Nanofiber Cavity Research Articles

Phase-engineered photon correlations in weakly coupled nanofiber cavity QED

Phase-engineered photon correlations in weakly coupled nanofiber cavity QED

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.

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.

Strong photon blockade in an all-fiber emitter-cavity quantum electrodynamics system

We present an all-fiber emitter-cavity quantum electrodynamics (QED) scheme which makes it possible to realize the strong photon blockade phenomenon in the strong coupling regime. To begin with, we study the emitter-cavity system where the two-level emitter and cavity are driven by a laser field under different driving strengths, respectively. Then, we theoretically derive that through driving the single-mode nanofiber cavity and a single two-level emitter simultaneously with two classical fields, the photon antibunching effect and the intracavity mean photon number will be tremendously enhanced. Compared with the systems driving an emitter or the cavity, respectively, the second-order correlation function at zero time decay ${g}^{(2)}(0)$ in our system can reach the minimum value as 0.0003, which is much smaller than a single-emitter-cavity QED system of emitter drive or cavity drive. In particular, under the condition of quantum interference model, the strong phenomenon of photon antibunching can be flexibly reached by controlling of the intensity relationship between the dual-pumping fields in the strong coupling regime. Our proposal can be achieved in the near future and has great potential applications in high-quality single-photon sources and quantum communication processing.

Single air-mode resonance photonic crystal nanofiber cavity for ultra-high sensitivity refractive index sensing

We propose a design of series-connected one-dimensional photonic crystal nanofiber cavity sensor (1-D PC-NCS) and one-dimensional photonic crystal nanofiber bandgap filter (1-D PC-NBF). The proposed structure can get a single air mode for refractive index sensing with its extinction ratio of 58.64 dB. It filters out the high order mode and reduces the interaction between signals. By 3D FDTD, the calculated sensitivity is 848.18 nm/RIU (RIU – refractive index unit). Compared with general silicon on-chip nanobeam cavity, the sensitivity is increased by eight times. The additional 1-D PC-NBF will not change the sensitivity and the position of the resonance wavelength. Therefore, the new design we propose addresses the issue of crosstalk, and can be applied to ultra-high sensitivity index-based gas sensing and biosensing without the need for complicated coupling systems.

Real-Time Observation of Single Atoms Trapped and Interfaced to a Nanofiber Cavity.

We demonstrate efficient interfacing of individually trapped single atoms to a nanofiber cavity. The cavity is formed by fabricating photonic crystal structures directly on the nanofiber using femtosecond laser ablation. The single atoms are interfaced to the nanofiber cavity using an optical tweezer based side-illumination trapping scheme. We show that the fluorescence of individual single atoms trapped on the nanofiber cavity can be readily observed in real-time through the fiber guided modes. From the photon statistics measured for different cavity decay rates, the effective coupling rate of the atom-cavity interface is estimated to be 34±2 MHz. This yields a cooperativity of 5.4±0.6 (Purcell factor=6.4±0.6) and a cavity enhanced channeling efficiency as high as 85±2% for a cavity mode with a finesse of 140. The trap lifetime is measured to be 52±5 ms. These results may open new possibilities for deterministic preparation of single atom events for quantum photonics applications on an all-fiber platform.

Photothermal tuning and stabilization of a photonic crystal nanofiber cavity.

We report the photothermal properties of a photonic crystal nanofiber (PhCN) cavity, which is fabricated using a femtosecond laser ablation technique, under ultra-high vacuum conditions. The results show that by launching a non-resonant guided light, the stopband of the PhCN, along with the cavity modes, can be tuned at a rate of 250GHz (∼0.5 nm)/mW. Moreover, due to the thermal self-locking effect of a resonant light, a cavity mode with a finesse of 25 can be tuned over a few free spectral ranges using only a few μW of guided light. As an enabling step, we demonstrate a dual-mode locking scheme to thermally stabilize a cavity mode to the atomic line for single atom-based cavity quantum electrodynamics experiments.

Coherent interaction of orthogonal polarization modes in a photonic crystal nanofiber cavity.

We show that coherent interaction between two sets of multiple resonances leads to exotic resonant effects, such as Fano-type resonances, optical analogue of electro-magnetically induced transparency, and avoided crossing between modes, under different coupling regimes. We experimentally demonstrate such resonant effects in a photonic crystal nanofiber cavity using two sets of cavity modes with orthogonal polarizations. The interaction between the modes arises due to intra-cavity polarization mixing. The observed line shapes are reproduced using a multiple-mode interaction model. Such spectral characteristics may further enhance the capabilities of the nanofiber cavity as a fiber-in-line platform for nanophotonics and quantum photonics applications.

High-Q, low-index-contrast photonic crystal nanofiber cavity for high sensitivity refractive index sensing.

We present the design of simultaneous high-quality (Q)-factor and high-sensitivity (S) photonic crystal nanofiber cavities (PCNFCs) made of single silica nanofiber that have a low-index contrast (ratio=1.45). By using the three-dimensional finite-difference time-domain method, two different resonant modes, dielectric mode (DM) and air mode (AM), are designed and optimized to achieve an ultrahigh figure of merit (FOM), respectively. Numerical simulations are performed to study the Q-factors and sensitivities of the proposed PCNFCs. It shows that for both DM- and AM-based PCNFCs, respectively, the Q-factors and sensitivities of Q∼1.1×107, S=563.6 nm/RIU and Q∼2.1×105, S=736.8 nm/RIU can be estimated, resulting in FOMs as high as 4.31×106 and 1.13×105, respectively. To the best of our knowledge, this is the first silica nanofiber cavity geometry that simultaneously features high Q and high S for both DM and AM in PCNFCs. Compared with the state of the art of nanofiber-based cavities, the cavity Q-factor to mode volume (V) ratio (Q/V) in this work has been improved more than two orders of magnitude. The demonstration of a high Q/V cavity in low-index-contrast nanofibers can open up versatile applications using a broad range of functional and flexible fibers. Moreover, due to the extended evanescent field and small mode volumes, the proposed PCNFCs are ideal platforms for remote ultra-sensitive refractive-index-based gas sensing without the need for complicated coupling systems.

Humidity induced inhibition and enhancement of spontaneous emission of dye molecules in a single PEG nanofiber

Intrinsic properties of a polyethylene glycol nanofiber are utilized for the first time to dynamically modify the spontaneous emission rate of encapsulated boradiazaindacene dye molecules with fluorescence lifetime imaging. Nanofibers, fabricated by the electrospinning technique, are exposed to relative humidity up to 80%. The spontaneous emission rate of the confined boradiazaindacene is observed to inhibit and enhance the nanofibers’ swelling. The Purcell factor is determined to demonstrate an oscillatory behavior resulting from the distinct characteristics of the mode volume, and the step-like increase of the quality factor of the nanofiber cavity due to the changes in the refractive index and fiber diameter.

Simultaneously high-Q and high-sensitivity slotted photonic crystal nanofiber cavity for complex refractive index sensing

We proposed and theoretically investigated a one-dimensional slotted photonic crystal nanofiber cavity (SPCNC) for complex refractive index sensing in a gaseous environment. Three-dimensional finite-difference time-domain simulations show that a high quality factor (Q) of 1.45×106 and a high sensitivity of 756 nm/refractive index unit (RIU) can be simultaneously achieved for the detection of the real part of the refractive index (RI) of the surrounding gas. In addition, we demonstrated that the SPCNC possesses the capability of detecting the change of the imaginary part of the RI. A high sensitivity of 1525 nm/RIU is obtained, and Q is found to decrease with increasing absorption strength but still remains as high as 1.1×104 for strong absorption. We believe our proposed SPCNC with excellent sensing performances, ultra-small mode volume, and compact footprint may offer the potential to develop multiplexed sensing techniques for applications in multielement mixture detection.

Composite device for interfacing an array of atoms with a single nanophotonic cavity mode

We propose a method of trapping atoms in arrays near to the surface of a composite nanophotonic device with optimal coupling to a single cavity mode. The device, comprised of a nanofiber mounted on a grating, allows the formation of periodic optical trapping potentials near to the nanofiber surface along with a high cooperativity nanofiber cavity. We model the device analytically and find good agreement with numerical simulations. We numerically demonstrate that for an experimentally realistic device, an array of traps can be formed whose centers coincide with the antinodes of a single cavity mode, guaranteeing optimal coupling to the cavity. Additionally, we simulate a trap suitable for a single atom within 100 nm of the fiber surface, potentially allowing larger coupling to the nanofiber than found using typical guided mode trapping techniques.

Nanofiber-based all-optical switches

We study all-optical switches operating on a single four-level atom with the $N$-type transition configuration in a two-mode nanofiber cavity with a significant length (on the order of $20$ mm) and a moderate finesse (on the order of 300) under the electromagnetically induced transparency (EIT) conditions. In our model, the gate and probe fields are the quantum nanofiber-cavity fields excited by weak classical light pulses, and the parameters of the $D_2$ line of atomic cesium are used. We examine two different switching schemes. The first scheme is based on the effect of the presence of a photon in the gate mode on the EIT of the probe mode. The second scheme is based on the use of EIT to store a photon of the gate mode in the population of an appropriate atomic level, which leads to the reduction of the transmission of the field in the probe mode. We investigate the dependencies of the switching contrast on various parameters, such as the cavity length, the mirror reflectivity, and the detunings and powers of the cavity driving field pulses. For a nanofiber cavity with fiber radius of 250 nm, cavity length of 20 mm, and cavity finesse of 313 and a cesium atom at a distance of 200 nm from the fiber surface, we numerically obtain a switching contrast on the order of about 67% for the first scheme and of about 95% for the second scheme. These switching operations require small mean numbers of photons in the nanofiber cavity gate and probe modes.

Manipulating single atoms and photons using optical nanofibers

We discuss how optical nanofibers, subwavelength-diameter fibers, can open new perspectives in quantum optical technologies theoretically and experimentally. Discussions are mainly focused on the manipulation of spontaneous emission for atoms around the nanofiber. We show that photons from single quantum emitters can be efficiently channeled into guided modes of the nanofiber. Especially by fabricating a cavity structure of the nanofiber, the channeling efficiency can be improved to exceed 80% although the cavity finesse is moderate. We discuss also how to realize such a nanofiber cavity experimentally.

Deterministic generation of a pair of entangled guided photons from a single atom in a nanofiber cavity

We investigate the controlled production of entangled guided photons. The technique is based on a two-stage vacuum-stimulated Raman process with a single atom in a nanofiber cavity. We examine the dependences of the two-photon emission probability and the entanglement concurrence on various parameters of the system, such as the radial and axial positions of the atom, the intensities and durations of the pump pulses, the one- and two-photon detunings, the cavity finesse, and the separation time between the pump pulses. We find that, due to the confinement of the guided cavity field in the transverse plane of the fiber and in the space between the cavity mirrors, the probability of generation of a pair of entangled guided photon can be high even when the finesse of the nanofiber cavity is moderate.

Cavity formation on an optical nanofiber using focused ion beam milling technique

We present the experimental realization of nanofiber Bragg grating (NFBG) by drilling periodic nano-grooves on a subwavelength-diameter silica fiber using focused ion beam milling technique. Using such NFBG structures we have realized nanofiber cavity systems. The typical finesse of such nanofiber cavity is F ∼ 20 - 120 and the on-resonance transmission is ∼ 30 - 80%. Moreover the structural symmetry of such NFBGs results in polarization-selective modes in the nanofiber cavity. Due to the strong confinement of the field in the guided mode, such a nanofiber cavity can become a promising workbench for cavity QED.

Triggered generation of single guided photons from a single atom in a nanofiber cavity

We study the deterministic generation of single guided-mode photons from an atom in the vicinity of a nanofiber with two fiber-Bragg-grating (FBG) mirrors. The technique is based on a cavity-enhanced Raman scattering process involving an adiabatic passage. We take into account the scattering of the pump field from the fiber, the multilevel structure of the atom, and the surface-induced van der Waals potential in describing the photon generation process. We find that, due to the confinement of the cavity field in the transverse plane of the fiber and in the space between the FBG mirrors, the probability of the generation of a single guided-mode photon can be close to unity even when the finesse of the nanofiber cavity is moderate. We show the possibilities of saturation and power broadening in the behavior of the number of photons emitted into the nanofiber.