Quantum Membrane Experiment

Quantum Membranes - Main People working on the experiment

 

Short description of the experiment

The focus of the group is the optomechanical coupling of a coherent laser beam and the vibrational behaviour of a small membrane. The membrane is of micrometer dimensions and have very pure vibrational modes which enable the coherent excitation of a vibration by light. The excitation is then stored in the vibration for several seconds and it is this coupling between the laser light and the vibrations that is studied.

This optomechanical coupling is so strong, and the micro membrane so small that new physics is available. This enables new ultra precise measurements of the position of the membrane on the order of a billionths of a meter and also presents the possibility of actively cooling the vibration of the membrane down using light, to even achieve a state where the vibration all but stops and the membrane starts to behave in a strongly quantum mechanical manner.

The optomechanical system with the mechanical part constituted by a micromembrane is only one type, and the archtype is a system consisting of two mirrors forming a cavity. One of these mirrors are then allowed to move due to temperature and the interaction with the light.

with a denoting the cavity field photons and the b operator denoting the vibrational phonon modes of the membrane. This expression is true if the laser field is strong. The optomechanical coupling rate is given by

 

GaAs cooling experiment

To investigate the cooling of a vibrational mode, a 160 nm thick GaAs membrane have been manufactured and studied. The membrane is roughly 1 x 2 mm. An optical cavity is created by placing a small curved mirror such that light can be confined between the mirror and the membrane.

The optomechanical system is as described above and as expected for other materials in the literature, but what is different for a membrane made of GaAs is that this material is a crystalline semiconductor with what is known as a direct bandgap. This means that the photons in the laser light can directly excite electrons from the electron gas in the material to a higher energy state by absorbing photons.  The excited electrons then decay back to their original energy by coupling to the vibrational modes of the material and heating the material. Additionally as the electrons are excited from the ground state they take up more space in the crystal lattice introducing local stress. In combination with the optical cavity the stress from the heating and the local expansion leads to a feedback on the macroscopic vibrations of the membrane that can cool them significantly. In the experiment cooling of a vibrational mode down to about 20 K from room temperature is achieved with very small light powers, demonstrating the power of this new cooling mechanism.

The work is described in the article submitted to a peer reviewed journal.

 

 

More information

Quantum optomechanics - throwing a glance. Review article in J. Opt. Soc. Am. B 27, A189-A197 (2010)
Optoelectronic cooling of mechanical modes in a semiconductor nanomembrane . GaAs cooling article.

Optical Cooling of an Electronic Circuit via a SiN Membrane

Radiation pressure cooling of micromechanical resonators in cavity optomechanical setups has attracted great attention during the last decade and impressive reduction of thermal vibrational noise of the mechanical resonators has been reported in several experiments [1]. In terms of the quality of the mechanical resonators used in the experiments, dielectric SiN membranes [2] have proved to be a very suitable choice due to their high mechanical Q-factors(exceeding 10^6 at room temperature!) and low optical absorption which are crucial for the effectiveness of radiation pressure cooling.

 

Motivation and Setup

In our experiment, we aim to use a SiN Membrane(50 nm thick) as a mechanical interface between optical and electrical modes. For this purpose, we want to couple a specific SiN membrane vibrational mode(around 1 MHz) to a capacitor and furthermore place the membrane in a high-finesse optical cavity [2] to couple it to light. In this way, the membrane acts as a mediator between the optical mode in the cavity and the electrical signal in the capacitor which may then be combined with an inductor to act as a resonant tank circuit. Current electronic detection schemes usually suffer from excess noise (amplifier etc) and in some cases sensitive circuits require cryogenic operation to eliminate the thermal noise. The aforementioned combined system may both be used for high sensitive shot-noise limited optical detection of electronic noise via optical readout of membrane’s vibrations and also for optical cooling of electro-mechanical modes in the rf domain. This scheme is expected to pave the way for crucial applications in narrow-band electronic circuits. The simplified version of the setup is shown in the figure to the left. The capacitor has a hole(not shown) in the middle that allows the passage of the optical beam.

 

 

 

Interdigitated Capacitor

For the electrical part of the project, we aim to use an interdigitated capacitor(fabricated at DTU Nanotech) and place the dielectric membrane on it at a specified distance to achieve electromechanical coupling. When one applies a voltage between the electrodes of the capacitor, the field polarizes the dielectric SiN membrane and due to the inhomogenity of the electric field distribution in the close vicinity of the fingers of the capacitor, the membrane feels the so-called Kelvin polarization force that attracts it to the electrodes [3]. This provides a very convenient way of coupling the membrane displacement to capacitance and thereby renders it possible to actuate it with an rf voltage [3,4].

Preliminary Results

Here is our first promising trials with the interdigitated capacitor(with the membrane on it) where we could actuate the membrane by an rf voltage on top of a DC voltage applied to the electrodes of the capacitor. The picture on the left shows the wide rf driven mechanical spectrum of the SiN Membrane (measured by a Vibrometer at DTU).

 

 

The fundamental mode is around 400 KHz as expected from the commercial, high stress membranes(Norcada). The picture to the left from the vibrometer scan shows the (1,2)610KHz eigenmode of the membrane. The very thin interdigitated capacitor fingers can also be seen.

 

Optical Cooling

As for the optical part of the setup, we plan to make use of the membrane in the middle approach [2] and utilize it for nearly shot-noise limited measurement of the mechanical motion and at the same time achieve radiation pressure cooling of the membrane. Passive radiation pressure cooling is a way of cooling where the cooling light is red detuned from the cavity resonance [1,2]. In a quantum mechanical picture, this condition favors the cooling process where mechanical quanta is removed from the membrane and scattered to the optical mode. The main idea of the project is to take advantage of these well established optical readout and cooling techniques and create a bridge to the rf electrical domain via a mechanical interface.

 

References