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the kwisp ( kinetic wisp detection ) force - sensor consists of a thin ( 100 nm ) dielectric membrane suspended inside a resonant optical fabry - perot cavity @xcite .
the collective force exerted by solar chameleons bouncing off the membrane surface @xcite will cause a displacement from its equilibrium position which can be sensed by monitoring the cavity resonant frequency .
since , in addition , the membrane is a mechanical resonator , the displacement sensitivity is enhanced by the mechanical quality factor of the membrane . for a detailed description of the kwisp force sensor see @xcite .
an absolute calibration of the kwisp sensor in terms of force has been obtained in the infn trieste optics laboratory by applying a known external force supplied by the radiation pressure of a laser beam ( _ pump beam technique _ ) .
this external force is modulated at a given frequency allowing one to explore the frequency region near the mechanical resonance of the membrane . here
we obtain a force sensitivity already at the 300  k thermal limit @xcite . in order to effectively use the kwisp sensor for chameleon detection
it is necessary to find a means of modulating the amplitude of the expected chameleon beam . by exploiting the ability of chameleons to reflect off any material surface when impinging at grazing incidence , and to correspondingly traverse it when at normal incidence @xcite
, we have originally devised and built a _
chameleon chopper _ prototype .
the chopper allows one to shift the expected chameleon signal away from the noisy region near zero frequency , eventually reaching , with a suitable high frequency chopper , frequencies near resonance .
we have used the prototype chopper , working at frequencies below 200 hz , for preliminary solar chameleon search runs , also taking advantage of the fact that the kwisp membrane orientation in space is such , that a hypothetical chameleon beam from the sun will reflect off it at grazing angles between 0 and 20 degrees for about 1.5 hours each day . in the following
we will briefly describe the sensor setup , the results from absolute calibration measurements , the chameleon chopper prototype and its use in preliminary solar runs .
the main element of the kwisp force sensor is a vacuum chamber containing an 85 mm long fabry - perot cavity made with two 1-inch diameter , 100 cm curvature radius , high - reflectivity , multilayer dielectric mirrors .
a si@xmath4n@xmath5 , 5x5 mm@xmath6 , 100 nm thick membrane is inserted inside the cavity and it is initially placed approximately midway between the two cavity mirrors ( _ membrane - in - the - middle _ configuration ) . the fabry - perot cavity is excited using a cw 1064 nm laser beam emitted by a nd : yag laser . a second , frequency doubled , cw beam at 532 nm emitted by the same laser is used as an auxiliary beam ( _ pump beam _ ) for alignment and for exerting a known external force on the membrane . when the sensor is in detection mode
the fabry - perot cavity is frequency locked to the laser using an electro - optic feedback loop @xcite .
the error signal generated by this loop is proportional to the instantaneous frequency difference between laser and cavity and its power spectrum contains the information on membrane displacements .
the pump beam , amplitude - modulated at a given frequency , is then injected into the cavity and it exerts a known force on the membrane by reflecting off it .
the intensity of the pump beam corresponds in our case to a net force of @xmath7 n. the presence of this force is detected as a peak in the measured spectrum of the error signal .
the membrane behaves as a mechanical oscillator and its fundamental resonant frequency and quality factor can be directly measured with the pump beam technique .
figure [ fig : resfreq ] shows a plot of several power spectra of the feedback loop error signal .
the peaks indicate the presence of the calibration force , while different peaks correspond to different excitation frequencies .
the peak with the largest amplitude occurs when the the pump beam modulation frequency matches the membrane mechanical resonance freqeuncy .
the background level in figure [ fig : resfreq ] gives a force sensitivity of @xmath2 , corresponding to @xmath3 in terms of displacement .
these values correspond to the thermal limit at 300 k @xcite .
to investigate the possible presence of a signal from a beam of chameleons emitted by the sun , it is necessary to impress a time modulation on it .
this can be done by exploiting the general chameleon property of traversing any material when impinging on it at right angles , and of reflecting off it when arriving at grazing incidence ( see @xcite for details ) .
we have designed and built a prototype chameleon chopper ( see figure [ fig : chamchop ] ) exploiting this property .
hz , corresponding to a chopping frequency of @xmath8 hz , as a grazing incidence surface is presented to the incoming beam 4 times each turn.,scaledwidth=50.0% ]    the chopper was placed in the proper position in order to intercept a hypothetical solar chameleon beam hitting the membrane at grazing incidence angles between 0 and 20 degrees , depending on the time of day .
data were then acquired by recording 40 s long power spectra of the feedback loop error signal .
a partial preliminary analysis of the solar data was conducted by computing for each spectrum the signal - to - noise ratio and by plotting the snr as a function of time .
a sample plot of this type is shown in figure [ fig : snr ] .
the kwisp force sensor now running in the infn trieste optics laboratory has been calibrated in absolute terms using a known force exerted by an auxiliary pump beam .
the measured sensitivity of @xmath9 , corresponding to @xmath3 in terms of displacement , is already at the 300 k thermal limit .
the _ chameleon chopper _
concept has been implemented in a working prototype @xcite .
this was used in combination with the force - sensor to conduct preliminary runs fro the detection of an hypothetical chameleon beam emitted from the sun .
analysis of the data from these runs is in progress @xcite .
the kwisp force sensor , once coupled to the x - ray telescope at cast , has the potential to access unexplored regions in the chameleon parameter space , possibly allowing a first glimpse at the nature of dark energy @xcite .
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the kwisp opto - mechanical force sensor has been built and calibrated in the infn trieste optics laboratory and is now under off - beam commissioning at cast . 
 it is designed to detect the pressure exerted by a flux of solar chameleons on a thin ( 100 nm ) si@xmath0n@xmath1 micromembrane thanks to their direct coupling to matter . 
 a thermally - limited force sensitivity of @xmath2 , corresponding to @xmath3 in terms of displacement , has been obtained . 
 an originally developed prototype chameleon chopper has been used in combination with the kwisp force sensor to conduct preliminary searches for solar chamaleons .