A 260 nm layer of organic bulk heterojunction blend of the polymer poly (3-hexylthiophene) (P3HT) and the
fullerene [6,6]-phenyl C61-butyric (PCBM) was spin-coated in between aluminum and gold electrodes
respectively on top of a laser inscribed azo polymer surface relief diffraction grating. Angle dependent surface
plasmons (SP) with a large band gap were observed in the normalized photocurrent by the P3HT-PCBM layer
as a function of wavelength. The highest increase in photocurrent was found to be 272% at a wavelength of
618 nm for a grating depth of 50 nm. The SP-induced photocurrents were also investigated as a function of
the grating depth and spacing.
We study the polarization conversion from horizontally polarized to vertically polarized light (or vice versa)
that occurs when light is incident on a cross-corrugated metal surface. The conversion is due to the optical energy
exchange between the surface plasmon (SP) resonances at the metal-dielectric interface of the surface. The angular and
wavelength dependence of the SP-induced polarization conversion was studied for cases where two perpendicular
surface relief diffraction gratings of different pitch are used to couple the light into and out of the plasmon resonance
modes. The polarization conversion occurs only when a plasmon mode is exited by one of the gratings and then the
energy in this mode is out-coupled by the second grating. For various grating combinations this occurs at specific
wavelength and incidence angles, otherwise no signal is obtained in a crossed polarizer set-up. An enhancement of the
signal (and therefore of the surface field) is also observed when a standing wave surface plasmon is generated when one
of the gratings produces counter propagating surface plasmons. A theoretical model based on graphical representations in
momentum space of the travelling light beam through the specimen was developed and successfully applied to explain
the experimental results.
Azobenzene polymer surface relief diffraction gratings were inscribed on relaxor ferroelectric Lead Lanthanum
Zirconate Titanate (PLZT) thin films having a (9/65/35) composition. Resonance structures were observed in the
reflected intensity spectra of a laser light beam onto these films, indicating coupling of the incident light. An ac electric
field was then applied along the thickness of the PLZT films and an electro-optic modulation signal was measured as a
function of the incidence angle of the laser beam. It was found that the resonance structures increased the modulation
signal of the PLZT thin films at constant field amplitude.
Surface relief structures are optically inscribed onto azobenzene containing thin films. We present a brief review of the
technique and results. The films act either as optical waveguides of as substrates upon which thin gold films are
deposited to support surface plasmon waves. Multiple gratings are inscribed to act as optical couplers and Bragg
gratings. Here, we report on the amplitude and phase changes that occur when the incident light is resonantly coupled
into waveguide or surface plasmon modes. We illustrate the method of obtaining the dispersion curve of the device. The
effects on amplitude are readily observed in the transmission or reflection spectra. In addition, since the structures can be
made such that the resonant TE and TM modes to be well separated, we can measure the phase changes of the
transmitted or reflected beams near resonance. A simple device showing polarization to wavelength modulation
encoding is presented.
A relief diffraction grating is holographically recorded in azo-polymer. The polymer layer under the grating serves as a waveguide for the light coupled by the grating. When a guiding condition is met, sharp minima appear in the transmitted spectrum and sharp maxima appear in the reflected spectrum. Since the resonance conditions for transverse electric (TE) and transverse magnetic (TM) polarized waves occur at different wavelengths, the effect can be used for fast wavelength encoding of a binary signal. The speed of such a device would be limited only by the speed of the encoding part, which changes the azimuth of the input linearly polarized light.
Surface relief gratings are optically inscribed on azobenzene polymer films. Multiple gratings can be inscribed on the same spot either simultaneously or sequentially by projecting a holographic pattern onto the surface. The grating pitch can range from 350 nm to 1.5 μm and depths can easily reach 400 nm. The inscription is done at room temperature, in air using a 100 mW/cm2 beam for 1 to 5 minutes. In the present study we inscribe two sinusoidal gratings and coat the surface with a 20 nm thin film of gold. Light incident onto this structured surface is coupled into a surface plasmon when the appropriate angle of incidence and wavelength are selected. The coupling condition can be observed in the reflection spectrum from the surface. A dispersion curve for the plasmon propagation can then be produced by studying the reflection spectrum as a function of the angle of incidence. By using two gratings as couplers, a surface plasmon standing wave can be produced for a selected incident light wavelength.
Azobenzene containing amorphous polymers are used to fabricate thin films that can act as optical waveguides.
Furthermore, stable surface relief structures such as gratings can be holographically inscribed onto the films using low and
medium power lasers. The grating depth and spacing can easily be controlled with the depth varying from 15 nm to several
hundred nanometers and spacing varying from 170 nm to 3 micrometers. These thin-film structures can then be designed to
act as optical devices such as optical couplers, optical filters and birefringence films. We will describe the method of the
fabrication ofthe surface relief structures and suinmarise the theories proposed for the mass polymer movement that produce
these structures.
Polarization holographic gratings were inscribed in azobenzene side chain polymer films. The polarization gratings were produced using two orthogonally circularly polarized beams and the resulting stable transmission gratings were studied using a low power HeNe laser. The gratings were observed to efficiently separate left and right circularly polarized light from the probe beam and two generate two elliptically polarized beams in the first order diffraction direction.
A slab waveguide is made by spin coating a thin film of azopolymer onto a glass slide. A surface grating is then optically generated and acts as optical coupler. The structure exhibits resonance coupling as a function of the incident probe wavelength and angle of incidence. The transmission of the film is observed to decrease by more than 50 percent at resonance. The wavelength at resonance in seen to vary linearly with the angle of incidence for small angles.
When surface gratings are inscribed on the surface of resonant cavities such as slab waveguides, sharp peaks can be observed in the transmission and reflection spectra of the structures. The peaks in the optical characteristics occur because the gratings act as couplers for light into the waveguides at specific wavelengths and angles of incidence. It has been found that stable surface gratings can readily be inscribed on polymers containing side groups such as poly{4'-[(2- (acryloyloxy)ethyl)ethylamino]-4-nitroazobenzene}. The gratings are made by direct exposure to an interference pattern and no further processing is required. The grating properties such as orientation, grating spacing and depth are easily controlled, and multiple gratings can be inscribed on one spot. Using various combinations of waveguides and surface gratings, structures which act as line filters and resonant reflectors are made and studied.
Copolymers containing rigid azobenzene and ester side groups linked by relatively short spacers to the main chain form amorphous films. Such films can be addressed with polarized laser light to produce oriented domains. Birefringence measurements and time dependent infrared spectroscopy are used to analyze the orientation. Two series of copolymers are analyzed in comparison. The first consists of copolymers of 4'-{[2-(methacryloyloxy)ethyl] ethylamino}4-nitroazobenzene (DR1M) and 4-{[2- ((2-methyl-1-oxo-2-propenyl)oxy)ethyl]oxy}benzoate (BEM), where both the azo and the ester rigid groups have similar dipole moments. The second series consists of copolymers of DR1M with a much less polar 4-phenyl-4- {[2-(2-(2-methyl-1-oxo-2- propenyl)oxy)ethyl]oxy}benzoate (NBEM). Birefringence studies clearly suggest that in poly(DR1M-co- BEM), the BEM groups are being oriented by 'sympathy' with the DR1M groups, while in poly(DR1M-co-NBEM) no such cooperative motion is apparent. Time dependent infrared spectroscopy can analyze separately the motions of each of the rigid groups, and it clearly shows that the BEM groups are being moved by the DR1M groups, while the orientation of the NBEM groups is much lower. Thus one can envisage materials where cooperative motion is present (polar groups), or where only the polar azobenzene groups are 'molecularly addressed' by the laser, with the non-polar rigid groups remaining relatively inert. The consequences to formation of surface gratings on these copolymer films are discussed.
Poly(methyl methacrylate) slab waveguide materials were prepared, incorporating covalently attached azobenzene side groups. Birefringence was rapidly photoinduced in the films with linearly polarized light from an Ar+ laser to define stable channel waveguides, and the irradiated regions were shown to be suitable for multimode guiding of light at 633 nm. This single-step photoinscription process gives a controlled refractive index variation up to 0.012 for step or graded index channels, and can be rapidly modulated or completely erased with irradiation from a circularly polarized Ar+ laser beam. Written waveguides are stable indefinitely. Coupling in and out of the waveguides can be achieved with diffraction gratings photoinscribed in the polymer film using interfering beams from the same Ar+ laser. These high efficiency volume and surface diffraction gratings are stable over time and light exposure at the guiding wavelength. The grating spacing can be controlled by the geometry of the interference pattern, and hence can be optimize for high efficiency in-coupling and out-coupling at any required angle. Gratings with fringe spacings from 350 nm to 2000 nm were photoinscribed, and shown to couple light in and out of birefringent channel waveguides photoinscribed in the same material. The reversibility of the channels allows the guides to be photoaddressed for switching and mode filtering.
Optical information can be stored, erased or overwritten on polymer thin films using polarized light. The information is in the form of localized dichroism and birefringence and is read using a low power probe beam. We have measured the efficiency of the writing beam in a number of polymer and copolymer films containing azobenzene dyes as side groups. In this paper we compare the results amongst the polymers we have synthesized. Preliminary results indicate that copolymers may be optimized to allow for the preparation of efficient thick films for erasable optical recording.
Optical information can be stored, erased, or overwritten on azo polymer thin films using polarized light. The information is in the form of localized dichroism and birefringence and is read using a low power probe beam. We have investigated this image storage phenomenon on a series of polyacrylates and polymethacrylates containing azo aromatic groups on the side chains. The monomers are obtained from commercially available azo dyes. The polymers are amorphous and have high glass transition temperatures. We present a model for the writing and erasing process and investigate the physical origins of the parameters which influence the properties of the recording medium.
Light which is emitted from a point source and is randomly scattered by an ensemble of particles is found to form an image of the point source. Results are presented for light which is emitted from a 25 micrometer pin hole and is then scattered by a suspension of .296 micrometer polystyrene spheres. This phenomenon is interpreted in terms of coherent multiple backscattering. The present study investigates the effects of particle density on the image resolution.
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