“…There has been much recent interest in polymeric materials for holographic gratings in the fields of information storage, waveguide coupling, and nonlinear optoelectronics. − Both volume refractive index and surface profile gratings can easily be formed, and polymer properties can be tailored to optimize the desired effect. Polymeric systems are commonly liquid crystalline, ,− chromophore doped, − or amorphous and covalently attached. − , The gratings in each case are produced with an interference pattern created from coherent laser light at a wavelength absorbed by the material. Depending on the polarization state of the incoming light, the interference is that of either intensity modulation or phase modulation, which can lead to either direct changes in absorption coefficient or refractive index or indirect optical changes involving molecular reorientation or mass transport. ,, If a photoresist film is irradiated, subsequent wet chemistry has also been shown to produce a surface profile grating .…”
A series of amorphous azobenzene-containing polymers were cast as thin films and shown to produce both reversible volume diffraction gratings and high-efficiency surface gratings by laser irradiation at an absorbing wavelength. The latter process involves localized mass transport of the polymer chains to a high degree, as atomic force microscopy reveals surface profile depths near that of the original film thickness. A mechanism for this phenomenon is proposed which involves pressure gradients as a driving force, present due to different photochemical behaviors of the azo chromophores at different regions of the interference pattern. The phase addition of the two beams in the interference pattern leads to regions of high trans-cis-trans isomerization by the absorbing azo groups, bordered by regions of low isomerization. As the geometrical isomerization requires free volume in excess of that available in the cast films, the photochemical reaction in these areas produces a laser-induced internal pressure above the yield point of the material. It is proposed that the resulting viscoelastic flow from these high-pressure areas to lower-pressure areas leads to the formation of the regularly spaced sinusoidal surface relief gratings observed by a number of research groups, but previously unexplained. This mechanism of photoinduced viscoelastic flow agrees well with the results of experiments investigating the effect of the polarization state of the interfering writing beams and the photochemical behavior of the chromophore, the free volume requirements of the induced geometric changes, and the viscoelastic flow of the material.
“…There has been much recent interest in polymeric materials for holographic gratings in the fields of information storage, waveguide coupling, and nonlinear optoelectronics. − Both volume refractive index and surface profile gratings can easily be formed, and polymer properties can be tailored to optimize the desired effect. Polymeric systems are commonly liquid crystalline, ,− chromophore doped, − or amorphous and covalently attached. − , The gratings in each case are produced with an interference pattern created from coherent laser light at a wavelength absorbed by the material. Depending on the polarization state of the incoming light, the interference is that of either intensity modulation or phase modulation, which can lead to either direct changes in absorption coefficient or refractive index or indirect optical changes involving molecular reorientation or mass transport. ,, If a photoresist film is irradiated, subsequent wet chemistry has also been shown to produce a surface profile grating .…”
A series of amorphous azobenzene-containing polymers were cast as thin films and shown to produce both reversible volume diffraction gratings and high-efficiency surface gratings by laser irradiation at an absorbing wavelength. The latter process involves localized mass transport of the polymer chains to a high degree, as atomic force microscopy reveals surface profile depths near that of the original film thickness. A mechanism for this phenomenon is proposed which involves pressure gradients as a driving force, present due to different photochemical behaviors of the azo chromophores at different regions of the interference pattern. The phase addition of the two beams in the interference pattern leads to regions of high trans-cis-trans isomerization by the absorbing azo groups, bordered by regions of low isomerization. As the geometrical isomerization requires free volume in excess of that available in the cast films, the photochemical reaction in these areas produces a laser-induced internal pressure above the yield point of the material. It is proposed that the resulting viscoelastic flow from these high-pressure areas to lower-pressure areas leads to the formation of the regularly spaced sinusoidal surface relief gratings observed by a number of research groups, but previously unexplained. This mechanism of photoinduced viscoelastic flow agrees well with the results of experiments investigating the effect of the polarization state of the interfering writing beams and the photochemical behavior of the chromophore, the free volume requirements of the induced geometric changes, and the viscoelastic flow of the material.
“…5,7 Holographic gratings in azobenzene-containing polymer films caused by the irradiation with red light or as a result of the "biphotonic" orientational procedure are also reported in some other papers. [10][11][12][13] In these studies the time gap between the two irradiation steps was on the order of seconds to minutes. Fei et al 10 Wu et al utilized nonpolarized UV light (300-500 nm) from a low-pressure mercury lamp (8 mW/cm 2 ) for the first excitation step of Methyl Yellow {4-(dimethylamino)azobenzene)} in polystyrene or poly(methyl methacrylate) and Ethyl Orange {potassium salt of 4-[4-(diethylamino)phenylazo]benzenesulfonic acid} in poly(vinyl alcohol) as well.…”
In contrast to the conventional photoorientation process with blue light, an orientation of 4-cyano-4′-alkoxyazobenzene side groups parallel to the electric field vector of the incident light is generated upon irradiating films of a liquid-crystalline side-chain polymer with linearly polarized red light. The polyester is characterized by smectic and nematic phases g24S X26SA34N46i and a strong tendency to form J-aggregates. The process requires a photochemical pretreatment by irradiation with UV light or an exposure to visible light of high power density to produce a certain concentration of the Z-isomer, which destroys any initial orientational order and J-aggregates. The orientation process is cooperative, whereas the light-induced orientation of the photochromic moiety causes an ordering of the alkylene spacers and even of the main-chain segments into the same direction. The most probable mechanism of this two-step process is the angular-selective transformation of the bulky Z-isomers to the rodlike E-isomeric formed by the red light. The aligned E-azobenzene side groups become strongly J-aggregated. Very high values of dichroism of about 0.8 and birefringence of about 0.3 were generated as a result of this combination of the photoinduced orientation process and the thermotropic selforganization, which take place simultaneously under the irradiation conditions. The process results in a uniaxial prolate order of the film, whereas conventional photoorientation leads to a biaxial oblate order. These two different three-dimensional orders have been characterized by FTIR polarization spectroscopy and exhibit also varying intermediate thermal stabilities.
“…Recently, holographic data storage in a side-chain liquid-crystalline polyester and in a host−guest system utilizing unusual photochemistry of azo dyes was reported. − Fei et al. investigated the azo dyes methyl orange (MO) and ethyl orange (EO) embedded in a poly(vinyl alcohol) (PVA) matrix …”
Section: Introductionmentioning
confidence: 99%
“…In the ideal case for two-photon holography, most of the molecules reach the chemically active T 2 state and do not isomerize directly from the lower T 1 state. However, a detailed investigation of the energy levels and lifetimes of the excitation states of methyl red 10 gives no indication for the population of higher triplet states. 1 Energy level scheme for the isomerization of azo dye molecules according to the data given in refs and . The two additional triplet states T 2 were proposed by Fei et al .…”
Section: Introductionmentioning
confidence: 99%
“…However, a detailed investigation of the energy levels and lifetimes of the excitation states of methyl red 10 gives no indication for the population of higher triplet states. 1 Energy level scheme for the isomerization of azo dye molecules according to the data given in refs and . The two additional triplet states T 2 were proposed by Fei et al . but have not been detected spectroscopically …”
Transient biphotonic optical storage in the guest−host system
4-amino-4‘-nitroazobenzene/polycarbonate is
accomplished by excitation of the azo dye molecules at 488 nm with an
argon ion laser and subsequent
writing at 633 nm with two interfering HeNe laser beams. The
mechanism of the observed refractive index
change is investigated by detailed kinetic studies. It is based on
the formation of cis isomers of the azo dye
by the argon ion laser. These are subsequently transformed back to
the trans isomers by fast photoinduced
cis−trans isomerization where the interference of the writing beams
is constructive and by slow thermal
cis−trans isomerization where the interference of the HeNe beams is
destructive, respectively. The cis
concentration produced by the exciting light is thus modulated with the
HeNe beams, resulting in a modulation
of the refractive index.
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