Photon maps

Fig. 30. Topographic STM image (a) and photon map (b) recorded on p-type 85% PSL (after [131]). (c) Profiles showing the correlation between the surface topography and the intensity of emitted light (after [139]).

On semiconductors light emission is induced by injection of electrons into the conduction band and subsequent band-to-band radiative recombination with holes (Fig. 38a). The process is reminiscent of electroluminescence or cathodolumines-cence and works with p-type substrates only (at n-type specimens no hole is available at the surface). Tunnel biases of 1.5-2 V are necessary in the case of GaAs, for instance. Figure 38b is a photon map of a GaAlAs/GaAs multiquantum well obtained by Alvarado et al. [140], The white stripes are regions where photons are emitted and correspond to the GaAs layers. The lateral resolution is about 1 nm and is limited by the diffusion distance of minority carriers. In Sec. 5.1 we have seen an example of the application of this technique in the case of porous silicon layers. 

Fig. 38. Photon emission at a semiconductor, (a) Principle of the process at a p-type semiconductor, (b) Photon map taken on a GaAlAs/GaAs multiquantum well. Electrons are injected into GaAlAs and recombine in p-GaAs, from which light is emitted (after [140]).

Beermann, J., Bozhevolnyi, S., Bordo, V., and Rubahn, H.-G. (2004). Two-photon mapping of local molecular orientations in hexaphenyl nanofibers. Opt.Commun., 237 423-429. 

It is possible with an STM tip to locally excite specific sites in the surface and observe the fluorescence from the excitation by recording spatially resolved and energy-resolved photon maps. If this can be done with sufficiently high spatial resolution, a detailed assignment of excitation energies to particular sites will be possible [130]. 

Figure 9.4 Cross sections through a stepped area of a Au surface and results of a model calculation, (a) Section of a STM topograph, (b) corresponding section of the photon map, (c) surface corrugation in the simulation and (d) calculated photon intensity. Section (b) has been low-pass filtered to decrease the photon shot noise.

Simultaneously recorded STM topography (A) and photon map (B) of hexagonally ordered silver nanoparticles on a Au substrate. Scan area 26 X10 nm. Graph (C) shows a topography cross-section along the dotted line in (A). The emission intensity shows maxima when the tip is between the particles indicating that the emission is caused by coupled plasmon modes between the particles. 

In absorption spectroscopy, the attenuation of light as it passes tln-ough a sample is measured as a function of wavelength. The attenuation is due to rovibrational or electronic transitions occurring in the sample. Mapping out the attenuation versus photon frequency gives a description of the molecule or molecules responsible for the absorption. The attenuation at a particular frequency follows the Beer-Lambert law,... 

Figure Bl.22.7. Left resonant seeond-hannonie generation (SHG) speetnimfrom rhodamine 6G. The inset displays the resonant eleetronie transition indueed by tire two-photon absorption proeess at a wavelength of approximately 350 mn. Right spatially resolved image of a laser-ablated hole in a rhodamine 6G dye monolayer on fiised quartz, mapped by reeording the SHG signal as a fiinetion of position in the film [55], SHG ean be used not only for the eharaeterization of eleetronie transitions within a given substanee, but also as a mieroseopy tool.

In the x-ray portion of the spectmm, scientific CCDs have been utilized as imaging spectrometers for astronomical mapping of the sun (45), galactic diffuse x-ray background (46), and other x-ray sources. Additionally, scientific CCDs designed for x-ray detection are also used in the fields of x-ray diffraction, materials analysis, medicine, and dentistry. CCD focal planes designed for infrared photon detection have also been demonstrated in InSb (47) and HgCdTe (48) but are not available commercially. 

Without considering photon noise, Le Louam and Tallon (2002) have found that the 4 LGSs 3D mapping system delivers a Strehl ratio of 80% on axis when a single LGS would have been limited to S f 10%. These numbers have to be compared with S f 85% obtained with the same MCAO device fed with a NGS. With a field of view of 100 arcsec, fhey gef S f 30% wifh little anisoplanatism. Performances are weakly dependent on errors in the altitude of the turbulent layers (which could be measured from the 3D mapping system, at the expenses of the linearity of the equations, since the interaction matrix depends on layer altitudes). Unsensed layers do not produce a significant anisoplanatism, as the central obscuration within which no measurement is available. 

Astronomical data is archived and released in the form of images (maps and outlines) and spectra (distribution of photons as a function of their energy). Duly classified, these constitute a huge database. The problem then is to give physical and astrophysical meaning to these cosmic archives, via an interpretation within the framework of the most relevant physical theory. 

Laiho, L. H., Pelet, S., Hacewicz, T. M., Kaplan, P. D., and So, P. T. C. 2005. Two-photon 3-D mapping of ex vivo human skin endogenous fluorescence species based on fluorescence emission spectra. J. Biomed. Opt. 10 1-10. 

---