Ultrafast nonlinear optical studies of organic, inorganic and nano materials:

1. Ultrafast Nonlinear optics of semiconductor nano particles

   With the increase in use of internet the amount of data transported from one place to other is increasing at large rate. Switches operating at faster rate are needed for future systems. The nonlinear optics may provide solution for faster switches as it offer potential of controlling a weak incident light by a strong light.  The response bandwidth of such optical control is limited by the response time of the interaction material which would be smaller than 100 fsec. The bandwidth of optical switch would thus reach beyond 1 Tera Hertz not attainable by any electronic switch.

   Materials with large nonlinear optical coefficient (nonlinear refractive index) are required. Large nonlinear refractive index would lead to devices with small size and operate with low input pump power. Various semiconductors, organic materials have been very well studied for their nonlinear optical properties. Currently nano materials are fabricated which have significantly different properties from the corresponding bulk materials. Therefore the investigation of nonlinear optical properties of nano particles, porous materials is an area of active research.

   We have extensively studied the third order nonlinear optical properties of CdS_xSe_1-x nano particle doped in the glass matrix using ultrafast lasers. Various nonlinear optical techniques such as nonlinear transmission, z-scan, pump-probe, degenerate four wave mixing and optical kerr shutter were employed to probe the nonlinear response. The third order nonlinear coefficients include nonlinear refractive index, two-photon absorption(TPA) coefficient, free carrier refraction( refractive index change by carriers generated by the TPA), and free carrier absorption.

   a) Femtosecond Z-scan: Over the last few years we have performed various experiments using z-scan technique. The experimental Z-scan data for semiconductor doped glasses containing CdS_xSe_1-x nano-particles obtained at wavelength of 1064 nm with the 100 fsec pulses was analysed. The Z-scan profiles with the fsec pulses were in complete contrast with those obtained for psec pulses. The Z-scan profile with the picosecond pulses shows a peak-valley profile indicative of negative fifth order nonlinearity dominated due to carriers generated by TPA. However the fsec Z-scan profile show a valley–peak profile indicative of the positive third order nonlinearity. The femtosecond Z-scan profiles are sensitive to both the third and the fifth-order nonlinearity. The difference between the psec and the fsec Z-scan profile is due to the fact that the fifth-order nonlinearity depends both on the fluence and irradiance whereas the third-order nonlinearity is proportional to the irradiance only. When the laser pulse width is reduced from psec to the fsec duration for the fixed irradiance the fsec pulse would have smaller fluence. Thus for the fsec pulse the contribution of the negative fifth-order nonlinear effect to the total nonlinearity would reduce in comparison to the positive third-order nonlinear effect.

   
   
   Figure: Typical open aperture Z-scan profile with(a) picosecond pulses (b) femtosecond pulses.

   The fsec Z-scan profiles were significantly affected by the magnitude of the two-photon absorption coefficient as was observed for the two samples OG550 and RG850. A large value of TPA coefficient in RG850 sample results in generation of more electron hole pairs and thus results a greater contribution to the negative fifth order nonlinearity. This results in the disappearance of positive peak at the positive z position due to positive third order nonlinearity. Complete understanding and analysis of fsec and  psec  Z-scan profile is thus achieved.

   b)  Pump-probe Z-scan experiment in semiconductor doped glasses: Over the past few years Z-scan has become an important method for determining the nonlinear refractive index and two-photon absorption (TPA) coefficients. In semiconductors, for example, the free carriers generated by TPA also contribute to absorption and refraction, and the effect is cumulative for carrier decay times longer than the pulse width. Therefore with conventional single beam Z-scan the interpretation of mechanisms of nonlinear response requires careful analysis and some additional experiments. We have for the first time performed a two beam pump-probe Z-scan to unambiguously measure the contribution of TPA generated free carriers alone in the nonlinear response of semiconductor doped glasses. This pump-probe experiment clearly establishes that for picosecond laser pulses free carrier nonlinearity is the dominant mechanism for SDG’s in contrast to single beam z-scan experiment.

   
   Figure: Pump-probe Z-scan experimental set up

   The experiments were performed with 30 picosecond pulses at 1.06 mm. An intense pump beam generates free carriers due to TPA. A weak probe beam (intensity 10 times less than that of the pump beam) was delayed by 100 ps with respect to the pump beam. This delay is much larger than the laser pulse width. Z-scan of the probe beam was performed. In the absence of pump beam no Z-scan signal of probe beam was observed. In the presence of pump beam a strong Z-scan signal of probe beam was observed. The absence of Z-scan signal without pump beam and large delay between pump and probe beams clearly indicate that the nonlinear refraction is entirely due to TPA generated free carriers. Transmission of probe beam was also recorded at different time delays indicating that the decay time of TPA generated carriers is much more than 200 ps. The magnitude of nonlinear refraction due to free carriers is being calculated taking into account the absorption of pump as well as probe beam by the free carriers.

   

2. Nonlinear Optical properties of porous Si

   Recently, in literature a large third order electronic nonlinearity in porous Si using Z-scan experiment with psec laser pulses is reported. The porous Si sample has a strong linear absorption of 10 cm^-1 at 1064 nm. The sample appeared highly transparent due to small thickness( 20 microns). The nonlinearity observed in the Z-scan signal was attributed to the bound electronic nonlinearity due to high linear transmission of the sample.

   Due to presence of linear absorption in porous Si the contribution to the nonlinear refraction by the free carriers cannot be neglected. We calculated the intrinsic nonlinearity and the effect of the change of refractive index by free carriers generated by linear absorption on the Z-scan signal for porous Si. Our calculations show that for the porous Si sample, the Z-scan signal could be attributed to the free carrier nonlinearity. The contribution due to the intrinsic third order nonlinearity is negligible. The conclusions drawn from our calculations are in complete contrast with the reported results.  We further show that the effect of the free carrier nonlinearity would be small for the Z-scan experiments performed with femtosecond pulses.

3. All-optical switching with metalloporphyrins

   Considerable research efforts are directed towards all-optical molecular devices for information processing and storage that offer advantages of small size and weight. Metalloporphyrins are interesting chromophores due to their large nonlinearity and amenability to tailoring of their properties by different techniques to develop better materials for device applications. Vanadium (IV)-oxide tetraphenylporphyrin (VOTPP) and Zinc (II) tetraphenylporphyrin (ZnTPP) show enhanced nonlinear optical properties. All-optical switch is a fundamental building block of information processing for future. All-optical switching in ZnTPP and VOTPP in toluene solution using pump probe method has been studied experimentally. A He-Ne laser of 633 nm was used as probe beam and a flash lamp pumped frequency doubled Q-switched Nd:YAG laser of 532 nm, 20 ns pulsewidth was used as the pump beam. Effect of variation of pump power and concentration of the medium on switching characteristics has been studied. Switching results have been explained by carrying out a theoretical analysis based on the rate equation model.

   
   
   Figure . All-optical switching in (a) ZnTPP and (b) VOTPP.
   

   Figures (a) and (b) show all-optical switching characteristics of ZnTPP and VOTPP, respectively. Switch ‘off’ and ‘on’ time for ZnTPP are ~200 ns and ~3.5 ms respectively and for VOTPP respective time are ~1.5 ms and ~2.5 ms. It is evident from Figs. 1(a) and 1(b) that the switch off time is much longer for VOTPP than ZnTPP. Further the effect of variation of pump energy on transmitted probe beam intensity was studied for different concentrations of VOTPP. Results indicates that there is an optimum value of concentration (43% linear transmission at 532 nm) for which maximum modulation of the probe beam (~30%) is achieved with 1.1 mJ pump energy.

   Rate equation approach has been used to analyze the observed switching characteristics of ZnTPP and VOTPP. Effective three state rate equation model is able to analyze experimental results for ZnTPP but not for VOTPP. Theoretical analysis to the experimental results indicated possibility of an intermediate state between S1 and T1 states having ~1.5 ms lifetime in case of VOTPP. Since the kinetic and spectral properties of metalloporphyrins can be tailored by different means, the switching characteristics can be optimized for desired applications. In the ideal case, where the probe beam does not get absorbed by the ground state, complete switching (100% modulation of the probe beam) can also be achieved by increasing the input intensity only. The switching operation can be made fast by decreasing the lifetime of triplet state. Metalloporphyrins based switches would be potentially useful in optical signal processing. They can provide an alternative to the thermo-optic, MEMS, and liquid crystal switches that operate in the ms range.

   All-optical switching in Pt:ethynyl complex has also been theoretically analyzed to achieve it at lower power with high switching contrast and low switching time. The transmission of a cw probe laser beam at 633 nm corresponding to the peak absorption of the T₁ state is switched by a pulsed pump laser beam at 355 nm that excites the molecules from the S₀ state. The switching characteristics have been shown to be sensitive to the variation of the pump pulse width, peak pumping intensity, transition times of S₁-S₀ and S₁-T₁ state and lifetime of triplet state. It has been shown that the transmission of the probe can be completely switched off (100% modulation) by a pulsed pump laser beam at 50 kW/cm2. Further, the results have been used to design all-optical NOT and the universal NOR and NAND logic gates with multiple pump laser pulses that are the basic building blocks of computing circuits.

   References:

   • C.P. Singh, K.S. Bindra, B. Jain and S.M. Oak, “All-optical switching characteristics of metalloporphyrins,” Optics Communication 245, pp. 407-414, 2005.
     
     
   • C.P. Singh, K. Kulshrestha and S. Roy, “High contrast all-optical switching with Pt:ethynyl complex,” Optik 117, pp. 499-504, 2006.
   
   
4. Effect of excitation pulse characteristics on numerical simulation of reverse saturable absorption

   Reverse saturable absorption (RSA) is one of the physical phenomena responsible for observation of optical limiting in many organic materials, particularly in molecules with center-symmetric structure and –electron conjugated system. RSA can be observed when the absorption cross-section of the excited state is larger than the ground state, resulting in decrease in the transmission of the medium with increase in fluence. We analyzed rate equations for reverse saturable absorption (RSA) process to understand the applicability and probable errors in using only time dependence of the excitation pulse as against both space and time dependence in numerical simulations of rate equations. Three-level approximation of the five-level rate equations has been considered under nano-second laser pulse excitations. The population density, nonlinear absorption and transmission have been calculated and compared for different approximations and for different excitation pulse spatial shapes. We show that the neglect of spatial characteristics of the excitation pulses in the rate equation analysis can lead to considerable errors in the numerical simulations of RSA in case of excitation fluences below the saturation of transmission. As an example simulation results for C60 solution in toluene excited by nanosecond laser pulses at 532 nm are shown in the following figure.

   
   
   Figure .Variation of transmission with input fluence, considering input pulse as a function of time only (squares), both space and time (circles) and steady-state approach (triangles).
   

   References:

   • C.P. Singh, K.S. Bindra, and S.M. Oak, “Analysis of effect of excitation pulse characteristics on numerical simulation of reverse saturable absorption,” Optics Communication 269, 223-229, 2007.
     
     
   • C.P. Singh, K.S. Bindra, and S.M. Oak, “Effect of optical pulse characteristics on analysis of reverse saturable absorption,” PHOTONICS 2006, pp. 223-229, 2007.
     
     

5. Nonlinear optical properties of many materials dissolved in organic solvents are studied using high intensity femtosecond laser pulses. It is essential to separate the solvent contribution from that of the material under study. Therefore it is important to study the nonlinear response of the pure solvents. Nonlinear refraction is an important parameter in the study of nonlinear optical properties of the medium due to its wide range of applications including optical switching, image processing etc. Nonlinear refraction coefficients of various media including common solvents, organic dyes, bulk semiconductors as well as semiconductor-doped glasses were investigated under excitation with picosecond laser pulses. For picosecond pulses both the electronic nonlinearity and that due to molecular reorientation Kerr effect contribute to the nonlinear refractive index (n₂). For femtosecond pulses only the electronic nonlinearity would contribute as the molecular reorientational life times are in general in pico-second time scales. Therefore the nonlinear refraction coefficient for organic solvents measured with femtosecond and picosecond pulses differ significantly even in the non-resonant regime. However, these parameters are not available for many solvents in case of femtosecond pulse excitation. Results of measurements of nonlinear refraction of various solvents under femtosecond laser excitation have been presented. The n₂ was measured by Z-scan technique using a Ti: Sapphire laser giving 100 femtosecond pulses at 800 nm operating at 1 kHz repetition rate.
   
   The Z-scan with various solvents was performed. All the solvents show valley peak profile in the closed aperture Z-scan indicative of positive nonlinearity and there is no signal in the open aperture profile (squares) indicative of absence of nonlinear absorption. The maximum intensity used was about 200 GW/cm². The values of n₂ were estimated by numerically fitting the experimental data. The femtosecond Z-scan experimental setup was highly sensitive and n₂ values as low as @ 10^-16 cm²/W that of the deionised water could be easily measured. Such a high sensitivity is generally not reachable with picosecond pulse excitation. This is due to fact that for femtosecond pulses the damage threshold is much higher and therefore higher input intensities can be used in the experiments. It was found that in the femtosecond pulse excitation regime the n₂ values are generally about one order of magnitude smaller than the picosecond pulse excitation regime. This is due to the different mechanisms of nonlinear response with the pico and femtosecond pulse excitations.

   
   
   Figure: Z-scan profile for the CS2. Circles and squares are the experimental data points for closed and open aperture scans respectively. The solid line is the theoretical fit to the closed aperture experimental data.
   
   

6. Study of optical limiting in laser dyes and nano particles

   Optical limiter is a device that strongly attenuates optical beam to a threshold level at high intensity while exhibits linear transmittance at low intensity. Such devices are used for protecting human eyes and optical sensors from damage due to exposure to intense radiation. The search for efficient optical limiters has lead to the study of various materials that exhibit nonlinear absorption. Materials that exhibit reverse saturable absorption (RSA) are of a great deal of interest over the past several years for their optical limiting properties. RSA can be observed when the absorption cross-section of the excited state is larger than the ground state, resulting in decrease in the transmission of the medium with increase in fluence. Optical limiting experiments on laser dyes namely LD700, Crystal violet, Methylene blue, HITC per-chlorate were carried out and results were compared with standard optical limiter fullerene C₆₀. Experiments were performed with Nd:YAG nanosecond laser pulses at 532 nm in two geometries; one by collecting whole transmitted beam and other by placing a 90 % linear transmission aperture in the far field. We found that the HITC- perchlorate dye show a better optical limiting compared to the reference sample C₆₀ while other samples show poor optical limiting than C₆₀. Z-scan experiments were also carried out on these samples simultaneously in closed and open aperture geometry. We observed dip in the open aperture indicative of nonlinear absorption(RSA) and in the closed aperture also we observed only valley with no clear peak. Our experimental observations indicate that the reverse saturable absorption is more dominant in HITC compared to C₆₀. The dyes were also studied using picosecond pulses. The comparison of piocsecond and nanosecond pulses optical limiting data indicates that apart from RSA thermal nonlinearity also contribute to the optical limiting process.

   
   
   Figure: Variation of output energy with input energy
   

   On the material front synthesis of nanoparticles of various materials has received much attention due to possibility of enhancement in the nonlinear optical properties for photonic devices. Optical limiting performance of nano particles is also being studied. Interesting optical limiting results have been obtained and would be reported shortly.

   Investigation of optical limiting in iron-oxide nanoparticles

   Optical limiting in iron-oxide nanoparticles of diameters ~30, ~45, and ~60 nm dispersed in toluene under exposure to nanosecond laser pulses at 532 nm was studied. In the low fluence region smaller size nanoparticles show better optical limiting compared to larger size nanoparticles. Further iron-oxide nanoparticles show better optical limiting than reference sample fullerene C₆₀.
   

   To understand the mechanism of optical limiting in iron-oxide nanoparticles we measured the scattering in the backward and forward direction of transmitted beam through the sample. Normalized scattering in iron-oxide nanoparticles is very large in the backward direction compared to C₆₀ and solvent toluene. The nonlinear scattering in the iron oxide nanoparticles could be attributed to the bubble formation.

   
   
   Fig . Variation of normalized scattered signal in the backward direction with variation in input fluence for iron oxide nanoparticles of 44 nm (squares), C60 (triangles) and pure toluene (circles). The crosses represent the scattering from C60 when each laser pulse is exposed to fresh position in the sample.

   The Z-scan experiments were also performed with nano-second and pico-second pulses at 532 nm to understand fluence or intensity dependence of the nonlinearity. Picosecond Z-scan profile was in complete contrast with that obtained for nano-second pulses, hence contribution to the optical limiting due to electronic nonlinearity could be neglected. In conclusion, nonlinear scattering is dominant mechanism responsible for observation of optical limiting in iron oxide nanoparticles. The absorption spectrum of the iron oxide nanoparticles does not show any sharp feature, these nanoparticles could be used as broad-band optical limiter. Optical limiting studies on various other nanoparticles are in progress.
   
   References:

   1. C.P. Singh, K.S. Bindra, G.M. Bhalerao and S.M. Oak, “Investigation of optical limiting in iron oxide nanoparticles,” Optics Express 16, pp. 8440-8450, 2008.
      
      
   2. C.P. Singh, K.S. Bindra, V. Shukla, G.M. Bhalerao and S.M. Oak, “Study on optical limiting in iron-oxide and copper nanoparticles,” International conference on Fiber Optics and Photonics (PHOTONICS 2008), Dec. 13-17, 2008, New Delhi.

   Z-Scan and optical limiting studies in Neutral red dye.

   The nonlinear optical response of an organic dye, neutral red (NR) was studied both in solution and solid film, made in methanol and polyvinyl alcohol respectively, using single beam Z-scan technique. The nonlinear optical absorption, refraction and optical limiting behaviour were investigated under excitation with nanosecond laser pulses at 532 nm. The open aperture Z-scans of the solution samples displayed a switch over from saturable absorption to enhanced absorption with increase in input intensity. Theoretical fit to the experimental data indicated that the dominant mechanism of nonlinear absorption is two-photon absorption. The closed aperture Z-scans of both the samples denoted positive nonlinearity, which was three orders larger in magnitude in solid film, compared to that in solution. The results of optical limiting experiments revealed that neutral red exhibited strong optical limiting of nanosecond laser pulses with a threshold lower than that of C₆₀ in toluene.

   
   
   Figure. Open aperture Z-scan experimental data (squares) of NR solution of 20 % linear transmission at 532 nm at I0 = 80 MW/cm2. Solid line shows theoretical fit to the experimental data.
   

   References

   1. M. George, C.I. Munera, C.P. Singh, K.S. Bindra and S.M. Oak, “Z-scan studies and optical limiting of nanosecond laser pulses in neutral red dye,” Optics & Laser Technology 40, pp. 373-378, 2008.

7. Synthesis of nano materials in liquids

   Efforts are made to synthesise the nano particles because of their exotic properties and offer wide range of applications in every branch of science, physics, chemistry and biology. The production of nano particles by laser ablation in liquids offer advantages over the chemical method. The nano particles of any solid material can be prepared in any solvent. Cu nanoparticles attracted considerable attention because of their catalytic, optical and conducting properties. We have synthesized Cu nano particles by laser ablation in water using a Q-switched Nd-YAG laser (1064 nm). The figure below shows the absorption spectra. The peak observed at 595 nm indicated the presence of Cu nano particles. This technique is very simple and size of the nano particles can be controlled by laser parameters, e.g. laser fluence, laser pulse duration, laser wavelength and surfactant.

   
   
   

8. Spintronics

   Magnetic thin films have been a topic of great current interest because of their novel physical properties and diverse technological applications. The magnetic properties depends strongly on the interface and thickness of films. In investigating magnetic thin films, the magneto-optical Kerr effect (MOKE) has been widely utilized as a tool in probing surface magnetism.
   
   When a plane polarized light interacts with a magnetic material the polarization gets changed by an amount proportional to the magnetization of the specimen. The effect in the reflection is known as Kerr effect and in the transmission it is known as Faraday effect. The MOKE manifests itself by the change of polarization and/or intensity of incident polarized light when it is reflected from the surface of a magnetized medium. The magnetooptical effects are shown to be proportional to the magnetization (M) in ferromagnets or a linear summation of sublattice magnetizations in ferrimagnets. Using the Kerr effect one, therefore, probes a quantity proportional to the magnetization in a surface layer, the thickness of which is determined by the optical absorption coefficient of the material at the wavelength used for the experiments. For metals in the visible region this surface layer thickness is typically 10-20 nm.

   
   
   Figure-1. Block diagram of the MOKE setup

   The experimental set up for MOKE is shown in the above figure. Two Glan-Taylor prisms polarizers, Intensity stabilized He-Ne laser (632.8 nm), Si photodiode as detector and Stanford make (model SR-830) Lock-in amplifier are used for the set-up. PEM operating at a frequency of 50 kHz is used for the modulation. The direction of the magnetization vector with respect to the reflection surface and the plane of incidence categorize MOKE. MOKE can be done in three configurations viz., longitudinal, polar and transverse. In the longitudinal geometry, the applied magnetic field will be parallel to the film surface and in the plane of incidence. In this geometry, the signal is sensitive to the in-plane component of the magnetization. In the polar geometry, the applied field will be perpendicular to the film surface and in the plane of incidence. The signal is sensitive to the out-of-plane component. In the transverse geometry the applied field will be perpendicular to the film surface and also to the plane of incidence. In the first two cases, there will be a change in the state of polarization due to magneto-optical effects and in the transverse mode it is the intensity, which changes in proportional to the magneto-optical interactions. We have used this setup for the measurement of hysteresis loop for the standard iron film. Hysteresis loop gives information about the magnetic behaviour and quality of the material i.e. whether the material is paramagnetic or ferromagnetic etc. Further modifications in the detector system are on for improving the sensitivity of the system.
   
   Computer controlled MOKE using He-Ne laser has been setup in our laboratory. We developed an indigenous photodiode circuit which provides the MOKE signal similar to that obtained with Peltier cooled PMT as shown in Fig. 2. We also, plan to study spin dynamics using pump-probe experiments with ultrafast lasers.

   

    

9. Manipulation of femtosecond pulses for control of linear and nonlinear interactions.

   The use of femtosecond lasers to control quantum mechanical systems emerged in the early 1990s. The field of quantum control has advanced rapidly over the years [1,2]. Experiments have ranged from the control of electronic dynamics, molecular vibrations and rotations, to influencing the selectivity and yield of high harmonic generation. The three most important enabling technologies in this field are lasers that generate ultrafast pulses, the Fourier transform pulse shapers that sculpt these pulses into useful control fields, and computer learning algorithms which search for control fields to give the desired results.
   
   In 1992, Judson and Rabitz proposed using a closed-loop apparatus with a learning algorithm to discover control fields that yield a target result [3]. Profile of the pulse can be shaped by either temporal or spectral manipulation. Manipulation of the ultrafast pulses (fs) would require a system capable of reacting at speeds in the attosecond regime. A Fourier transform pulse shaper circumvents this need by manipulating the pulses in the spectral domain. This is accomplished by the use of a grating and a lens to map the various frequency components, spatially, to a line (the Fourier transform), which passes through a modulator that may manipulate the phase and/or amplitude of each frequency component individually, to be reconstructed by a second, commensurate lens and grating pair (the inverse Fourier transform) as shown in the figure below.

   
   
   Figure 1. Pulse shaper mechanism
   
   
   
   Figure 2: Closed-loop learning control. The combination of a femtosecond laser system and a computer-controlled pulse shaper is used to generate specific electric fields E(t), which are irradiated onto a given quantum system, initiating photophysical or photochemical processes. After detection of the resulting photoproducts, a learning algorithm calculates modified electric fields based on the information from the experimental feedback signal and the user-defined control objective. The improved laser pulse shapes are tested and evaluated in the same manner. Cycling through this loop many times results in iteratively optimized laser pulse shapes and the desired control over the quantum system [4].

   For example, closed-loop learning loop is applied to control the energy transfer efficiency in a donor-acceptor macromolecule that mimics light harvesting system. By analyzing the pulse optimization process and optimal pulse features, it was found that a step phase function plays a crucial role [5].

   We are setting up femtosecond pulse shaping experiment in our lab.

   References:

   1. W. S. Warren, H. Rabitz, and M. Dahleh, Science, 259, pp. 1581(1993).
   2. Y. Silberberg, Annu. Rev. Phys. Chem. 60, pp 277 (2009).
   3. R.S. Judson and H. Rabitz, Phys. Rev. Lett. 68, pp. 1500 (1992).
   4. T. Brixner, N. H. Damrauer, G. Krampert, P. Niklaus and G. Gerber, J. Modern Opt., 50, pp.539(2003).
   5. D.G. Kuroda, C.P. Singh, Z. Peng and V.D. Kleiman, SCINECE 326, pp. 263 (2009).

       

10. A low cost, sensitive and high dynamic range CW laser power meter
    
    A cost effective CW laser power meter with a large dynamic range and high sensitivity based on a single photodiode was developed. The power meter consists of a photodiode, a current to voltage converter circuit, an offset balancing circuit, a microcontroller, an analog to digital converter, reed relays and an alphanumeric liquid crystal display. The high sensitivity and large dynamic range are achieved by the implementation of an analog background balancing circuit and auto ranging.
    
    The principle of operation of the power meter was based on the conversion of incident laser light to a proportionate current by a photodiode and a current to voltage converter circuit. This analog voltage was digitized by an ICL7135 ADC, converted to absolute power after calibration and displayed on a 2 lines × 16 characters alphanumeric LCD with the help of a atmel 89C4051 microcontroller.Fig.1. shows the laser power recorded by the power meter as a function of input laser power.The power meter can record absolute laser power levels as low as 1 pW. The dynamic range measured with a 200mW CW laser at a wavelength of 532 nm is 8 × 10¹⁰.

    
    
    Figure, The plot of laser power recorded by power meter as a function of input laser power.

    Papers in Journal:
    
    
1. Influence of free carrier refraction due to linear absorption on Z-scan study of porous Si,
   K.S. Bindra,
   Optics Commun. 246, 421, 2005.
   
   
2. All optical switching characteristics of metalloporphyrins,
   C.P. Singh, K.S. Bindra, B. Jain and S. M. Oak,
   Optics Commun. 245, 407, 2005.
   
   
3. High-contrast all-optical switching with Pt:ethynyl complex\
   Optik, Volume 117, Issue 11, 1 November 2006, Pages 499-504
   C.P. Singh, K. Kulshrestha and S. Roy
   
   
4. Analysis of effect of excitation pulse characteristics on numerical simulation of reverse saturable absorption
   C.P. Singh, K.S. Bindra, and S.M. Oak,
   Optics Commun. 269, 223, 2007.
   
   
5. Direct measurement of free carrier nonlinearity in semiconductor-doped glass with picosecond pump-probe Z-scan experiment,
   K.S. Bindra, C.P. Singh and S.M. Oak.
   Optics Commun. 271, 248, 2007.
   
   
6. Influence of multiphoton events in measurement of two-photon  absorption   cross-   sections and optical nonlinear parameters under femtosecond       pumping,
   R. Sailaja, P. B. Bisht , C.P. Singh, K.S. Bindra and S.M. Oak.
   Optics Commun., 277, 433, 2007.
   
   
7. Z-Scan studies and optical limiting of nanosecond laser pulses in neutral red dye,
   M. George, C. I. Muneera, C P Singh, K. S. Bindra , S. M. Oak
   Journal of Optics and Laser Technology, 40, 373, 2008.
   
   
8. Investigation of optical limiting in iron oxide nanoparticles”,
   C. P. Singh, K. S. Bindra, G. M. Bhalerao, and S. M. Oak
   Optics Express 16, 8440, 2008.
   
   
9. A sensitive and high dynamic range CW laser power meter”,
   S. Krishnan, K.S. Bindra and S.M. Oak
   Rev. Sci. Inst., 79, 125101, 2008.
   
   
10. Effects of thickness of beta barium borate and angle of non-collinearity on the fs pulse generation by optical parametric amplification”,
    A. Nautiyal, P. B. Bisht, K. S. Bindra and S. M. Oak
    Journal of Optics and Laser Technology, 41, 539, 2009.
In conferences:
1. Pump-probe Z-scan studies in semiconductor-doped glasses.
   K.S. Bindra, C.P. Singh, and S. M. Oak,
   National Laser Symposium, B.A.R.C., Mumbai, Jan 10-13, 2005 page 430.
   
   
2. All-Optical switching with Metalloporphyrins.
   C.P. Singh, K.S. Bindra, B. Jain and S. M. Oak,
   National Laser Symposium, B.A.R.C. Mumbai, Jan 10-13, 2005 page 131.
   
   
3. On numerical simulation of reverse saturable absorption.
       C.P. Singh, K.S. Bindra, and S.M. Oak,
   National Laser Symposium,  Dec, 2005, Vellor.
   
   
4. Design and operation of femtosecond noncollinear optical parametric amplifier(NOPA).
       A. Nautiyal, P.B. Bisht, K.S. Bindra, and S.M. Oak,
   National Laser Symposium,  Dec, 2005, Vellor.
   
   
5. Measurement of nonlinear refraction of various solvents under femtosecond pulse excitation.
   C.P. Singh, K.S. Bindra and S.M. Oak
   National Laser Symposium, Dec, 2006, RRCAT, Indore.
   
   
6. Development of a battery operated handheld power meter for a cw and pulsed femto second laser
   S. Krishnan, V.P. Bhange, K.S. Bindra, S.M.Oak
   National Laser Symposium, Dec, 2006, RRCAT, Indore.
   
   
7. Effect of optical pulse characteristics on analysis of reverse saturable absorption
   C.P. Singh, K.S. Bindra and S.M. Oak
   Eighth International Conference on “Optoelectronics, Fiber Optics and Photonics”, Photonics 2006, Dec. 13-16, 2006, University of Hyderabad, Hyderabad.
   
   
8. Generation of tunable femtosecond pulses with the noncollinear optical parametric amplifier fabricated at IIT Madras
   A. Nautiyal, K.S. Bindra, S.M. Oak and P.B. Bisht
   National Conference on “Progress on Tunable lasers for Ultrafast Process and Applications”, Dec. 21-22, 2006, IIT Madras, Chennai.
   
   
9. Nonlinear absorption and refraction studies on tartrazine solid films using Z-scan technique”,
   S.R. Suresh, P.G.L. Frobel, M. George, M. Devi, C.I. Muneera, A. C.P. Singh, K.S.      Bindra and S.M. Oak,
   National Conference on “Recent Trends in Optoelectronics and Laser Technology”, Apr.9-11, 2007, Univ. of Kerala, Thiruvananthapuram.
   
   
10. Development of sensitive photo diode based high dynamic range laser power meter using microcontroller”,
    S.R. Krishnan, K.S. Bindra and S.M. Oak,
    National Symposium on Instrumentation, (NSI-32) organised during           Oct  24-26, 2007 at   Tiruchengode- 637 209, TamilNadu.
    
    
11. Nonlinear optical studies in neutral red dye under nanosecond laser pulse excitation”,
    M. George, C.I. Muneera, C.P. Singh, K.S. Bindra and S.M. Oak,
    DAE-BRNS  National  Laser  Symposium, Dec. 17-20, 2007,  M.S. University of Baroda,  Vadodara.
    
    
12. Synthesis of Si nano-particles by laser ablation and optical limiting studies”,
    V. Shukla and K.S. Bindra,
    DAE-BRNS National Laser Symposium, Dec. 17-20, 2007, M.S. University of Baroda Vadodara.
    
    
13. A high dynamic range energy meter”,
    S. Krishnan and K.S. Bindra,
    National Symposium on Instrumentation, A.U. College of Engineering,  Visakhapatnam, during Dec. 8-10, 2008.
    
    
14. An optimized stepper motor drive for laser ablation experiments”,
    S. Krishnan, V. Shukla, K.S. Bindra, V.P. Bhange and S.M. Oak,
    National Symposium on Instrumentation, A.U. College of Engineering,  Visakhapatnam, during Dec. 8-10, 2008.
    
    
15. Study on optical limiting in iron-oxide and copper nanoparticles”,
    C.P. Singh, K.S. Bindra, V. Shukla, G.M. Bhalerao, and S.M. Oak,
    International conference on Fiber Optics and Photonics (PHOTONICS 2008),    Dec. 13-17, 2008, New Delhi.
    
    
16. Conical emission in b-barium borate by using femtosecond pulses”,
    S.A. Ali, A. Nautiyal, P.B. Bisht, V. Shukla, K.S. Bindra and S.M. Oak,   
    DAE-BRNS National Laser Symposium, Jan. 7-10, 2009, LASTEC, New Delhi.
    
    
17. Random laser based on Ag-nanoparticle suspension prepared by wet chemical method”S. K. Das, C.P. Singh, V. Shukla, T.S. Dhami and K.S. Bindra,
    DAE-BRNS National Laser Symposium, Jan. 7-10, 2009, LASTEC, New Delhi.
    
    
18. Effect of nonlinear absorption on fluorescence signal”,
    K.S. Bindra, C.P. Singh, S.M. Oak, R. Sailaja and P.B.  Bisht,
    DAE-BRNS National Laser Symposium, Jan. 7-10, 2009, LASTEC, New Delhi.
    
    
19. Synthesis of Cu nano-particles by laser ablation and optical limiting studies”,  V. Shukla and K.S. Bindra,
    DAE-BRNS National Laser Symposium, Jan. 7-10, 2009, LASTEC, New Delhi.
    
20. A photodiode based energy meter”,
    S. Krishnan and  K. S. Bindra,
    DAE-BRNS National Laser Symposium, Jan. 7-10, 2009, LASTEC, New Delhi.
    
    
21. Synthesis, characterization and optical limiting studies of Silicon nano-particles synthesized by laser ablation technique’
     V. Shukla and K.S. Bindra 
    5th DAE-BRNS National Symposium on Pulsed laser Deposition of Thin Films and Nanostructured Materials to be held at IIT Madras, Chennai, Dec. 2-4, 2009.
    
    
22. Nonlinear optical studies in semiconductor doped glasses under femtosecond pulse       excitation
    C.P. Singh, K.S. Bindra, and S.M. Oak.
    DAE-BRNS National Laser Symposium, Jan. 13-16, 2010, B.A.R.C., Mumbai.
    
    
23. Three dimensional polystyrene photonic crystals and their optical characteristics.
    K.S. Alee,  K. S. Bindra and D. Narayana Rao.
    DAE-BRNS National Laser Symposium, Jan. 13-16, 2010, B.A.R.C., Mumbai.
    
    
24. Generation and amplification of femtosecond pulses by using two stage non collinear optical parametric amplifier.
    S. Akbar Ali, P.B. Bisht, V. Shukla, K.S. Bindra and S.M. Oak
    DAE-BRNS National Laser Symposium, Jan. 13-16, 2010, B.A.R.C., Mumbai.