Submicron Period Gratings Research Articles

Spectral Fourier-microscopy of the periodic structures based on Ge2Sb2Te5

The method of spectral Fourier microscopy was used to study the reflection spectra with an angular resolution of submicron periodic gratings based on amorphous and crystalline Ge2Sb2Te5. The form of the dispersion curves of quasi-waveguide modes in the structures under study was established. The experimental data were compared with the calculations of dispersion curves in synthesized diffraction gratings. Reasonable agreement between theoretical and experimental data was obtained.

Femtosecond-laser-induced submicron grating periodic structures on As2S3 and As35Se65 glasses

Submicron periodic structures produced by femtosecond Ti:sapphire laser irradiation (1 kHz, 150 fs, 3–5 μm) on As2S3 and As35Se65 glasses at edges of damage holes are observed. Effects of changes in laser wavelength (λ), pulse number, power, and irradiation time on grating periodicity were studied. Regular groove-like ripples and distinct parallel submicron periodic gratings were seen in damaged areas. The damage grating period (Λ) is approximately λ/2n. The period increases with wavelength but is hardly influenced by irradiation time and pulse number. Samples with high refractive index and low bandgap are more prone to melting, and edge stripes are not obvious.

Fast dynamic interferometric lithography for large submicrometric period diffraction gratings production

This paper describes a successful technological transfer, from a state laboratory to an industrial company, for writing long and large submicron period gratings (potentially square-meter sized), implemented in an industrial direct laser beam writing equipment at 355 nm wavelength (Dilase 750 from KLOE SA company). The writing head, which has been inserted in the machine, consists of a phase mask which enables it to project a set of fringes pattern of small area onto a photoresist-coated substrate. As the substrate is continuously moving, one-dimensional or two-dimensional gratings can be fabricated over large areas limited only by the machine’s stage displacement range. The optical scheme is described from the beam shaping to the printing processes and the phase mask optimization. In order to demonstrate the technology, 600 nm period gratings of over 20 cm in length have been fabricated. Such submicron structures can be used in solar cell modules for photon trapping and as antireflection coatings.

On-the-fly writing of a long grating phase mask

A method is demonstrated for writing long grating phase masks, which can be used for patterning large-area (square meter size) submicron-period gratings. The method consist of illuminating a small area transmission grating phase mask by a continous wave transverse-electric-polarized collimated laser beam under the −1st order Littrow mounting to define a high-contrast interferogram composed of fringes. By sliding a long photoresist-coated substrate under this small area phase mask, gratings of arbitrary length may be written, with grating lines oriented in the scan direction. The patterning of uninterrupted gratings with lengths exceeding 300 mm is demonstrated.

Optical modeling of thin-film silicon solar cells with submicron periodic gratings and nonconformal layers

In thin-film silicon solar cells (TFSC) efficient light management is essential in order to increase energy conversion efficiency. The application of nano-scale periodic gratings (PG) is a promising method to enhance absorption in the absorber layers of TFSC since they can efficiently scatter the incident light. Carefully designed gratings give a possibility to increase the photocurrent over a wavelength range where silicon exhibits a weak absorption. Maxwell's equations solver was employed to carry out optical simulations of TFSC with PG. Atomic force microscopy (AFM) measurements demonstrate that film deposition smoothens the morphology of PG. In the simulations we used the results of AFM measurements to define the morphology of interfaces between the layers of TFSC. An optimum smoothing of interface roughness was determined that resulted in maximum absorption in thin-film silicon solar cells.

An optoelectronic device in bulk LiF with sub-micron periodic gratings fabricated by interference of 400 nm femtosecond laser pulses

Sub-micron periodic gratings of transparent materials are holographically fabricated by interference with the second harmonic (400nm) of a mode-locked Ti:sapphire oscillator–amplifier laser. As one optoelectronic application, a pulsed, room temperature laser action in bulk lithium fluoride is demonstrated, for the first time, in the green spectral region based on the laser-active F3+ colour centres utilizing a distributed feedback structure encoded by interference of 400nm femtosecond laser pulses. A lasing output with a linewidth of 1nm is obtained at approximately 539nm, which value reflects the selective laser resonator. Realization of green and red distributed feedback colour centre laser action based on the F3+ and F2 centres in LiF excited by a single wavelength can be expected.

Red and Green Distributed-Feedback LiF Color Center Lasers Fabricated by Interference of Femtosecond Laser Pulses

Sub-micron periodic gratings inside bulk lithium fluoride (LiF) are holographically fabricated by interference with the second harmonic (390 nm) of a mode-locked Ti:sapphire oscillator-amplifier laser. As one optoelectronic application, pulsed, room temperature distributed feedback (DFB) color center laser action is demonstrated, for the first time, in the green spectral region based on the F3+ color centers in LiF utilizing both a micro-structure encoded by femtosecond-laser-pulses, and higher gain of the laser-active centers produced by an e-ray irradiation at 200 K. A lasing output with a linewidth of 0.5 nm is obtained at approximately 537 nm, which value reflects the selective laser resonator. Realization of green and red DFB color center lasers based on the F3+ and F2 centers in LiF excited by a single wavelength can be expected.

Formation of dry etched gratings in GaN and InGaN

Sub-micron periodic gratings with pitch ~3,000A were formed in GaN and InGaN using holographic lithography and room temperature electron cyclotron resonance (ECR) BClg3/N2 dry etching at moderate microwave (500 W) and radio frequency (100 W) powers. The process produces uniform gratings without the need for elevated sample temperatures during the etch step.

Excimer laser ablation of polymers and glasses for grating fabrication

In this paper we describe studies carried out related to grating formation for optoelectronics applications using 248 nm and 193 nm excimer lasers. These include basic studies of UV laser induced incubation and ablation of various glasses, and an evaluation of laser-induced-periodic-surface-structures (LIPSS) and UV holographic techniques based on a phase mask for forming gratings on polymers and in fibres. Modelling and experiments show that the use of excimer laser illuminated phase masks in a contact printing mode or Talbot reimaging arrangement provide a particularly effective approach to forming submicron period gratings.

A pixellated grating array using photoelectrochemical etching on a GaAs waveguide

We have demonstrated and evaluated a grating array outcoupler fabricated by photoelectrochemical (PEC) etching, a manufacturable and practical approach for fabrication of grating-based III-V semiconductor waveguide devices. An array of submicron period gratings was etched into photolithographically delineated areas in a single PEC step. The fabricated devices are: 10-/spl mu/m wide rib waveguides with 0.35-/spl mu/m first-order outcoupling gratings; and 10-/spl mu/m wide rib waveguides with 10 /spl mu/m/spl times/10 /spl mu/m pixellated areas of gratings. Device characterization demonstrates the effectiveness of outcoupling grating fabrication using PEC and that the pixellated grating outcoupler is an effective and simple means of generating an optical beam array. >

Fabrication of Submicron Gratings for 1.5 μm InGaAsP / InP Distributed Feedback Lasers by Reactive Ion Etching Using C 2 H 6 / H 2

Etching conditions for submicron period gratings on substrates by reactive ion etching using a mixture have been systematically investigated. Etching rates and shapes of grooves in samples are assessed after being etched under various gas pressure conditions, flow ratios, and RF power densities. The pressure affects the angle of the etched side wall. The selective deposition of organic compound in the etching process can be controlled by the flow ratio. It was proven that the depth of the grooves are controllable up to 700 nm by adjusting the etching conditions. The technique has been applied to form gratings with a period of 240 nm for distributed feedback (DFB) lasers. The lasers have threshold currents ranging from 30 to 45 mA, which are comparable to those of DFB lasers fabricated by conventional wet chemical etching. Gratings with a period of 135 nm were also fabricated on an substrate by reactive ion etching, using a technique which halves periods of gratings.