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Lasers & Sources

Near-IR plastic semiconductor laser demonstration

Laser emission at 825nm is achieved from a self-assembled, cylindrical microcavity based on a semiconducting plastic.
10 November 2010, SPIE Newsroom. DOI: 10.1117/2.1201010.003260

Organic and polymeric materials provide a number of technical advantages over their inorganic counterparts. They offer great flexibility in molecular design and synthesis, and are amenable to inexpensive processing like solution casting. In stark contrast, inorganic materials often require high-temperature processing at expensive facilities. The potentially low cost has been a driving force behind commercialization of LEDs that use luminescent semiconducting polymers.

Following successful development of visible-light-emitting diodes, attempts have been made to take advantage of solution-processable polymer semiconductors for realization of light sources operating in the spectral region between 0.8 and 1.5μm, which is potentially important in applications associated with communication, sensing, and spectroscopy. Near-IR LEDs have been fabricated from a wide variety of polymeric semiconducting materials.1–6 However, laser emission in the near-IR from a polymer semiconductor remained to be shown.

We recently demonstrated a near-IR cylindrical microlaser.7 Such a device geometry offers strong potential for integration with passive components.8,9 Since some polymer waveguides exhibit low-loss communication windows in the near-IR regime, compact and inexpensive near-IR sources and amplifiers may find applications in photonic circuits and short-distance communication networks.

We employed the same material composition that was used for the first demonstration of a near-IR polymer LED.1 The luminescent layer is formed by a hole-transport polymer, poly(9-vinylcarbazole) (PVK), doped with an electron-transport material, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3, 4-oxadiazole (PBD), and a near-IR-emitting molecule, 2-(6-(p-dimethylaminophenyl)-2,4-neopentylene-1, 3, 5-hexatrienyl)-3-ethylbenzothiazolium perchlorate (available as LDS821 from Exciton Inc.). Figure 1 shows their molecular structures. Using a computer-controlled step motor, we immersed a piece of silica optical fiber with a diameter of 125μm in chloroform solution containing the specified amounts of PVK, PBD, and LDS821. A polymer layer was formed on the glass fiber surface owing to surface tension and adhesion effects as it was pulled from the solution. After evaporation of the solvent, the resulting PVK layer contained 30% by weight (wt%) of PBD and 1wt% LDS821.

Figure 1. Molecular structures of the materials used in our near-IR microlaser: poly(9-vinylcarbazole) (PVK), 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), and 2-(6-(p-dimethylaminophenyl)-2,4-neopentylene-1, 3, 5-hexatrienyl)-3-ethylbenzothiazolium perchlorate (LDS821). n: Denotes repeating cell.

We obtained laser emission from the polymer microcavity when it was transversely pumped at a wavelength of 532nm with a neodymium-doped yttrium aluminum garnet laser (pulse duration 4ns, repetition rate 10Hz). A certain fraction of photons in the cavity mode is coupled out through imperfect reflection due to the curvature at the polymer-air boundary. Figure 2 shows emission spectra taken at a series of pump-pulse energies (0.6, 12, and 94μJ/pulse from bottom to top, respectively). A single, spectrally narrow cavity mode at 825nm emerged around 3.5μJ/pulse. The threshold for laser operation is somewhat difficult to determine. However, since we identified the emergence of the cavity mode at 3.5μJ/pulse, we assumed it to lie near this value. Based on the spectral halfwidth of the laser emission line, the cavity quality factor of our cylindrical microresonator is Q=(2.7±0.1)×103.

Figure 2. Emission spectra (in arbitrary units, a.u.) of our microlaser taken below, above, and well above the lasing threshold, at 0.6, 12, and 94μJ/pulse, respectively. (inset) Schematic of the pump configuration.

In summary, we have demonstrated laser emission at a wavelength of 825nm from a self-assembled, semiconducting polymer microcavity. We will now further develop this technology. Such microcavity devices operating in the 0.8μm region may find application in the growing technologies associated with polymer photonic circuits and short-distance communication networks.

We are grateful to H. Barry, P. Hayden, P. Hough, and D. Wencel for excellent technical assistance. We also thank J. Costello and C. McDonagh for allowing us to use their facilities.

Takeyuki Kobayashi, Maroussia Vavasseur
Dublin City University
Dublin, Ireland

Takeyuki Kobayashi is a lecturer in physics. He holds a PhD in materials science from Keio University (Japan), and has worked at the University of Arizona and Trinity College Dublin (Ireland).

Maroussia Vavasseur is pursuing an engineering degree in applied physics at the Institut National des Sciences Appliqués de Toulouse (France). She worked on fabrication and characterization of polymer microlasers during her 12-week internship at Dublin City University.