Duchang Heo

Parametric study on optical properties of digital-alloy In(Ga1−zAlz)As/InP grown by molecular-beam epitaxy

Optical properties of digital-alloy InGaAlAs grown by molecular-beam epitaxy were parametrically investigated by 10-K-photoluminescence (PL) study on (In0.53Ga0.47As)n/(In0.52Al0.48As)n short-period superlattices (SPSs) in the range of n=1–5 monolayers. Two different peaks are resolved in PL spectra, and the higher energy peak (H) results from an excitonic transition while the lower energy peak (L) is related to a phonon-assisted transition. The H peak energies decrease monotonously as n increases, and it is in good agreement with the band-gap calculation with transfer matrix methods. It is found that two monolayer-period length (n=2) is the optimum one, where the PL intensity is largest and the ratio of L peak to H peak intensity is lowest. The various compositions (z) of digital-alloy In(Ga1−zAlz)As are prepared and their optical properties are investigated for z=0.2, 0.4, 0.6, and 0.8. The linewidths of 9-K-PL spectra for various z values are within the range of 10–15 meV, which are comparable to the b...

Study on InGaAsP-InGaAs MQW-LD with symmetric and asymmetric separate confinement heterostructure

We studied symmetric and asymmetric InGaAsP-InGaAs 1.55-/spl mu/m multiquantum-well (MQW) laser diodes (LDs) with highly p-doped layers in the two-step separate confinement heterostructure (SCH). The p-doping in p-SCH suppresses the electron overflow from the MQWs to p-SCH, but it is an origin of free carrier absorption loss. An additional InGaAsP layer inserted inside n-SCH makes asymmetric field distribution and, therefore, reduces the portion of optical field distribution in highly p-doped regions with high optical loss. Compared with symmetric structure, asymmetric SCH LD has low threshold current density, low internal loss, and high and flat slope efficiency with respect to temperature.

Design and Fabrication of 1.35-

We report full digital-alloy In(Ga1-zAlz)As/InP multiple-quantum well 1.35-μm laser diodes using molecular beam epitaxy. The wells and barriers consist of five pairs of InGaAs/InAlAs (1.5 nm/0.375 nm) and four pairs of InGaAs/InAlAs (0.66 nm/0.98 nm). The separate confinement layer consists of two 60 pairs of InGaAs/InAlAs (0.66 nm/0.98 nm) and makes for an optical confinement factor of 7.07%. We obtained a continuous wave of 200 mW from a single cleaved facet of 1.6-mm long broad area LDs, with 100-μm aperture width at 10°C, and high characteristic temperature T0 of 70 K. In this paper, we find that, with the MBE, the full digital-alloy technique makes bandgap engineering possible through the entire LD structure with only InGaAs/InAlAs short-period superlattices pairs.

\mu{\rm m}

High-power femtosecond (fs) lasers in the visible wavelength regime have numerous applications in areas including micro-machining, medical eye surgery, communication, spectroscopy, etc. To generate this laser beam, frequency conversion, especially second-harmonic generation (SHG), of near-IR lasers using nonlinear optical crystals is known to be the most standard technique. However, the use of a long-length crystal, which is preferred to achieve high SHG conversion efficiency for long-pulse or cw lasers, cannot be applied to the fs laser with broad linewidth due to the tight phase matching condition and the exacerbated walk-off effect. Thus the conditions of the nonlinear optical crystal should be optimized to achieve efficient SHG generation and hence, the high power visible fs laser pulses. There are many reports for the efficient SHG of the fs lasers but not many reports about influence of the crystal length on the SHG process, such as the pulse width and the linewidth and the conversion efficiency. Here, we report efficient SHG of femtosecond Yb lasers at 1 um by optimizing the conditions of nonlinear optical crystals. The SHG pulse and the conversion efficiency were numerically calculated to find the optimized conditions of the nonlinear optical crystals for the high power fs laser pulses with different pulse widths. Preliminary experiments were conducted using a Type I LBO crystal and the femtosecond Yb laser at 1 um, which was in good agreement with the theoretical results. The theoretical and the experimental results for LBO and BBO crystals will be reported in detail.

Laser Diodes With Full Digital-Alloy InGaAlAs MQW

Summary form only given, as follows. Summary form only given. We have studied optical and structural properties of a-SiN/sub x/ thin films prepared by plasma method. While there are several methods of preparing a-Si based alloy (a-SiN/sub x/), plasma methods, such as PECVD and sputtering, have been widely used because of its good film quality and low deposition temperature. a-SiN samples were prepared on c-Si substrates by PECVD at 270/spl deg/C with pure NH/sub 3/ and 5%-diluted SiH/sub 4/ in N/sub 2/ as a precursor, and plasma was generated by RF power. All the deposition conditions were fixed except plasma power. We have observed two photolummescence (PL) peaks at about 3.0 eV and 2.5 eV at room temperature by He-Cd laser with 3.82 eV excitation energy. The former peak is due to the defect structures in SiN/sub x/ and the latter small silicon cluster. With the increase of plasma power, the PL properties of green band were improved with the increase of intensity, and the narrowing of full width at half-maximum. This result is attributed to the enhancement of nano-crystal silicon because the plasma power enhances the formation of radicals such as SiH, SiH/sub 2/, and SiH/sub 3/from precursors.

Optimization of second harmonic generation in high power femtosecond Yb lasers (Conference Presentation)

We improved the performance of InGaAsP/InGaAs 1.55 /spl mu/m multi-quantum well laser diodes with differently highly p-doped layers in the two-step separate confinement heterostructure (SCH) by inserting additional InGaAsP layer in n-SCH, which makes asymmetric field distribution and reduces the optical loss in highly p-doped regions. To reduce the inevitable optical loss that maybe due to high-level doping, we inserted additional 100 nm InGaAsP layer in the 1st n-SCH, which shifts the field distribution from highly p-doped regions with high loss to intrinsic or n-doped region with relatively low loss, and improved the light-current characteristics.

Visible photoluminescence from a-Si based alloy (a-SiN/sub x/) thin films deposited by plasma enhanced CVD

In this presentation, the electrical and optical properties of In/sub 0.5/Ga/sub 0.5/As/GaAs QD LD grown with ALMBE are reported. In/sub 0.5/Ga/sub 0.5/As/GaAs QD LD was grown on [001]n-GaAs substrate. The total density of 3-stacked QD layers is /spl sim/5/spl times/10/sup 10//cm/sup 2/, and the average height and width of each QD are 7 and 45 nm, respectively. A room temperature photoluminescence (PL) peak is shown to have linewidth of 31.3 meV and the peak wavelength is about 1.24 /spl mu/m. Light-current curve of the broad area laser diode with 2 mm cavity length and 200 /spl mu/m width at temperature of 15 /spl deg/C was also shown. The current injection conditions were 1 /spl mu/s per 1 ms and the lasing threshold current is 2 A, which corresponds to a threshold current density of 500 A/cm/sup 2/. The external quantum efficiency is 40% while the internal quantum efficiency is 66% with a 45 cm/sup -1/ internal loss.

The study on InGaAsP/InGaAs MQW-LD with symmetric and asymmetric separate confinement heterostructure

A high-temperature characteristics of 1.52 /spl mu/m InGaAs ridge waveguide laser diode (LD) with InGaAs/InAlAs short period superlattices (SPSs) barriers is reported. The SPSs are grown by digital-alloy molecular beam epitaxy (MBE). There have been similar studies at the 0.98 /spl mu/m LD using GaAs/AlGaAs SPSs barrier (T/sub 0/=300 K) and 1.3 /spl mu/m InGaAsP LD using (InGaAsP)/sub 4//(InP)/sub 5/ SPSs barrier (T/sub 0/=90 K). On the basis of above context, It is reasonable to adapt SPSs to InGaAlAs/InGa(Al)As system. In this letter, a high-temperature characteristics of 1.52 /spl mu/m InGaAs LD with (InAlAs)/sub 0.4/(InGaAs)/sub 0.6/ SPSs barriers grown by digital alloy MBE technique is reported for the first time.