Gabriel Ramos-Ortiz

Förster energy transfer from a fluorescent dye to a phosphorescent dopant: a concentration and intensity study

We report on the long-range dipole-dipole Forster energy transfer between a fluorescent dye and a phosphorescent dopant in solid thin films. For our study we used 4,4′-N,N′-dicarbazole-biphenyl as a host and the organometallic complex fac tris(2-phenylpyridine) iridium(III) as a guest. By studying the photoluminescence efficiency of films with various host and guest concentrations and by comparing these results with those obtained from a numerical model, we determine a Forster radius of R0u2006∼u20063 nm.

Photorefractive polymers sensitized by two-photon absorption

We demonstrate the recording of holograms and their nondestructive readout in a photorefractive polymer, using two-photon absorption. Sensitivity is provided by the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into the photoconducting poly(N -vinylcarbazole) matrix. The holograms can be fully erased with a pulsed laser source but are insensitive to cw laser beams with the same wavelength. Studies of the field and intensity dependence of the diffraction efficiency indicate that the holograms are formed through the photorefractive effect.

Third-harmonic generation and its applications in optical image processing

We review potential applications of third-harmonic generation (THG) in polymer composites to implement optical image processing applications at the eye-safe and technologically relevant telecommunication bands. We discuss two examples, time-gated imaging through scattering media, and image recognition using Fourier-based techniques, where THG in a polymer composite offers significant advantages over typical holographic media, in that it produces signals that are non-degenerate in optical frequency and space, and allows the development of applications that are compatible with low-cost Si-based electronic components.

Ultrafast-pulse diagnostic using third-order frequency-resolved optical gating in organic films

We report on the diagnostic of ultrafast pulses by frequency-resolved optical gating (FROG) based on strong third-harmonic generation (THG) in amorphous organic thin films. The high THG conversion efficiency of these films allows for the characterization of sub-nanojoule short pulses emitting at telecommunication wavelengths using a low cost portable fiber spectrometer.

Direct imaging through scattering media by use of efficient third-harmonic generation in organic materials

We report on real-time, time-gated, direct imaging through scattering media with an attenuation of 14 mean-free paths by use of third-harmonic generation in the eye-safe and telecommunication-compatible near-IR spectral region (1550 nm).

Photorefractive Polymers with Non-Destructive Readout

Photorefractive polymers are suitable for real-time holographic applications. Since the recording and readout of a hologram is carried out with laser beams with the same wavelength, the readout process partially erases the stored information, a problem common to all current photorefractive materials and referred to as destructive readout. In this paper we describe photorefractive polymers that are sensitized by two-photon absorption. Holographic recording is achieved with high-intensity writing beams and readout using low light intensity, but high power beams. Using this nonlinear recording scheme, non-destructive readout was demonstrated.

Temperature dependence of the threshold for laser emission in polymer microlasers

We studied the temperature dependence of the laser emission threshold in microring polymer lasers. For microring lasers with diameters between 16 and 120 μm, a weak temperature dependence was observed when the temperature was varied between 300 and 10 K. These experimental results are explained within a four-level model. Our results suggest that a significant reduction of threshold can be achieved at low temperatures if the quality factor Q of the microcavities is improved.

Time-gated imaging through scattering media by using efficient THG in organic films

We report on efficient third-harmonic generation (THG) with large spectral and angular bandwidths in polymer thin films doped with conjugated organic molecules. Using the efficient THG exhibited by these organic films, we demonstrate real-time, time-gated, direct imaging of objects immersed in highly scattering media by using femtosecond pulses tuned at the eye-safe and telecommunication-compatible near-infrared spectral region (1550 nm). By using a low cost detection system we resolved THG time-gated images of objects immersed in media with a scattering attenuation of 14 mean-free paths, equivalent to an attenuation of six optical densities.

Temperature dependence of the laser emission threshold in organic semiconductor lasers

Summary form only given. Despite numerous reports of line narrowing and laser emission in optically pumped conjugated polymers, the demonstration of electrically injected polymer lasers remains a challenging task due to the high pump power requirement for laser emission. The current studies focus on the development of low threshold laser geometries such as whispering gallery mode lasers, and on a better understanding of the mechanisms that are responsible for line narrowing and gain in conjugated polymers. Laser emission in conjugated polymers is generally described in terms of a four level system analogous to that used for laser dyes. In this model, excitation starts mainly from the lowest vibrational level of the ground state S/sub 0/ (/spl nu/=0) (level I) and populates higher vibrational levels of the excited state S/sub 1/ (/spl nu//spl ne/0) (level II). After relaxation, emission occurs from the lowest vibrational level of the excited state S/sub 1/ (/spl nu/=0) (level III) to a higher vibrational level of the ground state S/sub 0/ (/spl nu//spl ne/0) (level IV). Previous studies have suggested that the threshold for line narrowing and laser emission was limited by the population of level IV at thermal equilibrium and that this threshold could be reduced significantly by lowering the temperature. We studied the temperature dependence of whispering gallery mode microring lasers using the conjugated polymer BEH:PPV as a gain medium.