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  1. J

    Thermocouple connection

    I want to measure the temperature of a metal stage where I am holding a sample. What I have done is to clamp both wires from a thermocouple against the stage using a bolt and washer. Supposedly this is not the best practice but I don't see why this would be a problem. I have read that you are...
  2. J

    Conceptual question on field/displacement

    I have a very basic problem in understanding the relationship between electric field and displacement. If a field is applied to a dielectric, it is clear that this will cause the material to polarize, and one can define the permittivity of the material to quantify the size of this effect...
  3. J

    How much time to spend on background theory?

    I am a materials science graduate student so I have not taken the full set of courses that a physics or applied physics graduate student would. It is very important to me to have a strong understanding of all applicable areas of physics - classical E&M, quantum mechanics, statistical physics...
  4. J

    Identity operator

    I am trying to follow a derivation in a book which is written without bra-ket notation, and presumably without the concept of state vectors. I can easily follow it if I may use the fact that \sum_{n}|\varphi_{n}\rangle\langle\varphi_{n}| is the identity operator. Analogously to the way I would...
  5. J

    Reflected wave extent into media

    This question relates to second harmonic generation from a sub 100 nm nonlinear film. When light impinges on an interface, some fraction is transmitted and reflected as described by the Fresnel equations. If I want to calculate the radiated second harmonic wave from a material, at first...
  6. J

    Use of I vs we in abstract

    Use of "I" vs "we" in abstract I'm writing an abstract for a poster which I am required to give for a fellowship. In general it is my understanding that no one would use "I" in an abstract, even if he or she were the sole author. But in this case, the purpose of the poster session is to...
  7. J

    Modelocked oscillator question

    I have a Ti:Sapphire oscillator which outputs something like 25 fs pulses at 100 MHz (KM-labs if anyone is familiar). The laser has two curved mirrors on either side of the Ti:Sapphire which direct the fluorescence to the end mirrors. Once it is lasing in CW, you translate one focusing mirror...
  8. J

    Can somebody explain what a discriminator is?

    I work in a laser lab and I'm constantly hearing about electronic components that I don't understand (my background is materials science). This is one example. Could someone either explain or point me to an introductory level reference on what a discriminator is in the context of light...
  9. J

    Indium tin oxide - thickness question

    I read that the thickness of an ITO film affects performance as a trade off between transparency and conductivity. How thick can ITO be and still remain reasonably transparent to visible light? I found a couple papers which characterize the transmittance but only out to 500 nm or so. Could it...
  10. J

    Density of states in anisotropic conduction band valley

    I need to calculate the density of states for a dispersion relation which is like the free electron dispersion, but with one effective mass in the kx, ky directions, and a different effective mass in kz. So I need to integrate the inverse gradient of E(k) over a surface of constant energy, ie...
  11. J

    Aligning a Michelson interferometer

    Are there any tricks for getting the two arms of a Michelson interferometer exactly equal, if all I have is a CW HeNe laser to align? The real experiment will be using the interferometer on short pulses, so I will only see interference when the path lengths are equal to within 30 microns or so...
  12. J

    How can light cause atoms to oscillate without being absorbed?

    If light travels through a perfectly transparent material, it will not be absorbed, and other than the reflection loss at the two interfaces, the energy that comes out will be equal to that which went in. Yet we know that the light field is causing all the atoms in its path to oscillate, thus...
  13. J

    What's special about the point group 432?

    Every non-centrosymmetric point group is piezoelectric, except 432. It is neither centrosymmetric nor piezoelectric. Why?
  14. J

    Trying to understand quasi-static processes

    In my thermo course, we made a distinction between quasi-static and reversible transitions (which apparently some don't make). Reversible means both system and surroundings need to be able to return to their initial state. Quasi-static only means that the system transitions to a new state by...
  15. J

    Detecting single photons - conceptual question

    Imagine I have a detector which is "single photon sensitive". What that means, I gather, is that if I shine a pulsed laser at the detector with just the right amount of attenuation, I should get to the regime where there is less than 1 photon arriving each time the laser fires a pulse, so the...
  16. J

    Electrons moving due to a field

    I am trying to understand what happens to the electrons in a solid when I apply a field. If I consider the free electron model at 0 K, I have a Fermi sphere and only those electrons at the Fermi energy have empty states which they can access. Then it is these electrons that are able to respond...
  17. J

    Orbital magnetic moment

    I am taking a class which discusses orbital angular momentum in a pseudo-quantum way, and it was explained that the orbital angular momentum is zero if the time average of the individual "L vectors" sum to zero. I am considering p-orbitals. The argument is that, if there are 3 electrons in the...
  18. J

    Optical tweezers

    Can someone give an intuitive explanation for this effect? I didn't go through the derivation carefully, but I know that the upshot of the math is that the dipole feels a force in the direction of the intensity gradient, so it goes to the focus of the laser. But I feel like there must be some...
  19. J

    Detecting hydrogen gas via IR spectroscopy

    Say I want to monitor the interface of an electrochemical cell via IR spectroscopy. If H2 gas is generated, there must be a characteristic IR signature to this event. How can I figure out what this is? Are there calculations for this? I'm sure this is in the literature somewhere, but I...
  20. J

    Question about refractive index and momentum

    I'm probably doing something stupid here... A wave travels from a medium into a second medium with a smaller refractive index. By definition the speed of the wave increases. Then lambda*f = v, so the wavelength also increases. According to de Broglie, this means the momentum goes down...
  21. J

    Help with physical understanding of AC signal

    My understanding of how AC current works is that the current changes direction, so essentially the charges are oscillating back and forth along the direction of the cable. Then since accelerating charges radiate, this produces an EM wave which is what carries the energy. But my problem is...
  22. J

    Estimating efficiency of sum frequency generation

    I want to estimate the efficiency of SFG in a BBO crystal for the situation when one pulse has much higher energy than the other (nJ compared to fJ), and the beams cross at an angle in the crystal. Can anyone recommend a resource that would help with this?
  23. J

    Could crystalline GeSbTe be considered a dielectric?

    I am interested in the recent theoretical prediction that a shocked crystal can be a source of coherent THz radiation. The computational study was done for a crystal of NaCl, but the authors say that basically any dielectric crystal would work, as the radiation is emitted due to polarization...
  24. J

    Shock waves in crystals - temperature effect

    If you induce a shock wave in a covalent crystal using an ultrashort laser pulse, is there any way to estimate the temperature rise in relation to the pressure? Say I want to induce pressures of 10's of GPa. Is there a general way to think of this problem? I know next to nothing about shock...
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