Near-infrared sensitivity enhancement of photorefractive polymer composites by pre-illumination

Abstract

Among the various applications for reversible holographic storage media, a particularly interesting one is time-gated holographic imaging (TGHI). This technique could provide a noninvasive medical diagnosis tool, related to optical coherence tomography. In this technique, biological samples are illuminated within their transparency window with near-infrared light, and information about subsurface features is obtained by a detection method that distinguishes between reflected photons originating from a certain depth and those scattered from various depths. Such an application requires reversible holographic storage media with very high sensitivity in the near-infrared. Photorefractive materials, in particular certain amorphous organic systems, are in principle promising candidate media, but their sensitivity has so far been too low, mainly owing to their long response times in the near-infrared. Here we introduce an organic photorefractive material -- a composite based on the poly(arylene vinylene) copolymer TPD-PPV -- that exhibits favourable near-infrared characteristics. We show that pre-illumination of this material at a shorter wavelength before holographic recording improves the response time by a factor of 40. This process was found to be reversible. We demonstrate multiple holographic recording with this technique at video rate under practical conditions.

Photophysical and Redox NIR‐Sensitivity Enhancement in Photorefractive Polymer Composites

“…In our earlier paper, we had reported S g (h ext 1 %) 19 cm 2 J À1 for the TPD ± PPV/PCBM composite (material 1) with I WB,ext 0.65 W cm 2 , I g 2 W cm À2 and E ext 60 V mm À1 [1] in a pulsed experiment (90 Hz write rate, during the first quarter of a cycle the gate beam was on while the write beams were on during the remaining time), which is a factor of % 3 higher than for the CW data presented here (S g 6.1 cm 2 J À1 ). There are two reasons for this discrepancy: On the one hand the sensitivity S g was found to increase when lowering I ext at a constant preillumination intensity (see leveling of t 50 in Figure 1 a for b < 1).…”

“…[1] We refer to this procedure as ™gating∫ in analogy with the two-color method known from inorganic PR materials [20] even though the mechanism discussed herein is different.…”

“…The excitons can dissociate under the influence of the poling field E ext , and the mobile charge carriers (mostly holes) are redistributed and become trapped in the dark regions, which leads to an internal space ± charge field E SC . The superposition of E ext and E SC acts on the NLO chromophores, modulating the refractive index of the bulk to replicate the interference pattern, as shown in Equation (1): [5,6] Dn $ E ext E SC r EO,eff (1) Here, Dn is the index modulation amplitude, r EO,eff is the effective EO coefficient, which by itself depends on number density of EO chromophores and the photorefractive molecular figure-of-merit of the chromophore. [6] The external diffraction efficiency h ext in a holographic experiment (see below for details) depends on Dn according to Equations (2 a) and (2 b).…”

Molecular design of photorefractive polymers

“…Typical PR quantities for different types of PR polymer composites in the literature are summarized in Table 1. 39,[41][42][43][44][45][46][47][48][49][50] Usually, diffraction efficiencies and response times have electric field and wavelength dependencies. Furthermore, glass transition temperatures (T g ) also affect these quantities.…”

“…Furthermore, glass transition temperatures (T g ) also affect these quantities. Therefore, as noted by a previous study, 49 a true comparison is difficult because of the different experimental conditions, such as film thicknesses, wavelengths employed, the applied electric fields and the T g s of the corresponding PR composites. The Kukhtarev 14 model predicts the space-charge field E SC :…”