The muscle you are referring to is known as your "VMO" and is recruited maximally at the very bottom of full squats as well as during terminal knee extension in a Petersen Step Up (google these).
By far one of the most effective recruiters of the superior and inferior fibers ( upper and lower parts) is the stationary split squat . . as you descend, press your hamstring onto the calf so it looks the dramatic lunge at the bottom. If you cannot keep your front foot flat on the floor while doing this, place it on a low riser until you have the requisite flexibility . .
4x8/8 at a 4010 tempo, rest 60 s between legs should more than suffice if done every 5 days.

Yes leg extensions are great for the tear drop but they are also pretty bad for your knees. Half rep walking lunges are pretty good too.
If you are going to leg extensions don't go too heavy. Stick in the 15 rep range and get a good 1-2 second squeeze at the top. Your legs will be crying in not time ;)

Me personally bloke, along with leg extension, I hit the machine leg press but I adjust the foot plate....Some people thinks it doesn't do much, but it hits muscles there that only really get hit with stuff like hill climbs.

Thats if your gym has a leg press that can be adjusted like that.

See how it goes for you if you can!

2 things that have helped me develop my teardrop are doing wide leg presses and turning my feet outwards when doing leg extensions.