Lasing threshold

Figure 10-14. Inset Phololumincsccncc spectrum for low excitation pulse energy EP Main part (a) displays the spectrum for pump pulse energies well below the lasing threshold while (b) shows the spectrum obtained lor excitation with a pump energy close to the lasing threshold (c) presents the single mode-lasing spectrum emitted when the device is pumped well above threshold. The dashed lines indicate the zero line which is arbitrarily shifted in case of (b) and (c).

Figure 10-15. Output vs. input energy characteristic of our laser device. The horizontal dashed curve indicates the zero line. A clear laser threshold behavior at an excitation pulse energy ol 1.5 nJ is observed. Below the lasing threshold only isotropic phololuminesccncc is entitled. Above threshold the device emits low divergence single mode laser emission perpendicular to the surface, as schematically shown in the inset. The laser light is polarized parallel to the grating lines.

Figure 12.10b depicts the emitted spectra from the m = 0 laser for various pumping levels above the lasing threshold. An SEM image of the resonator is shown in Fig. 12.10a. The emitted light consists primarily of a single wavelength...

Fig. 12.10 (a) SEM image of the circular Bragg nanocavity designed to support the m 0 mode in the 300 nm wide central pillar, (b) The evolution of the emitted spectrum from the device shown in Fig. 12.9a as a function of the pump intensity. Inset L L curve, indicating a lasing threshold of Pth 900 pW. (c) Calculated modal intensity profile of the nanocavity, (d) IR image of the emitted beam profile... 

A second measurement series illustrates the lasing action of the dye droplet, where consecutively loaded droplets are pumped at different average pump power. The measured spectra shown in Fig. 17.7 show the dye droplet output power vs. pump power. In the measurement sequence, the pump power was first increased from 150 to 900 mW and subsequently decreased again. The reproducibility of the obtained spectra and the lasing threshold are seen from Fig. 17.7, respectively. The experiments reveal no significant indication of neither bleaching nor evaporation. 

In our experiment, we first use R6G and LDS722 as the donor and acceptor. It is important to characterize the FRET signal of this pair in a linear regime below the lasing threshold to provide a performance reference for the OFRR FRET laser that we will investigate later. Figure 19.10 shows the characterization of their FRET behavior for varying acceptor/donor concentration ratios when the donor is excited with a low power CW laser at 532 nm. As shown in Fig. 19.10a, in the absence... 

Fig. 19.12 Characteristics of the nonradiative Forster transfer, (a) FRET spectra show that e 100%, 91%, and 63%, respectively, when the pump intensity is below, near, and above the R6G lasing threshold, (b) FRET spectra show a decreased s (61%, 41%, and 34%) when R6G concentration increases while LDS722 concentration and the pump power remain the same, (c) LDS722 emission as a function of its concentration. Reprinted from Ref. 20 with permission. 2008 International Society for Optical Engineering...

Films of this material can be optically pumped to induce amplified spontaneous emission at 535 nm, as shown in Figure 14. The lasing threshold (Eth) h the pump energy at which amplified spontaneous emission is observable, and depends strongly, among other factors, upon the lifetime of the polymer excited state. A longer excited state lifetime allows more emissive excitons to build up in... 

To overcome the lasing threshold, the light amplification has to be larger than unity. For this reason, the loss from the cavity has to be small. Whereas the length of one round-trip in the largest crystals we are using is 3 p.m, it is about... 

Fig. 9. The energy diagram of the C3 laser illustrates its Lunability by current injection. As explained by Tsang, the current to one of the half-lasers elevates it above its lasing threshold. Thus, its resonant modes are fixed as indicated in...

Lasing Threshold—The lowest excitation level at which a laser s output is dominated by stimulated emission rather than spontaneous emission. 

Ip is degassed and then distilled into a previously evacuated 23 cm long quartz cell with wedged (1.5°) anti-reflection coated windows epoxied on the ends. The cell is mounted in the laser cavity on X-Y-Z translation stages, and the I2 is frozen into the sidearm by dipping it in+a cold bath. The dye lasing threshold is measured, and the Ar laser is then set to the desired power. For one set of experiments, threshold pump power is near ( 11%) 550 mW, while for a second set the threshold is near 790 mW. 

C5.3 Gain coefficient and lasing threshold in GaN-based lasers C5.4 Theoretical and experimental results on GaN-based lasers... 

As in any other laser, the lasing threshold in a semiconductor laser diode is reached when the gain of the active material overcomes the losses of the laser cavity. These losses have two basic origins, namely the finite reflectivity of the mirrors mid distributed losses due to scattering and parasitic absorption in the active medium. In contrast to other lasers, the mirrors in typical semiconductor lasers are simply formed by cleaved or etched crystal facets. Therefore, the reflectivity (Fresnel reflectivity) is rather low, about 20% in the case of the nitrides. 

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