Wide wavelength range tunable guided-mode resonance filter based on incident angle rotation

A wide wavelength range tunable guided-mode resonance filter (GMRF) with high peak efficiencies, narrow linewidths, and low sidebands is experimentally demonstrated. The resonance wavelength can be tuned under TM polarized light illumination by rotating the angle of incidence. The GMRF composed of a one-dimensional grating layer and two waveguide layers on a glass substrate is designed with rigorous coupled-wave analysis. The grating structure of the GMRF is patterned by interference lithography with two ultraviolet laser beams. The reflection colours of fabricated GMRF can be shifted from blue to red by rotating the angle of incidence. The measured full width at half-maximums of the reflection peaks located at 450.6, 500.9, 551.0, 601.1, and 651.3?nm are 3.1, 3.9, 4.3, 3.8, and 4.1?nm, respectively. The corresponding sidebands of measured spectra are below 0.2. Compared with previous experimental studies on tunable structures, the narrow linewidths and low sidebands of the proposed GMRF are remarkably improved.     

Terahertz narrow-band filter based on rectangle photonic crystal

A terahertz filter with a channel drop cavity and a resonant reflection cavity in a two-dimensional photonic crystal is theoretically proposed. The channel drop cavity is used to trap photons at resonant frequency from the bus waveguide through coupling and emit them to a drop waveguide, while the resonant reflection cavity is used to realize wavelength selective reflection feedback in the bus waveguide. The transmission properties of the terahertz filter are simulated using the finite element method. It is found that a peak with the central frequency of 1.12 THz is existed in the transmission spectrum. The full width at half maximum of the passband is only 5 GHz, and the peak drop efficiency is up to 94.8%.     

Design considerations for narrow-band dielectric resonant grating reflection filters of finite length

An interference-waveguide approach is developed to predict the response of a resonant grating reflection filter and to provide a better understanding of the resonant process. An expression for the reflected field that accounts for all internal boundary reflections within the filter is developed. Under the assumption of an anti-reflective design, expressions characterizing the line shape of a filter of infinite length are first developed; then the effects of finite length on the response are determined. Expressions relating the length of the filter to the peak reflection efficiency and line width are developed. The degradation of the response as a function of filter length is evaluated. An equivalent waveguide representation is used to determine the location of the resonance as well as the spectral and angular linewidths of the filter. The minimum obtainable spectral linewidth for a filter of given length is determined to be on the order of deltalambda approximately lambda2/L. Rigorous analysis is used to verify the interference-waveguide approach.     

Wide-angle transmissive filter based on a guided-mode resonant grating

A wide-angle color filter for TE-polarization is proposed based on a guided-mode resonant grating selectively coated with a metal film. The effects of the structure parameters to the resonant transmission characteristics are discussed by the rigorous coupled-wave analysis. Its optimal structure was given for a green color filter with a peak transmission of 77.6%, a full width at half-maximum (FWHM) of ～120 nm, and a good angular tolerance up to ±40°. Compared to the similar structure with a nonresonant dielectric grating, it decreases the FWHM effectively for the resonance induced by the guided-mode resonant grating. The numerical results indicate that the designed structure has a good wide-angle transmission performance.     

F2-laser-machined submicrometer gratings in thin dielectric films for resonant grating waveguide applications

Surface-relief gratings with submicrometer modulation periods were ablated by F2-laser radiation in thin metal-oxide films to produce resonant grating waveguide structures. For 150 nm films of Nb2O5, grating amplitudes in the range of 5-50 nm could be reproducibly excised with a controlled exposure of a laser energy density and a number of pulses within a narrow processing window. Resonant coupling of 800 nm ultrashort pulsed laser light into the resulting grating waveguide structure is verified with reflection and transmission spectra and satisfactorily modeled by coupled-mode theory. The laser-fabricated grating waveguides are attractive for high damage threshold reflectors and biosensor applications.     

Electrically controlled integrated optical filter

The possibility of electric control over the spectral transfer function of an integrated optical filter comprising a reflection Bragg grating in a photorefractive waveguide is demonstrated for the first time. The filter has a spectral selectivity of 0.15 nm, ensures continuous tuning within ～0.1 nm, and can be electrically switched on and off. The proposed method of control is effective and has good prospects for practical applications.     

Embossed Bragg Gratings Based on Organically Modified Silane Waveguides in InP

Considering the large variety of applications for optical glass waveguide gratings, the effective production method of embossing for micropatterning, and the unique advantages of InP-based materials, we expect that hybridization of embossed optical glass waveguide gratings and InP substrates will inevitably lead to new applications in integrated optics. We present our preliminary results of research toward the development of solgel-derived glass waveguide gratings made by embossing on InP. Theoretically, the dependence of the stop-band FWHM and transmission contrast of the grating filter on the grating length, and the relationship between the Bragg grating's reflective wavelength and the dopant concentration in the solgel waveguide, are obtained. Experimentally, using organically modified silane, we solve the problem of mismatching of SiO(2) and InP, and successfully fabricate an embossed glass grating with a second-order Bragg reflection wavelength of 1580 nm and a FWHM of 0.7 nm fabricated upon a solgel waveguide on an InP substrate.     

Wideband tunable Q-switched fiber laser using graphene as a saturable absorber

An ultra-wide wavelength tuning range, which covers three different band regions consisting of the S-, C-, and L-bands, is proposed and demonstrated for a graphene-based Q-switched erbium-doped fiber laser using a tunable bandpass filter as the wavelength tuning and filtering mechanism. A 3?m length of erbium-doped fiber is used as the gain medium in a ring laser cavity configuration, with absorption coefficients of between 11 and 13 dB?m^?1 at 980?nm and about 18?dB?m^?1 at 1550?nm. The tuning range of the Q-switching pulses covers a wide wavelength range of 58?nm, which spans from 1512.5?nm to 1570.5?nm. In addition, the lasing and Q-switching thresholds are considerably low, with respective values of ～11.0?mW and ～18.4?mW. A repetition rate of 55.3?kHz is obtained at the maximum pump power of 100.4?mW, together with pulse width and pulse energy of 1.6?μs and 25.8?nJ, respectively.     

Multiline two-dimensional guided-mode resonant filters

A novel multiline filter using a two-dimensional guided-mode resonant (GMR) filter is proposed. The filter concept utilizes the multiple planes of diffraction produced by the two-dimensional grating. Multiple resonances are obtained by matching the guided modes in the different planes of diffraction to different wavelengths. It is shown that the location and the separation between resonances can be specifically controlled by modifying the periodicity of the grating and the other physical dimensions of the structure. This is in contrast to the one-dimensional GMR filters where the location of the resonances is material dependent. Two-line reflection filter designs with spectral linewidths less than 1 nm and a controllable spectral separation of up to 23% of the short resonance wavelength are presented using rectangular-grid grating GMR structures. Three-line filters are designed in hexagonal-grid grating GMR structures with two independently controllable resonance locations.     

Near-IR tunable laser with an integrated LiTaO3 electro-optic deflector

We report the successful demonstration of a near-IR tunable laser (1525.4-1558.2 nm) that uses an integrated LiTaO3 deflector in combination with a reflection grating as an electronically tunable filter. The electro-optic deflector is a unique integrated optical device and consists of a horn-shaped array of electro-optic prisms in series. The almost 33 nm of tuning covers a wavelength region of high interest to the communications industry (1527-1550 nm).     

A simplified broadband coupling approach applied to chemically robust sol-gel, planar integrated optical waveguides

A new generation waveguide spectrometer with broadband coupling capabilities has been developed. As opposed to previous devices, this attenuated total reflection (ATR) spectrometer is much simpler in design, is more chemically robust, and transmits light down to at least 400 nm. The attenuated total reflection element consists of a single-mode, planar integrated optical waveguide fabricated by dip-coating a approximately 300 nm thick, sol-gel composite layer on a glass substrate. A commercially available prism is used as the incoupler with an integral holographic diffraction grating acting as the dispersive outcoupling element. The transmission of narrow band-pass filters was used to compare the response of the waveguide spectrometer to that of a conventional transmission instrument. Spectral resolution was assessed by measuring the fwhm of various laser lines, which were found to range from 0.5 to 1.3 nm. The measured limits of detection for the waveguide spectrometer from 400 to 600 nm are 8.0 and 10.1 milliabsorbance units for TE and TM polarizations, respectively. Finally, to demonstrate the application of this technology to a molecular film confined to a solid-liquid interface, visible ATR spectra of an adsorbed submonolayer of horse heart cytochrome c were acquired. A procedure to correct the waveguide spectra for the wavelength dependence in ATR path length is described.     

Guided-mode resonant grating filter with an antireflection structured surface

We describe a new structure of guided-mode resonant grating (GMRG) filters with low sideband reflectance. This GMRG filter consists of a high-index thin film on an antireflective structured surface called "moth-eye structure." Since the high-index film undulates along the surface structure, the film acts as a modulated optical waveguide. An incident light wave satisfying a resonant condition is reflected by the GMRG filter, and nonresonant light waves pass through the filter. This GMRG filter is valid for reducing reflection of nonresonant light waves in a wide spectral range. The resonant reflection of this new filter was investigated by numerical calculation based on an electromagnetic grating analysis. In the case of a triangular antireflective surface structure whose thickness is 2x greater than its period, the sideband reflectance for nonresonant light waves was lower than 0.5% for TM-polarized light in a wide range of wavelengths.     

Numerical study on an asymmetric guided-mode resonant grating with a Kerr medium for optical switching

Optical switching effects of a guided-mode resonant grating (GMRG) with a Kerr medium have been simulated with the nonlinear finite differential time domain (FDTD) method. An asymmetric waveguide grating with a large second spatial harmonic component has been proposed for the optical switch. Resonant reflection occurs at both of the band-edge wavelengths. These wavelengths are used for the pump light and the probe light. The enhanced electric field of the pump light changes the resonant wavelength for the probe light as a result of the Kerr effect. We designed the GMRG with resonant wavelengths of 1489.6 and 1630 nm, which were used for the pump light and the probe light, respectively. When the grating material has a third-order susceptibility chi(3) of 8.5 x 10(-10) esu, the transmittance of the probe light changes from 0 to 80% by increasing the intensity of the pump light from 0 to 60 kW/mm2.     

Narrow-band Resonant Grating Waveguide Filters Constructed with Azobenzene Polymers.

Resonant grating waveguide structures were used to fabricate narrow-bandwidth optical filters. Azopolymer films were deposited on top of slab waveguides, and surface relief gratings were optically inscribed on them to be used as couplers. This technique is a simple one-step process and produces efficient gratings with high accuracy. Sharp resonant peaks are observed in the transmission and the reflection spectra of these structures. The thickness and the index of refraction of the waveguide can be accurately determined from these resonances by use of modal theory. These parameters are then used in the design of an optical filter. Bandwidths of less than 1 nm and a decrease in transmitted signal of 60% are reported. Measurement of these values was limited by the divergence of the probe beam.     

Narrowband, polarization-independent free-space wave notch filter

A two-dimensional-corrugated-slab-waveguide add/drop filter providing 100% resonant reflection at 1.55 microm wavelength for both TE and TM polarizations with identical FWHM is designed. The fabricated device exhibits a reflectivity spectrum of more than 95% peak reflection for both polarizations at 1.537 microm. The coupling scheme involves the TE0 guided mode only; it is made relatively tolerant by means of a double-sided crossed grating.     

Electro-optic polymer waveguide grating with fast tuning capability

A novel fast tunable electro-optic (EO) polymer waveguide grating is proposed and designed. Its resonant wavelength can be linearly tuned via the first-order EO effect with a high sensitivity of 6.1 pm/V. We find that the spectrum characteristics of EO polymer waveguide gratings depend strongly on many grating parameters, such as refractive-index modulation, modulation function, grating period, and period number. Material selection, fabrication technology, EO tuning ability, and polarization dependence of EO polymer waveguide gratings are also discussed. Such a waveguide grating not only overcomes the disadvantages of fiber-optic gratings, such as slow wavelength tuning ability and large-scale integration inconvenience, but also has many advantages, such as high resonant-wavelength tuning sensitivity, the same fabrication technology used for semiconductors, and polarization independence.     

Atomic nanofabrication using an ytterbium atomic beam

Abstract

Highly parallel and periodically narrow lines of ytterbium (Yb) atoms were successfully produced on a substrate using a near resonant laser light and direct-write atomic nanofabrication. Yb atoms are a promising material for nanofabrication using atom optics particularly due to their electric conductivity, the laser wavelength required for their manipulation, and the vapor pressure required for their fabrication. Collimated ¹⁷⁴Yb atoms were channeled into the nodes of an optical standing wave with dipole force and then deposited onto a substrate. We clearly observed a grating pattern of Yb atoms fabricated on a substrate with a line separation of approximately 200 nm after examining the surface of the substrate with an atomic force microscope. This is the first demonstration of nanofabrication using the atom-optical approach with Yb atoms.     

Atomic nanofabrication using an ytterbium atomic beam

Highly parallel and periodically narrow lines of ytterbium (Yb) atoms were successfully produced on a substrate using a near resonant laser light and direct-write atomic nanofabrication. Yb atoms are a promising material for nanofabrication using atom optics particularly due to their electric conductivity, the laser wavelength required for their manipulation, and the vapor pressure required for their fabrication. Collimated ¹⁷⁴Yb atoms were channeled into the nodes of an optical standing wave with dipole force and then deposited onto a substrate. We clearly observed a grating pattern of Yb atoms fabricated on a substrate with a line separation of approximately 200 nm after examining the surface of the substrate with an atomic force microscope. This is the first demonstration of nanofabrication using the atom-optical approach with Yb atoms.     

Effect of finite grating, waveguide width, and end-facet geometry on resonant subwavelength grating reflectivity

Resonant subwavelength gratings (RSGs) offer narrowband high reflectivity with low-reflectivity sidebands. Analysis with the commonly used rigorous coupled-wave analysis assumes an RSG with infinite lateral extent and illumination by plane waves. This analysis is performed with a finite-difference semivectorial high-order accurate two-dimensional Helmholtz code that is able to simulate the entire finite RSG structure in the dimension of the grating vector. We study the effect of finite beam size on RSG reflectivity, resonant wavelength, and spectral response width. Independently, we study the effect of a finite RSG by varying the waveguide length and number of grating periods while fixing the beam size. We show that the placement of the waveguide end facets relative to the termination of the grating has a significant effect on the reflectivity and response width.     

Resonant waveguide grating imager for live cell sensing

We report on a resonant waveguide grating imager for high throughput screening using live cells. This imager can generate a snapshot image of all biosensors in a 384-well microtiter plate with a time resolution of ～3 s and a spatial resolution of 80 μm. This imager is well tolerant to variability in plate configurations and cell confluency. The resonant wavelength and its shifts induced by cell responses at each pixel correlate well with cell confluency. Data filtration protocol can be used to improve assay quality for partially confluent cells.