Understanding the principle of photo diode signal generation

the electrons excited by photons in the process generates a photo current,

I'm with you so far.

Each photon absorbed in the detector generates an electron-hole pair, which is quickly swept out of the depletion region, forming a photocurrent.

and these charge carriers now oscillate like an antenna to emit microwave frequency

I think you have cause and effect reversed here.

You illuminated the photodiode with two lasers. These two lasers will create an interference pattern on the surface of the photodiode, although this pattern will be changing much too fast for the eye to see.

If the laser emission frequencies are separated by some difference $Delta f$, then the electric field at the photodiode surface will be fluctuating at this same rate. Seen at the quantum level, the rate of photon absorption will be fluctuating at this frequency also.

If $Delta f$ is in the microwave region (300 MHz - 300 GHz or so) then the rate of photon absorption will be varying at a microwave frequency (It typically takes special care to tune two lasers to be as near in frequency to each other as a few GHz). If the diode capacitance isn't high enough to damp out this variation, you'll also see this frequency in the photocurrent.

If you set up an oscilloscope to measure the photocurrent, and the oscilloscope is fast enough to measure microwave frequencies, then you should see a (quite noisy) microwave signal on your oscilloscope screen. Given the random fluctuations of frequency you're likely to see in this experiment, it might make more sense to measure the signal with a spectrum analyzer rather than an oscilloscope.

Main point: the current doesn't start oscillating after the absorption occurs. The rate of absorption depends on the interference between the two laser beams, and the photocurrent is just proportional to the photon absorption rate.