If that is a photoIC, a sensor which gives 0 or 5 V when it sensed a light, there is nothing you can do to adjust its sensitivy by using electronics components. All you can do is to add a light filters.
I think its output is already schmitt triggered so the output can only swing between 0 and 5 volts.
There is always another option ..
If you can't do much about the receiver (and it looks that you can't) try to reduce current to the transmitter diode and reduce the IR level at that end ..
Hey Keith, thank you for that. I am using an IR sensor to measure the blood flow in the finger, by placing the finger between the sensor and IR LED(transmitter). I need an IR sensor sensitive enough to detect the change in intensity of IR radiation caused by blood flow.
Could you suggest which sensor can i use?