Study on the Spatial Resolution of Imaging Technique for Absorption Loss Measurement of Optical Coatings

The uniformity of optical coatings becomes more and more important as large diameter optical devices are widely used. Absorption loss in optical components, particularly in optical coatings, is a limiting factor in high-power laser applications. This article analyzes the main factors, which affect the spatial resolution of three techniques for surface absorption loss measurement, including the photothermal deflection technique, the surface thermal lens technique, and the photothermal detuning technique. The influence of the size of the heating and probe beam on the photothermal detuning technique is studied in detail. Experiments are conducted to study the photothermal signal of the photothermal detuning technique for absorption measurement of the optical coating point by point. The results show that the main factors, which affect the spatial resolution of imaging measurements for absorption loss of coatings, are the heating beam size and the step accuracy of the sample translation stage. The heating and probe beam sizes has a significant impact on the application of the photothermal detuning technique. Experimental result shows that the photothermal detuning technique can be used for imaging of absorption loss measurements of optical coatings. The results provide theoretical and experimental supports for further application of the photothermal detuning technique.     

Investigation of nonlinear dynamics in CdS by time-resolved nondegenerate pump–probe system with phase object

The time-resolved nondegenerate pump–probe system with phase object is employed for investigation of nonlinear absorption and refraction dynamics in CdS. The 532?nm laser beam with 21?ps duration is used as the excitation and the laser beams of 600 and 680?nm with 10?ps duration from optical parametric generation are used for probing. The experimental results at both probe wavelengths show free-carrier absorption and large free-carrier refraction along with two-photon absorption and bound electronic optical Kerr effect. By numerically fitting the experimental data based on the nondegenerate pump–probe theory, the nondegenerate two-photon absorption coefficient, the nondegenerate Kerr coefficient, the free-carrier decay time, the free-carrier absorptive cross-section and free-carrier refractive coefficient at different wavelengths are all determined.     

Simultaneous Measurement of Thermal Diffusivity and Optical Absorption Coefficient of Solids Using PTR and PPE: A Comparison

Modulated photothermal radiometry (PTR) and a modulated photopyroelectric (PPE) technique have been widely used to measure the thermal diffusivity of bulk materials. The method is based on illuminating the sample with a plane light beam and measuring the infrared emission with an infrared detector (PTR) or the electric voltage produced by a pyroelectric sensor in contact with the sample (PPE). The amplitude and phase of both photothermal signals are recorded as a function of the modulation frequency and then fitted to the theoretical model. In this work, we compare the ability of modulated PTR and PPE to retrieve simultaneously the thermal diffusivity and the optical absorption coefficient of homogeneous slabs. In order to eliminate the instrumental factor, self-normalization is used, i.e., the ratio of the photothermal signal recorded at the rear and front surfaces. The influence of the multiple reflections of the light beam and the transparency to infrared wavelengths are analyzed. Measurements performed on a wide variety of homogeneous materials, transparent and opaque, good and bad thermal conductors, confirm the validity of the method. The advantages and disadvantages of both techniques are discussed.     

Theoretical Modeling of Obliquely Crossed Photothermal Deflection for Thermal Conductivity Measurements of Thin Films

A three-dimensional theoretical model has been developed to calculate the normal probe beam deflection of the obliquely crossed photothermal deflection configuration in samples which consist of thin films deposited on substrates. Utilizing the dependence of the normal component of probe beam deflection on the cross-point position of the excitation and probe beams, the thermal conductivity of the thin film can be extracted from the ratio of the two maxima of the normal deflection amplitude, which occurs when the cross-point is located near both surfaces of the sample. The effects of other parameters, including the intersect angle between the excitation and the probe beams in the sample, the modulation frequency of the excitation beam, the optical absorption and thickness of the thin films, and the thermal properties of substrates on the thermal conductivity measurement of the thin film, are discussed. The obliquely crossed photothermal deflection technique seems to be well suited for thermal conductivity measurements of thin films with a high thermal conductivity but a low optical absorption, such as diamond and diamond-like carbon, deposited on substrates with a relatively low thermal conductivity.     

Optimized Photothermal Lens Determination of Nonlinear Absorption

An optimized pump–probe mode-mismatched photothermal lens experiment aimed at determination of nonlinear absorption of an optical sample is reported. The pump beam generates a local thermal gradient or thermal lens that is tested by the probe light. The pump beam is tightly focused, and the probe beam is highly collimated. Changes in the probe light transmission through a small aperture located at some distance from the sample provide the signal. Scanning of the sample around the focal point yields a single-peaked Z-scan signature with a width several times larger than the pump Rayleigh range for linear absorption. If nonlinear absorption is dominant, the width of the peak is significantly smaller and of the order of the Rayleigh range of the pump field. If linear and nonlinear absorption are present simultaneously, a double-peaked Z-scan signature is obtained. In this situation, the linear and nonlinear absorption contributions can be easily separated and compared to each other for calibration purposes. Using the known values of linear absorption, nonlinear absorption coefficients can be estimated with good accuracy. The method is tested by studying nonlinear absorption in nitrobenzene and iron oxide water colloids. The values of the effective nonlinear absorption coefficients are determined. The physical origin of nonlinear absorption in both types of samples is also discussed.     

Microwave spectrum asymmetry in an optically pumped cesium beamresonator

This paper deals with a theoretical and experimental study that describes the effect of a frequency detuning of the laser used to achieve optical pumping in a small optically pumped cesium beam resonator. The ground state Zeeman sublevels with opposite angular momentum are unequally populated, leading to an asymmetrical microwave spectrum. The relative population asymmetry as a function of the laser frequency has a dispersion-like shape. Its dependence on laser intensity, applied magnetic field, and laser linewidth is demonstrated for a laser at 852 nm and tuned to the ²S_1/2F=3→²P_3/2F'=3 transition of the Cs D₂ line. Finally, the effect of a slight laser detuning on the Rabi pulling frequency shift is discussed     

Large Photothermal Effect in Sub‐40 nm h‐BN Nanostructures Patterned Via High‐Resolution Ion Beam

The controlled nanoscale patterning of 2D materials is a promising approach for engineering the optoelectronic, thermal, and mechanical properties of these materials to achieve novel functionalities and devices. Herein, high‐resolution patterning of hexagonal boron nitride (h‐BN) is demonstrated via both helium and neon ion beams and an optimal dosage range for both ions that serve as a baseline for insulating 2D materials is identified. Through this nanofabrication approach, a grating with a 35 nm pitch, individual structure sizes down to 20 nm, and additional nanostructures created by patterning crystal step edges are demonstrated. Raman spectroscopy is used to study the defects induced by the ion beam patterning and is correlated to scanning probe microscopy. Photothermal and scanning near‐field optical microscopy measure the resulting near‐field absorption and scattering of the nanostructures. These measurements reveal a large photothermal expansion of nanostructured h‐BN that is dependent on the height to width aspect ratio of the nanostructures. This effect is attributed to the large anisotropy of the thermal expansion coefficients of h‐BN and the nanostructuring implemented. The photothermal expansion should be present in other van der Waals materials with large anisotropy and can lead to applications such as nanomechanical switches driven by light.     

Super‐Resolution Nonlinear Photothermal Microscopy

Super‐resolution fluorescence microscopy enables imaging of fluorescent structures beyond the diffraction limit. However, this technique cannot be applied to weakly fluorescent cellular components or labels. As an alternative, photothermal microscopy based on nonradiative transformation of absorbed energy into heat has demonstrated imaging of nonfluorescent structures including single molecules and ~1‐nm gold nanoparticles. However, previously photothermal imaging has been performed with a diffraction‐limited resolution only. Herein, super‐resolution, far‐field photothermal microscopy based on nonlinear signal dependence on the laser energy is introduced. Among various nonlinear phenomena, including absorption saturation, multiphoton absorption, and signal temperature dependence, signal amplification by laser‐induced nanobubbles around overheated nano‐objects is explored. A Gaussian laser beam profile is used to demonstrate the image spatial sharpening for calibrated 260‐nm metal strips, resolving of a plasmonic nanoassembly, visualization of 10‐nm gold nanoparticles in graphene, and hemoglobin nanoclusters in live erythrocytes with resolution down to 50 nm. These nonlinear phenomena can be used for 3D imaging with improved lateral and axial resolution in most photothermal methods, including photoacoustic microscopy.     

Photothermal behavior of an optical path adhesive used for photonics applications at 1550 nm

A theoretical and experimental study of photothermal behavior in a commercially available optical path adhesive is described. Photothermal effects were examined for cw and pulsed laser radiation (~1 mus) at 1550 nm. A fiber-optic backreflection technique was used to measure the thermo-optic glass transition temperature of the adhesive. This transition temperature was then used to calibrate fiber-optic photothermal blooming and backreflection pump-probe experiments. Simple thermal models predict DT at 300 mW (cw) to be 65 degrees C and 53 degrees C at 100 W (pulsed). Experimental results are in reasonable agreement with theoretical predictions. The characteristic photothermal relaxation time after a 1-mus pulse for optical path adhesives is found to be 166 mus at the end of a fiber where the mode field diameter is 10.5 mum. Photothermally induced temperatures were found to be below the thermal degradation temperature of the adhesive even at powers as high as 1 W (cw) or 100 W (pulse).     

Optimizing modulation transfer spectroscopy signals for frequency locking in the presence of depleted saturating fields

A theoretical model of modulation transfer spectroscopy (MTS) that includes pump beam depletion is presented and experimentally verified with data covering visible iodine transitions at 532, 543, and 612 nm. This model is used to determine the values for pressure, interaction length, and saturation intensity that yield maximum MTS signals for frequency locking to iodine transitions. The approach is demonstrated for iodine transitions at 532, 633, and 778 nm, with the results showing that theoretically the frequency instability scales inversely to the absorption coefficient.     

Solvent influence on the nonlinear optical properties of molecular systems in the presence of degenerate and non-degenerate four-wave mixing

The nonlinear optical properties are studied theoretically of a molecular system immersed in a thermal bath and interacting with a four-wave mixing signal. A methodology based on cumulant expansions to obtain the average in the Fourier components associated with the coherences and populations, calculated by the optical stochastic Bloch equations, is employed. The system properties are analyzed as a function of the frequency detuning of the high-intensity pump beam and parameterized by the frequency detuning of the probe beam, both with respect to the Bohr frequency of the system. The longitudinal and transversal relaxation times and the bath effects, modeled as white noise, are also included as parameters in the calculation. Different cases, with degenerate and non-degenerate optical frequencies are examined and compared, resulting in differences related to the maximum in the population pulsation effects.     

“Thermal Mirror” Method for Measuring Physical Properties of Multilayered Coatings

In this study theoretical principles underlying the photothermal displacement (“thermal mirror”) method for measuring physical properties of opaque multilayered and functionally graded coatings with low thermal conductivity are analyzed. In this method, the specimen is locally heated by a power laser beam, and a two-dimensional transient temperature field is formed in a specimen. The physical basis for the photothermal displacement method is the non-stationary buckling and displacement of an irradiated surface due to a non-uniform thermal expansion. The surface is monitored by a low-power probe beam of a second laser, which is reflected from the specimen, i.e., the system operates as a convex “thermal mirror.” The photoinduced displacement varies with time, and the probe beam is reflected at a different angle depending on the slope of the displacement. The deflection angle is measured as a function of time by a position sensor, and the results of these measurements are compared with the theoretical dependence of the deflection angle on time and physical properties of a coating. This dependence was determined analytically from the solution of the two-dimensional thermal elasticity problem. It is shown that for the specimen composed of a substrate and a coating it is feasible to determine the properties of the coating, e.g., the thermal diffusivity and coefficient of linear thermal expansion provided that the analogous properties of the substrate are previously measured or otherwise known.     

Photothermal self-phase-modulation technique for absorption measurements on high-reflective coatings

We propose and demonstrate a new measurement technique for the optical absorption of high-reflection coatings. Our technique is based on photothermal self-phase modulation and exploits the deformation of cavity Airy peaks that occurs due to coating absorption of intracavity light. The mirror whose coating is under investigation needs to be the input mirror of a high-finesse cavity. Our example measurements were performed on a high-reflection SiO2-Ta2O5 coating in a three-mirror ring-cavity setup at a wavelength of 1064 nm. The optical absorption of the coating was determined to be α=(23.9±2.0)·10(-6) per coating. Our result is in excellent agreement with an independently performed laser calorimetry measurement that gave a value of α=(24.4±3.2)·10(-6) per coating. Since the self-phase modulation in our coating-absorption measurement affects mainly the propagation through the cavity input mirror, our measurement result is practically uninfluenced by the optical absorption of the other cavity mirrors.     

Detector effects in photothermal deflection experiments

We report on the theoretical analysis of a detector type influence on the normal deflection signal in photothermal experiments. Two cases are examined. In the first, the quadrant photodiode was considered as the detector; in the second the signal from the position detector, which measures the central moment displacement of the probe beam, was analyzed. Both analyses were carried out within the framework of the complex ray theory. The normal photodeflection signal was found to depend on the type of detector used in the photothermal deflection experiments for some parameters of its setup.     

All-optical switching in Pharaonis phoborhodopsin protein molecules

Low-power all-optical switching with pharaonis phoborhodopsin (ppR) protein is demonstrated based on nonlinear excited-state absorption at different wavelengths. A modulating pulsed 532-nm laser beam is shown to switch the transmission of a continuous-wave signal light beam at: 1) 390 nm; 2) 500 nm; 3) 560 nm; and 4) 600 nm, respectively. Simulations based on the rate equation approach considering all seven states in the ppR photocycle are in good agreement with experimental results. It is shown that the switching characteristics at 560 and 600 nm, respectively, can exhibit negative to positive switching. The switching characteristics at 500 nm can be inverted by increasing the signal beam intensity. The profile of switched signal beam is also sensitive to the modulating pulse frequency and signal beam intensity and wavelength. The switching characteristics are also shown to be sensitive to the lifetimes of$mmb pbf pR_bf M$and$mmb pbf pR_bf O$intermediates. The results show the applicability of ppR as a low-power wavelength tunable all-optical switch.     

Comparison of BaTiO3 optical novelty filter and photothermal lensing configurations in photothermal experiments.

Photorefractive BaTIO3 is used as an optical novelty filter to highlight the high spatial frequency components of the photothermal signal. A real-time phase grating recorded in BaTIO3 acts as a matched rejection spatial filter for the probe laser. This reduces the stationary background from the optical signals thereby increasing signal contrast ratios. Rejection of the monotonous stationary signal provides a powerful means of improving the photothermal signal. This paper describes the construction of this novel apparatus and the experiments performed in order to compare its performance with photothermal lensing results. A theory that explains photothermal signal filtering with BaTIO3 as an adaptive spatial frequency filter is presented. Results comparing the optical signals obtained in a photothermal lensing experiments and those obtained in the BaTIO3 optical novelty filter experiments are presented. The optical novelty filter signals demonstrate a remarkable improvement in the signal contrasts for moderate photothermal-induced phase shifts.     

Theoretical, experimental, and computational aspects of optical property determination of turbid media by using frequency-domain laser infrared photothermal radiometry

In this work, the optical and thermal properties of tissuelike materials are measured by using frequency-domain infrared photothermal radiometry. This technique is better suited for quantitative multiparameter optical measurements than the widely used pulsed-laser photothermal radiometry (PPTR) because of the availability of two independent signal channels, amplitude and phase, and the superior signal-to-noise ratio provided by synchronous lock-in detection. A rigorous three-dimensional (3-D) thermal-wave formulation with a 3-D diffuse and coherent photon-density-wave source is applied to data from model phantoms. The combined theoretical, experimental, and computational methodology shows good promise with regard to its analytical ability to measure optical properties of turbid media uniquely, as compared with PPTR, which exhibits uniqueness problems. From data sets obtained by using calibrated test phantoms, the reduced optical scattering and absorption coefficients were found to be within 20% and 10%, respectively, of the values independently derived by using Mie theory and spectrophotometric measurements.     

Measurement of laser-induced refractive index change of inverted ferroelectric domain LiNbO3

Optical nonlinearities of periodically poled LiNbO(3) crystals were investigated by the single beam Z-scan technique with a continuous wave (cw) laser beam at 532 nm. The nonlinear optical absorption coefficient and refractive index change are determined to be 8.1 x 10(-6) cm/W and 2.6 x 10(-4) at 0.5 MW/cm(2) light intensity, respectively. Both sign and magnitude of the measured refractive nonlinearity are considerably different from the Z-scan results in congruent LiNbO(3). The nonlinearities in the periodically poled LiNbO(3) induced by 532 nm continuous waves are believed to be mainly due to the photorefractive effect.     

Decorrelation with wavelength of laser intensity fluctuations after passage through a thermal plume

Abstract

Experiments are described in which two laser beams of different wavelength (mean λ=1550 nm, with variable detuning Δλ up to ±4%) are simultaneously transmitted through a thermal phase screen created by a convection heater. The work is motivated by applications such as differential absorption lidar (DIAL) in which beams of multiple wavelength are transmitted through the atmosphere. The experiment is optical fibre based with common transmit and receive optics being used for the two wavelengths, thus ensuring near-identical spatial modes. The far-field case is examined by placing the receiver at a focus of the beam, and reflective optics are used to avoid problems of chromatic aberration. A heterodyne detection scheme measures the intensity fluctuations at a point in the centre of the beam for each wavelength, and the degree of correlation between the two resulting intensity time series is determined as a function of detuning. The decorrelation increases as the square of the detuning, and the results are consistent with the predictions of theory in which it is assumed that the phase screen undergoes Gaussian phase fluctuations, and off-axis terms are ignored.     

Measurement of nonlinear absorption coefficients of organic materials by mode-mismatched Z-scan thermal lensing technique

We evaluate a new pump-probe mode-mismatched thermal lens (TL) scheme for the measurement of nonlinear absorption in nitrobenzene, benzene, and chloroform. In this new scheme the pump beam is focused in the presence of a collimated probe beam. Values of the nonlinear absorption coefficients of the materials studied for the wavelength of 532 nm are reported, and we compare the proposed technique with the well-known open Z-scan method.