Nanowire Lasers

Nanowire lasers Julia van der Burgt, 5575087 Paper for Photon Physics - University of Utrecht - Utrecht, NL April 22, 2016 Abstract Nanowire lasers are a special type of semiconductor lasers in which a semiconductor nanowire acts as the active medium and optical cavity. The dimensions are in the range of tens to hundreds of nm in diameter and a few to hundreds of µm in length. They can be grown as nearly defect-free single crystals, with distinct crystal planes as end facets that act as mirrors for the optical cavity. The light is confined in the nanowire, which acts as a wave guide and only supports certain modes. These modes are amplified through stimulated emission which leads to lasing. A unique property of nanowire lasers is that the laser threshold depends on the length and diameter of the wire. For the ZnO nanowires discussed in this paper a maximum output power of 0.65 mW was detected for optical pumping at 40 mW. The exact mechanism that causes lasing in ZnO is still under debate. Recent research has been focused on practical applications, to develop efficient and stable electrically pumped ZnO nanowire lasers. 1 Introduction A diode laser is electrically pumped by applying a for- ward bias to the p-n junction. Without the bias voltage a The first nanowire laser was observed by the group of depletion region exists around the p-n junction, in which Peidong Yang at Berkley university in 2001 [1]. They the diffusion of electrons and holes over the junction is demonstrated lasing in ZnO nanowires at room temper- in equilibrium with the diffusion potential. The exter- ature. Since then many different types of semiconductor nal bias voltage reduces this potential and electrons and nanowire lasers have been investigated. Semiconductor holes are injected in the conduction band and valence lasers are already widely used in all kinds of application band respectively. This gives population inversion in a and can be built on very small scale. Nanowire lasers are narrow region around the junction. This is called the ac- promising for even smaller applications. Due to their rel- tive region, in which electrons and holes can recombine atively simple, bottom-up production process they can while emitting a photon. When this recombination hap- be produced at low costs. pens through spontaneous emission, the properties are This paper is focused on the first type of nanowire laser, similar to that of a regular light emitting diode (LED). made of ZnO. First some background information, re- To get laser action, the light must be amplified through quired to understand the properties of ZnO nanowire stimulated emission. To achieve this, the rate of stimu- lasers, is given. This consists of an explanation of semi- lated emission must be larger than the rate of stimulated conductor lasers, which work fundamentally different absorption, i.e. a photon has more chance to be ampli- than other solid state and gas lasers, and a general de- fied than to be absorbed. It can be shown that this the scription of nanowire lasers. In the next section some case when properties of ZnO are discussed and the results of several EFc − EFv > hν > Eg; (1) researches to ZnO nanowire lasers are presented. Finally where E and E are the Fermi energy levels of the some recent findings for application of ZnO nanowire Fc Fv conduction and valence band respectively, hν the photon lasers are briefly mentioned. energy and Eg the band-gap energy. So photons with an energy between that of the band-gap and the difference 2 Background information in the Fermi levels will experience a net gain [2]. 2.1 Semiconductor lasers To achieve sufficient gain for laser action, many round trips through the medium are required. While in other A semiconductor laser is a laser of which the gain types of lasers highly reflective mirrors are required to medium consists of a semiconductor p-n junction. The build a Farby-Pérotoptical cavity, semiconductor lasers laser can be pumped optically or electrically to get pop- often have no mirrors. Instead, the semiconductor crys- ulation inversion. Electrically pumped semiconductor tal is cleaved along well-defined crystals planes perpen- lasers are called diode lasers and are widely used in all dicular to the cavity. Because the refractive index is kind of applications. For the gain medium a direct band- usually much higher than that of the surroundings (air), gap material is required. Most often used are III-V or a sufficient amount of the light is reflected from the ends II-VI compound semiconductors. to achieve an optical cavity. In addition a highly reflec- 1 Γgth equals the round-trip losses α: 1 1 Γ = α + α ; α = ln (2) gth w m m L R where the losses are split up in the waveguide losses αw and the mirror losses αm, which depends on cavity length L and mirror reflectivity R. While in conventional semiconductor lasers the wave guide losses dominate, the op- posite is the case for nanowire lasers. Due to the very small length of the nanowire and smaller reflection coef- ficient αm αw. This leads to an important and unique property of nanowire lasers: the threshold gain depends on the nanowire length and diameter. The length de- pendence is clear from eq. (2), but there is also a strong Figure 1: Schematical illustration of a diode laser [2] diameter dependense through R. The diameter of the nanowires is on the order of the wavelength of the light. This causes large diffraction losses at the end facets, be- tive coating can be applied to increase reflectivity. cause the light extends outside of the nanowire. This It can be shown, that gain is optimized by increasing part of the light is not reflected, and thereby the re- the photon density. In addition to the resonator cav- flectivity R depends on the diameter [5]. In addition a ity, photon density is also increased in a semiconductor threshold for the nanowire diameter was found by Zimm- laser by confinement in the active region. A thin layer ler et all. [5], who showed that for ZnO nanowires with of undoped active material is put between the p- and a diameter below 150 nm no lasing was achieved, inde- n-doped materials and by tuning the relative band-gaps, pendent of the length of the nanowire. photons and electrons are confined in the active material. A schematic illustration of the layout and components of 3 ZnO nanowire lasers a typical semiconductor laser are shown in fig. 1 [2]. The first laser action in a nanowire was demonstrated 2.2 Nanowire lasers with zinc-oxide nanowires. Since then much research has been performed on both further understanding of Nanowire lasers are based on the same working principle the ZnO nanowires and on making nanowire lasers of as the general semiconductor laser as described in the other types of materials. This section will focus on ZnO previous section. What makes it a nanowire laser is the nanowire lasers. First some properties of ZnO and why shape of the semiconductor material that forms the opti- it is a suitable material for nanowire lasers are discusses. cal cavity: a diameter of tens to hundreds of nanometres This is followed by a summary of the results of some and the length ranging from a few to hundreds of mi- more recent researches to ZnO nanowire lasers and the crons. With this size range quantum confinement usually specific properties found. The section ends with a rough does not play a role (opposed to quantum wires) but for calculation on the power output. the optical properties the dimensions are important. The high contrast in refractive index of the nanowire (usually in the range 2.5-3.5) and air makes them good optical 3.1 Properties of ZnO wave guides. The light travels back and forth, confined in The underlying principle of laser action in ZnO nan- transverse direction, and reflected at the ends. Because wires is not easily explained and actually still under de- nanowires are relatively easily grown as a single crystal bate. In [6] Vanmaekelbergh and van Vugt propose that through for example vapour transport, no cleaving of the at weak optical excitation and low temperature lumi- ends is required and nearly defect-free ends can be ob- nescense comes from excitonic recombination, while at tained [3]. The fabrication of nanowire lasers is much intense excitation and increased temperature the exci- simpler compared to the multi step, top-down process in tons are split and the material is better described by a which a conventional semiconductor laser is produced. plasma of holes and electrons. Here the focus will be on The nanowire acts both as the optical resonator and the the excitonic recombination. gain medium and can be grown in one bottom up pro- ZnO is known for its remarkably high exciton binding en- cess [4]. Although most common semiconductor lasers ergy of 60 meV [1], more than twice that of most other are electrically pumped, this has turned out to be diffi- III-V and II-IV compounds. The exciton binding energy cult to achieve in nanowires. Therefore the description is larger than the thermal energy at room temperature of lasing action in nanowires in the the following will be (∼25 meV), so at room temperature still most electrons based on optical pumping. and holes will be bound together. This is a favorable sit- The threshold for laser action is when the round-trip gain uation for laser action, because it leads to a narrow band 2 width.

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