Progress in photonics and indeed in many areas of science and technology has in many cases been closely linked to development in materials. This development has in some cases been initiated by theoretical considerations and requirements formulation, in other cases novel materials have opened up unexpected applications. Examples of the former are given, where materials meeting certain requirements would lead to quantum leaps in device performance.
DNA is emerging as a novel exciting photonic polymer material due to its unique double-helix structure and the ability to
act as a host capable to be aligned itself and capable of inducing orientation of nonlinear optical (NLO) chromophores.
Physical and optical properties of DNA are remarkably modified with the alteration of the nucleic acid counter-ions. We
determined optical properties of salmon-derived DNA and DNA complexed with cetyltrimethyl-ammonium (CTMA)
surfactant in solutions and films. Absorption coefficients derived for an average nucleotide formula weight indicated
DNA of high purity. Prism coupling measurements showed a large birefringence in refractive indices in the direction
parallel and perpendicular to the surface plane of the films indicating anisotropic alignment of DNA molecules. Almost
isotropic refractive indices were measured in DNA-CTMA films indicating disorder in orientation of DNA-CTMA
molecules in the films. Doping with about 5 wt% Disperse Red 1 (DR1) essentially did not change this very weak
birefringence in the DNA-CTMA films. Optical properties of DNA films were sensitive to environmental humidity while
the DNA-CTMA films were less susceptible to it. The Z-scan technique using femtosecond pulsed laser system was
employed to determine the NLO properties of DNA in solutions in the 530-1300 nm wavelength range.
A new capacitive test structure is used to characterize biopolymers at microwave frequencies. The new test structure is comprised of a parallel plate capacitor, combined with coplanar waveguide-based input and output feed lines. This allows microwave measurements to be taken easily under an applied DC electric field. The microwave dielectric properties are characterized for two biopolymer thin films: a deoxyribonucleic acid (DNA)-based film and a bovine serum albumin (BSA)-based film. These bio-dielectric thin-films are compared with a standard commercial polymer thin film, poly[Bisphenol A carbonate-co-4,4'(3,3,5-trimethyl cyclohexylidene) diphenol], or amorphous polycarbonate (APC).
Marine-based deoxyribonucleic acid (DNA), purified from waste products of the Japanese fishing industry, has recently become a new material of interest in photonics applications. The water soluble DNA is precipitated with a surfactant complex, hexadecyltrimethyl-ammonium chloride (CTMA), to form a water insoluble complex, DNA-CTMA, for application as a nonlinear optical material. In order to fabricate an all-DNA-based waveguide, it was necessary to crosslink the DNA-CTMA films. Crosslinking makes the films resistant to their initial solvent; this permits spin-coating of successive DNA-CTMA layers without solvent damage. A chromophore dye is added to the core layer to allow for an electro-optic coefficient to be induced through contact poling. Through contact poling, an electro-optic (EO) coefficient comparable to that in other polymers was demonstrated in crosslinked DNA-CTMA films with the chromophore dye Disperse Red 1. This EO effect allowed for the creation of the first all-DNA-based EO waveguide modulator. The performance of the modulator is described.
We report on a quantum dots-in-a-well infrared photodetector (DWELL QDIP) grown by metal organic vapor phase epitaxy. The DWELL QDIP consisted of ten stacked InAs/In0.15Ga0.85As/GaAs QD layers embedded between n-doped contact layers. The density of the QDs was about 9 x 1010 cm-2 per QD layer. The energy level structure of the DWELL was revealed by optical measurements of interband transitions, and from a comparison with this energy level scheme the origin of the photocurrent peaks could be identified. The main intersubband transition contributing to the photocurrent was associated with the quantum dot ground state to the quantum well excited state transition. The performance of the DWELL QDIPs was evaluated regarding responsivity and dark current for temperatures between 15 K and 77 K. The photocurrent spectrum was dominated by a LWIR peak, with a peak wavelength at 8.4 μm and a full width at half maximum (FWHM) of 1.1 μm. At an operating temperature of 65 K, the peak responsivity was 30 mA/W at an applied bias of 4 V and the dark current was 1.2×10-5 A/cm2. Wavelength tuning from 8.4 μm to 9.5 μm was demonstrated, by reversing the bias of the detector.
Ordered hexagonal arrays of isoporous films prepared from poly(9,9'-dihexylfluorene) and polystyrene grafted silica nanoparticles (Si-graft-PS) are presented. These close packed arrays were formed in areas of many square millimeters. The pore size varied from 2.9 - 8.5 μm, depending on the concentration of Si-graft-PS and the processing conditions. Solid state photoluminescence resulted in a significant red shift of up to 30 nm in these films compared to conventional processing techniques. These differences are attributed to induced aggregation of the polymers caused by polymer-solvent interactions. Interfacial properties were investigated, and it was found that hydrophobic surfaces (contact angles of up to 129°) were prepared because of high surface roughness. These ordered porous polymer films may find use in microelectronic and bio- and/or chemical sensor applications.
Optical limiting materials are developed for applications in protection of optical sensors against laser aggressions. We have studied functionalised macrocycles (thiacalixarenes) and alkynylplatinum(II) derivatives for optical limiting applications. Glass materials based on alkynylplatinum(II) derivatives and macrocycles were prepared through the sol-gel process. The molecular species were grafted to the matrix in order to maximise the concentration and the stability of the final solid-state material. The glass materials were cut and polished to approximately 1.5 mm. The glass materials show broadband optical limiting in the visible wavelength region, for nanosecond laser pulses.
We report the design of donor functionalized π-conjugated pyridine dicarboxamide based chromophores presenting TPE fluorescence properties. Their use for the complexation of lanthanides and the sensitization of europium(III) luminescence by two photon antenna effect is also described.
Preliminary results on the optical power limiting properties of platinum(II) acetylides containing triazole units are presented. It is shown that the triazole units give a positive contribution to the limiting abilities of the platinum(II) acetylide and that this modified chromophore could have potential use in sensor protection devices. Moreover, this paper discusses how the versatile building block 2,2-bis(methylol)propionic acid (bis-MPA) can be used advantageously to functionalize nonlinear optical (NLO) platinum(II) acetylides. The bis-MPA units can be used to prepare dendritic substituents offering site isolation to the chromophore leading to improved clamping. The bis-MPA functionalization also improves the solubility of the platinum(II) acetylides in many organic solvents. The preparation of solid-state optical power limiters, where the NLO chromophore is inserted in an optically transparent matrix, is addressed. Again, the bis-MPA unit can be employed to increase the number of accessible end-groups to which matrix-compatible species can be attached. It is concluded that the hydroxy-functional platinum(II) acetylides can be modified to fit almost any matrix, organic or inorganic. Finally, depending on functionalization, it is possible to prepare doped glasses where the chromophore is either embedded in the matrix, or covalently bonded to the matrix.
Single quantum dots (QDs), based on the InAs/GaAs material system have been characterized by micro-photoluminescence (μPL). The self-organized quantum dots studied are fabricated by the Stransky-Krastanov method, taking advantage of the strain caused by the lattice mismatch between InAs and GaAs. Well-defined narrow excitonic features from individual QDs are monitored in the μPL spectra, upon single or dual tunable laser excitation. The charge state of the quantum dot is revealed from these excitonic lines in the μPL spectra. However, by tuning the laser excitation energy, it is demonstrated that the charge state of the dot can be altered: The distribution of neutral and charged excitons is demonstrated to be extremely sensitive on the laser energy. In addition, with an additional infrared laser, striking changes are induced in the μPL spectra. The results achieved demonstrate the existence of two well-defined excitation energy regions for the main laser, in which the presence of the infrared laser will decrease or increase, respectively, the integrated dot μPL intensity. For excitation above the critical threshold energy of the main laser, the addition of the infrared laser will induce a considerable increase, by up to a factor 5, in the QD emission intensity. At laser excitation below the threshold energy, on the other hand, the QD emission intensity will decrease. This fact is due to reduced carrier capture efficiency into the dot as determined by the internal electric field driven carrier transport. In order to get further insight into the carrier capture process due to the electric field in the vicinity of the QD, the dots have also been subjected to an external electric field
In most optical experiments with QDs, electrically injected or photoexcited carriers are primarily created somewhere in the sample outside the QDs, e.g. in the barriers or in the wetting layer. Consequently, excited carriers undergo a transport in the wetting layer and/or barriers prior to the capture into the QDs. This circumstance highlights the crucial role of the carrier transport and capture processes into the dot for the performance and operation of the dot based devices such as QD lasers, QD infrared detectors and QD memory devices. This transport effect on the optical response of the quantum dots has been investigated by subjecting the carriers to an external electric field in μPL measurements. This external field is formed by application of a lateral field between two top contacts. It is demonstrated that the QD PL signal intensity could be increased several times (>5 times) by optimizing the magnitude of this external field.
The multi-photon absorption and optical power limiting (OPL) properties of two new thiophenyl-containing bis(ethynylaryl)bis(tributylphosphine) platinum(II) complexes (ATP1, ATP2) were studied. Thiophene units were introduced into the structure as an attempt to enhance the OPL properties. The two compounds have the thiophene rings either close to the Pt-atom (ATP1) or at the terminal ends (APT2). The measurement results were compared with those of Pt1 capped with a 2,2-bis(methylol)propionic acid (bis-MPA) dendrimer (Pt1-G1). As for Pt1-G1, both thiophenyl derivatives showed large inter-system crossing capabilities and triplet phosphorescence, indicating that these compounds have potential of enhancing the nonlinear absorption and specifically the OPL properties. The two-photon absorption cross sections of ATP1 and ATP2 was found to be in the same order of magnitude as that of Pt1-G1, i.e. between 10-20 GM, but is slightly larger for ATP1 than for ATP2. The fluorescence decay time of all compounds was found to be very short (sub nanosecond) and with quantum yields in the order of 10-3. The multi-photon induced phosphorescence was reduced with decreased pulse repetition frequency (prf) showing a population dependence of the triplet state with prf, correlating with the relatively long phosphorescence decay lifetime around 200 μs. OPL measurements at 532, 550 and 610 nm show that ATP1 has the same clamping level as Pt1-G1 at 532 nm and ATP2 has somewhat weaker OPL response than the other two.
This report presents a theoretical investigation of the excited states and photo-physical properties of collsion complex
between molecular oxygen (dioxygen) and free-base porphin. A highly symmetric collision complex of the C2v point
group is calculated by multi-configuration self-consistent field method (MCSCF) method with linear and quadratic
response wavefunctions. Spin-orbit coupling (SOC) is not strongly affected by complexation. Spin forbidden character
of singlet oxygen emission is overcome by relatively strong SOC inside dioxygen, and the main effect of intermolecular
interaction is to enhance the Noxon band (b1Σ+
g - a1Δg) in the O2 molecule. MCSCF calculation of the collsion complex
explain very high efficiency of triplet-singlet energy transfer and photodynamic therapy sensibilized by porphyrins. We
have shown that porphin strongly perturbs many singlet-singlet and singlet-triplet transitions in dioxygen including
enhancement of Herzberg bands in ultra violet region. Vibrational activity in infra red absorption is also predicted in
agreement with some recent experimental data.
Thermochromic metal oxides with a Mott transition, such as vanadium dioxide (VO2) exhibit an extensive alteration in
their infrared reflectivity when heated above the transition temperature. For VO2 the infrared reflectivity increases as the
material becomes more metal-like above the transition temperature at 68°C.
Given these dynamic electromagnetic properties in the IR-range, it is interesting to study the reflection of the material
also in other wavelength ranges. The microwave properties of VO2 as a function of temperature have been investigated
Measurements were made with an automated network analyzer combined with an electrical heating unit.
Reflection properties of VO2 in the microwave region were determined.
Above the transition temperature, an increase in the reflection of the surface was observed. The VO2 became more
metal-like in the whole measured microwave frequency range, as in the infrared region.
It is concluded that VO2 not only can be used to adapt the thermal emissivity of a surface but also to control the
microwave reflectivity. Possible applications are switchable radomes, switchable radarabsorbers and heat protection for
Novel single crystalline high-performance temperature sensing materials (quantum well structures) have been developed for the manufacturing of uncooled infrared bolometers. SiGe/Si and AlGaAs/GaAs quantum wells are grown epitaxially on standard Si and GaAs substrates respectively. The former use holes as charge carriers utilizing the discontinuities in the valence band structure, whereas the latter operate in a similar manner with electrons in the conduction band. By optimizing parameters such as the barrier height (by variation of the germanium/aluminium content respectively) and the fermi level Ef (by variation of the quantum well width and doping level) these materials provide the potential to engineer layer structures with a very high temperature coefficient of resistance, TCR, as compared with conventional thin film materials such as vanadium oxide and amorphous silicon. In addition, the high quality crystalline material promises very low 1/f-noise characteristics promoting an outstanding signal to noise ratio and well defined and uniform material properties, A comparison between the two (SiGe/Si and AlGaAs/GaAs) quantum well structures and their fundamental theoretical limits are discussed and compared to experimental results. A TCR of 2.0%/K and 4.5%/K have been obtained experimentally for SiGe/Si and AlGaAs/GaAs respectively. The noise level for both materials is measured as being several orders of magnitude lower than that of a-Si and VOx. These uncooled thermistor materials can be hybridized with read out circuits by using conventional flip-chip assembly or wafer level adhesion bonding. The increased bolometer performance so obtained can either be exploited for increasing the imaging system performance, i. e. obtaining a low NETD, or to reduce the vacuum packaging requirements for low cost applications (e.g. automotive).