A microscopic theory for excitonic nonlinearities and light propagation in semiconductor microcavities is applied to study normal-mode coupling (NMC) for varying electron-hole-pair densities. The nonlinear susceptibility of quantum confined excitons is determined from quantum kinetic equations including dephasing due to carrier-carrier and polarization scattering. The predicted disappearing of the normal-mode transmission peaks with negligible change in the NMC splitting agrees well with cw pump-probe measurements on samples showing remarkable splitting-to-linewidth ratios.[S0031-9007(96)01904-7] PACS numbers: 71.35.Cc, 71.36. + c, 73.20.Dx Atom-photon interaction in the strong-coupling regime is realized in high-finesse cavities where the coupling strength between the atom and the cavity mode exceeds both the cavity and atomic damping and the spontaneous emission rate. For the interaction of a single excited atom with an empty cavity in the strong coupling regime vacuum field Rabi oscillations have been predicted [1] and observed [2]. When the number of excited atoms is large compared to the number of cavity photons the quantum treatment of the system by means of master equations becomes equivalent to the semiclassical treatment of Maxwell-Bloch equations [3] and the resonance splitting can be assigned to the normal-mode coupling (NMC) of classical oscillators. Recently, NMC of quantum confined excitons in high-finesse semiconductor microcavities has been observed [4]. In contrast to experiments with preexcited atoms, the semiconductor excitons in the microcavity are coherently driven by the weak probe field. The coupled system of excitons and cavity field can be described in terms of new states similar to exciton polaritons. However, radiatively stable polaritons can exist only in bulk semiconductors whereas in quantum wells (QWs) the lack of momentum conservation in the growth direction leads to an intrinsic radiative lifetime [5,6]. Large cavity-polariton NMC splitting has been demonstrated in wide-gap II-VI semiconductor QW microcavities [7] and III-V semiconductor bulk microcavities [8] and NMC could even be observed at room temperature [9]. Recently, the nonlinear saturation of the NMC has been investigated experimentally [10,11].In this Letter we study the microscopic mechanisms for nonlinear saturation of QW excitons and the corresponding modification of the NMC in a microcavity. We compare the influence of broadening and reduction of the oscillator strength and study the transition from the so-called strong-coupling to the weak-coupling regime. Theoretical results are compared with pump-probe measurements on samples with remarkably large splitting-to-linewidth ratios. It turns out that carrier-carrier and polarization scattering has to be considered in order to explain the experiments.In most recent publications [12 -14] the theoretical description of QW polaritons in semiconductor microcavities was based on a linear dispersion theory or phenomenological exciton Hamiltonians. The broadening of the e...

Photoluminescence experiments from semiconductor quantum wells inside a microcavity are reported which exhibit a thresholdlike transition already below lasing. A fully quantum mechanical theory for an interacting system of photons and Coulomb-correlated quantum well electrons and holes inside a microcavity is presented. The experimental results previously attributed to bosonic condensation are explained consistently in terms of fermionic electron-hole correlations. [S0031-9007(97)04762-5]

Thank you for the opportunity to respond to Dr. Mannion's letter in which he describes that "epidural spread after posterior lumbar plexus block (PLPB) depends on the approach used," and suggests that the technique used by the author, and described by Capdevila and colleagues, 1 is preferable to the approach used in our patient 2 in minimizing epidural spread and thus adverse hemodynamic consequences, particularly in patients with severe aortic stenosis. Epidural spread with bilateral symmetrical 3 or unilateral 1 anesthesia after PLPB has been reported, but as acknowledged by Dr. Mannion the extent to which epidural spread occurs or factors that contribute to epidural spread after PLPB are still not defined. In our experience, and that of others, this is variable and mul-tifactorial and may be related to the position of the needle prior to the injection, unintended injection close to the intervertebral foramen via a medially directed needle , accidental epidural injection via a misplaced catheter, 4 large volume of local anesthetic used, and the presence of spinal deformity (e.g., scoliosis), 4 etc. Our patient did not have any obvious spinal deformity and we chose the L3 approach with no medial direction of the needle, as described by Parkinson et al., 3 because L3 has the longest lumbar transverse process and allows the injection to be made furthest away from the interverte-bral foramina, thus minimizing epidural spread. Parkinson et al. report bilateral symmetrical anesthesia in 16% of patients using the above approach with an initial injection volume of 0.5 mL·kg-1. 3 Interestingly, although the point of needle insertion, as described by Capdevila et al., 1 is more medial than in our patient, unilateral epidural anesthesia was seen in only 6.5% of patients, 1 suggesting that factors other than the approach may be responsible. Also of note is the lower initial volume and dose (0.4 mL·kg-1 of 0.2% ropivacaine) of local anesthetic used. 1 We used only 15 mL (0.3 mL·kg-1) of ropivacaine 0.5% for the PLPB and there was no clinical evidence of epidural anesthesia in our patient, 2 the drop in blood pressure in our patient notwithstanding. Moreover, the propofol infusion used for sedation during the procedure may also have been partly responsible for the drop in blood pressure. Therefore, before we can recommend a certain approach or technique of PLPB as being superior to another in causing less epidural spread or adverse hemodynamic consequences, further research is required.

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