In the presented work, we investigated the optical and thermal stability of upconversion nanoparticles based on the three widely used matrices (NaYF<sub>4</sub>, Y<sub>2</sub>O<sub>3</sub>, LaF<sub>3</sub>). Analysis of the upconversion emission as a function of pump power density in a wide range revealed a multi-stage functional dependence. The stages of linear growing, saturation and degradation with both reversible and irreversible characters were discovered. For matrices of nanoparticles with low-temperature stability (NaYF<sub>4</sub>), the dependence proves to be irreversible that could cause by a change in the structure and chemical composition of the matrix. Reversible dependence occurs in matrices with high-temperature stability (Y<sub>2</sub>O<sub>3</sub> and LaF<sub>3</sub>) and is caused by multiphonon nonradiative relaxation, which can be temperature-stimulated because of self-heating and low air-cooling of the crystal matrixes with low thermal conductivity.
We demonstrate a novel approach for electron-beam lithography (EBL) of periodic nanostructures. This technique can rapidly produce arrays of various metallic and etched nanostructures with line and pitch dimensions approaching the beam spot size. Our approach is based on often neglected functionality which is inherent in most modern EBL systems. The raster/vector beam exposure system of the EBL software is exploited to produce arrays of pixel-like spots without the need to define coordinates for each spot in the array. Producing large arrays with traditional EBL techniques is cumbersome during pattern design, usually leads to large data files and easily results in system memory overload during patterning. In Dots-on-the-fly (DOTF) patterning, instead of specifying the locations of individual spots, a boundary for the array is given and the spacing between spots within the boundary is specified by the beam step size. A designed pattern element thus becomes a container object, with beam spacing acting as a parameterized location list for an array of spots confined by that container. With the DOTF method, a single pattern element, such as a square, rectangle or circle, can be used to produce a large array containing thousands of spots. In addition to simple arrays of nano-dots, we expand the technique to produce more complex, highly tunable arrays and structures on substrates of silicon, ITO/ FTO coated glass, as well as uncoated fused silica, quartz and sapphire.
Here we present a device concept utilizing GaSb-based laterally-coupled DFB-lasers. Fabrication procedure to define the
ridge waveguide and the grating makes use of nanoimprint lithography. This technology addresses issues related to mass
fabrication and cost of the DFB-lasers. We demonstrate state-of-the-art devices on a range of wavelengths around 2 μm.
These lasers exhibit single-mode operation with a maximum side-mode suppression ratio of more than 55 dB and high
output power of ~25 mW.
We report the development of a nanoimprint lithography patterning method and inductively coupled plasma etching
recipe designed for GaSb-based semiconductor materials. The developed processes were used to fabricate edge-emitting
ridge-waveguide lasers and laterally-coupled distributed feedback lasers operating at 1945 nm. For ridge-waveguide
laser with 1 mm cavity length, a threshold current of 32 mA was measured. Side-mode suppression ratio in excess of 30
dB was measured for the distributed feedback lasers with 2 mW output power and the output wavelength was
temperature-tunable with a tuning coefficient of 0.16 nm /°C.
We investigate a novel nanofabrication process called soft ultraviolet (UV) nanoimprint lithography (NIL), for nanopatterning of compound semiconductors. We use flexible stamps with three layers and analyze their performance with wafers composed of III-V semiconductors. The developed stamp configuration is in many ways advantageous for the fabrication of precise gratings for various applications in photonics. We describe how to handle the deformation in both lateral and vertical directions by tuning the softness of the stamp and using a two step imprint process. As an application of the UV-NIL, we demonstrate a fabrication process for a laterally corrugated distributed feedback laser. Our laser fabrication process is free from regrowth and therefore easily adaptable to various material compositions and emission wavelengths. Because of the cost-effective full-wafer NIL, these lasers are attractive in various applications where low-cost, single-mode laser diodes are required. Our development work improves the design freedom of the NIL fabrication process of the laser diodes and improves the quality of the transferred patterns. To the best in our knowledge, this is the first demonstration of a single-mode laser diode fabricated by soft UV-NIL.
In this paper, we investigate a novel nanofabrication process called soft UV nanoimprint lithography, for nanopatterning of compound semiconductors. We use flexible stamps with three layers and analyze their performance with wafers composed of III-V semiconductors. The developed stamp configuration is in many ways advantageous for the fabrication of precise gratings for various applications in photonics. We describe how to handle the deformation in both lateral and vertical directions by tuning the softness of the stamp and using a two step imprint process.
As an application of the UV-NIL, we demonstrate a fabrication process for a laterally corrugated distributed feedback laser. Our laser fabrication process is free from regrowth and therefore easily adaptable to various material compositions and emission wavelengths. Due to the cost effective full wafer NIL, these lasers are attractive in various applications where low cost, single-mode laser diodes are required. Our development work improves the design freedom of the NIL fabrication process of the laser diodes, and improves the quality of the transferred patterns. To the best of our knowledge, this is the first demonstration of a single-mode laser diode fabricated by soft UV-NIL.
We show that local fields associated both with overall structural features and with unintended defects can be important in
the second-order nonlinear response of metal nanostructures. We first consider noncentrosymmetric T-shaped gold
nanodimers with nanogaps of varying size. The reflection symmetry of the T-shape is broken by a small slant in the
mutual orientations of the horizontal and vertical bars, which makes the sample chiral and gives rise to a different
nonlinear response for left- and right-hand circularly-polarized fundamental light. Measurements of achiral and chiral
second-harmonic signals as well as the circular-difference response exhibit a nontrivial dependence on the gap size. All
results are explained by considering the distribution of the resonant fundamental field in the structure and its interaction
with the surface nonlinearity of the metal. We also prepared arrays of ideally centrosymmetric circular nanodots.
Second- and third-harmonic generation microscopies at normal incidence were used to address polarization-dependent
responses of individual dots. Both signals exhibit large differences between individual dots. This is expected for second-harmonic
generation, which must arise from symmetry-breaking defects. However, similar results for third-harmonic
generation suggest that both nonlinear responses are dominated by strongly localized fields at defects.
We review our work done for topology optimization of passive photonic crystal component parts for broadband and
wavelength dependent operations. We show examples of low-loss topology-optimized bends and splitters optimized for
broadband transmission and demonstrate the applicability of topology optimization for designing slow-light and/or
wavelength selective component parts. We also present how the dispersion of light in the slow-light regime of photonic
crystal waveguides can be tailored to obtain filter functionalities in passive devices and/or to obtain <i>semi</i>-slow light
having a group velocity in the range ~(c<sub>0</sub>/15 - c<sub>0</sub>/100); vanishing, positive, or negative group velocity dispersion (GVD);
and low-loss propagation in a practical ~5-15 nm bandwidth.
Very low propagation losses in straight planar photonic crystal waveguides have previously been reported. A next natural step is to add functionality to the photonic crystal waveguides and create ultra compact optical components. We have designed and fabricated such structures in a silicon-on-insulator material. The photonic crystal is defined by holes with diameter 250 nm arranged in a triangular lattice having lattice constant 400 nm. Leaving out single rows of holes creates the planar photonic crystal waveguides. Different types of couplers and splitters, as well as 60, 90 and 120 degree bends have been realized. We have designed and fabricated components displaying more than 200 nm of useful bandwidth around 1550 nm. Design strategies to enhance the performance include systematic variation of design parameters using finite-difference time-domain simulations and inverse design methods such as topology optimization. We have also investigated a new device concept for coarse wavelength division de-multiplexing based on planar photonic crystal waveguides. The filtering of the wavelength channels has been realized by shifting the cut-off frequency of the fundamental photonic band gap mode in consecutive sections of the waveguide. Preliminary investigations show that this concept allows coarse de-multiplexing to take place, but that optimization is required in order to reduce cross talk between adjacent channels and to increase the overall transmission. In this work the design, fabrication and performance of these planar photonic crystal waveguide components are reviewed and discussed.