We present an entirely linear all-optical method of dispersion enhancement that relies on mode coupling between the orthogonal polarization modes of a single optical cavity, eliminating the necessity of using an atomic medium to produce the required anomalous dispersion, which decreases the dependence of the scale factor on temperature and increases signal-to-noise by reducing absorption and nonlinear effects. The use of a single cavity results in common mode rejection of the noise and drift that would be present in a system of two coupled cavities. We show that the scale-factor-to-mode-width ratio is increased above unity for this system and demonstrate tuning of the scale factor by (i) directly varying the mode coupling via rotation of an intracavity half wave plate, and (ii) coherent control of the cavity reflectance which is achieved simply by varying the incident polarization superposition. These tuning methods allow us to achieve unprecedented enhancements in the scale factor and in the scale-factor-to-mode-width ratio by closely approaching the critical anomalous dispersion condition.
We present an entirely linear all-optical method of dispersion enhancement using coupled cavities that leads to a substantial increase in system transmission in comparison with atom-cavity systems. This is achieved by tuning the system to an anomalous dispersion condition by under-coupling at least one of the cavities to the other. The intracavity anomalous dispersion is then associated with a dip in reflection (and in turn with a peak in transmission) rather than with an absorption resonance as in the case of the atomic vapor. We find that in contrast with the atom-cavity system where mode reshaping always contributes to the mode pushing, in coupled cavity systems reshaping of the mode profile can either contribute to or oppose the mode pushing, and even reverse it under appropriate conditions leading to a reduced scale factor in transmission. We demonstrate a method for further optimizing the transmission of both atom-cavity and coupled-cavity systems, but show that this leads to a more rectangular mode profile and a reduction in the scale factor bandwidth. We also derive the cavity scale factor in reflection for both atom-cavity and coupled cavity systems and show that in reflection the reshaping of the mode profile can either contribute to or oppose the mode pushing, but cannot reverse it.
We have measured mode pushing by the dispersion of a rubidium vapor in a Fabry-Perot cavity and have
shown that the scale factor and sensitivity of a passive cavity can be strongly enhanced by the presence of
such an anomalous dispersion medium. The enhancement is the result of the atom-cavity coupling, which
provides a positive feedback to the cavity response. The cavity sensitivity can also be controlled and tuned
through a pole by a second, optical pumping, beam applied transverse to the cavity. Alternatively, the
sensitivity can be controlled by the introduction of a second counter-propagating input beam that interferes
with the first beam, coherently increasing the cavity absorptance. We show that the pole in the sensitivity
occurs when the sum of the effective group index and an additional cavity delay factor that accounts for mode
reshaping goes to zero, and is an example of an exceptional point, commonly associated with coupled non-
Hermitian Hamiltonian systems. Additionally we show that a normal dispersion feature can decrease the
cavity scale factor and can be generated through velocity selective optical pumping.
We analyze the effect of a highly dispersive element placed inside a modulated optical cavity on the
frequency and amplitude of the modulation to determine the conditions for cavity self-stabilization and
enhanced gyroscopic sensitivity. We find an enhancement in the sensitivity of a laser gyroscope to rotation
for normal dispersion, while anomalous dispersion can be used to self-stabilize an optical cavity. Our results
indicate that atomic media, even coherent superpositions in multilevel atoms, are of limited use for these
applications, because the amplitude and phase filters work against one another, i.e., decreasing the
modulation frequency increases its amplitude and vice-versa. On the other hand, for optical resonators the
dispersion reversal associated with critical coupling enables the amplitude and phase filters to work together.
We find that for over-coupled resonators, the absorption and normal dispersion on-resonance increase the
contrast and frequency of the beat-note, respectively, resulting in a substantial enhancement of the
gyroscopic response. Under-coupled resonators can be used to stabilize the frequency of a laser cavity, but
result in a concomitant increase in amplitude fluctuations. As a more ideal solution we propose the use of a
variety of coupled-resonator-induced transparency that is accompanied by anomalous dispersion.
We show that the dynamics of photons in a ring laser gyro are adequately represented by the damped Rabi problem, and thus demonstrate a variety of photonic coherence phenomena analogous to those that occur in atoms. We discuss methods to circumvent the deleterious consequences and exploit the advantageous consequences of these effects. Specifically, we discuss the use of short pulses for the elimination of the gyro dead-band, the use of the dead-band locking frequency for the hypersensitive measurement of scattering and absorption, and the incorporation of fast and slow light media into the cavity for the enhancement of the gyro response.
We predict the propagation of slow and fast light in two co-resonant coupled optical resonators. In coupled resonators, slow light can propagate without attenuation by a cancellation of absorption as a result of mode splitting and destructive interference, whereas transparent fast light propagation can be achieved by the assistance of gain and splitting of the intracavity resonances, which consequently change the dispersion from normal to anomalous. The effective steady-state response of coupled-resonators is derived using the temporal coupled-mode formalism, and the absorptive and dispersive responses are described. Specifically, the occurrence of slow light via coupled-resonator-induced transparency and gain-assisted fast light are discussed.
An iterative method is applied to the analysis of N-coupled ring resonators. Splitting of the modes into N higher-Q modes occurs when the round-trip phase shifts in each ring are equal, in agreement with results for planar resonators and whispering gallery modes (WGMs) in coupled microparticles. This mode-splitting is, therefore, a universal phenomenon for resonant structures, and can lead to reduced thresholds for nonlinear optical effects.
Gold nanoparticle composites are known to display large optical nonlinearities. In order to assess the validity of generalized effective medium theories (EMTs) for describing the optical properties of metal nanoparticle composites, we have used the z-scan technique to measure the third-order susceptibility of gold nanoparticle composites across the entire range of fill fractions. These materials range from low concentration statistically random gold sols, to aggregated thin (two-dimensional) composite films, to quasi-bulk thin films above the percolation threshold. These measurements allow the nonlinearity of gold to be determined both directly and by deduction from applicable effective medium theories. We compare our results with predictions which ascribe the nonlinear response to a Fermi-smearing mechanism. We demonstrate that the nonlinear susceptibility changes sign due to a phase shift between the applied field and the local field, and that this sign change occurs at the percolation threshold. Further for films whose thickness is less than an optical wavelength we introduce a 2D form of the Maxwell Garnett model.
Polydiacetylenes (PDAs) are attractive materials for both electronic and photonic applications because of their highly conjugated electronic structures. They have been investigated for applications as both 1D conductors and nonlinear optical (NLO) materials. One of the chief limitations to the use of PDAs has been the inability to readily process them into useful forms such as films and fibers. In our laboratory we have developed a novel process for obtaining amorphous films of a PDA derived from 2- methyl-4-nitroaniline using photodeposition with UV light from monomer solutions onto transparent substrates. Photodeposition from solution provides a simple technique for obtaining PDA films in any desired pattern with good optical quality. This technique has been used to produce PDA films that show potential for optical applications such as holographic memory storage and optical limiting, as well as third-order NLO applications such as all-optical refractive index modulation, phase modulation and switching. Additionally, copolymerization of diacetylenes with other monomers such as methacrylates provides a means to obtain materials with good processibility. Such copolymers can be spin cast to form films, or drawn by either melt or solution extrusion into fibers. These films or fibers can then be irradiated with UV to photopolymerize the diacetylene units to form a highly stable cross-linked PDA-copolymer network. If such films are electrically poled while being irradiated, they can achieve the asymmetry necessary for second-order NLO applications such as electro-optic switching. On Earth, formation of PDAs by the above mentioned techniques suffers from defects and inhomogeneities caused by convective flows that can arise during processing. By studying the formation of these materials in the reduced-convection, diffusion- controlled environment of space we hope to better understand the factors that affect their processing, and thereby, their nature and properties. Ultimately it may even be feasibly to conduct space processing of PDAs for technological applications.