Method
The spectral loss and gain of the quantum dot material was obtained following the multi-section method of Thomson et al. [42]. The method allows one to
5. Quantum-dot Material - Theory and Characterisation 50
Thickness (µm) Repeats Description Composition and Doping
0.1 Cap GaAs p = 5x1020 0.015 Top Cladding Ga0.4Al0.6As p = 3 x1018 0.4 Ga0.4Al0.6As p = 1 x1018 0.5 Ga0.4Al0.6As p = 5 x1017 0.033 10 Waveguide GaAs 0.005 10 Quantum-well Ga0.85In0.15As 0.0008 10 Quantum-dots InAs 0.033 Waveguide GaAs 0.5 Lower Cladding Ga0.65Al0.35As n = 5 x1017 1 Ga0.65Al0.35As n = 1 x1018 0.02 GaAs n = 3 x1018 Substrate GaAs n+
Tab. 5.1: Material composition of quantum-dot material
obtain the gain and loss spectra via measurements from a single multi-section device. One major advantage of this is that any coupling loss in the mea- surement set-up remains constant for all measurements. The device consists of a broad-area ridge with five or six identical sections, each with their own separate contact for electrical excitation. Feedback is prevented by a long ab- sorbing region and an angled facet, thus avoiding lasing. In this experiment an un-cleaved facet was used instead of an angled facet. A schematic of the device is shown in figure 5.3(a).
50 µm wide stripes were defined using photo-lithography and the pattern transferred into the quantum-dot material using chemically assisted ion beam etching (CAIBE) with Chlorine chemistry. The ridges were etched to a depth of 2 µm, through the active region. Full-length contacts were laid down on the ridges using an electron beam evaporation step and a lift-off step. A third photo-lithography step was used to define 4µm wide slots perpendicular to the contacts at equal intervals (250µm) along the contacts, to create multi-section devices. The slots were then ion-milled (CAIBE etching at a low temperature with no chemical component) to electrically isolate each of the sections. The devices were then cleaved at the appropriate end and mounted p-side up on an earthed copper block (figure 5.3(b)). The device output was coupled through a lens into an optical spectrum analyser (OSA). An example of the emission
5. Quantum-dot Material - Theory and Characterisation 51
Fig. 5.3: (a) Schematic of multi-section device for measurement of gain and loss spectra (taken from [42]) and (b) Measurement set-up for multi-section device measurements
spectrum from a multi-section device taken by the OSA is shown in figure 5.4. The emission spectra shown are for each section pumped individually. Note how the peak of this emission shifts to longer wavelength as the excited section moves farther from the output facet. This is due to higher absorption of shorter wavelengths in the unpumped absorbing sections. The material gain peak is at a wavelength of 1280 nm in reality (see below). The emission spectrum is fairly broad (∼150 nm) allowing the gain and loss spectra to be measured over the same broad range.
Loss Spectrum
The loss spectrum can be obtained from a multi-section device by pumping each section independently and measuring the output emission spectrum of each (as in figure 5.4). Firstly the emission spectrum is taken with section 1 being pumped only. It is then measured, for the same current density, for section 2 only, section 3 only and so on. As each section is the same length, they act as similar sources of amplified spontaneous emission (ASE) at different distances from the output facet. The passive distance is increased as each section is successively pumped. The output intensity, It, can be related
5. Quantum-dot Material - Theory and Characterisation 52
Fig. 5.4: An example of the emission spectra, from a multi-section device, obtained by the optical spectrum analyser. The spectra shown are for each section pumped individually at 25mA
to the initial ASE intensity, I0, via
It=I0 e−αiL
(5.1) whereLis the passive length transmitted through andαi is the internal modal loss of the material. It is easily seen from equation 5.1 that a plot of ln(It) versus L should be a straight line plot with gradient −αi. Figure 5.5 shows two such plots for wavelengths of 1280 nm (peak emission wavelength) and 1325 nm. The scatter in the straight line is small, indicating low experimental error. Some error may have arisen from slightly differing section lengths. By analysing all the data from the OSA measurements in the same manner, a plot of internal loss, αi, versus wavelength can be obtained (figure 5.6). The absorption edge at∼1280 nm can clearly be seen in figure 5.6. The plot is less smooth to the left of the absorption edge. This is because signal strength is lower in this region (due to increased absorption) and this leads to an increase in uncertainty. The loss in the non-absorbing region is∼6 cm−1. This value is in good agreement with other measurements on the material [64]. Any loss in this spectral region arises from optical scattering and free-carrier absorption only.
5. Quantum-dot Material - Theory and Characterisation 53
Fig. 5.5: Logarithmic plot of Intensity versus passive transmission length for multi- section devices at (a) 1280 nm and (b) 1325 nm
Fig. 5.6: Loss spectrum of quantum dot material obtained via multi-section method