Technical details

Experimental setup


Schematic of our experimental set-up. External cavity diode laser is used as a probe for the confocal microscope in interferometric configuration.  One arm on the interferometer is used to actively stabilize the path length using a piezo and proportional integrator (PI) lockbox. 

Graphene is actuated by d.c. gate voltage while SiNx being heavier in mass is actuated with an external laser beam incident at an angle. Modulated signal from confocal microscope is detected using a balanced photo-detector and analyzed using a spectrum analyzer or lock-in amplifier. a.c. gate voltage is applied to parametrically drive the system.

Sample image


SEM image of our sample where graphene drums (G1 and G2) of 20 µm diameter are deposited on a SiNx surface. Lower panel shows cartoon image of our sample. Here X1, X2 and X3 are detection points on graphene drums and SiNx surface respectively when the sample is kept in the interferometric configuration and probed with a laser. 

Interaction between graphene and SiNx can be tuned


Fundamental mode of both the graphene drums can have resonance coupling with many high quality factor SiNx modes but not coupled to each other directly. Similarly, coupling among SiNx modes exist through graphene modes mediated interaction.

Graphene resonators having low mass and high tension have mode frequency which can be tuned by applying dc. voltage. Nevertheless, high mass SiNx mode frequency remains unchanged with increasing dc. voltage. Spectra detected on SiNx for increasing gate voltage shows effect of interaction of high quality factor SiNx modes with graphene modes. However, graphene drums does not interact directly but via SiNx modes. Varying gate voltage, graphene SiNx interaction strength can be tuned.

Instrumentation

PI lockbox: In one arm of the interferometric configuration of the experimental set-up, we use piezo and homemade PI lockbox circuit to reduce ambient vibrations by actively stabilizing the path length.  Also, the PI lockbox is used to lock the frequency of a laser using beat-note technique. 



Temperature and current controller: Also, we make temperature and current controller circuits to bring the laser frequencies to the desired level. 


External Cavity Diode Laser(ECDL): We bought laser components like laser diode [780 nm], TEC, thermistor,  piezoelectric chips, reflective gratings etc from Thorlabs and assemble them in our lab to build ECDL.