CARACTERES DE LA CIVILIZACIÓN HELENÍSTICA

considerations

Annular Bragg resonators with large index contrast Bragg reflectors were realized in a membrane of active quantum well (QW) InGaAsP semiconductor material. The broadband photoluminescence from InGaAsP QWs provides a means of probing the modal properties of ABR devices at wavelengths in the near infrared relevant to opti- cal telecommunication. In addition, optical gain within such a QW material allows for investigation of the performance of ABR microcavities as low-threshold lasers, a sub- ject which will be explored in greater detail in Chapter 5. The semiconductor medium used is illustrated in Fig. 4.2. The 250 nm thick InGaAsP membrane consisted of 6 strained InGaAsP QWs and their barrier layers (75 Å wells, 1% compressive strain; 120 Å barriers, 0.5% tensile strain,λg= 1.2µm), which were sandwiched between two 605 Å InGaAsP (λg = 1.2 µm) layers. The peak photoluminescence from the QWs occurred atλpeak = 1559 nm. All epitaxial layers were grown by metal-organic chem- ical vapor deposition (MOCVD) on an InP substrate [89]. The 250 nm InP sacrificial layer can be selectively removed to produce an air gap below the patterned InGaAsP membrane, as in the case of the 2D photonic crystals discussed above. However, in the ABR fabrication process, the InP sacrificial layer and 50 nm InGaAsP stop etch are used to facilitate transfer of the InGaAsP QW membrane to an alternative substrate, as will be described below.

The out-of-plane optical confinement was investigated using a custom one-dimensional

Figure 4.2: Schematic of the InGaAsP quantum well membrane epistructure used for the ABR devices. Where shown, the quantity in parentheses, i.e., (Q1.1), refers to the bandgap wavelengthλg of the InGaAsP alloy in the given layer.

modes of the 250 nm thick InGaAsP slab. Since the optical gain in the 1% com- pressively strained InGaAsP quantum wells is greatest for electric fields polarized in the plane of the quantum wells [91, 92], i.e., Er, and Eθ, the optical design focused upon the TE polarized ABR modes, which have field components Hz, Er, and Eθ. The TE polarized modes are expected to have the lowest laser threshold, given the compressively strained quantum well design [93, 94].

The refractive index of the various quaternary alloys in the epitaxial structure of Fig. 4.2 were calculated using the empirical formula in reference [95], assuming a wavelength of λ = 1.55 µm. Figure 4.3 plots the refractive index profile of the as-grown InGaAsP membrane structure along the growth axis (z-axis), superimposed with the transverse electric field profile Ex(z) of the fundamental TE polarized slab mode (electric field and x-axis parallel to the plane of the membrane). With the membrane in contact with the high-index (n_∼ 3.17 at λ = 1.55µm) InP substrate, the fundamental slab mode exists at the cut-off condition [65], where the modal effective indexnef f equals the refractive index of the substrate, i.e. nef f ∼nsubstrate = 3.17. As a result, the mode is broadly distributed and extends deep into the InP substrate. Furthermore, the mode has peak amplitude at the depth of the InGaAsP stop etch layer, and thus possesses a small overlap with the quantum wells in the center of the membrane. These factors predict that the while the InGaAsP semiconductor membrane remains clad from below by the high-index InP substrate, the optical modes of ABR structures patterned into the membrane will suffer from large substrate radiation losses and low optical gain.

In order to achieve strong out-of-plane optical confinement, the InGaAsP mem- brane must be clad by low-index material both above and below. As discussed above, the ABR geometry prohibits use of the suspended membrane configuration. There- fore, an epitaxial layer transfer technique [96, 97], using UV-curable Norland optical adhesive NOA 73 (Norland Products,n_∼1.56atλ = 1.55µm), was adopted to facil- itate transfer of the InGaAsP membrane to a transparent sapphire substrate. Figure 4.4 shows the fundamental TE polarized mode supported when the InGaAsP mem- brane is clad below by the NOA 73 adhesive and above by air. In comparison with

Figure 4.3: Fundamental TE polarized slab mode profile Ex(z)supported by the as- grown InGaAsP-InP epistructure, superimposed against the refractive index profile, evaluated at λ = 1.55 µm. The mode is nearly cut off, and extends deep into the high-index InP substrate.

Fig. 4.3, the optical mode is far more confined within the membrane, and has a peak amplitude overlapping with the quantum wells, as desired for maximizing the optical gain. The peak is shifted slightly towards the NOA 73 layer, due to the asymmetry in refractive index between the air and NOA 73 claddings. Numerical calculations con-

firmed that the transferred membrane supported only a single TE transverse mode, with nef f = 2.822. Therefore, an effective index of 2.8 has been assumed for the majority of the ABR numerical designs and experimentally demonstrated devices presented in Chapters 3 and 5, respectively.