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Quantum Cascade Surface-Emitting Photonic Crystal Laser |
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Research in semiconductor heterostructures has led to the development of a number of optoelectronic devices in which the flow of electrons is controlled with great precision. The quantum cascade (QC) laser, one product of such progress in electronic bandstructure engineering, operates based upon intraband optical transitions (within conduction band states, or subbands) where electrons flow through a semiconductor superlattice "staircase", emitting a photon at each step. Such devices are hence unipolar (single carrier), and thus operate in a fundamentally different manner than standard semiconductor lasers, which rely upon electron-hole recombination for light generation. Analogous to this electronic bandstructure engineering, optical dispersion engineering through the use of artificially patterned materials, or photonic crystals (PCs), has been used to influence the propagation of light with a steadily increasing amount of control. By introducing a photonic crystal microcavity as the source of optical feedback within a QC heterostructure, a laser combining aspects of both electronic and photonic bandstructure engineering is created. Such a device has a number of aspects that motivate its study. Because the intersubband transitions in QC lasers are naturally transverse magnetic (TM) in nature, traditional QC lasers are intrinsically only in-plane emitters, though surface emission would be a useful characteristic for many applications. The PC microcavity that we employ both acts as a source of optical feedback and as the means for diffracting light vertically from the chip to provide surface emission. In addition to enabling surface emission, ourdevices are greatly scaled down from standard QC devices, enabling miniaturization and on-chip integration of QC lasers, with potential applications such as multi-wavelength two-dimensional laser arrays for spectroscopy envisioned. In addition, PC QC lasers are an interesting system for research on photonic bandgap structures, as their unipolar nature, operation through electrical injection, and long emission wavelengths (and hence larger device feature sizes) are unique and advantageous aspects in comparison to previously studied interband PC lasers. In particular, the demonstration of an electrically-injected PC microcavity laser is an important step in the development of PC technology for practical applications. This work began as a collaboration between researchers in the Quantum Cascade laser group at Lucent Technologies' Bell Laboratories and our group at Caltech, and with the movement of members of the team to different institutions, now encompasses L'Institut d'Electronique Fondamentale, Universite Paris-Sud (Raffaele Colombelli), Harvard University (Federico Capasso and Mariano Trocolli), and Princeton University (Claire Gmachl). Our recent results, highlighted by the demonstration of an electrically-injected, surface-emitting, quantum cascade photonic crystal microcavity laser, are the focus of recent publications [1-5]. A very brief summary is given below. Principle of Operation Connected photonic lattices do not exhibit an in-plane band-gap for modes of TM polarization. Nevertheless, as long as the PC lattice is of high enough index contrast that strong optical feedback can be achieved, modal localization on the scale of a few lattice periods can be achieved. Indeed, a complete in-plane bandgap is not even required to form high quality (>10^4) factor, ultra-small mode volume resonances, as we discuss in our work on high-Q PC microcavities. Unlike that work, here we do not create defect states by introducing a localized perturbation to the dielectric lattice, but rather employ a band-edge state where the laser operates in a region of energy-momentum space with a high photonic density of states (i.e. small group velocity), as a result of the distributed feedback of the lattice. A schematic
detailing the major ingredients involved in the device's operation is
shown below. Injected electrons seed the cascaded photon generation process
in the QC heterostructure by which light is created. This light is confined
as a result of distributed bragg reflection (DBR) in-plane and total internal
reflection (TIR) in the vertical direction. Second-order diffraction via
the photonic lattice results in surface emission.
Figure 1. Operation of the PC QC laser
We use a combination of photonic bandstructure calculations and finite-difference time-domain (FDTD) simulations to design our PC cavities, which consists of a hexagonal lattice of air holes etched into a QC heterostructure operating at ~ 8 mm. For the PC band-edge states of interest to overlap the QC gain spectrum, a lattice spacing a~2.8 mm with a hole radius to lattice spacing ratio r/a=0.30 is appropriate. The PC patterns are created[4] by electron beam lithography, mask transfer to a dielectric oxide layer, and transfer into the heterostructure material by inductively-coupled plasma reactive ion etching. The deep etch through the vertical waveguide core region into the bottom cladding layer produces a high-index contrast semiconductor-air grating (Figure 2), reducing substrate radiation losses and ensuring that only a small number of PC periods (less than 8 optical wavelengths in diameter) are required to provide strong optical feedback, in contrast to traditional second-order grating based devices which typically employ a shallow etch (weak grating) and require several hundred periods of the lattice. Figure 2: SEM micrographs of an etched PC QC device After etching of the PC pattern, an insulating silicon nitride layer is deposited surrounding the PC cavities, and top and back metal contact layers are evaporated, with the etched sidewalls sufficiently vertical to prevent electrical shorting. In addition, a thin metal layer is evaporated on the surface of the cavities. This metal layer serves to create a bound surface plasmon mode in the vertical direction of the waveguide, allowing for a less deep etch than what would be necessary if a standard vertical waveguide (semiconductor top and bottom claddings) was employed. Devices are cooled to 10K and electroluminescence measurements are performed, with light collected vertically from the top surface of the devices. Results Laser emission has been achieved (operating in pulsed mode with 50 ns pulse width at 5 kHz repetition rate), and is seen to tune with the hole radius and lattice spacing of the PC cavity in accordance with simulation predictions. A series of spectra from lasing devices, all fabricated on the same semiconductor chip, are shown in Figure 3. As described in Ref. [5], a careful analysis of the experimental data (spectral information, far-field emission measurements and polarized intensity measurements) and numerical simulations shows a close correspondence between theory and simulation, and provides a unique identification of the lasing mode.
Current device performance is limited to low temperature operation, with relatively high threshold current densities (though still within a factor of ~ 2 of those found in stripe lasers fabricated in the same material). Future work on this topic will focus on improvements in the performance of current devices, particularly by reducing laser thresholds through more efficient electrical current injection schemes. Applications in spectroscopy and nonlinear optics will be considered, as will extension of the work to different QC materials systems and far-field engineering of the laser emission pattern. References [1] R. Colombelli, K. Srinivasan, M. Troccoli, O. Painter, C. Gmachl, D.M. Tennant, A.M. Sergent, D.L. Sivco, A.Y. Cho, and F. Capasso "Quantum cascade surface-emitting photonic crystal laser," Science, v302 (5649), pp. 1374-1377, Nov. 21, 2003 (pdf). (R Colombelli et al., 2003, Sciencexpress 1090561). [2] A. Tredicucci, "Marriage of two device concepts," Science, v302 (5649), pp. 1346-1347, Nov. 21, 2003 (pdf). [3] R. Colombelli, K. Srinivasan, M. Troccoli, O. Painter, C. Gmachl, F. Capasso, D.M. Tennant, A.M. Sergent, D.L. Sivco, A.Y. Cho, "Quantum cascade photonic crystal surface emitting injection laser," CLEO Post-Deadline CThPDC1, Baltimore MD, May 2003 (talk). [4] R. Colombelli, K. Srinivasan, M. Troccoli, O. Painter, C. Gmachl, D.M. Tennant, A.M. Sergent, D.L. Sivco, A.Y. Cho, and F. Capasso "Fabrication technologies for quantum cascade photonic-crystal microlasers," Nanotechnology, Vol. 15, pp. 675-681, 2004 (pdf). [5] K. Srinivasan,
O. Painter, R. Colombelli, C. Gmachl, D.M. Tennant, A.M. Sergent, D.L.
Sivco, A.Y. Cho, M. Troccoli, and F. Capasso, "Lasing mode pattern
of a quantum cascade photonic crystal surface-emitting microcavity laser,"
App. Phys. Lett., v84 (21), pp. 4164-4166, May. 24, 2004 (pdf). Collaborators Please visit our Collaborators page for a listing of our co-workers on this project. Acknowledgements Kartik Srinivasan thanks the Hertz Foundation for its graduate fellowship support. Questions? Please contact Kartik Srinivasan or Oskar Painter if there are any questions. |
| Painter Research Group, August 25, 2004 . |
