Change Of Effective Refractive Index Research Articles

Beam quality evolution in large-mode-area specially doped laser fiber through bend-induced effective refractive index change

The effect of bending in a specially doped large-mode-area (LMA) gain fiber on the beam quality of the laser and amplifier has been studied through simulation and experimentation. The effect on the overlap between the fundamental mode (FM) and the doping region due to bend-induced refractive index change was studied theoretically by varying the bend radius. Bend radius of the gain fiber in the range of 5–8 cm was used to study the evolution of beam quality at the amplified output. The numerical simulation of the overlap between the FM and the dopant distribution in the core of the gain fiber for different bending radii is well matched to the experimentally measured beam quality of the amplified output for the respective bending of the gain fiber. The master oscillator (having M 2 = 1.1) was successfully amplified to 35 W maintaining the near-diffraction-limited beam quality (M 2 = 1.05) using a confined doped LMA gain fiber with a bend radius of 8 cm. However, the use of a uniform doped LMA gain fiber with similar bend configuration degrades the beam quality (M 2 = 1.37) at the amplified output power of 31.3 W.

Monolithically Integrated Tunable Three- Section All-Active DBR Laser Diodes With 30 nm Tuning Range

We present a monolithically integrated three-section all-active AlGaInAs/InP tunable distributed Bragg reflector (DBR) ridge waveguide laser diode that uses thermal tuning mechanism and mode-hopping tuning principle to achieve wide-range wavelength tuning. It is similar to the traditional three-section DBR laser structure, including a gain section, a phase section, and a DBR grating section. The difference is that the phase section and the DBR section incorporate the same active quantum well structure as the gain section. In this case, weak effective refractive index changes of the three active sections affected by the current injection will occur since the carrier density is clamped by the threshold gain condition of the DBR laser diode. Despite this limitation, a noncontinuous tuning up to ∼30 nm (1564–1594 nm) can be observed using only current injection at a constant temperature of 25 °C. Significant thermal effects occurred during the tuning process. In addition, a discontinuity is introduced in the tuning because it does not contain a passive region for phase adjustment. However, the device still provides a stable single longitudinal mode output with nine channels, a wavelength spacing of about 4 nm, and a side mode suppression ratio (SMSR) of more than 30 dB. Furthermore, the DBR laser diode can be easily fabricated utilizing the conventional distributed feedback laser diode fabrication process without secondary epitaxy and heater metal. The all-active DBR laser diode will be expected to be applied in the coarse wavelength division multiplexing (CWDM) systems with the advantage of low cost.

Cost-effective silicon-photonic biosensors using doped silicon detectors and a broadband source.

We propose and demonstrate a cost-effective, microring-based, silicon photonic sensor that uses doped silicon detectors and a broadband source. Shifts in the sensing microring resonances are electrically tracked by a doped second microring, which acts as both a tracking element and a photodetector. By tracking the power supplied to this second ring, as the sensing ring's resonance shifts, the effective refractive index change caused by the analyte is determined. This design reduces the cost of the system by eliminating high-cost, high-resolution tunable lasers, and is fully compatible with high-temperature fabrication processes. We report a bulk sensitivity of 61.8 nm/RIU and a system limit of detection of 9.8x10-4 RIU.

Suppressing the mechanochromism of flexible photonic crystals.

Photonic crystal slabs (PCS) are a promising platform for optical biosensing. Yet, flexible applications based on PCS for biosensing have been limited, as the mechanical properties influence the optical ones. Here, we show the suppression of the mechanochromism effect for flexible PCS. We obtained flexible photonic crystal slabs by sputtering of a dielectric 100 nm Nb2O5 high refractive index layer onto a flexible nanostructured polydimethylsiloxane (PDMS) substrate with 370 nm grating period. The PCS exhibit a guided mode resonance at around 650 nm. We demonstrate that these flexible photonic crystal slabs show less than 0.5 nm resonance shift for 4% strain and call them stabilized PCS (sPCS). We compare this to a resonance shift of ∼21 nm for ∼4% strain of a flexible photonic crystal with a flexible nanoparticle high index layer (mechanochromatic PCS, mPCS). This high resonance shift is expected from the Bragg equations, where 4% grating period change correspond to approximately 4% change of the resonance wavelength (i.e., ∼26 nm at a resonance wavelength of 650 nm), if changes in the mode effective refractive index are neglected. In a stretch series we obtain color-to-strain dependencies of 4.79 nm/% strain for mPCS and 0.11 nm/% strain for our stabilized sPCS. We analyze the suppression of the mechanochromism with detailed microscopy results. We observe that fissures and fractures form in the rigid waveguiding layer of the sPCS upon mechanical stress. An algorithm based on Holistically-Nested Edge Detection (HED) is used for automated counting of cracks. Rigid photonic crystal cells with sizes on the order of 10 µm to 100 µm are formed that explain the stable optical properties. Even more stable optical properties with less than 0.03 nm wavelength shift per 1% strain are demonstrated for sPCS with an additional dielectric 100 nm SiO2 low index layer beneath the Nb2O5 waveguide layer decoupling the waveguide further from the flexible PDMS substrate.

Piezoelectric‐Effect Enhanced Perovskite Plasmonic Nanolasers

AbstractHerein, the piezoelectric effect of CsPbBr3 perovskite is utilized for the first time to enhance the nanolasers performance based on the flexible poly(ethylenenaphthalate) PEN/single‐crystal‐Au/MgF2/CsPbBr3 (ScAu/M/CPB) structure. The dynamic‐modulated plasmonic lasing mode blueshift is achieved by the piezoelectric polarization effect (PPE)‐induced effective refractive index (neff) change by varying the applied strain (ε). In particular, taking advantage of the increased cavity mode frequency, the lasing threshold (Pth) reduces from 4.5 µJ cm−2 (ε = 0) to 1.5 µJ cm−2 (ε = −1.02%). The ε‐dependent carrier dynamics results reveal the nature of ultralow Pth being PPE‐induced spontaneous emission (SE) decay rate enhancement, leading to the reduction in radiation loss of the plasmonic cavity. The corresponding enhancement mechanism is put forward to elucidate the principle of piezoelectric‐induced dynamic modulation nanolasers characteristic. This work demonstrates that coupling of piezoelectric effect and plasmonics is an effective route to enhance the performance of plasmonic nanolasers.

Multi-angle holographic characterization of individual fractal aggregates.

Holographic particle characterization uses quantitative analysis of holographic microscopy data to precisely and rapidly measure the diameter and refractive index of individual colloidal spheres in their native media. When this technique is applied to inhomogeneous or aspherical particles, the measured diameter and refractive index represent properties of an effective sphere enclosing each particle. Effective-sphere analysis has been applied successfully to populations of fractal aggregates, yielding an overall fractal dimension for the population as a whole. Here, we demonstrate that holographic characterization also can measure the fractal dimensions of an individual fractal cluster by probing how its effective diameter and refractive index change as it undergoes rotational diffusion. This procedure probes the structure of a cluster from multiple angles and thus constitutes a form of tomography. Here we demonstrate and validate this effective-sphere interpretation of aspherical particles' holograms through experimental studies on aggregates of silica nanoparticles grown under a range of conditions.

Optimal photonic crystal slabs for modulators based on transitions between photonic bands

The real part of refractive index is important for the phase shift of light propagating in a dielectric medium. Modulation of the refractive index can be used for electro-optic modulators based on Mach–Zehnder interferometer. In this study we simulate Si-based 2D photonic crystal slabs and by comparing the performance of different lattice types and structure of holes, in respect to the value of effective refractive index change when modulating the refractive index of Si, we find the most promising structures for Si photonics electro-optic modulators based on transition between photonic bands.

Highly Sensitive Refractive Index-Based Sensor for DNA Hybridization Using Subwavelength Grating Waveguide

A biosensor for detecting DNA hybridization via refractive index sensing has been demonstrated using a subwavelength grating (SWG) waveguide. Waveguide dimensions are selected by parametric optimizations to increase the mode-analyte overlap with considerations of typical silicon-on-insulator fabrications technology. DNA layer is added with a linker layer on silicon pillars of the SWG waveguide to optimize and detect the DNA hybridization. Some essential characteristics such as mode overlap factor, change in effective refractive index, waveguide sensitivity, and shift in resonance wavelength are computed for the different dimensional parameters of the SWG waveguide in DNA hybridization along with mode field intensity and normalized power using the finite element method. The DNA hybridization is shown by variation in normalized power of interacting light in cladding region of the waveguide for 0% to 100% fractional change of dsDNA and ssDNA. Waveguide sensitivity, shift in resonance wavelength, device sensitivity, and intrinsic limit of detection are obtained to ∼ 0.157, 7.01, 605 nm/RIU, and 1.30 × 10−4 RIU, respectively, for the optimized structure of SWG waveguide in sensing of DNA hybridization, which could be suitable aspects for detecting DNA hybridization.

High sensitivity hydrogen sensor based on tilted fiber Bragg grating coated with PDMS/WO3 film

A hydrogen sensor based on tilted fiber Bragg grating (TFBG) coated with PDMS/WO3 composite film was proposed. The WO3 powder can be adhere well to the surface of TFBG by the flexible PDMS film. The exothermic reaction between WO3 and hydrogen made the effective refractive index of the cladding of the TFBG change, and the intensities of cladding modes coupled from the core mode changed accordingly. The proposed fiber hydrogen sensor showed the sensitivity of 0.596 dB/% under the hydrogen concentrations from 0% to 1.53%. The average response time and recovery time of the sensor were 93 s and 107 s, respectively. Moreover, the TFBG based sensor can be made inherently temperature self-compensation by referencing all cladding-mode wavelengths to the wavelength of the core mode resonance of the grating, which is isolated from the effective refractive index changes of the fiber surroundings.

AuNPs assisted all-fiber compact phase modulator and its application in erbium-doped fiber laser

A compact, low-loss, simply prepared all-fiber phase modulator based on the plasmonic absorption of gold nanoparticles decorated on the surface of non-adiabatic microfiber was demonstrated By applying 808 nm pump laser of different powers to the phase modulator, the effective refractive index changes resulting in phase shift of the interference spectrum at 1550 nm. The proposed device can also be utilized as an all-optical switch with the rising and falling edge time of 4.69 ms, 1.99 ms, respectively Furthermore, a tunable wavelength erbium-doped fiber laser was designed by inserting the proposed AuNPs-based phase modulator as a wavelength-tunable filter into a ring cavity. The output center wavelength of the laser was tuned from 1560.23 nm to 1559.29 nm. The proposed AuNPs-based phase modulator has advantages of compact size, low-loss, and simple preparation, which make it can be anticipated to have great potential in the application of all-optical systems.

Super continuum generation in water doped with Homeopathic medicines

We have carried out systematic studies to investigate the effect on supercontinuum generation in water using 40 fs laser pulses when doped with Homeopathic medicines. We perform these studies using five series of medications with different levels of dilution (10-30 to 10-100000). We measure supercontinuum spectra that span from 400-1050 nm. We monitor the area under the curve in the range 450-750 nm for each sample at a fixed incident laser energy. Our observations indicate that the yield of supercontinuum generation, in water containing Homeopathic medicine is significantly different from that obtained in water containing plain ethanol. The measurement for different dilutions shows up to 7 times standard deviation variation in the yield of supercontinuum generation? Even though linear absorption in the UV-visible region does not show any significant difference for different Homeopathic medicines, the supercontinuum yield which depends on the effective nonlinear refractive index changes with different samples.

Porous Silicon Optical Devices: Recent Advances in Biosensing Applications.

This review summarizes the leading advancements in porous silicon (PSi) optical-biosensors, achieved over the past five years. The cost-effective fabrication process, the high internal surface area, the tunable pore size, and the photonic properties made the PSi an appealing transducing substrate for biosensing purposes, with applications in different research fields. Different optical PSi biosensors are reviewed and classified into four classes, based on the different biorecognition elements immobilized on the surface of the transducing material. The PL signal modulation and the effective refractive index changes of the porous matrix are the main optical transduction mechanisms discussed herein. The approaches that are commonly employed to chemically stabilize and functionalize the PSi surface are described.

Phase change materials in photonic devices

Recent developments in reconfigurable photonic devices highly rely on the effective refractive index change enabled by phase change materials (PCMs) as either dielectric surroundings or constituting materials. This universal characteristic, tunable refractive index, is shared among various kinds of PCMs and has been successfully utilized to achieve multilevel modulations for both free-space and integrated photonics. In this Perspective, we briefly recapitulate the fundamental mechanisms of phase transitions for three dominant PCMs. The progress in integrating different PCMs with on-chip silicon photonics and periodic antenna arrays are reviewed and analyzed in parallel. We also discuss the existing problems of PCM photonics, for example, the compatibility with commercial production line, the stability issue, and accessibility of the stimuli. In the end, we provide the outlook for the improving material engineering of PCM and multi-functional PCM-based photonics devices.

Electrically Tunable Plasmonic Biosensors Based on Cavity-Coupled Structure With Graphene

In this research, we proposed a highly tunable hybrid plasmonic biosensor for identifying molecule fingerprints of proteins in the infrared range. The device is composed of a photonic band-gap structure and a cavity. The plasmonic energy is confined inside the cavity to enhance the light-analyte interactions. A layer of graphene is placed on the bottom surface of the cavity that is filled with analytes for detection. Our device has a high value of effective sensitivity that is defined as the ratio of the resonant frequency shift to the change of effective refractive index when the cavity is loaded with an analyte. The effective refractive index takes into account the effects of both the cavity structure and the optical property of the analyte. Another feature of our device is that a wide-ranging electrical tunability can be realized by applying various values of bias voltage to the graphene. The resonant frequency can be tuned for sensing different analytes without redesigning and refabricating the device. In addition, the graphene tunability can improve the sensitivity in detecting the non-targeted analytes. A prototype was designed based on photonic theory and fabricated via bottom-up lithography techniques. The research results are expected to be beneficial for nanometer-range bio-identifications and infrared optical sensors.

Temperatures effects on cascaded Mach-Zehnder interferometer structures

To increase bandwidth and number of channels per fiber for more than one wavelength in the same fiber the dense wavelength division multiplexing (DWDM) technology has been utilized. One of the devices that are important in DWDM is an optical interleaver. This paper discussed the effects of temperatures in the DWDM interleaver by using the Mach-Zehnder interferometer (MZI) structures which is arranged in two-stage cascaded MZI and the three-stage cascaded MZI geometries. The main consequences of increase temperature inside the fiber optics are the change of effective refractive index in the material of silica fiber due to the thermo-optics effects. In our analysis we have used the transfer matrix method to investigate the wavelength dependence of output power to the temperatures changes that varies from 30oC to 430ºC. In the calculation we have used the C-Band range wavelength which is around 1530 to 1565 nm. It has been shown that the change of temperatures may shift the wavelength inside the MZI output power in linear manners. These effects may be used to tune wavelength transmission inside the MZI structures to suit the ITU-T defined grid.

Design and simulation of strip loaded and rib waveguide with integration of 2D material

Objective- To design Graphene-Silicon based rib waveguide and reduce the losses in the strip in order to meet the requirement for ultra-fast & ultrahigh optical bandwidth communication and computing in integrated optical devices. Method –Propagation losses and effective refractive index are the two key parameters. In order to meet the objective, the effects of Graphene for manufacturing passive devices/components in the field of Integrated Photonic like integrated optical waveguide have been analysed by measuring the changes in propagation losses and effective refractive index of the silicon photonics devices for operating at different wavelengths. Findings- We have presented the design and simulation of SOI (Silicon-on-Insulator) platforms with 2D layer materials (graphene) which has been used along with their mode of propagation, effective refractive index (ne f f ), propagation losses (dB/cm) and varying wavelength range for optimum performance. In addition to this, we have also calculated the boundary limit for both the speed and bandwidth. We also reported the development of Silicon rib waveguide, Graphene-Silicon based rib waveguide and Ge on SOI with graphene later at the top of strip waveguide.Minimum loss of strip waveguide is 2.9 dB/cm which has been obtained for Mid-IR wavelength generally used for high power mid- IR sensing.

SiO2-SnO2:Er3+ planar waveguides: Highly photorefractive glass-ceramics

For different applications in laser photonics and integrated optics, photorefractive materials are surely of great interest and play an important role in miniaturization of devices and fabrication of functional photonic structures such as gratings and waveguides. In this work, high photorefractivity of sol-gel derived SiO2-SnO2:Er3+ glass-ceramic planar waveguides, with negative effective refractive index change, is demonstrated. Through the photorefractivity investigation employing the UV pulsed KrF excimer laser (λ = 248 nm), the glass-ceramic planar waveguides show fast photorefractive response after a cumulative dose of only 0.3 kJ/cm2. A higher SnO2 content in the glass-ceramics leads to a greater change of saturated effective refractive index: Δneff = - (2.60 ± 0.20) x 10-3 at 1550 nm on the TE0 mode, for the 30 mol% SnO2 planar waveguides and Δneff = - (2.00 ± 0.20) x 10-3 on the 1550 nm TE0 mode, for the 20 mol% SnO2 ones. To investigate the structural modifications involved in the photorefractivity of the SiO2-SnO2:Er3+ glass-ceramics, XRD and Raman characterizations are employed. The grating fabrication by direct UV writing on the SiO2-SnO2:Er3+ glass-ceramic planar waveguide as well as an important reduction in the employed energy is demonstrated.

Ultra-broad bandgap metal-insulator-metal waveguide filter with symmetrical stubs and defects

An ultra-broad bandgap plasmonic based filter that consists of a metal-insulator-metal waveguide with symmetrical stubs and defects is designed and investigated for the first time. A tunable wide bandgap can be achieved by introducing the defects into the stubs and a larger bandgap width about 1.35 times compared to its counterpart without defects is acquired. It is found that both the defects and number of periods play the crucial roles to affect the bandgap performance, which is attributed to the change of effective refractive index and plasmon resonance modes in the proposed structure. The proposed plasmonic filter has potential to be a promising alternative structure to realize in integrated optical circuits for optical trapping and tunable devices.

Defect modes of two-dimensional photonic crystals composed of epsilon-near-zero metamaterials

The defect modes of a two-dimensional square array of epsilon-near-zero material (air) circular rods in air (an epsilon-near-zero material) were investigated in this study. A local defect was created by varying the radius of an epsilon-near-zero material (air) rod at the center of the array. It was found that the frequencies of the defect modes can be tuned to certain values in a wide range of a photonic band gap. However, the effective refractive index change due to the variations in the radius of an epsilon-near-zero material (air) defect rod was very small. The energy density of the defect modes was found to be strongly confined near the local defect rod but rarely inside the epsilon-near-zero material.

Multi-parameter sensing based on surface plasma resonance with tungsten disulfide sheets coated

A tungsten disulfide (WS2) coated surface plasmon resonance (SPR) sensor based on gradient pitch Mach-Zehnder interferometer (GP-MZI) for measuring ethanol vapor concentration is proposed and verified by experiments. Under continuous CO2 laser heating, a MZI based on GP helix structure is fabricated by twisting single mode fiber (SMF), which can excites multi-order cladding modes. A gold film is deposited on the surface of the GP helix structure by a magnetic sputtering coating machine. WS2 film is coated on the gold film of the GP helix structure, which increases the evanescent field strength of the twisted structure surface and enhances the interaction between SPR wave and ethanol molecules. Since the absorption of ethanol molecules by WS2 sheets will cause the change of effective refractive index (RI) of WS2 film, the intensity of transmission signal can be adjusted accordingly. For multi-order cladding modes, the effective RI and the effective thermo-optic coefficient vary with the modal order, so the RI and temperature sensitivity of different modal orders are also different. So, the ethanol vapor concentration, relative humidity (RH), and temperature can be simultaneously measured by monitoring the intensity of those dips with the resolution of ± 0.030 mg/L, ±0.035%RH, and ± 0.010 ℃, respectively. This sensor structure provides a promising platform for multi-parameter sensing applications.