New optical methods to study director orientation in liquid crystals

Summary We report new optical methods useful to investigate the orientational order of nematic liquid crystals. The first one uses a linear optical technique and a variable-thickness cell to get a precise measurement of the anchoring energies at the boundaries. The second one exploits the nonlinear self-phase modulation to evaluate the elastic anisotropy of a nematic liquid crystal. Examples are reported of measurements performed on hybrid aligned liquid crystals. Also with GNSM and CISM, Unità di Cosenza.     

Spatial filtering in the detection of thermal transverse phase modulation of laser beams

We study the application of spatial filtering to the far field detection of a probe beam with transverse phase modulation induced by the refractive index distribution generated in an optically thin film by the steady state absorption of a pump beam. To this end we introduce a mathematical model able to describe the phenomenon and we study the dependence of the detected signal on the radius and the position of a circular stop operating as a spatial filter. We show, by computer calculations, that spatial filtering allows enhancement of signal/background ratio as has already been shown for the case of pulsed thermal lens spectroscopy. We give an explicit expression of the detected signal as a function of sample properties that can be used for the interpretation of measurements.     

Laser cleaning of gilded wood: A comparative study of colour variations induced by irradiation at different wavelengths

There is a growing interest by art conservators for laser cleaning of wood artworks, since traditional cleaning with chemical solvents can be a source of decay, due to the prolonged action of chemicals after the restoration. In this experiment we used excimer and Nd:YAG lasers, emitting radiation in the ultraviolet (248 nm), visible (532 nm) and near infrared (1064 nm), to investigate the effect of laser interaction on gilded wood samples at different wavelengths. Increasing fluence levels were tested to assess threshold values both for surface damage and colour changes. Detailed colorimetric analyses of the irradiated samples show that cleaning effectiveness is related to the emission wavelength, the fluence and the number of pulses.     

Photoacoustic analysis of liquid crystals' theramal parameters

Summary The general theory of photoacustic signals is adapted to the detection of liquid crystals' thermal parameters. The thermal-diffusion length to optical-absorption length ratio is given as a simple function of the phase, while the sample to gas effusivity ratio is given as a simple function of phase and normalized amplitude. A method allowing the determination of each thermal parameter separately is presented. Since it does not require strong thermal gradients in the sample, it is particularly well suited to the study of pretransitional effects in liquid crystals. To speed up publication, the authors of this papers have agreed to not receive the proofs for correction.     

Laser beam manipulation by composite material electro-optic devices

Liquid crystals are anisotropic materials whose optical properties can be easily controlled applying an electric field. For this reason, they are widely used in electro-optical devices. Besides other applications, liquid crystals electro-optical devices are suitable for laser beam manipulation. In this paper, we present some devices developed or studied in our laboratory: a twisted nematic liquid crystal (TN-LC) polarization rotator, a TN-LC protected light sensor, a polymer dispersed liquid crystal (PDLC) light modulator, a PDLC optoelectronic polarizer and a polymer liquid crystal (PoLiCryst) light modulator.     

Optoelectronic polarizer by PDLC

We present a new device based on Polymer Dispersed Liquid Crystals (PDLC). Applying a transverse low frequency electric field, the scattering cross section increases or decreases depending on the polarization of the impinging light. Depending on the choice of the refractive index of the polymeric matrix, such a device can be operated as a polarizer, or as an inverse shutter. In the first case one polarization component of an unpolarized beam is transmitted, while the other is scattered. In the second case an impinging polarized beam is totally scattered when an external electric field is applied and becomes partially transmitted when the external field is switched off.     

Study of thermally induced optical bistability in a twisted nematic liquid crystal

We report the observation and study of optical bistability in the power transmitted by a twisted nematic liquid crystal bounded by crossed dichroic polarizers. A detailed study of the dependence of the phenomenon on the sample initial temperature has been performed. The effect is a good test for a simple theoretical model which describes the isotropic droplet formation in a nematic sample under laser illumination. A good agreement is found between experimental results and theoretical predictions. Applications of the effect to realize temperature sensors are envisaged.     

Experimental results on the photophoretic motion and radiometric trapping of particles by irradiation with laser light

An experimental observation of the photophoretic motion of micro-sized particles suspended in air has been performed using the radiometric force from a continuous laser beam. The irradiation was used to trap the particles in a confined optical field and to levitate them. Complex photophoretic motion of the particles was observed in the focus of the beam.     

Experimental confirmation of a laser-induced temperature field model by means of microholographic interferometry

Summary The experimental validation of a mathematical model of the temperature field generated by the laser irradiation of a sample is shown. The test was performed on water, a material with well-known thermodynamic parameters. The experimental data were obtained by means of holographic interferometry. The validated model, in connection with the microholographic method, can be used to determine the thermodynamic properties of different materials.

---

Riassunto Si presenta la validazione sperimentale di un modello matematico dei campi di temperatura generati dall’irraggiamento laser di campioni. Il test è stato eseguito sull’acqua, un materiale dai parametri termodinamici ben noti. I dati sperimentali sono stati ottenuti mediante interferometria microolografica. Il modello validato, congiuntamente al metodo microolografico, può essere usato per la caratterizzazione termica dei materiali.

---

Резюме Экспериментально докаэывается справедливость математической модели температурного поля, образованного при лазерном облучении образца. Проверка проводилась на воде, т.е. материале с хорошо известными термодина-мическими параметрами. Экспериментальные данные получены с номощью голографической интерферометрии. Предложенная модель может быть использована для определениа термодинамических свойств различных материалов.