CW Photoacoustic Facility
Pulsed PA Facility
PA Spectrometer

Phthalocyanine Structure

 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Liquid Crystal

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Liquid Crystal

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 Dye Doped Polymer

  
  
  

  

 
 
 
 
 
 
 
 

 

 
 
 
 
 

 Ceramics

 
 
 
 
 
 
 

 

 
 

InP Wafer

  Home I Research I Photoacoustics I Work at ISP CW Photoacoustic Facility 

Continuous Wave Photoacoustic Facility

The continuous wave photoacoustic set-up consist of a Liconix 5000 series Argon ion laser as the excitation source with a power stability of +/_ 0.005 mW/h and can be operated at five different wavelengths. The laser beam is modulated using a electro mechanical chopper. The periodic pressure variation produced in the photoacoustic cell is detected using a highly sensitive miniature electric microphone and the output is processed using a SR 510 lock-in-amplifier.          

Continuous Wave PA Set-up

 

We are using the continuous wave photoacoustic facility to evaluate the important thermophysical parameters of many materials which have potential applications in photonics industry. Some of the material systems that we have already studied is given below

 

Liquid crystals
In recent years, considerable efforts have been made in the synthesis and characterization of liquid crystals and their mixtures with a wide nematic range. The unique properties, such as lower viscosity, increased temperature range, larger birefringence and dielectric anisotropy, of low-molecular weight nematic compounds segregate them from other liquid crystals. The wide range of a particular phase, especially the nematic phase, is an essential requirement in device fabrication. Apart from their applications in display devices, nematic liquid crystals are few among the most attractive nonlinear optical materials. Furthermore, the addition of organic dyes to nematic liquid crystals has been shown to be the source of myriad very interesting and complex nonlinear optical properties. The dye molecules added to the liquid crystals would become orientated along particular directions due to the long range orientational order and intricate hydrodynamic properties of liquid crystals. One of the major advantages of dye-doped nematic liquid crystals is that we can control the nonlinear optical properties of the liquid crystals at any wavelength by an appropriate dye selection. From an application viewpoint, dye-doped liquid crystals have been investigated as possible candidates for optical cavities as well as for spatial filtering, photothermal self-phase modulation, holograms and for optical recording. Thermal measurements play an important role in locating and characterizing the different phases and phase transitions in liquid crystal compounds. We are using photoacoustic technique on many liquid crystals and liquid mixtures for thermal characterization. This method is very simple, less time consuming and can be performed using optical radiation at any wavelength, and hence this is a very promising tool for the thermal characterization of liquid crystals.
  

Phase Transitions in Liquid Crystals

 
Liquid crystal polymers
The synthesis of polymer systems which successfully combine the unique properties of low molecular weight liquid crystals and high molecular weight polymers with their ability to form films, fibers, coatings etc offers great promise in the field of data storage and display technology. Comb-shaped polymers carrying mesogenic groups in their side chain are a class of materials that combine the properties of polymers with those of liquid crystals. The phase transitions between the crystalline, smectic, nematic and isotropic phases in liquid crystalline polymers (LCPs) are similar to those occurring in low molecular weight LCs. In side chain LCPs the liquid crystal moieties are attached to the polymer backbone through a flexible spacer, which enable them to move freely and therefore behave like a classical low molecular weight LC in their optical, electrical and magnetic characteristics. The structure and length of the spacer as well as the structure of the mesogen and the type of backbone have a pronounced effect on the type of phase transition and the transition temperatures in these materials. One of the most recent and important applications of LCPs is in the field of optical data storage. The thermo-optic recording is achieved by incorporating a dye into the LCP, which on irradiation with laser beam absorbs the light energy and re-radiates it as heat. Consequently the LCP is locally heated into the isotropic phase and a change in the optical density takes place on cooling . Usually very thin films (5-10 Ám) are required for the optical data storage. The speed at which the laser treated area cools back into the glassy or liquid crystalline phase is determined by a parameter called the thermal effusivity of the thin film. This quantity measures essentially the thermal impedance of the sample, which is the capability to exchange heat with the environment. This relevant thermophysical parameter is determined using photoacoustic technique in different liquid crystal polymers, which may find applications in photonics industry.
 
Dye doped polymers
Studies on photo-induced bleaching of organic dyes impregnated in solid matrices have great importance due to the various applications of these
materials, such as for active laser elements, passive Q-switches, optical data storage, photonic displays, optical waveguides etc. Suitable materials for different applications can be prepared by properly selecting the type of solid matrix and the dye incorporated into it. In all the above applications, a thorough knowledge of the photostability of the dyes incorporated in a solid matrix is necessary. We have successfully used photoacoustic techinique to study the photodegardation mechanism of Rhodamine 6G molecules in poly methyl methacrylate matrix. The same technique can be used to study similar systems. We also studied the bleaching of cresyl violet in polyvinyl alcohol due to irradiation by a He-Ne laser. Our studies show a decrease in PA signal amplitude with irradiation time. This information is very useful since cresyl violet can be used as a potential medium for holographic recording and information storage.
 

PA Studies on Doped Polymers

 
Organic conductors
Recently metal phthalocyanines and metal naphthalocyanines have attracted much attention because of their potential application in electronic devices, optical data storage, photoconductors and sensors.They are also very good candidates for high density optical data recording media due to their thermal and chemical stability and facility for synthetic modification. An essential requirement for the optical data recording material is the long wavelength absorption. Metal naphthalocyanines absorb at a longer wavelength than the phthalocyanines because of their additional benzoannulation. In order to use these materials for optical data recording, the thermal properties are of considerable importance. We have evaluated the the thermophysical parameters of many metal phthalocyanines and metal naphthalocyanines using photoacoustic technique.
 
Ceramics
One of the recently studied ceramic material system is lanthanum doped lead zirconate-titanate (PLZT). This belongs to the transparent ferroelectric ceramics which have potential applications in solid state optoelectronic devices due to the unique electro-optic and photo-electric proparties. Thermal diffusivity of this ceramic has been studied using photoacoustic technique, which is found to increase with increase in lead oxide content. The observed increase in thermal diffusivity of PLZT with an increase in lead oxide content is supposed to be due to the appearance of addi-tional defects other than the point defects introduced in the material due to the nonstoichiometry of lead oxide. 
 
III-IV Semiconductors
III-V compound semiconductors have potential applications in photonics industry. For the application point of view, the thermophysical parameters of these compounds are extremely important and these parameters can be evaluated using the photoacoustic technique. The photoacoustic signal is strongly depend upon the interplay of the sample optical absorption coefficient for the incident radiation, the light into heat heat conversion efficiency, as well as how the heat diffuse through the sample. So this technique can be used to to extract information about non radiative deexcitation processes and to measure the thermal proparties.In the case of semiconductors, the photoacoustic signal provides us with additonal information regarding the carrier transport proparties like carrier diffusion coefficient, surface recombination velocity and bulk recombination time. We are using open photoacoustic cell detection technique to study the bulk semiconductor and layered semiconductor samples. 
 

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