B. C. Forget

High resolution photothermal imaging of high frequency phenomena using a visible charge coupled device camera associated with a multichannel lock-in scheme

We have developed a technique using a photothermal microscope from which we can make a thermal image of an electronic component working at a “high frequency” using a charge coupled device (CCD) camera and a multichannel lock-in scheme. To do this, we have created an electronic “stroboscope”: the frequency F of the thermal signal induced by a high frequency electrical excitation and the frequency of the light F+f that illuminates the device are next to each other; the signal reflected at the surface of the device whose amplitude is proportional to the variation of reflectivity and hence to the variation of temperature and whose frequency is the blinking one f is analyzed by a visible CCD camera. Amplitude and phase images of the high frequency thermal phenomenon can then be made. Moreover, this technique presents a great advantage: the spatial resolution is better than 1 μm. The amplitude and phase images presented show, with a very good spatial resolution, Joule and Peltier heating of a polycrystalline si...

Photorefractive detection of tagged photons in ultrasound modulated optical tomography of thick biological tissues

We present a new and simple method to obtain ultrasound modulated optical tomography images in thick biological tissues with the use of a photorefractive crystal. The technique offers the advantage of spatially adapting the output speckle wavefront by analysing the signal diffracted by the interference pattern between this output field and a reference beam, recorded inside the photorefractive crystal. Averaging out due to random phases of the speckle grains vanishes, and we can use a fast single photodetector to measure the ultrasound modulated optical contrast. This technique offers a promising way to make direct measurements within the decorrelation time scale of living tissues.

Pulsed acousto-optic imaging in dynamic scattering media with heterodyne parallel speckle detection

We present a new detection scheme for acousto-optic tomography based on pulsed-wave ultrasound and illumination combined with heterodyne parallel speckle detection. This setup can perform tomographies inside several-centimeter-thick scattering samples. Test experiments confirm the suitability of this method for performing tomographies inside various types of optically scattering media, including liquids.

Heterodyne detection of multiply scattered monochromatic light with a multipixel detector

A new technique is presented for measuring the spectral broadening of light that has been multiply scattered from scatterers in motion. In our method the scattered light is detected by a heterodyne receiver that uses a CCD as a multipixel detector. We obtain the frequency spectrum of the scattered light by sweeping the heterodyne local oscillator frequency. Our detection scheme combines a high optical etendue (product of the surface by the detection solid angle) with an optimal detection of the scattered photons (shot noise). Using this technique, we measure, in vivo, the frequency spectrum of the light scattered through the breast of a female volunteer.

Theoretical description of the photorefractive detection of the ultrasound modulated photons in scattering media

Acousto-optic imaging of thick biological tissues can be obtained in real-time with an adaptive-wavefront holographic setup, where the holographic media is a self-developping photorefractive crystal. As a consequence, the interference signal resulting from the acousto-optic effect can be easily collected with a high etendue and fast single photodetector. We present a statistical model of the field propagating through the scattering media and show why the various acoustic frequency components contained in the speckle output pattern are uncorrelated. We then give a detailed description of the signal measured through the photorefractive effect, in order to explain the quadratic pressure response observed for the two commonly used configurations setup e.g an amplitude or a phase modulation of the ultrasound.

Electronic diffusivity measurement in silicon by photothermal microscopy

In this letter we demonstrate that a photothermal microscopy experiment can be used to determine the electronic diffusivity (or carrier mobility) in the same way it is now widely used to measure locally thermal diffusivity of various nonsemiconductor materials. The main difficulty lies in the fact that in order to separate thermal and carrier diffusion, the experiment must be performed for a relatively large distance between the pump and probe beams. Photothermal signals are therefore rather weak and great experimental care must be taken. We present and discuss experimental results on Si.

Microscopic measurements of the local heat conduction in polycrystalline diamond films

Abstract It is generally known that the microstructure of polycrystalline CVD diamond samples has a strong impact on their thermal properties. Despite the fact that nowadays layers can be deposited with macroscopic thermal conductivities or diffusivities rivalling those of type II natural diamonds, the samples are highly thermally inhomogeneous and sometimes show local values differing by up to two orders of magnitude. To examine these phenomena more closely, we present a microscopic photothermal measuring method for the local thermal diffusivity. We demonstrate the feasibility of diffusivity measurements at a sample surface of ca. 20 × 20 μm. We show results obtained on a reference sample of known diffusivity and present measurements on a small single crystal diamond, a local measurement at the substrate side of a CVD diamond layer, and a measurement of the diffusivity inside a microcrystal at the growth side of a CVD diamond layer.

Auger recombination in modulated photoreflectance characterization of silicon wafers

We have used photoreflectance signal as a function of modulation frequency to characterize electronic transport properties in silicon wafers. Due to the high densities of free carriers generated in such an experiment, we propose a theoretical model which takes into account Auger recombination. This nonlinear recombination process is usually neglected in the interpretation of photothermal phenomena. Our experimental results, in good accordance with theoretical prediction, show that this nonlinear recombination process not only cannot be neglected, but can be the dominant recombination process for certain experimental configurations, particularly for modulation frequencies up to 100 kHz and pump power ranging from 1.5 kW/cm2 to 9 kW/cm2.

Micron scale photothermal imaging

Abstract In this paper, we present various experimental results which demonstrate the ability of photothermal methods to perform heat diffusion imaging at the micron scale. Photoreflectance microscopy measurements have been performed on a composite sample presenting a strong anisotropy of thermal properties and on microelectronic devices. We will then introduce the CCD array photothermal microscopy which permits the acquisition of a complete image in a single shot, without moving the sample. A stroboscopic technique must be used in order to adjust to the relatively low frame rate of the CCD camera. We will discuss two cases: homodyne and heterodyne detection.

Nonlinear recombinations in photoreflectance characterization of silicon wafers

Abstract We have used the photoreflectance signal as a function of the modulation frequency to characterize electronic transport properties in silicon wafers. Due to the high densities of free carriers generated in such an experiment, we propose a theoretical model taking into account Auger recombination which is usually neglected in the interpretation of photothermal phenomena. We will also consider nonlinear recombination processes in the bulk and also, in the case of implantation monitoring, at the surface of the sample. Our experimental results, in good accordance with theoretical predictions, show that this nonlinear recombination process not only cannot be neglected, but can be the dominant recombination process for certain experimental configurations.