Laser microscopy

A laser system generates a beam consisting of light of two wavelengths, one of which is in the visible spectrum. The light of the other wavelength is ultraviolet and is generated from the visible light as a harmonic of it. The ultraviolet light is focussed, by reflection only, onto a target. Most of the visible light is separated out of the beam by dichroic mirrors but a very small residue is allowed to illuminate the target so that the exact position at which the ultraviolet light impacts the target can be monitored by means of a microscope.

The present invention relates to laser microscopy in which laser-generated 
light in a non-visible part of the spectrum is focussed in a microscope 
onto a specimen under investigation. This may be, for example, for the 
purpose of ablating a small particle of material for mass-spectrometer 
analysis or for causing a small sample of material to fluoresce for 
spectral analysis as a result of radiation. 
One such application of the invention is in the analysis of the elements 
present in small inclusions (typically in the range of 10-20 microns in 
size) embedded in a material such as quartz. The operator selects the 
inclusion to be analysed and then uses ultra-violet laser-generated light 
firstly to ablate the material above the inclusion itself, the material of 
which is then transferred in a stream of inert gas such as argon to the 
mass-spectrometer. 
Since the ultra-violet light is invisible to the eye, it is difficult to 
obtain accurate registration of the focussed laser beam with the 
inclusion. It has previously been proposed to use a separate source of 
visible light to illuminate the target area but it has not been found 
possible to align this second source sufficiently accurately. 
In accordance with the present invention, there is provided laser 
microscopy apparatus comprising a laser system for generating a laser beam 
comprising light of two wavelengths, the light of one wavelength being in 
the visible spectrum, the light of the other wavelength being outside the 
visible spectrum, the light of shorter wavelength being derived from the 
light of longer wavelength as a harmonic thereof, a first 
wavelength-dependant reflector in the beam path for separating almost all 
of the visible light from the non-visible light so as to produce two light 
beams extending in different directions, and a microscope having a 
reflective, non-refractive objective for forming an image of a specimen 
under investigation, an eyepiece for examining the image and a reflector 
in the microscope positioned to receive the beam of non-visible light and 
to divert it onto the objective for focussing by the latter on a specimen 
under investigation. 
With such an arrangement, a small amount of visible light travels in the 
beam of non-visible radiation and is focussed onto the target with exactly 
the same precision as the non-visible light. This is made possible by the 
fact that there is no refraction of the light along its path from the 
laser to the target specimen. 
Advantageously the reflector in the microscope is a further 
wavelength-dependant reflector positioned obliquely on the optical axis of 
the microscope between the objective and the eyepiece and constructed to 
transmit almost all of the visible light incident upon it both in the 
direction of the beam of non-visible light and in the direction of the 
optical axis of the microscope. 
With this arrangement, the operator sees by means of the visible light the 
exact spot on the target which is being intensely illuminated by the 
non-visible light. Although the proportion of light reaching the user's 
eye after the transmission by the two wavelength-dependant reflectors in 
series is extremely small, it is found to be sufficient to be detected by 
the human eye. 
Conveniently, the objective is of the kind which comprises a convex mirror 
facing the eyepiece and a concave mirror facing the convex mirror and 
having an aperture on the optical axis of the microscope.

The apparatus shown in the drawing comprises a laser oscillator 1 of the 
Md:YAG type which is pumped by flash lamps in the normal manner to provide 
pulses of light of wavelength 1064 nm. In order to increase the intensity 
of the pulses, their duration is shortened in known manner by means of a Q 
switch, for example of the Pockels type. Further, an appropriate aperture 
is used to insure the TEM001 mode. 
The pulsed beam 2 from the laser oscillator 1 is adjusted in size by means 
of a telescope 3 and the 1064 nm light is twice turned through 90.degree. 
by means of appropriate dichroic mirrors 4 and 5 into a laser amplifier 6 
again of the Nd:YAG type which is itself pumped by a further discharge 
lamp (not shown). The emerging beam 7 of high intensity pulses is then 
passed through a first frequency doubler 8 in which the high intensity of 
the pulses results in the generation, effectively by distortion, of green 
light of wavelength 532 nm. 
The beam 9 emerging from the first frequency doubler 8 passes through an 
oblique dichroic mirror 10 which reflects the light of wavelength 1064 nm 
laterally along a path 11 towards a beam stop where it is absorbed. The 
pulses of wavelengths 532 nm continue into a second frequency doubler 12, 
thereby generating pulses of ultra-violet light of wavelength 266 nm. Both 
of the frequency doublers 8 and 12 may be of the potassium dihydrogen 
phthalate type. 
The beam of light 13 leaving the second frequency doubler 12 contains light 
of wavelengths 532 nm and 266 nm. The beam 13 is passed through a further 
oblique dichroic mirror 14 which reflects almost all of the light of 
wavelength 532 nm along a path 15 to a beam stop where it is absorbed. In 
conventional apparatus, a refractive prism would be used in place of the 
mirror 14 as a result of which the beam 16 would contain no light of 
wavelength 532 nm. As a result of the use of a dichroic mirror, however, a 
small proportion of the light of wavelength 532 nm is transmitted in 
addition to the wavelength of light 266 nm. The beam 16 then strikes a 
further oblique dichroic mirror 17 which is arranged to reflect 
substantially all of the ultra-violet pulses of wavelength 266 nm as well 
as a small proportion of the residue of light of wavelength 532 nm. The 
resulting beam 18 is directed onto an oblique dichroic mirror 19 on the 
optical axis of a microscope 20. The mirror 19 directs the ultra-violet 
light downwards on path 21 together with a small proportion of the 
relatively small amount of green light of wavelength 532 nm in the beam 
18. 
The pulse beam 21 is directed down onto a reflecting objective 22 which 
consists of a small convex spherical mirror 23 on the optical axis facing 
upwards and a larger concave mirror 24 facing downwards and having a 
central aperture 25 on the optical axis of the microscope. 
The objective 22 thus focusses, by reflection, the ultra-violet light of 
wavelength of 266 nm (as well as the green light of much lower intensity) 
onto a specimen S under investigation. The focussed green and ultraviolet 
light are inevitably coincident, since no refraction is involved. 
The green light scattered by the specimen S travels back to the objective 
22 in the normal manner and in leaving the objective 22 passes through the 
dichroic mirror 19 and an ultra-violet filter 25 to a binocular eyepiece 
26 of the microscope through which the operator can accordingly determine 
with great precision the point on the sample S which is being eradiated by 
the ultra-violet light. 
In the case of ablation microscopy, the sample S is mounted in an enclosure 
having a silica window to admit the laser light. A turbulent stream of 
inert gas such as argon is directed through the enclosure and thence to a 
mass spectrometer which may be of the inductively coupled plasma type.