  Biological lasers have attracted vast attention because of their potential
medical application prospects, especially the low threshold biological laser,
which can be used for ultrasensitive biological detection while ensuring that
its luminous gain medium is not damaged by the high-energy pump light. By
coupling the low concentration green fluorescent protein (GFP) solution with a
high Q whispering gallery mode microbubble resonator, we managed to fabricate a
miniature GFP laser with ultralow lasing threshold of 500 nJ/mm^2. The energy
used to excite the GFP can be reduced to 380 fJ, two orders of magnitude lower
than that of the lowest excitation energy GFP laser known. The Q value of the
optical cavity in this biological laser is 5.3 x 10^7, the highest among GFP
lasers at present. We further confirmed the long-term stability of the working
characteristics of GFP laser for the first time and found that its optical
characteristics can be maintained for at least 23 days. Finally, we measured
the effects of different concentrations of fluorescent protein on the laser
threshold. The data show that this biological laser can be used for a highly
sensitive detection of GFP concentration.
