G.T. Liu

Low-threshold current density 1.3-μm InAs quantum-dot lasers with the dots-in-a-well (DWELL) structure

The wavelength of InAs quantum dots in an In/sub 0.15/Ga/sub 0.85/As quantum-well (DWELL) lasers grown on a GaAs substrate has been extended to 1.3-/spl mu/m. The quantum dot lasing wavelength is sensitive to growth conditions and sample thermal history resulting in blue shifts as much as 73 nm. The room temperature threshold current density is 42.6 A cm/sup -2/ for 7.8-mm cavity length cleaved facet lasers under pulsed operation.

The influence of quantum-well composition on the performance of quantum dot lasers using InAs-InGaAs dots-in-a-well (DWELL) structures

The optical performance of quantum dot lasers with different dots-in-a-well (DWELL) structures is studied as a function of the well number and the indium composition in the InGaAs quantum well (QW) surrounding the dots. While keeping the InAs quantum dot density nearly constant, the internal quantum efficiency /spl eta//sub i/, modal gain, and characteristic temperature of 1-DWELL and 3-DWELL lasers with QW indium compositions from 10 to 20% are analyzed. Comparisons between the DWELL lasers and a conventional In/sub 0.15/Ga/sub 0.85/As strained QW laser are also made. A threshold current density as low as 16 A/cm/sup 2/ is achieved in a 1-DWELL laser, whereas the QW device has a threshold 7.5 times larger. It is found that /spl eta//sub i/ and the modal gain of the DWELL structure are significantly influenced by the quantum-well depth and the number of DWELL layers. The characteristic temperature T/sub 0/ and the maximum modal gain of the ground-state of the DWELL structure are found to improve with increasing indium in the QW It is inferred from the results that the QW around the dots is necessary to improve the DWELL lasers /spl eta//sub i/ for the dot densities studied.

Transition dipole moment of InAs/InGaAs quantum dots from experiments on ultralow-threshold laser diodes

Semiconductor ultralow-threshold InAs quantum-dot lasers are investigated operating at 1230–1250 nm at room temperature (laser threshold range is of 16–83 A/cm2 for ground-state emission). The dependence of gain on current is derived from measurements of the threshold current as a function of the cavity length. The ground-state gain appears at very low current: the inversion threshold of ∼13 A/cm2 is a record low value. Analysis of these data for diodes of different molecular beam epitaxial-grown wafers leads to a squared dipole moment of the transition of ∼9.2×10−57 C2 m2 that corresponds to the length of elementary dipole of ∼0.6 nm.

Very low threshold current density room temperature continuous-wave lasing from a single-layer InAs quantum-dot laser

Continuous-wave (CW) lasing operation with a very low threshold current density (J/sub th/=32.5 A/cm/sup 2/) has been achieved at room temperature by a ridge waveguide quantum-dot (QD) laser containing a single InAs QD layer embedded within a strained InGaAs quantum well (dot-in-well, or DWELL structure). Lasing proceeds via the QD ground state with an emission wavelength of 1.25 /spl mu/m when the cavity length is longer than 4.2 mm. For a 5-mm long QD laser, CW lasing has been achieved at temperatures as high as 40/spl deg/C, with a characteristic temperature T/sub 0/ of 41 K near room temperature. Lasers with a 20 /spl mu/m stripe width have a differential slope efficiency of 32% and peak output power of >10 mW per facet (uncoated).

150-nm tuning range in a grating-coupled external cavity quantum-dot laser

An antireflection (AR) coated single-stack quantum-dot (QD) laser in a grating-coupled external cavity is shown to operate across a tuning range from 1.095 /spl mu/m to 1.245 /spl mu/m. This 150-nm range extends from the energy levels of the ground state to excited states. At any wavelength, the threshold current density is no greater than 1.1 kA/cm/sup 2/. This large tunable range is the product of the rapid carrier filling of the higher energy states under a low pumping current and homogeneous broadening in the QD ensemble. The possibility of a larger tuning range is discussed with the further improvement of the AR-coating.

1.58-μm lattice-matched and strained digital alloy AlGaInAs-InP multiple-quantum-well lasers

A versatile, digital-alloy molecular beam epitaxy (MBE) technique is employed to grow lattice-matched and strained AlGaInAs multiple-quantum well (MQW) 1.58-/spl mu/m laser diodes on InP. A threshold current density as low as 510 A/cm/sup 2/ has been demonstrated for broad area lasers with 1% strained AlGaInAs MQWs, which is the best MBE result in this material system. A single facet pulsed power as high as 0.56 W is obtained. It is also found that the efficiency and internal loss of digital alloy AlGaInAs QW devices are comparable to lasers grown by conventional MBE.

One and three-stack quantum dot lasers with very low threshold current density

Summary form only given. Quantum dot lasers with the dots-in-a-well (DWELL) structure have demonstrated very low threshold current densities. In particular, DWELL lasers with a single layer of InAs dots in an In/sub 0.15/Ga/sub 0.85/As well have demonstrated threshold current densities as low as 16 A-cm/sup -2/, which is the lowest threshold current density of any semiconductor laser. However, there are potential limitations to these lasers. Even though the ground state lasing threshold current density is very low, the ground state also has low saturated modal gain (/spl sim/4.5 cm/sup -2/). At a cavity length or 4 mm, the first excited state emission can be observed at high current levels. When the cavity length is further decreased increasing the cavity loss, lasing is purely from excited states. Also, the average T/sub 0/ value of these 1-DWELL lasers is only 45 K between 10/spl deg/C and 80/spl deg/C, lower than that reported by other groups for QD lasers. Three DWELL layers are used to increase the ground state gain and T/sub 0/ value. A quantum well laser is also grown to study limitations on the T/sub 0/ value of the DWELL structure.

Excitation Wavelength and Saturation Effects on Gallium Nitride Photoluminescence

The authors report on the excitation wavelength and power dependence of LPMOCVD grown GaN photoluminescence (PL). A mode-locked Ti:Sapphire laser that is doubled to provide a tunable wavelength range of 350--500 nm is used to pump the GaN samples. Special attention has been paid to the yellow ({approximately} 567 nm) luminescence. Different power dependence characteristics of the yellow emission are observed when pumping above and below the bandgap. A measurement of the photoluminescence excitation spectrum of the yellow peak ({approximately} 567 nm) is also performed and this spectrum shows a peak at room temperature at 3.36 eV (369 nm), an additional broad peak at 77 K and only the broad peak at 7 K. An excitation model is used to explain the behavior of the yellow emission, suggesting the neutral donor serves as a key species for the yellow emission.

Carrier lifetime and radiative recombination in quantum dot LEDs

We have measured the differential carrier lifetime as a function of current density for QD LED samples and used this data to calculate the functional relationship between the carrier lifetime, carrier density and radiative efficiency. The results indicate that carrier filling on the different dot energy levels has a strong influence on the radiative behavior of the devices and that the radiative rate coefficient, B, for different QD levels can vary considerably.

Gain in ultra-low-threshold InAs/InGaAs quantum dot lasers

Semiconductor lasers are grown on a GaAs substrate by MBE containing self-assembled InAs quantum dots (QDs) in an InGaAs quantum well, the so-called dot-in-a-well (DWELL) structure. The QDs are /spl sim/15 nm in diameter in the basal plane and /spl sim/7 nm in height. Several wafers are investigated in detail. The in-plane dot density is either 2.5x10/sup 10/ or 7.5x10/sup 10/ cm/sup -2/. The ground-state emission wavelength is 1230-1250 nm at room temperature, and the spectral FWHM is from 40 to 75 nm. We obtained and analyzed the dependence of the gain on current density in ultra-low-threshold laser diodes with DWELL quantum-dot structures.