Debabrata Goswami

Indian Institute of Technology Kanpur | IIT Kanpur · Department of Chemistry

Microscopic thermodynamic studies can elucidate specific molecular interactions. In this work, we report the microscopic thermodynamics in binary liquid mixtures, which elucidate the role of molecular structure in nonlinear solvent response using femtosecond optical tweezers (FOT). We obtain the excess thermodynamics property of mixing in various N...

Any optimization algorithm programming interface can be seen as a black-box function with additional free parameters. In this spirit, simulated annealing (SA) can be implemented in pseudo-code within the dimensions of a single slide with free parameters relating to the annealing schedule. Such an implementation, however, necessarily neglects much o...

We study the influence of pH on the thermophysical properties of monohydric and polyhydric alcohols through a femtosecond laser-induced thermal lens (TL) spectroscopic technique. Relative changes in the thermal signatures of monohydric and polyhydric alcohols are identified at different pH levels using both dual-beam Z-scan and time-resolved TL mea...

Any optimization alogrithm programming interface can be seen as a black-box function with additional free parameters. In this spirit, simulated annealing (SA) can be implemented in pseudo-code within the dimensions of single slide with free parameters relating to the annealing schedule. Such an implementation however, neglects necessarily much of t...

Amid complex-looking functionals in quantifying coherence, we set forth a simple and easy-to-evaluate approach: Principal diagonal difference of coherence (C_PDD), which is non-negative, self-normalized, and monotonic (under any incoherent operation). The numerical study of coherence evolution shows that the response of C_PDD is better than both th...

The structure and solvation behavior of binary liquid mixtures of Methanol (MeOH) and N, N-Dimethylformamide (DMF) are explored by ascertaining their intermolecular interactions with either Rhodamine-B (RhB) or Rhodamine101 (Rh101) dye through steady-state absorption, emission, and two-photon induced fluorescence. Specifically, in the present inves...

Photothermal processes primarily occur in millisecond timescales and are typically not ascribed to femtosecond lasers. However, femtosecond processes that rely on measurements with high repetition rate lasers must contend with cumulative thermal effects, which accumulate ultrafast interaction residuals into millisecond timescales. Consequently, the...

For studying any event, measurement can never be enough; “control” is required. This means mere passive tracking of the event is insufficient and being able to manipulate it is necessary. To maximize this capability to exert control and manipulate, both spatial and temporal domains need to be jointly accounted for, which has remained an intractable...

Evanescent wave-induced fluorescence spectroscopy (EWIFS) is a widely used technique for probing the interfacial behavior of different complex media in investigations of samples in the physical, chemical, and biological sciences. This technique takes advantage of the sharply decaying evanescent field, established following total internal reflection...

The structure and solvation behaviour of binary liquid mixtures of Methanol (MeOH) and N, N-Dimethylformamide (DMF) are explored by ascertaining their intermolecular interactions with either Rhodamine-B (RhB) or Rhodamine101 (Rh101) dye through steady-state absorption, emission, and two-photon induced fluorescence. Specifically, in the present inve...

Measurements of nonlinear optical (NLO) properties of different binary mixtures having carbon disulfide (CS2) as the common component, namely CS2-acetone, CS2-cyclopentanone, CS2-toluene, and CS2-carbon tetrachloride (CCl4), are carried out by using the z-scan technique. Open-aperture z-scan (OAZS) and close-aperture z-scan (CAZS) experiments are p...

Using thermal lens (TL) spectroscopy, we distinguish monohydric alcohols based on their molecular characteristics and thermophysical properties. We use both dual-beam Z-scan and time-resolved TL measurements to identify thermal signatures of the representative homologous series of aliphatic, cyclic, and aromatic alcohols. While, in aliphatic alcoho...

Probing transient states in molecules having vibronic transitions with femtosecond laser pulses often results in coherent oscillations either in the ground state, the excited states, or both. We find such coherent oscillations are highly solvent-dependent and provide a holistic overview of the pump-probe experiments for ultrafast dye dynamics at in...

We present a microscopic study of water–dimethyl sulfoxide (DMSO) binary mixtures using optical tweezers and thermal lens techniques. Binary mixtures of DMSO with water show anomalous behavior due to the specific hydrogen bonding ability of DMSO. We use a tightly focused femtosecond laser at a low average power to optically trap microspheres with d...

For studying the selective excitation of a particular fluorophore in fluorescence microscopy, the effect of the laser pulse chosen is immense. To study the ramifications of this selection, we present a thorough study of one-color stimulated emission depletion of the two-photon fluorescence of selective fluorophores by changing the wavelength and de...

Though a well-verified approach of quantifying quantum coherence is l 1 -norm, this is not normalized for a multi-qubit (multi-level) system. We propose a modification to show that the altered expression normalizes a multi-qubit (multi-level) system coherence with numerical studies.

To highlight the advantage of femtosecond optical tweezers over conventional tweezers, we trapped 1µm diameter polystyrene beads suspended in water using a Ti:Sapphire laser.

We report Thermal Lens measurements on the binary liquid mixtures of Methanol and Phenol with femtosecond pulse excitations. The photothermal response is found to be strongly dependent on relative concentrations and the constituents’ molecular properties.

Effective outcome from dynamic live-cell-imaging requires utilization of a probe with high emission intensity and low photobleaching. It would be preferable to achieve such properties at a low power of the applied laser to avoid any probable damage to biological cells or tissue. Most of the used small-molecule fluorophores have been reported to sho...

An obituary on Arthur Ashkin (1922–2020) who was awarded the 2018 Nobel Prize for Physics at the age of 96.

Alcohols are widely used in the industry depending upon their physical and chemical properties as a solvent, ingredients in cosmetics, medicine & fuel. Time-resolved thermal lens spectroscopy is a highly sensitive spectroscopic method for investigating the photo-thermal properties of chemicals. Its principles are based on the localized heat accumul...

Typical single-beam optical tweezers use continuous wave (CW) lasers, which can be explained through force balancing the light pressure from a tightly focused laser beam used for trapping microscopic particles. Recent years have also seen a surge in single-beam optical trapping research with high-repetition-rate femtosecond lasers that has shown ce...

We explore the characteristics of two-photon processes that undergo two-photon transitions with unresolvable intermediate states. We demonstrate how such two-photon processes show all the possible attributes of two-photon transitions in terms of their dependence on the laser phase, polarization, and intensity. We prove that the temporal overlap of...

Non-contact localized laser heating-based thermal lensing has emerged as a technique for probing the heat transport in liquids. A mode-mismatched dual-beam pump-probe spectroscopic technique was employed to investigate the photothermal response and modes of heat dissipation in methanol and binary mixtures of methanol with polar (water, methanol (Me...

Spatiotemporal control aspects of pulsed laser experiments rely on the ability to modulate the shape of the generated pulses efficiently. Drawing from current state-of-the-art theoretical aspects of computational simulations to reduce the sim-to-real bottlenecks, we devise a novel schematic for the generation of on-the-fly calibrated pulse trains w...

Vibronic transitions of a molecular system in solution upon probing with ultrashort laser pulses (<40 fs), often creates coherent oscillations. These are either in the ground or excited state or in both the ground and excited states. Such vibrational coherence or coherent oscillation produced in the ground or excited states are often highly solvent...

Systematic study of conventional and femtosecond optical tweezers demonstrates an interplay of thermal and optical nonlinearity due to tight focusing. Specifically, Brownian motion of the 250nm radius trapped bead is measured using a quadrant photodiode.

We use the femtosecond laser-induced thermal lens spectroscopy for sensitive detection of the liquid-gas phase interface, which can provide an accurate size measure of aerosols. Mapping microscopic aerosol distribution is critical for COVID-19 transmission.

We performed a dual beam Z-Scan experiment to examine the thermal lensing effects in CBNP nanofluids. The photothermal characteristics and heat dissipation dynamics were observed for these nanofluids at different levels of their linear absorption.

Simulating real-time scenarios to predict optical trapping behavior for continuous wave (CW) as well as femtosecond pulsed lasers is a challenging problem. The challenging nature of the problem arises from the fact that, for a tightly focused laser, one must also include the optical Kerr effect and the thermal nonlinearity. We show here the distinc...

This study deals with the effects of different sets of binary solvents on the Two-Photon Induced Fluorescence (TPIF), a non-linear process, of the Rhodamine-6G (Rh6G) dye, which is a well-known xanthene dye. This work examines the importance of inter-molecular interactions, which results in the modulation of the TPIF of the Rh6G. In this work, we h...

In this report, we explored the substitution of triple cations at B-site of ABX3 perovskite architecture in inorganic-organic hybrid perovskite materials in order to improve its stability over moisture and heat, which is considered as the paramount obstacles for commercialization of perovskite solar cell. In the current investigation we synthesized...

We report results establishing that detection of the liquid-liquid interface is possible through femtosecond pump-probe spectroscopy. In particular, we use femtosecond laser pulses to pump and probe the dynamics of infra-red dye molecules in a liquid pair that can form an interface. We demonstrate that an interface formation can be detected from su...

Simulating real-time scenarios to predict optical trapping behavior for continuous wave (CW) as well as femtosecond pulsed lasers is a challenging problem. This is especially so, because, for a tightly focused laser, one must also include optical Kerr effect as well as thermal nonlinearity. We show here the distinct differences between CW and femto...

Fluid systems exhibit thermal lens effects due to laser irradiation accompanied by convection in contrast primarily conductive heat dissipation in solids. The presence of a significant convective mode modifies the temperature gradient in fluids resulting in a deviation of the experimental results from theories that are based on pure conduction. Her...

Insights into the morphology of nanoclusters would facilitate the design of nano-devices with improved optical, electrical, and magnetic responses. We have utilized optical gradient forces for the directed self-assembly of colloidal clusters using high-repetition-rate femtosecond laser pulses to delineate their structure and dynamics. We have ratif...

The effect of the repetition rate of femtosecond laser pulses on the two-photon absorption and nonlinear refraction of pure organic liquids is presented using the conventional Z-scan technique. Such a study provides a way to determine the nature of light-matter interaction, explicitly enabling the identification of the linear versus nonlinear regim...

We show that the near-infrared-absorbing polymethine cyanine dyes exhibit nonlinear absorption (NLA) in the visible region, where they have negligible or very low linear absorption. In chloroform solvent, the studied cyanines were found to be photo-unstable and exhibited photo-degradation under prolonged exposures as compared to methanol and dimeth...

Optical tweezers work on the principle that microscopic particles may be immobilized by the application of an intense photon flux, which may be attained under tight focusing conditions. To elucidate the behaviour and mechanism of this tweezing action, herein we perform numerical studies and investigate the intensity distribution at the focusing spo...

We have shown experimentally the successful engineering of femtosecond pulse shaping at a 76 MHz repetition rate input pulse with an acousto-optic modulator (AOM). High repetition rate (HRR) femtosecond laser pulse shaping using an AOM in the Fourier plane was incomprehensible because of its intrinsic 100 kHz acoustic update limit. We demonstrate a...

Since so long the thermal effects have not been much explored in the optical trapping theory. In this paper we are establishing the effects of optically induced thermal nonlinearity in the medium of optical trapping in Rayleigh regime for both continuous wave and pulsed laser. For single beam optical tweezers high numerical aperture (N.A.) objectiv...

We use time-resolved thermal lens (TL) experiments to examine the convective heat transfer at microscale in the first eight members of the homologous series of primary alcohols. TL measurements enable a direct study of these primary alcohols without adding any chromophore as a function of varying heat loads created via femtosecond laser pulses at 1...

We illustrate control of a polarized laser optical trapping potential landscape through the nonideal mixing of binary liquids. The inherent trapping potential asymmetry (ITPA) present in the trapping region results from the asymmetric intensity distribution in focal volume due to the high numerical aperture objective lens. Experimentally, we show t...

On Tuesday, 02 October 2018, Arthur Ashkin of the United States, who pioneered a way of using light to manipulate physical objects, shared the first half of the 2018 Nobel Prize in Physics. The second half was divided equally between Gérard Mourou of France and Donna Strickland of Canada for their method of generating high-intensity, ultra-short op...

Here we are presenting the numerical simulation of the effects of varying pulse widths on the temperature profile of the pulsed laser when it interacts with the three separate mediums (Water, Water Ethanol mixture and Water Methanol mixture). Here we are also presenting the effects of repetition rate on the cumulation of temperature rise for consec...

Spectrally resolved photon echo spectroscopy in the off-resonance condition is reported for the first time to study the coherence and population dynamics of CdSe quantum dots. In this case, the information related to the system dynamics can be inferred indirectly. This is especially useful when such dynamical information might be hidden under the a...

A novel on-the-fly calibration method of optical tweezers is presented, which enables in situ control and measure of absolute temperature and viscosity at nanoscale dimensions. Such noncontact measurement and control at the nanoscale are challenging as the present techniques only provide off-line measurements that do not provide absolute values. Ad...

A new class of two-photon active and solvatofluorochromic dyes for the determination of ER polarity is reported. The fluorescent colour expands almost the entire visible spectrum. One of the derivative...

A numerical method to understand and analyze the laser-induced heating in various solvents have been described in this paper. We have developed the model comparing heating in continuous wave (CW) laser and pulsed laser for pure water, binary mixtures of water in ethanol and methanol. This comparative study has been done by using numerical simulatio...

We present ultrafast-depopulation-dynamics of higher-lying-excited-states using femtosecond fluorescence up-conversion techniques for two near-infrared (NIR) tricarbocyanine dyes (IR144 and IR140) in primary alcohols. With visible excitation wavelengths, such dyes show two distinct emission-bands with large peak wavelength difference: one at the vi...

Rhodamine group of molecules are widely used dyes for imaging of biological molecules. Application of these dyes however includes a limitation that these molecules absorb in the visible range of the spectrum, which does not fall in the ‘biologically transparent window’ (BTW). Two photon absorption (TPA) process could come up with an alternate solut...

Using degenerate pump probe technique at 800 nm, Ground State Vibrational Coherence (GSVC) of a cyanine dye (IR780) is explored in various solvents. Five representative solvents were studied of which GSVC is present in three solvents: Methanol (MeOH), Ethanol (EtOH) and Dichloromethane (DCM). The maximum number of damped oscillations were observed...

A facile synthesis and detailed photophysical investigation of E/Z-isomerization of fluorescent diphenylamine tethered stilbene derivatives (DPASs) under white light exposure have been carried out to understand the effect on fluorescence, electrochemical properties and photo-stability under various activation/deactivation pathways. In solution stat...

A new method of utilizing photothermal effect at nano-volume dimensions to measure viscosity is presented here that can, in turn, provide the surrounding temperature. Our measurements use high repetition rate, low average power, femtosecond laser pulses that induce photothermal effect that is highly influence by the convective mode of heat transfer...

Ultrafast depopulation dynamics of highly excited states of near-infrared (NIR) tri-carbocyanine dyes is presented in primary alcohols using the femtosecond fluorescence upconversion technique. Pronounced solvent dependence is observed in the dynamics of the highly excited states. Typically, the tri-carbocyanine dyes have strong absorption and fluo...

We report a structure of ferrocene-pyrene conjugate (1) comprising electro and photo-active dual-signaling units. In particular, 1 upon interaction with Cu(II), displays selectively one-photon fluorescence quenching, but it shows two-photon absorption (TPA) cross-section 1230 GM (at 780 nm). Further, 1 displayed two irreversible oxidative waves at...

We have investigated the concentration dependent of two-photon induced fluorescence (TPIF) in methanolic solution of Rhodamine 6G and Rhodamine B dye using 120 fs laser pulses at 780 nm, 76 MHz repetition rate. TPIF study of these dyes was compared with their respective one photon fluorescence intensity. We have shown the effect of chopper on TPIF...

We present here the control and measurement of temperature rise using femtosecond optical tweezers at near infrared (NIR) region. Based on our theoretical development, we have designed our experimental techniques. The high temporal sensitivity of position autocorrelation and equipartition theorem is simultaneously applied to elucidate temperature c...

Third order nonlinearities of Mn(III)-Phthalocyanine chloride in dimethyl-sulphoxide under 50 fs pulses, operating at 94 MHz, by eliminating cumulative thermal effects have been investigated and reported by us. Modifications were done in data acquisition during Z-scan experiment, which included recording of time evolution waveform traces in an osci...

Spatiotemporal imaging of sum-frequency generation process through second-order nonlinear optical interaction in a nonlinear crystal under femtosecond pulsed illumination is presented. Two focal points in the spatial dimension that result from use of widely separated wavelengths (780 and 1560 nm) including their appropriate intensity ratios are cap...

Using the close-aperture Z-scan technique, the pure nonlinear refractive index (n2) of carbon disulfide is measured with a 76 MHz repetition rate femtosecond laser. Strong interference of thermal effects exists with high-repetition-rate lasers that result in negative values of n2. We remove the thermal effect completely by continuously increasing t...

Ultrafast pump-probe technique has been invoked to understand the liquid:liquid interface by using a cyanine dye dissolved in dimethyl sulfoxide (DMSO), interfaced with neat diethyl-ether (Et2O). We show that the dynamics of the dye gets slow down on moving from the bulk dye solution in DMSO towards the interface with neat-diethyl ether. Hence we g...