Reflected Probe Beam Research Articles

Topological localized region of Goos-Hänchen shifts in reflection and transmission

In this manuscript, the Goos-Hänchen shift of the transmitted and reflected probe beam is controlled and modified. Topologically localized regions are reported in the G-H shift by introducing the position and orbital angular momentum dependency in the Rabi frequencies of the control fields. The system parameters along with the azimuthal quantum number are used to control and modify G-H shift as well as the topological regions. Crater and peak type topological regions are reported in the G-H shift of the reflected as well as transmitted beam. The number of localized regions are reported to strictly follow the general formula 2l2. The maximum shift in the reflected spectrum is reported to be Sr/λ=−140, where as in the transmitted spectrum it is only St/λ=−15. The modified results might have significant applications in optical waveguide switches and sensors.

Adaptive telescope for confocal photothermal microscopy of irregular surfaces

An adaptive telescope was developed to perform measurements using the confocal photothermal microscopy technique. The telescope automatically aligns to reinject the reflected probe beam as the measurement scans over irregular surfaces. The system uses DVD lenses that adapt to surfaces through controlled movements. Alignment algorithms are presented along with measurements on complex surfaces.

Dynamics of the blue pump-induced ultrafast insulator-to-metal transition and relaxation in VO2/TiO2 and VO2/TiO2:Nb thin films

We study the ultrafast time resolved response of 30 nm films of VO2 on a TiO2 substrate when 3.1 eV (400 nm wavelength) pump pulses were used to excite the insulator to metal transition (IMT). We found that the IMT threshold for these samples (≤30µJ/cm2) is more than 3 orders of magnitude lower than that generally reported for a more traditional 1.55 eV (800 nm wavelength) excitation. The samples also exhibited unusual reflectivity dynamics at near-threshold values of pump fluence where their fractional relative reflectivity ΔR/R initially increased before becoming negative after several hundreds of picoseconds, in stark contrast with uniformly negative ΔR/R observed for both higher 400 nm pump fluences and for 800 nm pump pulses. We explain the observed behavior by the interference of the reflected probe beam from the inhomogeneous layers formed inside the film by different phases of VO2 and use a simple diffusion model of the VO2 phase transition to support qualitatively this hypothesis. We also compare the characteristics of the VO2 films grown on undoped TiO2 and on doped TiO2:Nb substrates and observe more pronounced reflectivity variation during IMT and faster relaxation to the insulating state for the VO2/TiO2:Nb sample.

Photothermal mirror Z-scan spectrometry of opaque samples

This contribution introduces a pump-probe photothermal mirror Z-scan method to measure the thermal quantum yield, the thermal diffusivity, and other photothermal parameters of an opaque solid. The focusing of a pump beam of light onto the sample generates thermoelastic surface distortions. The distorted surface acts as a mirror affecting the diffraction pattern of a reflected probe beam yielding the experimental signal. Scanning the focusing lens produces a single peak photothermal mirror Z-scan signature. The amplitude and time evolution of the signal determines the sample’s photothermal properties. The method is used to analyze gallium arsenide and silicon plates, obtaining good agreement with previous studies.

Manipulation of Goos–Hänchen shifts in a HCN→HNC isomerization with permanent dipole moments

The behavior of the Goos-Hanchen shifts of the reflected probe light beam at the interface of a three-layer cavity with a three-level Λ-type molecular system is theoretically investigated. The molecular system is HCN→HNC isomerization with nonzero permanent dipole moments (PDMs). The effect of electromagnetically induced transparency (EIT) induced by PDMs in molecules is studied for (1+1)- and (1+2)-transition processes. Thus, the optical response of the medium is modified by the quantum coherence due to nonzero PDMs. Moreover, the reflected probe light field for both transitions experiences superluminal light propagation. Goos–Hanchen shifts of the reflected probe light beam are substantially modified by the refractive index of the interactive medium as well as incident angle. The effect of the coherent coupling field intensity on absorption, dispersion, group index, and the Goos–Hanchen shifts of the reflected probe beam is also discussed.

Solid-state framing camera operating in interferometric mode.

A high speed solid-state framing camera has been developed which can operate in interferometric mode. This camera measures the change in the index of refraction of a semiconductor when x-rays are incident upon it. This instrument uses an x-ray transmission grating/mask in front of the semiconductor to induce a corresponding phase grating in the semiconductor which can then be measured by an infrared probe beam. The probe beam scatters off of this grating, enabling a measure of the x-ray signal incident on the semiconductor. In this particular instrument, the zero-order reflected probe beam is attenuated and interfered with the diffracted orders to produce an interferometric image on a charge coupled device camera of the phase change induced inside the semiconductor by the incident x-rays.

Temporal evolution of low-coherence reflectrometry signals in photoacoustic remote sensing microscopy.

Recently, a new noncontact reflection-mode imaging modality called photoacoustic remote sensing (PARS) microscopy was introduced providing optical absorption contrast. Unlike previous modalities, which rely on interferometric detection of a probe beam to measure surface oscillations, the PARS technique detects photoacoustic initial pressures induced by a pulsed laser at their origin by monitoring intensity modulations of a reflected probe beam. In this paper, a model describing the temporal evolution from a finite excitation pulse is developed with consideration given to the coherence length of the interrogation beam. Analytical models are compared with approximations, finite-difference time-domain (FDTD) simulations, and experiments with good agreement.

Determination of thermal diffusivity of opaque materials using the photothermal mirror method

A pump-probe photothermal mirror (PTM) method has been developed to determine the thermal dif- fusivity of opaque solid samples. The method involves the detection of the distortion of a probe beam whose reflection profile is affected by the photoelastic deformation of a polished material surface induced by the absorp- tion of a focused pump field. We have measured the time dependence of the PTM signal of Ti, Al, Cu, Sn, Ag, and Ni samples. We show theoretically and experimentally that the time derivative of the signal in the first micro- seconds is proportional to the square root of the thermal diffusivity coefficient. The method affords a simple calibration and efficient interpretation of experimental data for a sensitive determination of the thermal diffusivity coefficient for materials. We demonstrate the applicability of the technique by measuring the thermal diffusivities of wadsleyite (β-Mg2SiO4) and diopside (MgCaSi2O6), two important minerals relevant to geophysical studies. © 2014 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1.OE.53.12.127101) beams. 9-12 A useful signal can be generated by measuring the transmission of the reflected probe light through a small aperture located at some distance from the sample. The PTM signal is defined as the relative change of the probe light transmission through the aperture. In the continuous wave regime of excitation, the signal grows as the square root of time in the first microseconds after the start of illu- mination. The signal reaches a stationary value when thermal diffusivity equilibrates the intake of thermal energy, yielding stationary values for the induced thermal gradients. The steady state situation is reached within a few milliseconds for metals, whose thermal diffusivities based on the simul- taneous resolution of the thermoelastic equation for the sur- face deformations range from 10 −5 to 2 × 10 −4 m 2 s −1 . The theoretical model used to explain the PTM method is based on the simultaneous resolution of the thermoelastic equation for the surface deformations and the heat conduction equa- tion. The thermal surface deformation yields a phase shift of the reflected probe beam of light. Diffraction theory provides the value of the probe wavefront at the detector location. The method affords a simple nondestructive determination of the photothermal properties of the sample. In this work, we have developed a simple and practical PTM technique to measure the thermal diffusivity of opaque samples. Based on the developed theoretical model for the PTM effect, we numerically show that the time derivative for the first microseconds of growth of the PTM signal divided over its stationary value is linearly proportional to the square root of the thermal diffusivity coefficient. We have studied the time dependence of the PTM signal for six different met- als: Ag, Cu, Ni, Al, Sn, and Ti. By measuring the time deriv- atives of these dependences, we demonstrate the validity of the theoretical predictions. We provide a calibration curve of the values of the time derivatives of the PTM signal that we have subsequently used to determine the thermal diffusivities of wadsleyite (β-Mg2SiO4) and diopside (MgCaSi2O6), two important minerals relevant to geophysical studies.

Simultaneous Measurement of Absorbance and Quantum Yields for Photocurrent Generation at Dye-Sensitized Single-Crystal ZnO Electrodes

It is often assumed that the photoresponse or incident photon-to-current conversion efficiency (IPCE) spectrum of a sensitized semiconductor electrode is directly correlated with the amount of sensitizing species present on the semiconductor surface. In reality, the various forms of adsorbed species, such as dye aggregates or dye molecules bound to different adsorption sites, such as terrace edges, can have significantly different electron injection yields and carrier recombination rates. To provide information about the amounts of the various adsorbed dye species and their effectiveness as sensitizers, we report the simultaneous acquisition of IPCE and attenuated total reflectance (ATR) UV-vis spectra for a thiacyanine dye bound to a single-crystal oxide semiconductor electrode surface. ZnO single crystals were fashioned into internal-reflection elements to act both as a waveguide for the internally reflected probe beam for UV-vis spectra and as the substrate for dye sensitization using dyes with distinct spectral signatures for monomers and aggregates. Strong agreement was observed between the quantum efficiency and ATR UV-vis spectra, suggesting that, under the conditions employed, both monomers and aggregates of the dye studied generate photocurrent with the same efficiency.

Electrically controlled Goos-Hänchen shift of a light beam reflected from the metal-insulator-semiconductor structure

We proposed a scheme to manipulate the Goos-Hänchen shift of a light beam reflected from the depletion-type device via external voltage bias. It is shown that the lateral shift of the reflected probe beam can be easily controlled by adjusting the reverse voltage bias and the incidence angle. Using this scheme, the lateral shift can be tuned from negative to positive, without changing the original structure of the depletion-type device. Numerical calculations further indicate that the influence of structure parameters and light wavelength can be reduced via readjustment of the reverse bias. The proposed structure has the potential application for the integrated electronic devices.

Cross-phase modulation and population redistribution in a periodic tripod medium

The cross-Kerr effect is studied for two weak beams, probe and trigger, propagating in an atomic medium in a tripod configuration, dressed by a strong standing wave coupling beam in a regime of electromagnetically induced transparency. The nonlinear phase shifts for both transmitted and reflected probe beams induced by the trigger's presence are found to depend on the probe detuning, the control beam's intensity, the relaxation rates and, in particular, on the redistribution of the population among the atomic levels. Such a quantitative analysis indicates that the transmitted and reflected probe beam components and their respective phase shifts can be easily controlled and optimized.

A micro-optical modulation spectroscopy technique for local strain measurement

We provide a high throughput method of performing optical modulation spectroscopy, such as photoreflectance or other spectroscopy techniques which include photoluminescence on a micrometric resolution scale of the order of 10 μm. The spectroscopic technique is designed for strain induced by process in silicon wafers. The optical system is optimized using a polarizing beamsplitter in conjunction with a single Fresnel rhomb for the provision of an optimum separation of the reflected probe beam with minimal optical losses. In addition, a rapid detection system is used that allows the spectrum to be acquired within few seconds.

Advances in optical carrier profiling through high-frequency modulated optical reflectance

As indicated by the ITRS roadmap, obtaining accurate information on the electrically active dopant profile for sub-30-nm structures is a key issue. Presently, however, there is no conventional, probe-based (destructive) technique available satisfying the ITRS targeted depth (3%) and carrier level (5%−10%) reproducibility and accuracy. In this work, the authors explore the promising capabilities of nondestructive photomodulated optical reflectance (PMOR) techniques, based on the localized (micrometer beam size) detection of variations in the reflectivity of the sample, due to thermal and plasma (excess carrier) effects as can be generated by a modulated pump laser such as the Therma-Probe® (TP) system. Earlier and more recent work using low modulation (1 kHz) frequencies has shown that it is possible, but rather tedious, to extract the electric junction depth (at about 1018 cm−3) and carrier concentration of chemical vapor deposition grown (CVD) (boxlike) structures based on so-called power curves (where the reflected power of the probe laser is plotted versus the power of the pump laser). In this work the authors focus on high-frequency (1 MHz) PMOR, which gives two (instead of one) independent signals, i.e., the amplitude (A) and phase angle (ϕ) of the reflected probe beam. It has been proposed earlier and is confirmed in this work that a single simple measurement allows for the direct and easy extraction of the junction depth (Xj) and carrier concentration (N) of boxlike profiles. Furthermore, the shape of the so-called three-dimensional PMOR offset curves (A and ϕ versus offset), where the distance of the pump relative to the probe beam is varied over several micrometers, might help to obtain information on more complex profiles. The principles allowing for the extraction of arbitrary carrier profiles, with nanometer depth resolution and carrier concentrations between 1018 and 1021 cm−3, from offset curves will be discussed and evidence for the proposed ideas will be given for homogeneously doped material and CVD boxlike structures based on FSEM device simulations.

Reflective thermal lens detection device

A reflective thermal lens detection device was developed for realizing a portable and sensitive detector for a microsystem. An aluminum mirror was formed on the main plate of a microchip, and a reflected probe beam was detected with a single pick-up unit. The background signal due to light absorption of the aluminum mirror was 60 times reduced when the microchannel and the mirror were separated with an interval of 600 microm. The tilt angle of the microchip significantly affected the precision of the measurement. Then a quadrant photodiode was used to detect the center of gravity of the reflected probe beam to regulate the tilt angle within +/-0.05 degrees , and this value was enough to achieve 1% CV (coefficient of variance) precision in the measurements. The limit of detection (LOD) was 60 nM for xylene cyanol solution, and the absorbance was 9.4 x 10(-6) AU. About 40 times higher sensitivity was obtained in comparison with a spectrophotometer.

Imaging of surface phonons on a sphere

Surface phonon wavepackets are generated on aluminium-coated transparent glass balls on irradiation with ultrashort light pulses. Delayed ultrashort probe light pulses are used to image the surface phonon propagation in real time by simply monitoring the modulation in the reflected probe beam intensity caused by the photoelastic effect. Imaging was achieved in the region of the phonon source and at the opposite pole of the sphere. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface phonon imaging through the photoelastic effect

Surface phonon imaging is very useful for testing thin structures, in particular because it allows film thicknesses or elastic constants to be derived. We present a method for imaging surface phonons based on optical reflectivity measurements combined with excitation using picosecond optical pulses. Surface phonons are generated in a thin aluminium film on a transparent glass substrate. The induced strain modulates the refractive index of the film and substrate through the photoelastic effect. Probe laser pulses incident from the film or substrate side can be used to image surface phonon propagation in real time by simply monitoring the modulation in the reflected probe beam intensity. When we image from the substrate side, surface-skimming longitudinal bulk phonons are clearly imaged in addition to the surface phonons. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Holographic surface gratings in iron-doped lithium niobate

Surface gratings associated with holographic volume gratings in photorefractive crystals of iron-doped lithium niobate have been studied using diffraction of a reflected probe beam and high-resolution phase-shifted interferometric profilometry. Both techniques show that the surface gratings exist in the form of periodical corrugations of the same period as that of the volume grating. The maximum amplitude of the periodical surface relief measured by both techniques is close to 6.5 nm. We also demonstrated that the periodical electric forces on the surface were capable of assembling polystyrene microspheres along the fringes of the grating. Large amplitude of the periodic electric field (1.6×104 V/cm) is associated with the photogalvanic effect.

Optoacoustic imaging using a three-dimensional reconstruction algorithm

A novel three-dimensional (3-D) computational algorithm to reconstruct 3-D optoacoustic images from two-dimensional (2-D) pressure distributions generated at the sample surface is presented. The 2-D pressure distributions were measured as images at different delay times after the excitation laser pulse. The pressure images were captured with a gated CCD camera as the local pressure induces intensity changes of a reflected probe beam at the surface of the irradiated sample. The illumination time was 10 ns and the resolution of the surface pressure image was 20 /spl mu/m. The reconstruction algorithm is based on the decomposition into plane waves. The algorithm was tested in the back projection of simulated pressure transients of three sources, and applied to different biological systems. Furthermore the algorithm was compared with the time of flight back projection algorithm. Optoacoustic images with a depth resolution of 15 /spl mu/m and a lateral resolution of 100 /spl mu/m are presented.

Integrated optical chemical vapor microsensors

We have recently demonstrated that an integrated optical waveguide chemical vapor microsensor may be fabricated on a glass support using a laser source capable of photopolymerizing a commercial dimethacrylate ester resin, to which a photocatalyst was added to the starting material. It was shown that this type of integrated sensor responded selectively to those vapors that could either be solubilized by the polymer structure or could produce an acid-base surface reaction. We have extended this microfabrication technique to another commercial oligomer material to which the photocatalyst is already incorporated into the starting material. Planar integrated optical waveguide structures were readily photopolymerized on glass substrates using a 100 W incoherent mercury arc source. These structures, when exposed to a series of organic vapors including water vapor, showed a selective response to only acetone and chloroform vapors. The apparent selectivity in these two cases arises from a partial solubilization of these condensed vapors into the photopolymerized waveguide structure, resulting in a slight swelling and hence a change in the optical path of the total internally reflected probe beam. Vapor concentration studies in a nitrogen-vapor dynamic flow system indicated detection levels of acetone and chloroform at 10 000 ppm at atmospheric pressure and room temperature.