Apparatus and method for investigating a sample

An apparatus for investigating a sample, the apparatus comprising:

• means for irradiating the sample with a first beam of electromagnetic radiation configured to excite an optically non-linear process within the sample such that radiation with a frequency of less than that of the first beam is generated within the sample;
• means for irradiating the sample with a second beam of electromagnetic radiation;
• and a detector for detecting a change in the second beam after it has been reflected from or transmitted through the sample.

An apparatus according to any preceding claim, further comprises inducing means for inducing an effective second order non-linearity from a third order non-linearity.

An apparatus according to any preceding claim, wherein the apparatus further comprises means for investigating an area of the sample and imaging means for producing an image of the said area of the sample.

An apparatus according to any preceding claim, wherein the first and second beams have a frequency in the range from 100 THz to 1000 THz.

An apparatus according to any preceding claim, wherein the first and second beams are pulsed beams.

An apparatus according to any preceding claim, wherein both the first and second beams are linearly polarised.

An apparatus according to any preceding claim, wherein the first beam is polarised at a different angle to that of the first beam.

An apparatus according to any preceding claim wherein the first and second beams overlap at a point in the sample which is to be investigated.

An apparatus according to an preceding claim, wherein the apparatus further comprises means for deriving the refractive index of the sample.

An apparatus according to any preceding claim, wherein the detector comprises means for separating the polarisation of the second beam into orthogonal components and means for measuring the relative magnitudes of the two orthogonal components.

A method for investigating a sample, the method comprising the steps of:

• irradiating the sample with first beam of electromagnetic radiation configured to excite an optically non-linear process within the sample such that radiation with a frequency of less than that of the first beam is generated within the sample;
• irradiating the sample with a second beam of electromagnetic radiation;
• and detecting a change in the second beam after it has been reflected from or transmitted through the sample.

An apparatus as hereinbefore described with reference to any of the accompanying figures.

A method as hereinbefore described with reference to any of the accompanying figures.

An apparatus for investigating a sample, the apparatus comprising:

• means for exciting a second order optically nonlinear process in a predetermined region of the sample;
• means for irradiating the sample with a second beam of electromagnetic radiation, the second beam passing through or close to the predetermined region;
• and a detector for detecting a change in the polarisation of the second beam after the second beam has been reflected from or transmitted through the sample.