparticle band-gap renormalization in semiconducting CNTs is found to scale as $\ensuremath{-}1/(2{h}_{a})$, with ${h}_{a}$ the apparent nanotube height, and it can exceed half an eV. Also, the binding energy of excitons is reduced dramatically---by as much as $75%$---near the surface. Compensation between quasiparticle and excitonic effects results in small changes in the optical gap. The important role played by the nanotube screening response in establishing these effects is emphasized and a simple electrostatic model with no adjustable parameters explains the results of state-of-the-art calculations and generalizes them to a large variety of CNTs.

Prolonged spontaneous emission and dephasing of localized excitons in air-bridged carbon nanotubes

The bright exciton emission of carbon nanotubes is appealing for optoelectronic devices and fundamental studies of light-matter interaction in one-dimensional nanostructures. However, to date, the photophysics of excitons in carbon nanotubes is largely affected by extrinsic effects. Here we perform time-resolved photoluminescence measurements over 14 orders of magnitude for ultra-clean carbon nanotubes bridging an air gap over pillar posts. Our measurements demonstrate a new regime of intrinsic exciton photophysics with prolonged spontaneous emission times up to T1=18 ns, about two orders of magnitude better than prior measurements and in agreement with values hypothesized by theorists about a decade ago. Furthermore, we establish for the first time exciton decoherence times of individual nanotubes in the time domain and find fourfold prolonged values up to T2=2.1 ps compared with ensemble measurements. These first observations motivate new discussions about the magnitude of the intrinsic dephasing mechanism while the prolonged exciton dynamics is promising for applications.

Brightening of excitons in carbon nanotubes on dimensionality modification

Artificially reducing the effective dimensionality of carbon nanotubes from one to zero dimensions increases the luminescence quantum yield of excitons confined in zero-dimensional-like states to ∼18%, which is over one order of magnitude larger than that of intrinsic one-dimensional excitons (∼1%). This finding will help realize future nanoscale photonic devices.