Spontaneous emission factor in oxide confined vertical-cavity lasers

Abstract

We report on measurements of the spontaneous emission factor for oxide-confined InGaAs vertical cavity surface emitting lasers. The spontaneous emission factor is determined as a function of the active layer volume from the measurement of small-signal harmonic distortion at threshold. For a 3ϫ3 m oxide aperture device we obtain spontaneous emission factor of 4.2•10 Ϫ2 at room temperature. © 1997 American Institute of Physics. ͓S0003-6951͑97͒03501-8͔The possibility of controlling spontaneous emission in semiconductor microcavities has attracted considerable attention due to predictions of essentially thresholdless lasing and enhanced modulation bandwidth. 1 The spontaneous emission factor ␤ has been measured for buried mesa VCSELs with dielectric mirrors, 2 gain-guided VCSELs, 3,4 optically pumped lasers with planar resonators, 5 and for air-post mesa VCSELs at low temperatures. 6 The ␤ factor of oxideconfined VCSELs was estimated from the spectral and angular width of the spontaneous emission. 7 The results reported thus far differ significantly from structure to structure and the measurement conditions, with the highest values of ␤ ϳ10 Ϫ2 observed at low temperatures. 5,6 The enhancement in the value of ␤ is expected to significantly affect the laser performance when the lateral dimensions of the cavity are reduced to less than 1 m. 8,9 Unfortunately, VCSEL size scaling is limited by increasing optical loss and consequently the threshold carrier density. In addition, the enhancement of spontaneous emission in a microcavity formed by distributed Bragg reflectors is limited by the finite width of the reflection band and the optical field penetration into the mirrors. 10 Oxide confined VCSELs are characterized by significantly reduced internal loss and have already demonstrated record performance levels in terms of the threshold current and power conversion efficiency. 11 Lasers with the lateral cavity dimensions below ϳ1 m appear feasible.Devices used in our measurements are based on three InGaAs quantum wells in the active region, and are designed to emit at 980 nm. Two quarter-wavelength Ga 0.02 Al 0.98 As layers, one above and one below the active region, are partially oxidized to form the current aperture. Devices with the aperture sizes in the range of 3-25 m exhibit threshold currents from 0.27 to 3.3 mA, and threshold voltages of 2.6-1.4 V.The conventional method of estimating the ␤ factor is based on the rate equations fitting the measured light-current ͑L-I͒ curve. 12 When applied to VCSELs, this technique is likely to be inaccurate. In very small devices, the temperature of the active region, and therefore the threshold and efficiency, are dependent on the drive current as a result of self-heating, and these effects require careful compensation. Larger index guided devices lase in multiple transverse modes ͓see Fig. 1͑b͔͒, and the modal distribution is also dependent on the drive current, even though calculations usually assume it to be constant.An alternative technique is used here to determine ␤. It i...

Enhancement of spontaneous emission factor in selectively oxidized vertical-cavity lasers with double oxide layers

“…y l.6x 10.2, 5.8x10 , and 1.5x103 are obtained from (20) for 3x3, 5x5 and' lOx 10 .tm2 VCSELs, respectively, using an equivalent radius to get the same area as the square aperture. The numerical results agree very well with the measurements of 5x103, and 1x103 for the 5x5 and lOx 10 pm2 VCSELs [12]. For the 3x3 .trn2 VCSEL, the measured value of 4.2x 102 is about 2.5 times as larger as the value predicted from our model, this disagreement can be attributed to the accuracy of the cavity size as indicated in [12].…”

“…For the VCSELs with the double 40-nm outside and 25-nm inner oxide layers, as in Fig. 1, and with double Ai4 oxide layers as in [12], we have the index step AN= 0.0225, 0.0745, and 0.164, respectively. In Fig.…”

“…Under the effective index approximation, we model the lateral waveguide of the selectively oxidized VCSELs as a cylindrical waveguide in the cylindrical polar coordinate with the following refractive index distribution , (12) where a is the radius ofthe oxide confined current aperture. Under the weak guiding approximation AN<< Na, the wave functions can be expressed in terms of linearly polarized LPIM modes by Bessel function J1 and The cut-off condition w-0 (ii = v v) yields (13) where 0 is the azimuthal angle, k 2it!X is the free space wave number, is x-direction propagation constant, and (14) (15) (16) for LP11 modes.…”

“…Verical-cavity surfaceemitting lasers (VCSELs) are good candidates for the investigation and realization of microcavity laser characteristics. The spontaneous emission factor of VCSELs has been measured based on the rate equation fitting the measured light-current curve [6][7][8][9], small-signal harmonic distortion at threshold [10][11][12], and other techniques [1 3-15]. Theoretical and experimental results show that y is inversely proportional to the laser cavity volume, and reducing the cavity volume is the main method to increase y.…”

Photonic crystal nanobeam lasers

“…The second factor accounts for coupling to all (non-lasing) cavity modes that have a spectral overlap with the emitter. This coupling, either due to splitting of mode degeneracy [13][14][15] or large mode volume, 18,19 has been the origin of low β factor. This second contribution can be greatly suppressed in small mode volume photonic crystal nanobeam cavity that has a free spectral range (spacing between the lasing mode and the closest cavity mode) much larger than the half-linewidth of the electron transition.…”