Densely Packed VCSEL Arrays Tailored for Optical Particle Manipulation

Abstract

Recently, the use of vertical-cavity surface-emitting lasers (VCSELs) as light sources for optical particle trapping and manipulation has gained increasing interest, owing to high beam quality, low cost and the possibility to fabricate two-dimensional, monolithic laser arrays (see Refs. in [1]). Also, typical emission wavelengths are in the near infrared spectral region, which is important for damage-free manipulation of biological material. However, mainly standard VCSEL arrays designed for data communication have been applied so far, which show a large pitch of typically 250 µm and necessitate the use of additional microlens arrays. Furthermore, for a drastic miniaturization of the trapping setup towards the so-called integrated optical trap, the typical multimode output beam profile of VCSELs is inappropriate [1]. Therefore, we have fabricated arrays of top-emitting VCSELs with pitches down to 24 µm and an only 2 µm wide gap between the devices. For enhanced single-mode emission, the inverted surface relief technique [1] was applied on some devices. The output facets then contain a shallow, circular etch in their center. To guarantee steep mesa etching and an accurate alignment of oxide aperture, p-contact ring and relief, a novel fabrication process has been developed, based on multiple resist layers. At first, relief and p-contact ring are structured within the same exposure step, so selfalignment is achieved. Afterwards, two subsequent resist steps with large alignment tolerance enable selective wetchemical relief etching and metallization. Finally, the p-contact metal serves as mask for reactive ion etching of the mesa, such that the following oxidation step leads to a self-aligned oxide aperture. In Fig. 1, a scanning electron microscope (SEM) image of an array with 24 µm pitch and 3.4 µm surface relief diameter is presented, showing almost vertical mesa edges and exact alignment. The process is completed by surface passivation and bond pad metallization. Fig. 2 Operation characteristics of an array containing 15 individual devices without relief (left) and of three relief devices driven in parallel (right). The inset shows the optical spectra of all three devices at 30 mA.The left side of Fig. 2 presents the output characteristics of 15 individually addressable devices forming an array with 8 µm oxide apertures but without relief. Threshold currents of around 1.6 mA and maximum multimode output powers of about 8 mW with only minor variations are observed. Since we plan to implement an optical particle sorting scheme where an individual addressing of lasers is not required [2], also arrays with devices connected in parallel were fabricated in order to decrease the number of contacts significantly. The right side of Fig. 2 shows the light versus current curve of three jointly lasing devices, which have a 6 µm wide oxide aperture and 3.4 µm relief diameter. A mean threshold current of 1.7 mA and a high maximum output power of 3.8 mW for each device can be deduced. The inset shows the optical spectra of all ...