m8ta
use https for features.
text: sort by
tags: modified
type: chronology
{1466}
hide / / print
ref: -0 tags: optical gain media lasers cross section dye date: 06-13-2019 15:13 gmt revision:2 [1] [0] [head]

Eminently useful. Source: https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-974-fundamentals-of-photonics-quantum-electronics-spring-2006/lecture-notes/chapter7.pdf

Laser Dye technology by Peter Hammond

  • This paper is another great resource!
  • Lists the stimulated emission cross-section for Rhodamine-6G as 4e-16 @ 550nm, consistent with the table above.
  • At a (high) concentration of 2mMol (1 g/l), 1/e penetration depth is 20um.
    • Depending on the solvent, there may be aggregation and stacking / quenching.
  • Tumbling time of Rhodamine 6G in ethanol is 20 to 300ps; fluorescence lifetime in oscillators is 10's of ps, so there is definitely polarization sensitive amplification.
  • Generally in dye lasers, the emission cross-section must be higher than the excited state absorption, σ eσ \sigma_e - \sigma^\star most important.
  • Bacteria can actually subsist on rhodamine-similar sulfonated dyes in aqueous solutions! Wow.

{1186}
hide / / print
ref: -0 tags: voltage sensitive dyes fluorescent protein date: 01-02-2013 05:08 gmt revision:0 [head]

PMID-20622860 Imaging brain electric signals with genetically targeted voltage-sensitive fluorescent proteins.

  • Interesting: Most fluorescent fusion proteins form intracellular aggregates during long-term expression in mammalian neurons, although this effect appears to be minimal in Aequorea victoria–derived fluorescent proteins.
  • See also {1185}

{1182}
hide / / print
ref: -0 tags: optical recording voltage sensitive dyes redshirt date: 01-02-2013 03:17 gmt revision:3 [2] [1] [0] [head]

PMID-16050036 Imaging brain activity with voltage- and calcium-sensitive dyes.

  • Voltage-sensitive dyes are well suited for measuring synaptic integration, as:
    • Electrodes are too blunt to effectively record these fine, < 1um diameter structures.
    • The surface area to volume ratio is highest in the dendrites
    • Voltage-sensitive dyes also permeate internal membranes not subject to voltage gradients, hence this does not contribute to the signal, leading to a decreased ΔF/F\Delta F / F .
  • Dominant experimental noise is shot noise, statistical -- see {1181}.
  • modern dyes and imagers can reliably record single action potentials; spatial averaging yields similar resolution as electrical recording.
  • They performed optical recording of Aplysia sensory ganglia, and discovered following light tail touch: "It is almost as if the Aplysia nervous system is designed such that every cell in the abdominal ganglion cares about this (and perhaps every) sensory stimulus. In addition, more than 1000 neurons in other ganglia are activated by this touch..."
      • These results force a more pessimistic view of the present understanding of the neuronal basis of apparently simple behaviors in relatively simple nervous systems.
  • Used calcium imaging on olfactory glomeruli of mice and turtles; measurements were limited by either shot-noise or heart/breathing artifacts.
  • Confocal and two-photon microscopes, due to their exchange of spatial resolution for sensitivity, are not useful with voltage-sensitive dyes.
    • The generation of fluorescent photons in the 2-photon confocal microscope is not efficient. We compared the signals from Calcium Green-1 in the mouse olfactory bulb using 2-photon and ordinary microscopy. In this comparison the number of photons contributing to the intensity measurement in the 2-photon confocal microscope was about 1000 times smaller than the number measured with the conventional microscope and a CCD camera.
  • By the numbers, quote: Because only a small fraction of the 10e16 photons/ms emitted by a tungsten filament source will be measured, a signal-to-noise ratio of 10e8 (see above) cannot be achieved. A partial listing of the light losses follows. A 0.9-NA lamp collector lens would collect 0.1 of the light emitted by the source. Only 0.2 of that light is in the visible wavelength range; the remainder is infrared (heat). Limiting the incident wavelengths to those, which have the signal means, that only 0.1 of the visible light is used. Thus, the light reaching the
preparation might typically be reduced to 1013 photons/ms. If the light-collecting system that forms the image has high efficiency e.g., in an absorption measurement, about 1013 photons/ms will reach the image plane. (In a fluorescence measurement there will be much less light measured because 1. only a fraction of the incident photons are absorbed by the fluorophores, 2. only a fraction of the absorbed photons appear as emitted photons, and 3. only a fraction of the emitted photons are collected by the objective.) If the camera has a quantum efficiency of 1.0, then, in absorption, a total of 10e13 photoelectrons/ms will be measured. With a camera of 1000 pixels, there will be 10e10 photoelectrons/ms/pixel. The shot noise will be 10e5 photoelectrons/ms/pixel; thus the very best that can be expected is a noise that is 10e−5 of the resting light (a signal-to-noise ratio of 100 db). The extra light losses in a fluorescence measurement will further reduce the maximum obtainable signal-to-noise ratio.

{1179}
hide / / print
ref: -0 tags: optical coherence tomography neural recording squid voltage sensitive dyes review date: 12-23-2012 21:00 gmt revision:4 [3] [2] [1] [0] [head]

PMID-20844600 Detection of Neural Action Potentials Using Optical Coherence Tomography: Intensity and Phase Measurements with and without Dyes.

  • Optical methods of recording have been investigated since the 1940's:
    • During action potential (AP) propagation in neural tissue light scattering, absorption, birefringence, fluorescence, and volume changes have been reported (Cohen, 1973).
  • OCT is reflection-based, not transmission: illuminate and measure from the same side.
    • Here they use spectral domain OCT, where the mirror is not scanned; rather SD-OCT uses a spectrometer to record interference of back-scattered light from all depth points simultaneously (Fercher et al., 1995).
    • Use of a spectrometer allows imaging of an axial line within 10-50us, sufficient for imaging action potentials.
    • SD-OCT, due to some underlying mathematics which I can't quite grok atm, can resolve/annul common-mode phase noise for high temporal and Δphase\Delta phase measurement (high sensitivity).
      • This equates to "microsecond temporal resolution and sub-nanometer optical path length resolution".
  • OCT is generally (intially?) used for in-vivo imaging of retinas, in humans and other animals.
  • They present new data for depth-localization of neural activity in squid giant axons (SGA) stained with a voltage-sensitive near-infrared dye.
    • Note: averaged over 250 sweeps.
  • ΔPhase>>ΔIntensity\Delta Phase &gt;&gt; \Delta Intensity -- figure 4 in the paper.
  • Use of voltage-sensitive dyes improves the resolution of ΔI\Delta I , but not dramatically --
    • And Δphase\Delta phase is still a bit delayed.
    • Electrical recording is the control.
      • It will take significant technology development before optical methods exceed electrical methods...
  • Looks pretty preliminary. However, OCT can image 1-2mm deep in transparent tissue, which is exceptional.
  • Will have to read their explanation of OCT.
  • Used in a squid giant axon prep. 2010, wonder if anything new has been done (in vivo?).
  • Claim that progress is hampered by limited understanding of how these Δphase\Delta phase signals arise.