Surface Emitting Semiconductor Lasers Vertical cavity surface emitting laser, diameter = 2-3 micrometer Anke Kuijk

Energy bands Inversion current: Semiconductor lasers work a little bit different wave functions overlap and energy levels form energy bands. valence band completely filled conduction band empty Electrically excited: electrons from the valence to the conduction band. Decay: recombining with holes -> radiation equivalent to spontaneous emission in atoms. N-type: electrons p-type: holes Fermi enery: probability of finding a electron with this energy is ½. A p-n junction: excess electrons along the edge of the n-type material flow into the p-type material, filling the holes in the barrier region. flow ceases when a space charge builds up owing to the extra doped electrons and holes -> voltage V stops the electron flow. Fermi levels of the p and n doped materials must be the same in order to produce an equilibrium. p n junction effectively shifts the valence and conduction bands of the p type material to a higher value relative to the n type material. positive voltage attached to the p type material (forward bias): energy barrier reduced significantly and current readily flows Nonradiative decay: heating Homojunction diode: one type of p-type material and one type of n-type material. The threshold electrical current density is extremely high. Can only operate at very low temperatures. Heterojunction lasers are the solution. The charge carriers are confined to a much smaller region than in homojunction lasers, so the heat deposition is much lower while the threshold current density is still at a sufficiently high level for laser output. Electrical pumping of semiconductors ‘pumped’ by flowing an electrical current across the junction region where the p type and n type materials are joined together. electrons and holes recombine at a rate that is of the order of 109 per second. j current density nC density of electrons in the conduction band d diffusion length or the distance over which gain can occur tR recombination time The output power can be reduced by by making a heterojunction, or a quantum well, which will reduce it to 200 A/cm2.

Quantum wells The tickness of the heterostructure laser active region is from 0.1 to 0.2 micrometer, which lowers the threshold current required for laser action because the thickness comprising the region of high resistance is small. If that thickness is reduced to dimensions of a few (tens) of nanometers then the energy levels exhibit quantum type behaviour. For such a narrow active layer, the structure is referred to as aquantum well. To calculate the energy, the particle in a box problem. Quantum cascade lasers do not operate on transitions from the conduction band to the valence band. Instead they operate on transitions between quantized conduction band states of a multi-quantum-well device. The injector injects an electron into the active region at the energy of the n = 3 level. A foton is emitted as the elctron makes a transition down to the n = 2 level. Then it is injected again into the next active region. The wavelength of the emitted foton can be varied by changin the width of the quantum well.

Surface emitting laser GSEL Grating VCEL Vertical cavity Materials: GaAs 635 – 870 nm InP 1.55 μm ZnSe 460 – 520 nm GaN 405 – 525 nm Semiconducting lasers emitting in a direction normal to the axis of the laser gain medium are referred to as surface emitting lasers (SEL). They can be fabricated in large two-dimensional arrays. two different types of structures: Grating vertical cavity GSEL If a second order grating is fabricated within the laser then the first-order diffraction occurs in a direction perpendicular to the grating surface, thereby providing the laser output as indicated in the figure VCSEL extremely short cavity length yields a very large longitudinal mode spacing, thereby allowing only a sinle longitudinal mode to fall within the gain bandwidth of the laser. In this laser, the gain length is limited to very short dimensions. High reflectivity (Bragg reflector) mirrors are therefore necessary to allow the gain to exceed the losses within the cavity Thin metal films as well as alternating high and low-index dielectric layers have been used to create such mirrors. The overall quantum-well gain region is tapered, using oxide materials to narrow the cross-sectional area of current flow and thereby reduce total current requirements. This also allows the current density to be increased in the central region, increasing the effective gain while reducing the overall current. GaAs: CD 780 AlGaAs DVD 640 AlGaInP InP: long distance fiber optics ZnSe: GaN: DVD 405 (blue) plastic fiber communications 525

Characteristics Gain coefficient: 5.000 – 10.000 m-1 Inherent distributed loss: 2.000 m-1 Stimulated emission crossection: 10-19 Power emitted: milliwats to watts VCSEL: 2 mW Relaxation oscillation decay rate: ~ 109 Mode: single longitudinal Homogeneous broadening Gain coefficient: 15.3 p. 596 Is very high, so even for a typical gain length of 1 mm or less, the gain per pass is large enough to overcome the large inherent distributed loss ( of the order off 2.000 m-1) within the gain medium. Output fluctuations Semiconductor lasers have a very short optical cavity length which makes their cavity decay time very short. This causes output fluctuationsThe recovery time of the excited state population inversion is significantly longer than the laser cavity decay time. p. 278: Relaxation oscillation decay rate is 2 x 109 /s. Homogeneous: broadening due to dephasing collisions. Broadening that occurs when every atom of the same species making the same transition produces an identical emission lineshape and width. Large mode spacing -> single mode usually

Near-field scanning optical microscopy composed of a VCSEL cavity with the microtip mounted on the top facet, and a PIN detector integrated into the back of VCSEL structure. A very weak evanescent light is emitted from the aperture of microtip, interacting with the sample. Evanescent wave reflected by the sample, transformed into the propagating one by the microtip, and is then propagated into the VCSEL cavity. Principle of SNOM detection system is based on power modulation detected by PIN detector power modulation: evanescent interaction leads to a change in effective reflectivity of SNOM probe SNOM head plays the double role of the light emitter and the near-field detector. New SNOM sensor using optical feedback in a VCSEL-based compound-cavity C. Goreckia, S. Khalfallah, H. Kawakatsu, Y. Arakawa Sensors and Actuators A 87 (2001) 113±123

Output power semiconductor laser GaAs based L 300 μm λ 600 nm η 3.4 σ 10-19 m2 g0 104 α 2000 m-1 A 10-10 m2 topt 0.75 Isat 3.3 GW/m2 Popt 150 mW