About the group
Leading researcher: Prof. Lian Nedelchev, PhD
Theme: Photoanisotropic materials; Polarization-selective holographic optical elements; Polarization and surface relief gratings with applications in photonics.
Azobenzene (AB)-containing materials are a class of optical materials intensively studied in recent decades due to their potential use as optical information recording media, optical switches and sensors, as well as in polarization holography and photonics. Due to their unique photochromic behavior, ABs can be used to induce controlled changes in the physicochemical, mechanical, electronic and optical properties of materials. In practice, the photoisomerization process is used to switch a material between two different states or phases.
Photoisomerization of azobenzenes can be used to convert electromagnetic radiation energy into mechanical energy by inducing reversible structure and volume changes in the material. At the molecular level, only active chromophores with a dipole moment parallel to the axis of polarization of the light are induced by linearly polarized light. This selectivity is due to the highly anisotropic structure of the trans- azobenzenes and ultimately leads to anisotropic clustering of the chromophores and birefringence in the material. Most azobenzenes are known to isomerize and exhibit a photoorientation effect. Due to photoorientation with linearly polarized light, azobenzene molecules preferentially absorb light polarized along the long axis of the molecule. In practice, this means that the absorption of molecules perpendicular to the polarization axis of the incident light is negligible compared to that of molecules located along the axis. Repeated, highly efficient transitions in azochromophores between trans and cis isomers lead to a reorientation perpendicular to the direction of polarization of the incident light. The resulting anisotropy, inducing a large and constant in-plane birefringence, can be observed in the polarized absorption spectra of the film. The course of the orientational anisotropy is followed by measuring the transmission of a low-power sample beam through the polarizer/sample/analyzer configuration. The birefringence Δn can be calculated by the formula below, where d is the film thickness, λ is the wavelength of the probe beam, I is the signal of the probe beam passed through the birefringent specimen (the polarizer and the analyzer are placed perpendicular to each other, and I0 is the signal through a non-irradiated sample (parallel polarizer and analyzer).
Holographic recording in this kind of materials is inextricably linked to the phenomenon of photoinduced mass transport and the formation of surface relief gratings (SRGs). The ability to create large-amplitude SRGs in azo materials has become essential because of the extremely high diffraction efficiency that is easily achieved using a one-step recording process and the ability to erase and reconfigure gratings at will.
Therefore, not only research on the realization of holographic recording, but also the formation of SRGs itself is attracting great attention as a new, important tool in the field of photonics and micro/nanotechnology. Another significant application of azo materials is as photoactive polymer matrices in nanomedicine in the form of supports for bioactive molecules in dosage forms. In the last few years, nanostructured azofilms have found application as (bio)sensors in a number of fields of medicine and science studying living organisms. Among the abundance of nanostructures currently available, SRGs offer key opportunities for applications in biosensing, such as portable in situ detectors, due to their inherent property to absorb in the fingerprint region, as well as their compatibility with collinear optical shapes and the possibility of easy integration into other micro-technology platforms such as microfluidics.
Leading researcher: Prof. Lian Nedelchev, PhD
Theme: Photoanisotropic materials; Polarization-selective holographic optical elements; Polarization and surface relief gratings with applications in photonics.
Azobenzene (AB)-containing materials are a class of optical materials intensively studied in recent decades due to their potential use as optical information recording media, optical switches and sensors, as well as in polarization holography and photonics. Due to their unique photochromic behavior, ABs can be used to induce controlled changes in the physicochemical, mechanical, electronic and optical properties of materials. In practice, the photoisomerization process is used to switch a material between two different states or phases.
Photoisomerization of azobenzenes can be used to convert electromagnetic radiation energy into mechanical energy by inducing reversible structure and volume changes in the material. At the molecular level, only active chromophores with a dipole moment parallel to the axis of polarization of the light are induced by linearly polarized light. This selectivity is due to the highly anisotropic structure of the trans- azobenzenes and ultimately leads to anisotropic clustering of the chromophores and birefringence in the material. Most azobenzenes are known to isomerize and exhibit a photoorientation effect. Due to photoorientation with linearly polarized light, azobenzene molecules preferentially absorb light polarized along the long axis of the molecule. In practice, this means that the absorption of molecules perpendicular to the polarization axis of the incident light is negligible compared to that of molecules located along the axis. Repeated, highly efficient transitions in azochromophores between trans and cis isomers lead to a reorientation perpendicular to the direction of polarization of the incident light. The resulting anisotropy, inducing a large and constant in-plane birefringence, can be observed in the polarized absorption spectra of the film. The course of the orientational anisotropy is followed by measuring the transmission of a low-power sample beam through the polarizer/sample/analyzer configuration. The birefringence Δn can be calculated by the formula below, where d is the film thickness, λ is the wavelength of the probe beam, I is the signal of the probe beam passed through the birefringent specimen (the polarizer and the analyzer are placed perpendicular to each other, and I0 is the signal through a non-irradiated sample (parallel polarizer and analyzer).
Holographic recording in this kind of materials is inextricably linked to the phenomenon of photoinduced mass transport and the formation of surface relief gratings (SRGs). The ability to create large-amplitude SRGs in azo materials has become essential because of the extremely high diffraction efficiency that is easily achieved using a one-step recording process and the ability to erase and reconfigure gratings at will.
Therefore, not only research on the realization of holographic recording, but also the formation of SRGs itself is attracting great attention as a new, important tool in the field of photonics and micro/nanotechnology. Another significant application of azo materials is as photoactive polymer matrices in nanomedicine in the form of supports for bioactive molecules in dosage forms. In the last few years, nanostructured azofilms have found application as (bio)sensors in a number of fields of medicine and science studying living organisms. Among the abundance of nanostructures currently available, SRGs offer key opportunities for applications in biosensing, such as portable in situ detectors, due to their inherent property to absorb in the fingerprint region, as well as their compatibility with collinear optical shapes and the possibility of easy integration into other micro-technology platforms such as microfluidics.
