Group Refractive Index Research Articles

Surface plasmon hole burning via doppler broadening

The hole burning phenomenon is investigated in a five-level atomic configuration at the interface of cesium and gold in the presence of Doppler broadening. The group index and refractive index of surface plasmon polaritons (SPPs) becomes equal in the hole burning region, which display that the phase and group velocities of SPPs are matched. The propagation length during absorption of SPPs decreases with the decrease of absorption, while there are increases with amplification of SPPs. This study could have significant applications in optical storage devices, telecommunication, imaging, and enhancement of plasmonster memory.

Accurate measurements of phase refractive index of soft contact lenses.

We describe a measurement approach for the bulk phase refractive index of hydrogel and silicone hydrogel (soft) contact lenses in solution at multiple wavelengths with an accuracy of +/- 0.001, and sensitivity to change of 0.0002. The method uses time-domain low-coherence interferometry to obtain group refractive index (GRI) for contact lenses between 530 to 670 nm in 10 nm increments. The measured GRI dispersion curve is then mathematically converted to the phase refractive index values. The approach is based on a point measurement using focused beam, and therefore does not require flattening of the lens for the measurements. We discuss practical implications of the method for the quality control in manufacturing of ophthalmic optics.

Lasing in low-dimensional single crystals of hexyl-substituted thiophene/phenylene co-oligomer

We report on the fabrication and characterization of low-dimensional single crystals of hexyl-substituted thiophene/phenylene co-oligomer, 5,5′-Bis(4′-n-hexyl-4-biphenylyl)-2,2′-bithiophene (BP2T-Hx). X-ray diffraction measurements of the single crystals reveal that BP2T-Hx molecules crystallize in a monoclinic form. Temperature dependence of photoluminescence (PL) decay profiles of the BP2T-Hx crystal results in a PL quantum yield of 39%. Above an excitation threshold of 262 μJ cm−2, amplified spontaneous emission is obtained in a disk-shaped crystal of BP2T-Hx based on the self-waveguiding effect of fluorescence light. Owing to two-dimensional light confinement supported by high group refractive index of 3.4 and high quality factor of 3800, the one-dimensional needle crystal with a cavity length of 70 μm realizes a Fabry–Pérot lasing at a threshold of 186 μJ cm−2 which is 29% lower than that of the disk-shaped crystal.

Triple mode coupling effect and dynamic tuning based on the zipper-type graphene terahertz metamaterial

We construct a laminated metamaterial structure of zipper-type monolayer graphene with silica, which is equipped with a distinctively dual plasmon-induced transparency (PIT) phenomenon. The graphene pattern in this structure craftily connects with the electrode so that we can dynamically control the PIT response (via regulating the bias voltage that applied between the electrode and the substrate to control the graphene Fermi energy), in which the range of voltage-regulate is calculated theoretically, proving to be a feasible measure for PIT response-tune. Then, the tuning discipline for this structure is summarized from the calculated result of the coupled mode theory deduced theoretical model and the simulation result. It is found that the structure possesses a great tuning performance within the tuning range we studied; also, as the structure ameliorates the monolayer graphene absorbance from 2.3% to about 50% within a broad dynamic frequency tuning range, the absorption peak frequency modulation depth is up to 45.46%. Besides, the group refractive index is discussed for reflecting the system capability of slow light, and the maximum of the coefficient can catch up to 595. Even if the impact of dielectric light- absorbance to slow light is taken into account, the slow light index at the transparent window is still as high as 200.

All‐Optical Spin–Orbit Coupling of Light in Coherent Media Using Rotating Image

AbstractThe possibility of all‐optical spin–orbit coupling (SOC) of light is investigated based on a rotating spinor image traveling through an electromagnetically induced transparency (EIT) medium. It is shown that the paraxial evolution of the spinor image composed of two Laguerre–Gaussian (LG) modes with different frequencies can be analogous with the quantum dynamics of a spin‐1/2 particle with strong and tunable SOC governed by the Pauli equation, where the spin‐up and ‐down states have different effective masses. Using realistic EIT parameters with cold atoms, both the radial inhomogeneity of the strong control field and the atomic density distribution with comparable size are considered. The results confirm that the large group refractive index varying in the radial dimension mimicking the central potential can greatly enhance the spin–orbit interaction, leading to visible spatial quantization of the oppositely oriented spin states, equivalent to the two LG modes.

Electromagnetically induced transparency in an all-dielectric nano-metamaterial for slow light application.

Slow light technique has significant potential applications in many contemporary photonic device developments for integrated all-optical circuit, such as buffers, regenerators, switches and interferometers. In this paper, we present an efficient coupling mechanism of an electromagnetically induced transparency like (EIT-like) effect in an all-dielectric nano-metamaterial. This EIT-like effect is generated by destructive interference between a radiative Fabry-Perot (FP) mode and a dark waveguide (WG) mode, which is based on a combined structure of a dielectric grating and multilayer films. The dark WG mode is excited by guided mode of dielectric grating instead of radiative FP mode. In analogy to the molecular transition process, the FP mode, guided mode and WG mode are denoted by excited states of |1〉, |2〉 and |3〉. The two coupling pathways of the EIT-like effect in our metamaterial are |0〉 → |1〉 and |0〉 → |2〉 → |3〉 → |1〉, where |0〉 is the ground state. The simulated resonant wavelength of WG mode is consistent with theoretical result. We further confirm this EIT-like effect through a two-oscillator coupling analysis. We achieve a group refractive index of 913.6 by adjusting these two modes coupling of the EIT-like effect, which is useful for developing slow light device. This work provides a valuable solution to realize electromagnetically induced transparency in all-dielectric nanomaterial.

Tunable slow light effect based on dual plasmon induced transparency in terahertz planar patterned graphene structure

We have studied a simple novel graphene ribbon structure. A very excellent and prominent dual graphene plasmon induced transparency phenomenon could be achieved by the destructive interference resulted from the excited plasmonic modes in terahertz band. Using the simple relationship between graphene and applied voltage, a good tunable effect of this structure can be achieved. The transmission of this proposed structure is theoretically investigated by using the equivalent resonator coupled mode method. The theoretical data from our proposed method are in good agreement with the numerical simulation results. Moreover, utilizing the high dispersion property, we have also researched the slow light effect for this proposed system. The results of theoretical research have indicated that the group refractive index of our proposed structure can maintain an excellent numerical value. This investigation can play a significant role in the tunable graphene-based slow light devices.

Simulation analysis of the optical properties of a liquid crystal-filled microstructured optical fiber

The optical properties of a liquid crystal-filled microstructured optical fiber (LC-MOF) were investigated using the finite element method. Liquid crystal E7 was designed to be infiltrated into six small air holes at the neighbourhood of silica core. Simulation results showed that the designed LC-MOF supported a continuous transmission in the silica core over a wide wavelength range. The optical properties of core modes such as refractive index, birefringence, confinement loss, model area, group refractive index, group birefringence, and zero dispersion wavelength, were illustrated. The structure parameters of LC-MOF and temperature could be used to adjusted the optical properties. The designed LC-MOF, providing a way for filling high refractive index materials in MOF air holes, has potential applications in optical fiber modulators and sensors.

Simultaneous measurement method of the physical thickness and group refractive index free from a non-measurable range.

An optical method to resolve the non-measurable thickness problem caused by the overlap of optical path differences within a specific thickness range when measuring the physical thickness of a sample using a spectral-domain interferometer is proposed and realized. Optical path differences can be discerned by inserting a correction glass piece into the measurement path, thus increasing the measurement optical path length. To verify the proposed method, 0.2-mm-thick N-BK7 glass was used as a sample, with physical thickness and group refractive index measurements conducted according to three different correction glass elements with corresponding nominal thicknesses of 3.0 mm, 3.5 mm, and 4.0 mm. Through uncertainty evaluations according to the correction glass used, the physical thicknesses of the sample were found to be in good agreement within measurement uncertainties of less than 100 nm, results comparable to those of previous works which did not use any correction glass.

A Theoretical Study on Rib‐Type Photonic Wires Based on LiNbO3 Thin Film on Insulator

AbstractA theoretical study on rib‐type photonic wires (PW) based on lithium niobate (LiNbO3)‐on‐insulator (LNOI) using finite element method is performed. Aiming at 1.55 µm wavelength, effective refractive indices of several lower‐order transverse‐electric/magnetic (TE/TM) modes in the central rib region and the fundamental mode in the slab waveguide in the side region are calculated as a function of geometric parameters of the PW. By comparing effective refractive index of the first‐order mode in the central rib region with that of the fundamental mode in the slab waveguide, single‐mode condition in the central rib region is determined in terms of geometric parameters of the PW. Electric field distributions of fundamental TE and TM modes are also simulated and discussed. In addition, dispersion relation of effective group refractive index of the fundamental mode in the central rib region is calculated and discussed in comparison with those of ridge‐type LNOI PW, traditional Ti‐diffused LiNbO3 strip waveguide, and bulk LiNbO3 single‐crystal. For a fundamental TE mode in a rib‐type LNOI PW with a LiNbO3 film thickness 700 nm, a rib width 1 µm and an etching depth 100–200 nm, nearly zero group index dispersion in the wavelength range 1.31.7 µm can be realized.

Simultaneous Measurement of Humidity and Temperature with Cytop-reduced Graphene Oxide-overlaid Two-mode Optical Fiber Sensor

A polymer-overlaid two-mode microfiber knot resonator (TMM-KR) was designed to measure relative humidity (RH) and temperature simultaneously in a living condition. The device is composed of two main optical features: two-mode microfiber (TMM, slow-varying term) and two-mode knot resonator (TMKR, fast-varying term). After fabricating a TMM by non-adiabatically tapering a conventional single mode fiber (SMF), we designed C-rGO-overlaid TMM as a polymer-overlaid optical sensor to evaluate the response and recovery times. The effective group refractive index between fundamental and cladding modes, HE11 and HE12, respectively, of the TMM-KR reduced to nearly zero by optimizing the waist diameter of the optical microfiber, and the sensitivities of the proposed sensor remarkably enhanced to 0.603 nm/%, and -0.79 nm/oC, for RH and temperature, respectively. Our calculated and experimental results demonstrated that the proposed micro-structure of the C-rGO-overlaid TMM-KR will provide optical properties that can be applied for practical use of environment sensing.

All-optical spin-orbit coupling of light using electromagnetically induced transparency

Analysis indicates that tunable spin-orbit coupling (SOC) can be introduced into the paraxial Schr\odinger-type equation for a spinor image traveling through a rotating electromagnetically induced transparency (EIT) medium. As a fundamental example, we show that, due to a radial gradient of large group refractive index, the SOC can lead to spatial separation of the oppositely oriented spinor states in the probe image, mimicking the fine-structure splitting of a spin-$\frac{1}{2}$ quantum harmonic oscillator described by the Pauli equation. Such a result enables a direct manipulation and visualization of spin-orbit physics by all-optical means based on EIT slow-light media. Possible experimental implementations of our model with ultracold atoms are discussed.

Dual dynamically tunable plasmon-induced transparency in H-type-graphene-based slow-light metamaterial.

An H-type-graphene-based slow-light metamaterial is proposed to produce a dual plasmon-induced transparency phenomenon, which can be effectively modulated by Fermi level, carrier mobility of graphene, and the medium environment. The data calculated by coupled mode theory and results of numerical simulation show prominent agreement. In addition, both the simplicity and continuity of the units of graphene-based metamaterial are extraordinary advantages. Furthermore, the slow-light characteristics of the proposed structure show that the group refractive index is as high as 237, which is more competitive than some other slow-light devices.

Application of optical coherence tomography and optical path length method for monitoring corneal thickness and refractive index change during corneal cross-linking.

Corneal cross-linking (CXL) using UVA irradiation with a riboflavin photosensitizer has emerged as a new treatment paradigm for corneal ectatic disorders. The thickness threshold for protection of intraocular structures has often been challenged with ongoing developments, and corneal thinning becomes an important safety concern, especially for patients with thin corneas. In this study with an ex vivo bovine eye model, we monitored corneal thinning and corneal refractive index changes using optical coherence tomography (OCT) integrated with an adaptation of the optical path length method. CXL experiments were performed based on the standard protocol that includes removal of the corneal epithelium to facilitate diffusion of riboflavin into the stroma. The corneal stromal thickness and group refractive index were measured by a 1310nm Fourier-domain OCT imaging system at three critical points of the procedure: immediately after epithelial removal, after 30min riboflavin instillation, and after 30min UVA irradiation with continuing instillation. We found that the refractive index of the bovine cornea changed significantly from epithelial removal to riboflavin instillation and UVA irradiation, increasing from 1.377±0.005 (mean±standard deviation) after de-epithelization to 1.387±0.003 after 30min instillation and 1.388±0.008 after subsequent irradiation. The corneas also underwent a considerable decrease (10%-20%) in stromal thickness with thinning of 95±29 μm (mean±standard deviation) after riboflavin instillation and a further decrease (∼5%) with thinning of 42±19 μm after UVA irradiation. Our study highlights the importance of corneal thickness monitoring during CXL, especially after riboflavin instillation when the decrease is the largest, to avoid delivering endothelial cytotoxic doses. An increase in refractive index heightens the concern for corneal thinning and the need for careful monitoring as a safety precaution.

Research on the effects of dual pumping on fast light propagation in an Er3+-doped optical fiber

In this paper, we propose a general theory for the coherent population oscillation effect in an -doped optical fiber under dual-frequency laser pumping at room temperature. We aim at comparing the effect of dual pumping (1480 nm and 980 nm) with single pumping of 980 nm and single pumping of 1480 nm on time advancement. According to the numerical calculations, we analyze the relationships between relative modulation attenuation and modulation frequency, plus fractional advancement and modulation frequency under the dual-frequency pumping laser with co-propagation. Compared to single pumping, a greater change in the group refractive index can be found. The maximum time advancement of 0.24 ms under a dual-frequency pump laser with co-propagation is obtained. Moreover, we demonstrate the influence of the pump ratio M on time advancement with modulation frequencies of 40 Hz, 100 Hz and 200 Hz. The ability to control the group velocity of light could have an important application for photonics.

Optimizing the spectro-temporal properties of photon pairs from Bragg-reflection waveguides

Bragg-reflection waveguides (BRWs) fabricated from AlGaAs provide an interesting nonlinear optical platform for photon-pair generation via parametric down-conversion (PDC). In contrast to many conventional PDC sources, BRWs are made of high refractive index materials and their characteristics are very sensitive to the underlying layer structure. First, we show that the design parameters like the phase matching wavelength and the group refractive indices of the interacting modes can be reliably controlled even in the presence of fabrication tolerances. We then investigate how these characteristics can be taken advantage of when designing quantum photonic applications with BRWs. We especially concentrate on achieving a small differential group delay between the generated photons of a pair and then explore the performance of our design when realizing a Hong–Ou–Mandel interference experiment or generating spectrally multi-band polarization entangled states. Our results show that the versatility provided by engineering the dispersion in BRWs is important for employing them in different quantum optics tasks.

Absorption and slow-light analysis based on tunable plasmon-induced transparency in patterned graphene metamaterial.

We propose a novel simple patterned monolayer graphene metamaterial structure based on tunable terahertz plasmon-induced transparency (PIT). Destructive interference in this structure causes pronounced PIT phenomenon, and the PIT response can be dynamically controlled by voltage since the existence of continuous graphene bands in the structural design. The theoretical transmission of this structure is calculated by coupled mode theory (CMT), and the results are highly consistent with the simulation curve. After that, the influence of the graphene mobility on the PIT response and absorption characteristics is researched. It is found that the absorption efficiency of our designed structure can reach up to 50%, meaning the structure is competent to prominent terahertz absorber. Moreover, the slow-light performance of this structure is discussed via analyzing the group refractive index and phase shift. It shows that the structure possesses a broad group refractive index band with ultra-high value, and the value is up to 382. This work will diversify the designs for versatile tunable terahertz devices and micro-nano slow-light devices.

Active whispering-gallery-mode optical microcavity based on self-assembled organic microspheres

The self-assembled organic microspheres of (E)-3-(4-(dip-tolylamino)phenyl)-1-(4-fluoro-2-hydroxyphenyl)prop-2-en-1-one (DTPHP) function as active whispering-gallery-mode (WGM) resonators with high group refractive index.

Calculation of Axial Length Using a Single Group Refractive Index versus Using Different Refractive Indices for Each Ocular Segment: Theoretical Study and Refractive Outcomes

To investigate the difference between the segmented axial length (AL) and the displayed AL on an optical low-coherence reflectometry (OLCR) biometer and to compare the refractive prediction errors calculated using the segmented and displayed ALs. Retrospective case series. Four thousand nine hundred ninety-two eyes from 4992 patients in the theoretical study and 1758 eyes from 1758 patients in the refractive prediction error comparison. First, we calculated the segmented AL as the sum of geometrical ocular segments converted from the optical path length (OPL) in each medium. To convert the OPL to a geometrical distance in each medium, we used 4 sets of group refractive indices. Then, the mean absolute prediction error (MAE) was calculated with the displayed AL and segmented AL using 6 intraocular lens power formulas: Olsen, Barrett Universal II (Barrett), Haigis, Hoffer Q, Holladay 1, and Sanders-Retzlaff-Kraff trial (SRK/T). Segmented AL, difference in AL (segmented AL minus displayed AL), MAE, and percentage of eyes within 0.5 diopter (D) of error. The segmented ALs were up to 0.29 mm longer in short eyes and 0.50 mm shorter in long eyes. The differences in ALs were correlated negatively with the displayed ALs (r values, -0.941 to -0.913; P < 0.001). The MAEs were significantly lower using segmented ALs for all formulas except the Olsen in both the entire group and the long eye subgroup (AL, ≥26 mm) and for the Holladay 1 and Hoffer Q in the short eye subgroup (AL, ≤ 22 mm). Use of segmented ALs produced a greater percentage of eyes within 0.5 D of error for all formulas except the Olsen and Haigis for the entire group, for long eyes, and for the Holladay 1 in short eyes. The segmented ALs were longer in short eyes and shorter in long eyes compared with the displayed ALs calculated with a single group refractive index for the entire eye. The refractive accuracy with segmented ALs was improved in short eyes with the Hoffer Q and Holladay 1 formulas and in long eyes with all formulas except the Olsen formula.

Correlation gating quantifies the optical properties of dynamic media in transmission.

Quantifying light transport in turbid media is a long-standing challenge. This challenge arises from the difficulty in experimentally separating unscattered, ballistic light from forward scattered light. Correlation gating is a new approach that numerically separates light paths based on statistical dynamics of the optical field. Here we apply correlation gating with interferometric near-infrared spectroscopy (iNIRS) to separate and independently quantify ballistic and scattered light transmitted through thick samples. First, we present evidence that correlation gating improves the isolation of ballistic light in a thick, intrinsically dynamic medium with Brownian motion. Then, from a single set of iNIRS transmission measurements, we determine the ballistic attenuation coefficient and group refractive index from the time-of-flight (TOF) resolved static intensity, and we determine the reduced scattering and absorption coefficients from the diffusive part of the TOF resolved dynamic intensity. Finally, we show that correlation gating is applicable in intrinsically static media in which motion is induced externally. Thus, for the first time, to the best of our knowledge, the key optical properties of a turbid medium can be derived from a single set of transmission measurements.