Yen-Chen Lin

Magnetostructural relationship in the spin-crossover complex t-{Fe(abpt)2[N(CN)2]2}: polymorphism and disorder phenomenon.

t-{Fe(abpt)(2)[N(CN)(2)](2)} [abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] is an intriguing spin-crossover system that crystallizes in two polymorphs. Polymorph A is paramagnetic; its crystal structure consists of a single molecule located at the center of inversion symmetry. Polymorph B, on the other hand, exhibits a rather complicated two-step-like spin transition; its crystal structure consists of two symmetry-independent molecules. The crystal structure of polymorph B has been derived in the different spin states: above the high-temperature step (300 K), between the two steps (90 K), below the incomplete low-temperature step (50 K), in the light-induced metastable state (15 K), in the thermally quenched metastable state (15 K), and after relaxation from the quenched state (15 K). The correlation between the structure and magnetic properties is precisely established, allowing the complicated magnetic behavior of polymorph B to be well understood. A unique order-disorder phase transition, resulting in a modulation of the metastable state structures, is detected for the first time on such spin-transition compounds. The modulation of the structure originates from a particular ordering of the dicyanamide ligand at one of the two Fe sites.

On the Radiation Profiles and Light Extraction of Vertical LEDs With Hybrid Nanopattern and Truncated Microdome Surface Textures

The n-side-up vertical light emitting diodes (LEDs) have the advantage of carving the surface of the thick n-GaN layer to improve light extraction and to adjust radiation profiles. In this paper, a two-step surface patterning is employed with the focus on understanding angular light diffractions from both nanopatterns and truncated microdomes. Light output enhancement of the LEDs with hybrid surface texturing is investigated experimentally and theoretically. The results suggest that light is diffracted through the grating effect and curved sidewalls when interacting with truncated microdomes, resulting in a maximum enhancement 64° away from the normal surface. On the other hand, nearly omni-directional enhancement was found from the randomly scattered nanopatterns. As for the hybrid structure, since guided modes in the semiconductor layers are diffracted by either nanopatterns or microdomes, the percentage increase of light extraction from the hybrid structure is approximately the linear superposition of both types of surface textures. The results suggest an interesting guideline to improve LED light output and to adjust angular radiation with the mutlistep surface patterning.

Light induced modulated structure of the spin crossover compound {Fe(abpt)2[N(CN)2]2}

{Fe(abpt)2[N(CN)2]2} is an intriguing spin crossover system which crystallizes in two separate phases. Phase A has already been investigated; its crystal structure consists of a single molecule sitting on a center of symmetry. Phase B is the subject of the present structural analysis and presents two independent molecules in the asymmetric unit. We have observed a unique order-disorder phase transition at very low temperature induced either by laser light irradiation or through rapid thermal quenching from room temperature. The corresponding metastable phase exhibits an incommensurate modulation of the crystal structure, disappearing at the HS to LS relaxation temperature (52 K). This modulation is most probably related to a particular ordering of the dicyanamide ligands.

Far-field self-focusing and -defocusing radiation behaviors of the electroluminescent light sources due to negative refraction

In recent years, researchers have demonstrated negative refraction theoretically and experimentally by pumping optical power into photonic crystal (PhC) or waveguide structures. The concept of negative refraction can be used to create a perfect lens that focuses an object smaller than the wavelength. By inserting two-dimensional PhCs into the peripheral of a semiconductor light emitting structure, this study presents an electroluminescent device with negative refraction in the visible wavelength range. This approach produces polarization dependent collimation behavior in far-field radiation patterns. The modal dispersion of negative refraction results in strong group velocity modulation, and self-focusing and -defocusing behaviors are apparent from light extraction. This study further verifies experimental results by using theoretic calculations based on equifrequency contours.

Correlation of angular light profiles of light-emitting diodes to spatial spontaneous emissions from photonic crystals

Typically, photonic crystal light-emitting diodes employ shallow nanostructures which only higher-order optical modes can be interacted with. Here, both the shallow photonic crystals and nanohole arrays (etched through active layers) are fabricated, which serve to diffract, respectively, higher and lower optical modes in the active layer. Our results indicate that the photon lifetime can be controlled by adjusting the geometry of shallow nanostructures and nanohole arrays. The angular emission profiles are thus determined by the dominance of higher- and lower-order mode quality factors in the band structure.

Polarization ratio enhancement of a-plane GaN light emitting diodes by asymmetric two-dimensional photonic crystals

Fabricating photonic crystals (PhCs) on GaN based non-polar light emitting diodes (LEDs) is an effective way to increase light extraction and meanwhile to preserve or improve polarization ratio. In this work, a-plane GaN LEDs with two-dimensional PhCs were demonstrated. With the E // m polarized modes (which mean the optical polarization with the electric field parallel to m-axis) as the target of diffraction, we matched E//m modes to the photonic bands and aligned E//c modes to fall within the photonic band gap. The results show stronger E//m but weaker E//c mode diffractions on both c- and m-axes. At the vertical direction, the polarization ratio is enhanced from 45.8% for the planar device to 52.3% for the LEDs with PhCs.

Spatially adjusted spontaneous emissions from photonic crystals embedded light-emitting diodes

In this work, the angular light output enhancements of LEDs were investigated from the spontaneous emission and light scattering of devices with different photonic crystal (PhC) geometries. The emitted photon coupled into a leaky mode is differentiated by the manipulation of the quality factor in various spatial frequencies. Therefore, light extraction in this light-emitting device is determined by the modal extraction lengths and the quality factor obtained from the measured photonic bands. Furthermore, the higher- and lower-order mode spontaneous emissions are affected by the nonradiative process in the PhC structures with different periods. In our cases, the photonic crystal device with the largest period of 500 nm exhibits the highest lower-order mode extraction and quality factor. As a result, a self-collimation behavior toward the surface-normal is demonstrated in the 3D far-field pattern of such a device. We conclude that, with the coherent light scattering from the PhC region, the spontaneous emission of the material and spatial behavior of the extracted mode can be both managed by the proper design of the device.

Control of self-collimated light-emitting diodes with negative refraction by photonic crystal nanohole arrays

Negative refraction was demonstrated in the visible wavelength range by two-dimensional (2D) photonic crystals inscribed at the peripheral of a GaN-based light-emitting diode (LED). Self-collimated behaviors in TE polarization were observed in the far-field measurement.

Far-field self-focusing and -defocusing radiation behaviors of the nanostructure LED due to negative refraction

Negative refraction was demonstrated in the visible wavelength range by two-dimensional (2D) photonic crystals inscribed at the peripheral of a GaN-based light-emitting diode (LED). Self-collimated behaviors in TE polarization were observed in the far-field measurement.

Mode interactions in a GaN-based light emitting diode with surface photonic crystals and nanoholes

Photonic crystals (PhCs) were typically fabricated on the light emitting surface of light emitting diodes (LEDs) to improve light extraction, which is regarded as the weak coupling between the laterally propagated light in the epi-layers and the surface nanostructure. This work demonstrates GaN-based LEDs with the PhC structure on the mesa surface and nanohole arrays surrounding the light emitting mesa. Our new device (SHLED) shows a 56% higher optical output power than the planar structure (PLED), as compared with the 40% improvement of the surface PhC device (SLED) over PLED. The output power of SHLED is higher than that of SLED due to the enhanced diffraction of low order modes propagated in the lateral direction, in addition to the higher order mode light diffraction from the surface PhCs. From the relative angular spectra, the interaction of in-plane optical wave with the nanoholes (which are etched through MQWs) is much stronger than that with surface PhCs, suggesting an efficient light diffraction to the surface normal by nanoholes.