Room-temperature Single-photon Emission Research Articles

Hexagonal Boron Nitride for Next-Generation Photonics and Electronics.

Hexagonal boron nitride (h-BN), an insulating 2D layered material, has recently attracted tremendous interest motivated by the extraordinary properties it shows across the fields of optoelectronics, quantum optics, and electronics, being exotic material platforms for various applications. At an early stage of h-BN research, it isexplored as an ideal substrate and insulating layers for other 2D materials due to its atomically flat surface that is free of dangling bonds and charged impurities, and its high thermal conductivity. Recent discoveries of structural and optical properties of h-BN have expanded potential applications into emerging electronics and photonics fields. h-BN shows a very efficient deep-ultraviolet band-edge emission despite its indirect-bandgap nature, as well as stable room-temperature single-photon emission over a wide wavelength range, showing a great potential for next-generation photonics. In addition, h-BN is extensively being adopted as active media for low-energy electronics, including nonvolatile resistive switching memory, radio-frequency devices, and low-dielectric-constant materials for next-generation electronics.

Room temperature single-photon emission from InGaN quantum dot ordered arrays in GaN nanoneedles

GaN-based single-photon sources have received immense attention for applications in quantum technologies. An isolated semiconductor quantum dot (QD) is an attractive and proven choice. Most experimental demonstrations involve epitaxial growth or etching of the QD embedded in a vertical nanopillar/nanowire structure. Here, we demonstrate room-temperature single-photon emission from an InGaN QD embedded in a GaN nanoneedle. The nanoneedle is tapered at the bottom and is formed by a succession of inductively coupled plasma reactive ion etching and crystallographic direction-dependent wet etching techniques. The nanofabrication process steps yield reproducible and uniform-sized QDs in the needle structures. Cross-sectional SEM images show needles are centered at the center of a hexagonal base, which confirms a good crystalline property of the QD. Micro-photoluminescence measurements on a single QD manifest a short time constant for radiative decay channels without any prominent non-radiative decay path. Second-order correlation measurements confirm the antibunching of the emitted photons. Higher spectral purity and smaller value of the second-order correlation are maintained up to a good excitation power, indicating the usefulness of the methodology for quantum technologies.

Photochemical spin-state control of binding configuration for tailoring organic color center emission in carbon nanotubes

Incorporating fluorescent quantum defects in the sidewalls of semiconducting single-wall carbon nanotubes (SWCNTs) through chemical reaction is an emerging route to predictably modify nanotube electronic structures and develop advanced photonic functionality. Applications such as room-temperature single-photon emission and high-contrast bio-imaging have been advanced through aryl-functionalized SWCNTs, in which the binding configurations of the aryl group define the energies of the emitting states. However, the chemistry of binding with atomic precision at the single-bond level and tunable control over the binding configurations are yet to be achieved. Here, we explore recently reported photosynthetic protocol and find that it can control chemical binding configurations of quantum defects, which are often referred to as organic color centers, through the spin multiplicity of photoexcited intermediates. Specifically, photoexcited aromatics react with SWCNT sidewalls to undergo a singlet-state pathway in the presence of dissolved oxygen, leading to ortho binding configurations of the aryl group on the nanotube. In contrast, the oxygen-free photoreaction activates previously inaccessible para configurations through a triplet-state mechanism. These experimental results are corroborated by first principles simulations. Such spin-selective photochemistry diversifies SWCNT emission tunability by controlling the morphology of the emitting sites.

Quantum Light Emission from Coupled Defect States in DNA-Functionalized Carbon Nanotubes.

Solid-state single-photon sources are essential building blocks for quantum photonics and quantum information technologies. This study demonstrates promising single-photon emission from quantum defects generated in single-wall carbon nanotubes (SWCNTs) by covalent reaction with guanine nucleotides in their single-stranded DNA coatings. Low-temperature photoluminescence spectroscopy and photon-correlation measurements on individual guanine-functionalized SWCNTs (GF-SWCNTs) indicate that multiple, closely spaced guanine defect sites within a single ssDNA strand collectively form an exciton trapping potential that supports a localized quantum state capable of room-temperature single-photon emission. In addition, exciton traps from adjacent ssDNA strands are weakly coupled to give cross-correlations between their separate photon emissions. Theoretical modeling identifies coupling mechanism as a capture of band-edge excitons. Because the spatial pattern of nanotube functionalization sites can be readily controlled by selecting ssDNA base sequences, GF-SWCNTs should become a versatile family of quantum light emitters with engineered properties.

(Invited) Quantum Light Emission from Coupled Defect-States in DNA-Functionalized Carbon Nanotubes

Solid-state single-photon sources are the essential building block for quantum photonics and quantum information technologies. The report here illustrates an advanced perspective of implanting coupled fluorescent quantum defects in single-wall carbon nanotubes (SWCNTs) through chemical reactions between the nanotube sidewall and the guanine nucleotides in single-stranded DNA (ssDNA) that coats the nanotube. Through low-temperature photoluminescence spectroscopy and photon correlation experiments, we have revealed that multiple guanine defects within a single ssDNA strand couple together to form a cumulative deep trapping potential supporting a localized quantum state that is capable of room-temperature single-photon emission. Helical wrapping of multiple ssDNA strands in single nanotubes allows a series of exciton localization sites to be created along the length of the SWCNTs. Such unique exciton trapping potential landscape gives rise to previously inaccessible intrinsic emission characteristics including coupling of multiple spatially separated quantum emitters. Our joint experimental and computational studies together reveal quantum emission properties from multiple spatially patterned covalent defects in SWCNTs, identify capture of the band-edge exciton as the underlying cause of the cross-correlated photon emission and open a distinctive prospect for tailoring chemically altered nanotubes as quantum light emitters.

Bright room temperature near-infrared single-photon emission from single point defects in the AlGaN film

Near-infrared (NIR) single-photon source plays a key role in a wide range of applications in quantum technology. In particular, in quantum communication, the NIR wavelength operation perfectly matches the relatively low-attenuation transmission window of the optical fiber, which attracts more and more research interest. Here, we report the room temperature single-photon emission from single point defects in the aluminum gallium nitride (AlGaN) film. The obtained single-photon emission covers from 720 to 930 nm and exhibits highly linear polarization and high photon brightness. This may provide a platform for future integrated on-chip quantum photonic devices.

Understanding radiative transitions and relaxation pathways in plexcitons

Summary Molecular aggregates on plasmonic nanoparticles have emerged as attractive systems for the studies of polaritonic light-matter states, called plexcitons. Such systems are tunable, scalable, easy to synthesize, and offer sub-wavelength confinement, all while giving access to the ultrastrong light-matter coupling regime, promising a plethora of applications. However, the complexity of these materials prevented the understanding of their excitation and relaxation phenomena. Here, we follow the relaxation pathways in plexcitons and conclude that while the metal destroys the optical coherence, the molecular aggregate coupled to surface processes significantly contributes to the energy dissipation. We use two-dimensional electronic spectroscopy with theoretical modeling to assign the different relaxation processes to either molecules or metal nanoparticle. We show that the dynamics beyond a few femtoseconds has to be considered in the language of hot electron distributions instead of the accepted lower and upper polariton branches and establish the framework for further understanding.

Controlling Defect-State Photophysics in Covalently Functionalized Single-Walled Carbon Nanotubes.

ConspectusSingle-walled carbon nanotubes (SWCNTs) show promise as light sources for modern fiber optical communications due to their emission wavelengths tunable via chirality and diameter dependency. However, the emission quantum yields are relatively low owing to the existence of low-lying dark electronic states and fast excitonic diffusion leading to carrier quenching at defects. Covalent functionalization of SWCNTs addresses this problem by brightening their infrared emission. Namely, introduction of sp3-hybridized defects makes the lowest energy transitions optically active for some defect geometries and enables further control of their optical properties. Such functionalized SWCNTs are currently the only material exhibiting room-temperature single photon emission at telecom relevant infrared wavelengths. While this fluorescence is strong and has the right wavelength, functionalization introduces a variety of emission peaks resulting in spectrally broad inhomogeneous photoluminescence that prohibits the use of SWCNTs in practical applications. Consequently, there is a strong need to control the emission diversity in order to render these materials useful for applications. Recent experimental and computational work has attributed the emissive diversity to the presence of multiple localized defect geometries each resulting in distinct emission energy. This Account outlines methods by which the morphology of the defect in functionalized SWCNTs can be controlled to reduce emissive diversity and to tune the fluorescence wavelengths. The chirality-dependent trends of emission energies with respect to individual defect morphologies are explored. It is demonstrated that defect geometries originating from functionalization of SWCNT carbon atoms along bonds with strong π-orbital mismatch are favorable. Furthermore, the effect of controlling the defect itself through use of different chemical groups is also discussed. Such tunability is enabled due to the formation of specific defect geometries in close proximity to other existing defects. This takes advantage of the changes in π-orbital mismatch enforced by existing defects and the resulting changes in reactivities toward formation of specific defect morphologies. Furthermore, the trends in emissive energies are highly dependent on the value of mod(n-m,3) for an (n,m) tube chirality. These powerful concepts allow for a targeted formation of defects that emit at desired energies based on SWCNT single chirality enriched samples. Finally, the impact of functionalization with specific types of defects that enforce certain defect geometries due to steric constraints in bond lengths and angles to the SWCNT are discussed. We further relate to a similar effect that is present in systems where high density of surface defects is formed due to high reactant concentration. The outlined strategies suggested by simulations are instrumental in guiding experimental efforts toward the generation of functionalized SWCNTs with tunable emission energies.

Room-temperature single photon emitters in cubic boron nitride nanocrystals

Color centers in wide bandgap semiconductors are attracting broad attention for use as platforms for quantum technologies relying on room-temperature single-photon emission (SPE), and for nanoscale metrology applications building on the centers’ response to electric and magnetic fields. Here, we demonstrate room-temperature SPE from defects in cubic boron nitride (cBN) nanocrystals, which we unambiguously assign to the cubic phase using spectrally resolved Raman imaging. These isolated spots show photoluminescence (PL) spectra with zero-phonon lines (ZPLs) within the visible region (496–700 nm) when subject to sub-bandgap laser excitation. Second-order autocorrelation of the emitted photons reveals antibunching with g2(0) ∼ 0.2, and a decay constant of 2.75 ns that is further confirmed through fluorescence lifetime measurements. The results presented herein prove the existence of optically addressable isolated quantum emitters originating from defects in cBN, making this material an interesting platform for opto-electronic devices and quantum applications.

Mod(n-m,3) Dependence of Defect-State Emission Bands in Aryl-Functionalized Carbon Nanotubes.

Molecularly functionalized single-walled carbon nanotubes (SWCNTs) are potentially useful for fiber optical applications due to their room temperature single-photon emission capacity at telecommunication wavelengths. Several distinct defect geometries are generated upon covalent functionalization. While it has been shown that the defect geometry controls electron localization around the defect site, thereby changing the electronic structure and generating new optically bright red-shifted emission bands, the reasons for such localization remain unexplained. Our joint experimental and computational studies of functionalized SWCNTs with various chiralities show that the value of mod(n-m,3) in an (n,m) chiral nanotube plays a key role in the relative ordering of defect-dependent emission energies. This dependence is linked to the complex nodal characteristics of electronic wave function extending along specific bonds in the tube, which justifies the defect-geometry dependent exciton localization. This insight helps to uncover the essential structural motifs allowing tuning the redshifts of emission energies in functionalized SWCNTs.

Photochemical Patterning of Fluorescent sp3 Quantum Defects on Carbon Nanotube Thin Films

Fluorescent sp3 quantum defects are an emerging subject of interest due to their remarkable optical properties, including chemically tunable emission in the shortwave infrared, environmentally selective sensitivity, and room-temperature single-photon emission. Many potential applications, such as bioimaging, chemical sensing, and quantum information processing, will benefit from the ability to place different types of defects at locations where, and only where, they are needed. In this presentation, we will discuss our progress in patterning fluorescent sp3 defects on thin films of single-walled carbon nanotubes. We show it is possible to trigger the defect-generating reactions by exciting the nanotubes with light to create arrays and artificial patterns of fluorescent defects.

Narrow-band single-photon emission through selective aryl functionalization of zigzag carbon nanotubes.

The introduction of sp3 defects into single-walled carbon nanotubes through covalent functionalization can generate new light-emitting states and thus dramatically expand their optical functionality. This may open up routes to enhanced imaging, photon upconversion, and room-temperature single-photon emission at telecom wavelengths. However, a significant challenge in harnessing this potential is that the nominally simple reaction chemistry of nanotube functionalization introduces a broad diversity of emitting states. Precisely defining a narrow band of emission energies necessitates constraining these states, which requires extreme selectivity in molecular binding configuration on the nanotube surface. We show here that such selectivity can be obtained through aryl functionalization of so-called 'zigzag' nanotube structures to achieve a threefold narrowing in emission bandwidth. Accompanying density functional theory modelling reveals that, because of the associated structural symmetry, the defect states become degenerate, thus limiting emission energies to a single narrow band. We show that this behaviour can only result from a predominant selectivity for ortho binding configurations of the aryl groups on the nanotube lattice.

Magneto-PL Spectroscopy in Aryl Functionalized CNTs

Recent developments in single wall carbon nanotube (SWCNT) functionalization chemistry introducing sp 3 defect sites via covalent attachment of chemical functional groups have made SWCNTs a promising material system for a variety of technologies. These defect states boost photoluminescence (PL) quantum yields and open new possibilities for SWCNTs as unique photon sources in emerging applications. In 2015, solitary oxygen defect states incorporated onto SWCNTs were shown to serve as room temperature (RT) single photon emitters at ~1300 nm [1]. More recently, this RT single photon emission (SPE) was further extended to cover 1550 nm telecommunication wavelength band via aryl sp 3 defect sites in SWCNTs of larger diameters [2]. The SPE of aryl sp 3 are also characterized with ultrahigh single-photon purity and enhanced emission stability approaching the shot-noise limit. While these properties make sp3 defects of SWCNTs very promising material of quantum information technology, establishing the functionalized SWCNTs as transformative materials for quantum technologies demands a detailed understanding and control of their fine electronic structure and quantum coherence properties. Aiming to meet this challenge here, we performed single defect magneto-PL optical spectroscopy on sp 3 defects of aryl functionalized SWCNT. Our work reveals indications that the defects possess complex excitonic fine structure that may open a route toward manipulating spin and/or angular momentum of the defect bound exciton for quantum information technology applications.

Constraining Photoluminescent Defect States in Chirality-Sorted Covalently Doped Single-Walled Carbon Nanotubes

Recent developments in carbon nanotube surface chemistry are leading to the emergence of unexpected new optical behaviors. Specifically, introduction of covalently-bound defect sites into the nanotube structure generates a new photoluminescent (PL) emitting state that is significantly red-shifted from the normal excitonic emitting state that results from optical excitation.1 These new states have been shown to significantly increase emission quantum yields and introduce new functionality including room-temperature single photon emission, near-IR photon upconversion, and multifunctionality generated by localization of excitons at these defect sites.2,3,4 The new optical behaviors present a broad range of unexplored photophysics yet to be fully understood. Of particular interest is the use of emerging functionalization chemistries to introduce sp3 defects that have highly tunable properties. An important challenge arising from the functionalization chemistry is that seemingly similar and structurally simple functionalization can give rise to a broad diversity in observed PL spectral features.1,3,5 Low-temperature spectroscopy along with quantum chemistry calculations demonstrated that these multiple emissive defect states are likely due to the occurrence of multiple possible, chemically distinct, binding configurations on the nanotube surface.6 Following these results, we hypothesize that the diverse emissive states are also influenced by the interplay of nanotube chirality, structure of the dopant, and surface structures on the tube. In this regard, we aim to control the number of emissive states through control of these factors. We have studied a number of different chiral structures for aryldiazonium functionalization, ranging from so-called near-armchair to near-zigzag structures of varying diameter. We have demonstrated that zigzag carbon nanotubes tend to constrain photoluminescent defect states more efficiently than other chiralities. Density functional theory (DFT) calculations suggest that symmetry of the zigzag tube is the primary factor in creating a less diversified emission spectrum. Our findings on controlling the defect peak diversity could enhance the predictable functionality of these doped carbon nanotubes.

(Invited) Aryl-Functionalized Single-Walled Carbon Nanotubes Embedded into Metallo-Dielectric Antennas

Creating exciton localization via covalent sp3 functionalization of single-walled carbon nanotubes (SWCNTs) is a promising route to enhance the optical quantum yield and can furthermore lead to single-photon emission at room temperatures [1]. Of recent interest are aryl-functionalized SWCNTs that are protected by polyfluorene, for which we have recently shown that a variety of chemically and energetically distinct defect state exist that have their origin in topological variation in the chemical binding configuration of the monovalent aryl groups [2]. Here we extend our study by embedding aryl-functionalized SWCNTs into metallo-dielectric antennas that provide broadband coupling with near-unity light collection efficiencies by trapping air gaps on chip that form cavity modes. Scalable implementation is realized by employing polymer layer dry-transfer techniques that avoid solvent incompatibility issues, as well as a planar design that avoids solid-immersion lenses, resulting in a narrow output cone of ±15° that enables a priori fiber-to-chip butt coupling. In these devices the exciton emission from the aryl-functionalized sides (E11*) remains spectrally sharp with linewidth values near the resolution limit of about 35 meV. Interestingly, we find that the pump-induced dephasing for E11* exciton (0D exciton) is significantly reduced by more than 300% as compared to the E11 exciton (1D exciton), thereby preserving exciton coherence even into the high pump power regime, as needed for practical devices in quantum photonics. We will discuss how these findings provide a foundation to build unified descriptions on emergence of novel optical behavior from the interplay of covalent dopants, dispersants, and excitons in SWCNTs. [1] He, X. et al. Tunable room-temperature single-photon emission at telecom wavelengths from sp3 defects in carbon nanotubes. Nature Photonics 11, 577–582 (2017). [2] He, X. et al. Low-Temperature Single Carbon Nanotube Spectroscopy of sp3 Quantum Defects, ACS Nano, Articles ASAP (2017); DOI:10.1021/acsnano.7b03022

Room-temperature single-photon emitters in titanium dioxide optical defects.

Fluorescence properties of crystallographic point defects within different morphologies of titanium dioxide were investigated. For the first time, room-temperature single-photon emission in titanium dioxide optical defects was discovered in thin films and commercial nanoparticles. Three-level defects were identified because the g(2) correlation data featured prominent shoulders around the antibunching dip. Stable and blinking photodynamics were observed for the single-photon emitters. These results reveal a new room-temperature single-photon source within a wide bandgap semiconductor.

Single-Photon Emitters in Boron Nitride Nanococoons.

Quantum emitters in two-dimensional hexagonal boron nitride (hBN) are attractive for a variety of quantum and photonic technologies because they combine ultra-bright, room-temperature single-photon emission with an atomically thin crystal. However, the emitter's prominence is hindered by large, strain-induced wavelength shifts. We report the discovery of a visible-wavelength, single-photon emitter (SPE) in a zero-dimensional boron nitride allotrope (the boron nitride nanococoon, BNNC) that retains the excellent optical characteristics of few-layer hBN while possessing an emission line variation that is lower by a factor of 5 than the hBN emitter. We determined the emission source to be the nanometer-size BNNC through the cross-correlation of optical confocal microscopy with high-resolution scanning and transmission electron microscopy. Altogether, this discovery enlivens color centers in BN materials and, because of the BN nanococoon's size, opens new and exciting opportunities in nanophotonics, quantum information, biological imaging, and nanoscale sensing.

Tunable and high-purity room temperature single-photon emission from atomic defects in hexagonal boron nitride

Two-dimensional van der Waals materials have emerged as promising platforms for solid-state quantum information processing devices with unusual potential for heterogeneous assembly. Recently, bright and photostable single photon emitters were reported from atomic defects in layered hexagonal boron nitride (hBN), but controlling inhomogeneous spectral distribution and reducing multi-photon emission presented open challenges. Here, we demonstrate that strain control allows spectral tunability of hBN single photon emitters over 6 meV, and material processing sharply improves the single photon purity. We observe high single photon count rates exceeding 7 × 106 counts per second at saturation, after correcting for uncorrelated photon background. Furthermore, these emitters are stable to material transfer to other substrates. High-purity and photostable single photon emission at room temperature, together with spectral tunability and transferability, opens the door to scalable integration of high-quality quantum emitters in photonic quantum technologies.

Nanotube chemistry tunes light

Room-temperature single-photon emission at several wavelengths in the near-infrared, including the telecom window, is realized by organic colour centres chemically implanted on chirality-defined single-walled carbon nanotubes.

Tunable room-temperature single-photon emission at telecom wavelengths from sp3 defects in carbon nanotubes

Generating quantum light emitters that operate at room temperature and at telecom wavelengths remains a significant materials challenge. To achieve this goal requires light sources that emit in the near-infrared wavelength region and that, ideally, are tunable to allow desired output wavelengths to be accessed in a controllable manner. Here, we show that exciton localization at covalently introduced aryl sp3 defect sites in single-walled carbon nanotubes provides a route to room-temperature single-photon emission with ultrahigh single-photon purity (99%) and enhanced emission stability approaching the shot-noise limit. Moreover, we demonstrate that the inherent optical tunability of single-walled carbon nanotubes, present in their structural diversity, allows us to generate room-temperature single-photon emission spanning the entire telecom band. Single-photon emission deep into the centre of the telecom C band (1.55 µm) is achieved at the largest nanotube diameters we explore (0.936 nm). Single-photon emission with 99% purity is generated from sp3 defects in carbon nanotubes (CNTs) by optical excitation at room temperature. By increasing the CNT diameter from 0.76 nm to 0.94 nm, the emission wavelength can be changed from 1,100 nm to 1,600 nm.