Optical Nanofiber Research Articles

Photothermal phase modulation in a gas-immersed optical nanofiber

We report the observation and theoretical analysis of photothermal phase modulation in an optical nanofiber (NF) immersed in light-absorbing gas. The phase modulation arises from the contrasting photothermal processes experienced by the mode field within and outside the NF, which exhibits significant wavelength and pressure dependence in the nanoscale waveguides. By designing the diameter of the NF, the photothermal phase modulation can be readily controlled, and a nullification of phase modulation is achieved at a specific wavelength. Experiments on NFs with different diameters demonstrated pressure-controllable phase modulation up to 0.058π rad/mW, potentially allowing the development of nanoscale all-optical modulators and sensors with optimal performance.

Fabrication and characterization of optical nanofiber tips

Fabrication and characterization of optical nanofiber tips

Channeling of fluorescence photons from quantum dots into guided modes of an optical nanofiber tip

We demonstrate the channeling of fluorescence photons from quantum dots (QDs) into guided modes of an optical nanofiber tip (ONFT). We deposit QDs on the ONFT using micro/nano fluidic technology. We measure the photon-counting and emission spectrum of fluorescence photons that are channeled into guided modes of the ONFT. The measured emission spectrum confirms the deposition of QDs on the ONFT. We perform numerical simulations to determine channeling efficiency (η) for the ONFT and a single dipole source (SDS) system. For the radially oriented SDS at the center of the facet of the ONFT, we found the maximum η-value of 44% at the fiber size parameter of 7.16, corresponding to the ONFT radius of 0.71 µm for the emission wavelength at 620 nm. Additionally, we investigate the SDS position dependence in transverse directions on the facet of the ONFT in view of keeping experimental ambiguities. The present fiber inline platform may open new avenues in quantum technologies.

Heterodyne Raman dispersion spectroscopy with harmonic demodulation for hydrogen detection

A heterodyne scheme for stimulated Raman dispersion gas spectroscopy with phase-sensitive harmonic measurement is demonstrated. It is accomplished by optically manipulating the Raman transition through pump modulation and harmonically extracting the associated change on the beatnote generated by the sidebands of a Stokes beam. Theoretical formulation describing the spectral signal is developed and verified experimentally. Experiments on a 5-cm-long, 700-nm-diameter optical nanofiber have demonstrated a noise equivalent concentration of 24 ppm with 99 s integration time and a dynamic range of four orders of magnitude for hydrogen detection. This new approach provides a sensitive spectroscopic tool with the potential for widespread gas sensing applications.

Slot waveguide enhanced asymmetric photonic crystal nanofiber cavity for fiber-coupled single photons

We report a slot waveguide-enhanced asymmetric photonic crystal optical nanofiber (ONF) cavity to realize cavity quantum electrodynamics. We show that the device can strongly enhance the spontaneous emission of a single quantum emitter leading to a Purcell factor as high as 106 and enables single-photon coupling efficiency as high as 86% into fiber-guided modes. The introduction of the slot enhances the Purcell factor by six times as compared to the ONF cavity structure without slot, and the asymmetric cavity design enables unidirectional coupling of single photons. The cavity is designed to minimize the losses leading to a scattering-limited Q-factor and one-pass loss estimated to be 6388 and 1.2%, respectively. This fiber-coupled single-photon device may open advanced possibilities and applications for quantum information processing.

Numerical investigation of plasmon-enhanced emission from a nanofiber coupled single photon emitter

This study explores the enhancement of emission from a single photon emitter in a quantum communication network by coupling the source with an optical nanofiber and leveraging gold nanoparticles for Purcell enhancement. Large Purcell enhancements of more than 50 times were recently reported experimentally, but the understanding of important issues, including the maximum Purcell factor and limits to quantum efficiency due to ohmic losses, is still lacking. Our findings reveal that the reported experimental results are reasonable, and confirm that such composite devices provide a promising route for high-efficiency single photon sources coupled to an optical fiber-based quantum communication network.

Optomechanical feedback cooling of a 5 mm long torsional mode

We report three orders of magnitude optical cooling of the fundamental torsional mode of a 5 mm long, 550 nm diameter optical nanofiber. The rotation of the nanofiber couples to the polarization of guided laser fields. We use a weak laser probe to monitor the rotation and use feedback to modulate the polarization of an auxiliary drive laser providing torque. Our results present a tool for the optomechanical control of large-scale torsional resonators, with metrological applications and potential implications for studying macroscopic objects in quantum states.

Excitation of 87Rb Rydberg atoms to nS and nD states (n≤68) via an optical nanofiber

Cold Rydberg atoms are a promising platform for quantum technologies, and combining them with optical waveguides has the potential to create robust quantum information devices. Here, we experimentally observe the excitation of cold rubidium atoms to a large range of Rydberg S and D states through interaction with the evanescent field of an optical nanofiber. We develop a theoretical model to account for experimental phenomena present such as the AC Stark shifts and the Casimir–Polder interaction. This work strengthens the knowledge of Rydberg atom interactions with optical nanofibers and is a critical step toward the implementation of all-fiber quantum networks and waveguide quantum electrodynamics (QED) systems using highly excited atoms.

Excitation of 87Rb Rydberg atoms to nS and nD states (n≤68) via an optical nanofiber

Cold Rydberg atoms are a promising platform for quantum technologies, and combining them with optical waveguides has the potential to create robust quantum information devices. Here, we experimentally observe the excitation of cold rubidium atoms to a large range of Rydberg S and D states through interaction with the evanescent field of an optical nanofiber. We develop a theoretical model to account for experimental phenomena present such as the AC Stark shifts and the Casimir–Polder interaction. This work strengthens the knowledge of Rydberg atom interactions with optical nanofibers and is a critical step toward the implementation of all-fiber quantum networks and waveguide quantum electrodynamics (QED) systems using highly excited atoms.

Spin-orbit interaction in nanofiber-based Brillouin scattering.

Angular momentum is an important physical property that plays a key role in light-matter interactions, such as spin-orbit interaction. Here, we investigate theoretically and experimentally the spin-orbit interaction between a circularly polarized optical (spin) and a transverse vortex acoustic wave (orbital) using Brillouin backscattering in a silica optical nanofiber. We specifically explore the state of polarization of Brillouin backscattering induced by the TR21 torso-radial vortex acoustic mode that carries an orbital angular momentum. Using a full-vectorial theoretical model, we predict and observe two operating regimes for which the backscattered Brillouin signal is either depolarized or circularly polarized, depending on the input pump polarization. We demonstrate that when the pump is circularly polarized and thus carries a spin angular momentum, the backscattered signal undergoes a handedness reversal of circular polarization due to opto-acoustic spin-orbit interaction and the conservation of overall angular momentum.

Optical nanofiber stretcher.

Piezoelectric stretching of optical fiber is a technique that enables the creation of optical delays of a few picoseconds; this is useful in a variety of applications in interferometry or optical cavities. Most commercial fiber stretchers involve lengths of fiber of a few tens of meters. Using a 120-mm-long optical micro-nanofiber, we can create a compact optical delay line that achieves tunable delays of up to 19 ps at telecommunication wavelengths. The high elasticity of silica and the micron-scale diameter allow this significant optical delay to be achieved with low tensile force while keeping the overall length short. We successfully report both static and dynamic operation of this novel, to the best of our knowledge, device. It could find application in interferometry and laser cavity stabilization, where short optical paths and strong resistance to the environment would be required.

Manipulation of polarization topology using a Fabry–Pérot fiber cavity with a higher-order mode optical nanofiber

Optical nanofiber cavity research has mainly focused on the fundamental mode. Here, a Fabry–Pérot fiber cavity with an optical nanofiber supporting the higher-order modes (TE01, TM01, HE21 o , and HE21 e ) is demonstrated. Using cavity spectroscopy, with mode imaging and analysis, we observed cavity resonances that exhibited complex, inhomogeneous states of polarization with topological features containing Stokes singularities such as C-points, Poincaré vortices, and L-lines. In situ tuning of the intracavity birefringence enabled the desired profile and polarization of the cavity mode to be obtained. We believe these findings open new research possibilities for cold atom manipulation and multimode cavity quantum electrodynamics using the evanescent fields of higher-order mode optical nanofibers.

Collective Radiation of a Cascaded Quantum System: From Timed Dicke States to Inverted Ensembles.

The collective absorption and emission of light by an ensemble of atoms is at the heart of many fundamental quantum optical effects and the basis for numerous applications. However, beyond weak excitation, both experiment and theory become increasingly challenging. Here, we explore the regimes from weak excitation to inversion with ensembles of up to 1000 atoms that are trapped and optically interfaced using the evanescent field surrounding an optical nanofiber. We realize full inversion, with about 80% of the atoms being excited, and study their subsequent radiative decay into the guided modes. The data are very well-described by a simple model that assumes a cascaded interaction of the guided light with the atoms. Our results contribute to the fundamental understanding of the collective interaction of light and matter and are relevant for applications ranging from quantum memories to sources of nonclassical light to optical frequency standards.

Evanescent field trapping and propulsion of Janus particles along optical nanofibers

Small composite objects, known as Janus particles, drive sustained scientific interest primarily targeted at biomedical applications, where such objects act as micro- or nanoscale actuators, carriers, or imaging agents. A major practical challenge is to develop effective methods for the manipulation of Janus particles. The available long-range methods mostly rely on chemical reactions or thermal gradients, therefore having limited precision and strong dependency on the content and properties of the carrier fluid. To tackle these limitations, we propose the manipulation of Janus particles (here, silica microspheres half-coated with gold) by optical forces in the evanescent field of an optical nanofiber. We find that Janus particles exhibit strong transverse localization on the nanofiber and much faster propulsion compared to all-dielectric particles of the same size. These results establish the effectiveness of near-field geometries for optical manipulation of composite particles, where new waveguide-based or plasmonic solutions could be envisaged.

Photothermally Induced Birefringence in an Optical Nanofiber for All‐Optical Polarization Manipulation

AbstractThis study reports the experimental observation of photothermally induced birefringence (PIB) in an optical nanofiber (NF). The PIB originates from the interaction between the azimuthally asymmetric evanescent field of a guided pump beam in mode and the gas molecules surrounding the NF. Light absorption of gas molecules results in azimuthal inhomogeneity in gas density, which induces birefringence of the NF waveguide. The swift heat transfer and molecular transportation in sub‐micrometer scale accounts for the fast response of the PIB with a measured rising and falling time within 9 and 70 ns, respectively. With a 12‐mm‐long NF, phase retardance as large as 3π is achieved. A theoretical model based on the rate equation is also developed for studying the photothermal dynamics and gas kinetics in PIB generation. This novel model is well consolidated by the numerical simulations, with the results agreeing well with the experiments. In consideration of the extremely low insertion loss, fast response, and reconfigurable principal axes of birefringence, the PIB can be used for all‐optical polarization manipulation. In addition, NF with PIB can also be an interesting platform for studies of gas–surface interaction, light–sound interaction controlling, sub‐micrometer scale heat and mass transfer, and molecular spectroscopy.

Bright and Polarized Fiber In-Line Single-Photon Source Based on Plasmon-Enhanced Emission into Nanofiber Guided Modes

We demonstrate a bright and polarized fiber in-line single-photon source based on plasmon-enhanced emission of colloidal single quantum dots into an optical nanofiber. We show that emission properties of single quantum dots can be strongly enhanced in the presence of single gold nanorods leading to a bright and strongly polarized single-photon emission. The single photons are efficiently coupled to guided modes of the nanofiber and eventually to a single-mode optical fiber. The brightness (fiber-coupled photon count rate) of the single-photon source is estimated to be $12.2\ifmmode\pm\else\textpm\fi{}0.6$ MHz, with high single-photon purity [${g}^{2}(0)=0.20\ifmmode\pm\else\textpm\fi{}0.04$] and degree of polarization as high as 94--97 %. The polarized and fiber-coupled single photons can be implemented for potential applications in quantum photonics.

Spin-orbit interaction through Brillouin scattering in nanofibers

Spin-orbit interactions (SOI), describing the transfer of a spin degree of freedom to an orbital angular momentum (OAM), have been widely explored in recent opto-acoustic studies for applications mainly in spintronics and for topological insulators [1]. We report the observation of SOI by Brillouin scattering in an optical nanofiber. Specifically, we describe the transfer of a spin degree of freedom from light incident to the nanofiber to an acoustic vortex with a topological charge of order 2 in the form of OAM. Coupled with the phase matching condition for the energy conservation during Brillouin scattering, it results in a backscattered wave with a spin opposite to the incident wave. This observation allows considering applications of opto-acoustic Brillouin memory based on polarization conversion through a SOI [2].

Plasmon-enhanced single photon source directly coupled to an optical fiber

A bright source of fiber-coupled, polarized single photons is an essential component of any realistic quantum network based on today's existing fiber infrastructure. Here, we develop a Purcell-enhanced, polarized source of single photons at room temperature by coupling single colloidal quantum dots to the localized surface plasmon-polariton modes of single gold nanorods, combined on the surface of an optical nanofiber. A maximum enhancement of the photon emission rate of 62 times was measured corresponding to a degree of polarization of $85%$, and a brightness enhancement of four times in the fiber mode. Evanescent coupling of photons to the nanofiber guided modes ensures automatic coupling to a single mode fiber. Our technique opens the way to realizing bright sources of polarized single photons connected to fiber networks using a simple composite technique.

Synthesis, characterization, TD-DFT method, and optical properties of novel nanofiber conjugated polymer

The nanofiber polyimine blend [1 H-Pyr+Ben]B thin films were fabricated from pyrrole-2,5-dicarbaldehyde [1 H-Pyr] and benzidine [Ben] by placing a glass substrate in front of the target of an electro spinner. Fourier Transform Infrared Spectroscopy (FTIR), UV-Vis optical properties, and laser photoluminescence PL characterization techniques were used to study [1 H-Pyr+Ben]B films. In addition, time-dependent density functional theory (TD-DFT), optimization via TD-DFTD/Mol3, and Cambridge Serial Total Energy Bundle (TD-FDT/CASTEP) were used to perform the geometrical study. FTIR spectra from TD-DFT indicate the interaction between two monomers to form blended polyimine nanofiber thin films. The molecular weight of the produced polymer was evaluated using the static light scattering method and the Surface morphology of the synthesized polyimine. In the wavelength range of 190 – 800 nm, the optical properties of [1 H-Pyr+Ben]B were considered. The direct energy band gap was found to be changed from 2.31 eV for [1 H-Pyr+Ben]B to 1.49 eV for the indirect energy band gap. The refractive index nmax= 3.07 eV is 1.45 for the nanofiber [1 H-Pyr+Ben]B thin film. The emission peak at λmax=559.90 nm was figured in the laser photoluminescence spectra of [1 H-Pyr+Ben]B films. The investigated optical energy bandgap nanofiber blend is advantageous for some energy storage and solar cell applications.

Optimization of nanofiber gratings for efficient single-photon collection

We report on a simulation of a nanophotonic cavity constructed by designing periodic holes on an optical nanofiber to realize light–matter interaction. The cavity is designed using finite-difference time-domain simulations to maximize the coupling of spontaneous emission from a quantum emitter into fiber-guided modes. We systematically analyze the dependence of spontaneous emission on the quantum emitter position, polarization, and the grating strength (number of periods). We show that coupling efficiencies as high as 87% and 83% can be realized for a dipole emitter placed at the center of the nanofiber with polarization perpendicular (x-pol) and parallel (y-pol) to the hole-axis, respectively. This system may attract various quantum photonic applications based on single-photon sources.