Quantum cascade lasers with integrated plasmonic antenna-array collimators

School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
Optics Express (Impact Factor: 3.53). 12/2008; 16(24):19447-61. DOI: 10.1364/OE.16.019447
Source: PubMed

ABSTRACT We demonstrated in simulations and experiments that by defining a properly designed two-dimensional metallic aperture-grating structure on the facet of quantum cascade lasers, a small beam divergence angle can be achieved in directions both perpendicular and parallel to the laser waveguide layers (denoted as theta perpendicular and theta parallel, respectively). Beam divergence angles as small as theta perpendicular=2.7 degrees and theta parallel=3.7 degrees have been demonstrated. This is a reduction by a factor of approximately 30 and approximately 10, respectively, compared to those of the original lasers emitting at a wavelength of 8.06 microm. The devices preserve good room temperature performance with output power as high as approximately 55% of that of the original unpatterned lasers. We studied in detail the trade-off between beam divergence and power throughput for the fabricated devices. We demonstrated plasmonic collimation for buried heterostructure lasers and ridge lasers; devices with different waveguide structures but with the same plasmonic collimator design showed similar performance. We also studied a device patterned with a "spider's web" pattern, which gives us insight into the distribution of surface plasmons on the laser facet.

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    ABSTRACT: A slot plasmonic waveguide is promising solution as a replacement of electrical interconnects in the future optical integrated circuits. In this contribution we consider a set of compact solutions for coupling the infrared light from free space to the plasmonic slot waveguide. We systematically study various designs: dipole antennas outside the waveguide, antennas inside the waveguide and bow-tie antennas in the slots. Plasmonics provides groundbreaking solutions for extreme light concentration [1], that allows subwavelength light guiding [2]. Among the most promising is a plasmonic slot waveguide [3] that simultaneously allows for subdifraction mode confinement and reasonable propagation length of tens of micrometers. Moreover, metallic parts can serve as electrodes modulating the properties of the dielectric environment in the slot. From the other side, small size of the slot mode makes difficult to couple light from free space or an optical fiber into the plasmonic waveguide. Plasmonic nanoantennas, besides the applications for photodetectors [4], lasers [5] and light-emitting diodes [6] improvement, have been recently proposed as couplers to the plasmonic slot [7,8] and nanowire [9] waveguides. In this contribution we consider compact nanoantenna based solutions for efficient coupling of infrared light from free space to the plasmonic slot waveguide. We systematically study various designs: dipole antennas outside the waveguide, antennas inside the waveguide and bow-tie antennas in the slots (see Fig. 1). Figure 1. Sketch of the regarded nanoantenna coupler designs, white slits are air; (a) dipole antennas outside a waveguide, (b) antennas inside a waveguide, (c) bow-tie antennas in slots. The dipole antenna (Fig. 1(a)) and bow-tie antenna (Fig. 1(c)) are placed externally to the gap. They excite plasmons in the gap, which then propagate in the slot waveguide. The antennas inside the waveguide (Fig. 1(b)) act in a similar manner but from the inside of the slot waveguide. Connecting several antennas gives an opportunity to capture radiation from larger area. Simulation of the nanoantenna couplers is done in the CST Microwave Studio. The antennas are excited with a beam, which is Gaussian in one direction and homogeneous in the other (analogue of a beam focused with a cylindrical lens). For characterization of the nanocoupler performance we define antenna's figure of merit (AFOM) equal to the product of the coupling efficiency and the excitation spot area. AFOM is related to an effective area from which the antenna gain signal. Figure 2. Antenna's figure of merit (AFOM) for (a) dipole antennas outside a waveguide, (b) antennas inside a waveguide and (c) bow-tie antennas in slots. The results are shown for different number of antennas: zero (black squares), one (orange circles), two (green triangles) and three (red stars) Dipole antennas outside the waveguide and antennas inside the waveguide show similar AFOM (see Fig. 2(a) and (b)). Bow-tie antennas in the slot show better performance with AFOM reaching 1.4 µm 2 for three connected antennas (see Fig. 2(c)). In all cases several connected antennas allow for more efficient coupling into the slot waveguide. However, fast saturation effects are observed for the first two cases. During the presentation the results of fabrication and experimental characterization will be shown. Pros and cons of each system will be reported. The nanoantenna based couplers can provide a compact and efficient interface between optical fibers and plasmonic slot waveguides as well as directive in-chip communication systems.

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