http://arxiv.org/pdf/1012.2139v1
*Coupled-Mode Theory of Field Enhancement in Complex Metal Nanostructures* Referenced is a simplified sphere based math model of the cascade dipole amplifier. On Mon, May 13, 2013 at 10:09 PM, Axil Axil <[email protected]> wrote: > This post describes the mechanism that produces large power concentrations > in a multi-nanoparticle system where the particles vary widely in particle > sizes. > > First, let’s set the table. > > A cascade amplifier is any diode constructed from a series of amplifiers, > where each amplifier sends its output to the input of the next amplifier in > a daisy chain. > > Coherent anti-Stokes Raman scattering acts like such a cascade amplifier, > except that dipoles tuned to various resonant frequencies drive thermal > power to higher power concentration levels zero loss factors. > > In detail, coherent anti-Stokes Raman scattering, also called Coherent > anti-Stokes Raman scattering spectroscopy (CARS), is a form of spectroscopy > used primarily in chemistry, physics and related fields. > > It is sensitive to the same vibrational signatures of dipoles as seen in > Raman spectroscopy. Unlike Raman spectroscopy, CARS employs multiple photo > harmonics. > > It produces a signal in which the emitted waves are coherent with one > another. As a result, CARS is orders of magnitude stronger than spontaneous > Raman emission. > > CARS is a N-order nonlinear optical process involving multiple coupled > dipole sources. > > These dipoles interact and generate a coherent optical signal at the > anti-Stokes frequency. The high order harmonic is resonantly enhanced when > the frequency difference between the low order pumps and the dipoles > coincides with the frequency of a Raman resonance, which is the basis of > the technique's intrinsic vibrational contrast mechanism. > > Multiple nanoparticles of various sizes interact each with their > respective dipole resonant frequencies. > > The CARS process can be physically explained by using either a classical > oscillator model or by using a quantum mechanical model. > > Classically, the Raman active vibrator is modeled as a (damped) harmonic > oscillator with a characteristic frequency. In CARS, these oscillators are > not driven by a single optical wave, but by the different resonant > frequencies between the dipole pumps and the high order harmonic. > > This driving mechanism is similar to hearing the low combination beat tone > when striking two different high tone piano keys: your ear is sensitive to > the difference frequency of the high tones. Similarly, the Raman oscillator > is susceptible to the difference frequency of multiple optical waves. When > the difference frequency approaches beat resonance, the system of dipole > oscillators are driven very efficiently. > > While intuitive, this classical picture does not take into account the > quantum mechanical energy levels of the dipole. Quantum mechanically, the > CARS process can be understood as follows. Our dipole is initially in the > ground state, the lowest thermal energy state of the system. The pump > dipole excites the dipole chain to a virtual vibrational state. > > A virtual state is not an eigenstate of the dipole and it cannot be > occupied but it does allow for transitions between otherwise uncoupled real > states. If a dipole is simultaneously present along with the pumps, the > virtual state can be used as an instantaneous gateway to address a > vibrational eigenstate of the dipole. > > The joint action of the pumps and the Stokes has effectively established a > coupling between the ground state and the vibrationally excited state of > the system. > > The system is now in multiple states at the same time: it resides in a > coherent superposition of states. > > This promotes the system to a virtual state. Again, the molecule cannot > stay in the virtual state and will fall back instantaneously to the ground > state under the emission of a photon at the anti-Stokes frequency. The pump > dipoles are no longer in a superposition, as it resides again in the lowest > thermal state, the ground state. > > In the quantum mechanical model, energy is deposited in the dark mode > highest resonant system during the CARS process. The molecule acts like a > medium for converting the frequencies of the multiple resonant waves into a > CARS signal (a parametric process). There are, however, related coherent > Raman processes that occur simultaneously which do deposit energy into the > high order resonant cavity at high efficiency. > > The maximum sustained energy level achieved in this smallest resonant > cavity in the cavity chain is determined when losses from the cavity equals > input energy levels. > > > >
