Young-Gu Ju

Polarization stabilization of vertical‐cavity top‐surface‐emitting lasers by inscription of fine metal‐interlaced gratings

The polarization of vertical‐cavity top‐surface‐emitting lasers (VCSELs) is stabilized by inscribing a fine metal‐interlaced grating into the topmost λ/4 layer of the distributed Bragg reflector (DBR). In this scheme, thin Al/AuZn films are deposited on the sidewalls of the grating defined by the ion milling technique. These metal films enhance polarization anisotropy of the top DBR. Grating periods of 0.9, 1.2, and 1.5 μm are tested and VCSELs with the 1.2‐ or 1.5‐μm grating show good polarization selectivity over the entire range of operating current. The direction of the dominant polarization is orthogonal to the grating line, and the polarization extinction ratio exceeds 40:1 on the average. This polarization discrimination scheme works well for all sizes of VCSELs tested.

CW operation and threshold characteristics of all-monolithic InAlGaAs 1.55-μm VCSELs grown by MOCVD

We demonstrate an all-monolithic metal-organic chemical vapor epitaxy (MOCVD)-grown 1.55-/spl mu/m vertical-cavity surface-emitting laser operating continuous wave up to 35/spl deg/C. The structure is based on the InAlGaAs-InP material system grown by a single step of MOCVD. Wet oxidation of a strained In/sub 0.4/Al/sub 0.6/As layer is used for the current confinement. The threshold current, threshold voltage and the external quantum efficiency at room temperature are about 1.6 mA, 2.3 V, and 5.4%, respectively.

Numerical Analysis of High-Index Nano-Composite Encapsulant for Light-Emitting Diodes

We used two-dimensional finte-difference-time-domain (FDTD) software to study the transition behavior of nano-particles from scatterers to an optically uniform medium. We measured the transmission efficiency of the dipole source, which is located in the high refractive index medium (index=2.00) and encapsulated by low index resin (index=1.41). In an effort to compose index-matched resin and to reduce internal reflection, high-index nano-particles are added to low-index resin in simulations of various sizes and densities. As the size of the nano-particles and the average spacing between particles are reduced to 0.02 λ and 0.07 λ respectively, the transmission efficiency improves two-fold compared to that without nano-particles. The numerical results can be used to understand the optical behavior of nano-particles and to improve the extraction efficiency of high brightness light-emitting diodes (LEDs), through the use of nano-composite encapsulant.

Transverse mode control by etch-depth tuning in 1120-nm GaInAs/GaAs photonic crystal vertical-cavity surface-emitting lasers

Transverse mode control by etching depth tuning is demonstration from long wavelength (1120 nm) photonics-crystal vertical-cavity surface-emitting lasers. The non-degenerate single-transverse mode was obtained in 12 /spl sim/ 18-pair-etched PC-VCSELs over the entire operating current range.

Densely packed pie shaped vertical-cavity surface-emitting laser array incorporating a tapered one-dimensional wet oxidation

Wavelength-division-multiplexing (WDM) photonic integrated emitter (PIE) vertical-cavity surface-emitting laser (VCSEL) arrays are fabricated using a post growth wet oxidation technique. High-density integration of WDM VCSEL arrays is possible by combining the technique of one-dimensional oxidation and large-scale tapered oxidation. Eight channels are integrated into a circle of 60 /spl mu/m in diameter. Seven channels are found to operate as lasers. The lasing wavelengths range from 823 to 836 nm corresponding with the distance between the VCSEL mesa and the tuning trench. The successful demonstration of incorporating wet oxidation into the wavelength control of the PIE VCSEL array opens a new way of fabricating mask-defined densely packed WDM VCSEL arrays.

High-finesse AlxOy/AlGaAs nonabsorbing optical cavity

We report the measured finesse value of ∼390 in nonabsorbing AlxOy/AlGaAs cavities. The nonabsorbing cavity consisting of a bottom AlxOy/AlGaAs distributed Bragg reflector (DBR), an AlxOy spacer layer, and a top AlxOy/AlGaAs DBR is prepared by a wet-oxidation process. The measured resonance linewidth agrees well with that of calculation, indicating very small overall losses in the cavity. The wet-oxidation process does not seem to degrade the interface of the epitaxial layers significantly. The lower bound of maximum achievable reflectivity from the AlxOy DBR is estimated to be >99.95%, assuming an average interface roughness of about 0.6 nm. The maximum achievable finesse of this type of cavity is expected to be larger than that of the all-epitaxial counterpart assuming the same roughness.

Spontaneous emission factor of oxidized vertical-cavity surface-emitting lasers from the measured below-threshold cavity loss

A method to estimate the spontaneous emission factor β is proposed and applied to the 780 nm oxidized vertical-cavity surface-emitting lasers. The proportionality of the measured cavity loss multiplied by optical power to injected current is used. Our results agree better with theoretical calculations than those of conventional light-current curve fitting. The spontaneous emission factor of 0.0021 is obtained for a 2-μm-square device at room temperature. Since only the below-threshold information of cavity loss and output power are used in our method, the obtained β values are independent of any complex and unexpected above-threshold effects such as thermally induced mode-size contraction, as they should be.

Micro‐cavity design of bottom‐emitting AMOLED with white OLED and RGBW color filters for 100% color gamut

— Two optical structures used for a bottom-emitting white organic light-emitting diode (OLED) is reported. An RGBW color system was employed because of its high efficiency. For red, green, and blue (RGB) subpixels, the cavity resonance was enhanced by the use of a dielectric mirror, and for the white (W) subpixel, the mirror was removed. The optical length of the cavities was controlled by two different ways: by the thickness of the dielectric filter on top of the mirror or by the angle of oblique emission. With both methods, active-matrix OLEDs (AMOLEDs) that reproduced a color gamut exceeding 100% of the NTSC (National Television System Committee) standard were fabricated. More importantly, the transmission of a white OLED through R/G/B color filters was significantly higher (up to 50%) than that of a conventional structure not employing a mirror, while at the same time as the color gamut increased from ∼75 to ∼100% NTSC.

Analysis of metal-interlaced-grating vertical-cavity surface-emitting lasers using the modal method by modal expansion

It is analyzed by the modal method by modal expansion (MMME) that the strong polarization discrimination of a vertical-cavity surface-emitting laser (VCSEL) can be effectively obtained by using the metal-interlaced grating on the topmost layer of the VCSEL. The MMME is used to overcome the problems coming from the large permittivity discontinuities at GaAs-Al-air-Al interfaces. The analyses predict the correct direction of dominant polarization and the mode-suppression ratio. The calculated transmission efficiencies explain the characteristics of the VCSELs below threshold. It is found that the difference in phase and amplitude between the two orthogonal polarization generated by the grating plays important roles for the selection of a dominant polarization through mirror reflectivities and mirror losses. In addition, the influence of various thin metal layers on polarization stabilization is demonstrated.

A smartphone-based fluorescence microscope utilizing an external phone camera lens module

We constructed a smartphone-based fluorescence microscope by adding a phone camera lens module to the outside of a smartphone camera. The external lens module forms a relay system in conjunction with the built-in phone camera. The system benefits from the small pixel size of the smartphone and other advantages associated with the phone camera lens module such as its low cost, small volume, and low weight. To realize the fluorescence microscope function, the filters and illumination source were integrated into a simple mechanical structure with a focusing adjustment scheme. Based on an image analysis, the resolution of the microscope was approximately 2.5 µm and the field of view was as large as 1.2 mm×1.2 mm. The fabricated microscope was used successfully to visualize the fluorescent paint and the low concentration quantum dots on a slide glass. The microscope obtained fluorescence images of cells stained with carboxyfluorescein succinimidyl ester (CFSE). This demonstration of a smartphone-based fluorescence microscope supports the feasibility of applications such as point-of-care diagnostic equipment based on fluorescence methods.