Exciton polariton emission from a resonantly excited GaAs microcavity

Abstract

Coherent emission efficiency in a 100Å GaAs SQW microcavity was enhanced one order when pumped resonantly at 10 K, compared to the off-resonant excitation. The usual kink observed in the exciton emission linewidth as well as in the emission intensity in relation to the pump power, changes smoothly instead of the usual abrupt kink observed in the off-resonant microcavity laser. In addition, polarization measurements show a correlation relationship between the pump light polarization and the cavity emission polarization.Cavity quantum electrodynamics effects (CQED) in a semiconductor microcavities with a volume of ∼ λ 3 have been studied intensively in the last decade by many groups [1][2][3][4][5]. In contrast to the usual laser, which displays a well defined kink in the input-output (I/O) power curve, there is a dramatic change for this behavior in a cavity with high Q factor, i.e. high mode coupling efficiency (β). This β factor has been enhanced from 1%[1, 2] in a Fabry Perot microcavity laser to 5% in a hemispherical cavity [3], and even more in a whispering gallery mode cavity [4]. In addition, Rabi splitting effects were observed in the cavity reflectance spectrum, characterizing a weak coupling regime in a planar microcavity [5]. All these effects were observed by off-resonant excitation techniques. On the other hand, changing to resonant excitation process, exciton-polariton (e-p) emission was observed for special pump beam incidence angle [6,7,8]. The emission nature of the e-p has been intensively discussed in the literature due to the possibility of generation of exciton condensed states [9][10][11][12], and also to explore the coherence property in a mesoscopic system [13].We have studied experimentally a single quantum well (SQW) GaAs microcavity laser by off and resonant excitation techniques. The white light reflectance, photoluminescence and polarization characteristics of the cavity emission was measured at 10 K. A Rabi Splitting of the cavity mode was observed in the low intensity white light reflectance, and such coupling regime was studied for higher pump intensity, and for different wave vector excitations. The optical properties of the resonantly excited cavity show a non linear laser regime, showing us differences in relation to the offresonant laser. These results are discussed in terms of the exciton polariton emission compared to a bare exciton laser, which depends on the excitation condition angle close or far from the "magic" incidence angle [10,11] of the pump light.The GaAs planar microcavity structure was grown by molecular beam epitaxy (MBE). The cavity is formed by a 100Å GaAs SQW between two spacer layers of Al x Ga 1−x As (x = 0.3) with a thickness of λ/n. The cavity layers are sandwiched between two distributed Bragg reflector (DBR) mirrors formed by 29.5 (below) and 24 (upper) pairs of Al x Ga 1−x As (x = 0.2)/AlAs layers with thickness of λ/(4n) (n stands the respective refraction index of each layer). The cavity was designed in order to get the resonance window around 800nm,...

Bipolariton laser emission from a GaAs microcavity

“…In SMC the polariton laser can operate with almost no population inversion [13], and can reach high values as shown by Yamamoto et al [14]. Our results for the LB emission showed a larger kink in the I/O data curve and also in the linewidth data, compared to our previous behavior observed for the polariton laser [12]. These observations show qualitatively one inversion population behavior for this bipolariton laser.…”

“…The usual kink in the linewidth and the I/O curve at Pth is not clear enough here because the LB mode is too close the LP mode and we could not separate both spectra clearly for lower excitation pump power. The LP emission that was always above the threshold has a value of 100 µeV FWHM whose behavior have been described by Cotta et al [12]. The interesting point here is that the bipolariton linewidth narrowing never reach the polariton linewidth, even in the previous measurement, where the polariton laser emission goes down for larger detuning.…”

“…In the I/O curve should be possible to quantify β by the inverse amplitude of the kink, however our data intensity resolution were limited by the spectrum resolution ( Fig. 1(a)) and it is possible only to preview one β in the order of previous results for microcavity laser [12].…”

Bipolariton laser emission from a GaAs microcavity

“…The usual kink in the linewidth and the I/O curve at Pth is not clear enough here because the LB mode is too close to the LP mode and we could not separate both spectra clearly for lower excitation pump power. The LP emission that was always above the pump threshold has a value of ∼ 100 µeV FWHM whose behavior have been described by Cotta et al [6]. The interesting point here is that the bipolariton linewidth narrowing never reach the polariton linewidth, even in the previous measurement, where the polariton laser emission goes down for larger detuning.…”

“…More details on the sample can be found in Ref. [6]. The experiment was performed using a liquid helium cold finger cryostat held at 7 K. A continuous wave (CW) Ti:Sapphire tunable laser with linear polarization and a variable angle of incidence with respect to the cavity normal direction was used to excite the sample.…”