R. Gupta

SEMICONDUCTING POLYMER DISTRIBUTED FEEDBACK LASERS

We have fabricated photopumped distributed feedback lasers by spin-casting thin films of the semiconducting polymer poly(2-butyl, 5-(2′-ethyl-hexyl)-1,4-phenylenevinylene) over gratings in silicon oxide. The lasers have two modes that each have a linewidth of 0.2 nm. The lasing wavelength was tuned from 540 to 583 nm by adjusting the period of the gratings.

Low-threshold amplified spontaneous emission in blends of conjugated polymers

Low thresholds (∼500 W/cm2) for amplified spontaneous emission(ASE) are reported in films of soluble poly(paraphenylene vinylene)-based conjugated polymer blends. Efficient Forster energy transfer from the absorbing host polymer to the emitting guest polymer is observed. Emission in the blends originates predominantly from the guest polymer. The large spectral shift between the absorption and emission wavelengths lowers the self-absorption losses and results in low ASE thresholds. Initial results show an enhancement in photoluminescencequantum efficiency of the blends.

Time-resolved Förster energy transfer in polymer blends

Abstract Sub-picosecond spectroscopy and pump–probe experiments show Forster energy transfer in blends from larger gap (blue or green-emitting) host polymers poly(2,3-diphenyl-5-hexyl-1,4-phenylenevinylene) (DP6-PPV) or poly[2-(meta-2′-ethylhexoxyphenyl)-1,4-phenylenevinylene) (m-EHOP-PPV) to the smaller gap, red-emitting guest polymer poly(2,5-bis(2′-ethylhexoxy)-1,4-phenylenevinylene) (BEH-PPV). The dynamics of the stimulated emission (SE) and photoinduced absorption (PA) of the blends indicate that 10–20 ps are required for complete energy transfer. Quantitative measurements of energy transfer rates give a Forster interaction range of 3–4 nm, 1.4 times longer than the theoretical values as calculated from the spectral overlap. We attribute this difference to delocalization of the excited state. Insufficient spectral overlap between the emission of the host and absorption of the guest is shown to be the cause for the absence of energy transfer in a blend with poly(2,5-bis(cholestanoxy)-1,4-phenylenevinylene) (BCHA-PPV) as the guest polymer.

Low threshold distributed feedback lasers fabricated from blends of conjugated polymers: Reduced losses through Förster transfer

We report on low threshold photopumped amplified spontaneous emission (ASE) in blends of derivatives of poly (p-phenylene vinylene) and distributed feedback (DFB) lasers fabricated from the same materials. Forster energy transfer is used to spectrally shift the emission away from the absorption band, thereby reducing the optical losses. The concentration of the guest polymer in the blend determines the rate of energy transfer and, thereby, has a strong effect on the observed emission spectrum. The photoluminescence quantum efficiencies (ηPL) of the blends are higher than the ηPL of the pure emissive species (ηPL decreases with increasing amount of the guest polymer in the blends). The blends exhibit optical losses ∼3 cm−1 compared to ∼85 cm−1 in pure host. As a result, the ASE thresholds are reduced from 5000 W/cm2 in the host to 200 W/cm2 in the blends. The corresponding lasing threshold in the DFB structures is 100 W/cm2.

Förster transfer based Amplified Spontaneous Emission in conjugated polymer blends

Abstract Low thresholds for Amplified Spontaneous Emission (ASE) have been achieved in blends of PPV based conjugated polymers. These blends use Forster energy transfer from the host polymer to the guest polymer to shift the emission far away from the absorption edge of the blend in order to reduce the losses in the films. ASE thresholds as low as 500 W/cm 2 have been observed in the blends, more than an order of magnitude lower than the ASE thresholds of either of the two components of the blends. Blends have higher photoluminescence efficiencies and lower optical loss at the emission wavelength of the guest polymer.

Characterization of semiconducting polymer laser materials and the prospects for diode lasers

Abstract By measuring the gain and loss in thin film planar waveguides using a standard technique developed for inorganic laser materials, we show that the narrow-line emission from photopumped waveguides of the conjugated polymer poly(2-butyl-5-(2′-ethyl-hexyl)-1, 4-phenylenevinylene) (BuEH-PPV) results from amplification of spontaneous emission (ASE). The narrowed linewidth of the ASE spectrum is determined by gain saturation. The techniques presented offer a simple and useful way for evaluating conjugated polymer as materials for solid state lasers. The prospects for making polymer diode lasers are discussed.

Temperature dependence of amplified spontaneous emission in conjugated polymers

The photo-luminescence emission of conjugated polymers exhibits a red-shift as the temperature is reduced. The gain curve red-shifts along with the emission and hence, the wavelength for amplified spontaneous emission (ASE) red-shifts on lowering the temperature. A similar, but not as pronounced, red-shift is also observed in optical absorption of these materials. The increased Stokes shift between the absorption and emission leads to lower self-absorption losses and hence to lower thresholds for ASE at lower temperatures.

Time-resolved Foerster energy transfer in molecular and polymeric guest-host systems

Sub-picosecond spectroscopy and ultrafast pump-probe experiments spectrally and temporally resolve the Forster energy transfer in blends from larger gap host to the smaller gap guest organic materials. The dynamics of the stimulated emission and photoinduced absorption of the polymer blends indicate that 10 - 20 ps are required for complete energy transfer. The Forster interaction ranges suggested by quantitative measurements of energy transfer rates are compared with the theoretical values as calculated from the spectral overlap. We discuss the effect of the excited state delocalization. The energy transfer dynamics in small organic molecule blends have a longer time scale (1 ns), corresponding to the much longer lifetimes of the organic dyes.