METHOD AND APPARATUS FOR DETECTING STRAIN SENSITIVE FEATURES IN A SAMPLE

Abstract: The invention of this application relates to a method and apparatus for detecting a strain-sensitive spectral feature in a sample. The method comprises the steps of providing a sample (12) containing a strain-sensitive spectral feature, generating a first modulating field of radiation in the sample, propagating a second field of radiation (10) through a region of the sample containing a well-defined wavefront of the first radiation whereby the amplitude or phase of the second radiation is modulated, and analysing the modulated radiation emerging from the sample so as to detect modulations representing the existence of strain-sensitive features in the sample. The invention is characterised in that the first modulating field is an ultrasonic field of a frequency greater than 1 MHz, and the second field of radiation is a light beam with a spot size smaller than the ultrasonic wavelength and with a spectral linewidth narrower than that of the spectral feature expected to be detected.

Description: The invention of this application relates to the detection of strain-sensitive spectral features in samples. Strain-sensitive spectral features in samples are portions of optical absorption lines that shift, split, or broaden under the influence of an applied ultrasonic field. One class of strain-sensitive spectral features are spectral holes, which consist of narrow depressions or dips in the inhomogeneously broadened absorption lines of centres in solids at low temperatures. Persistent spectral hole formation, or hole burning, has been produced by photochemical processes in organic and inorganic systems as well as by nonphotochemical or photophysical mechanisms in glasses and crystals. In addition to providing important basic information about guest-light and guest-host interactions, persistent spectral holes can be used to store digital data in a frequency domain optical storage system, described in United States Patent Specification No. A - 4,101,976. Because holes burned on nanosecond time scales are usually shallow (see for example, Romagnoli, et al., Journal of the Optical Society of America B: Optical Physics, Vol. 1, 341 (1984)), high sensitivity methods for the detection and/or observation of spectral holes at a high speed are essential. A variety of optical techniques are available for the detection of specific spectral features including spectral holes. The most elementary methods include transmission spectroscopy and fluorescence excitation with narrow band tunable lasers. These techniques suffer from the limitation that they do not have zero background, so that detection of shallow holes is limited by the ability to accurately remove large baselines. Other detection techniques have been devised for the detection of weak spectral features which utilise indirect, external modulation to achieve high sensitivity and/or zero background. For example, frequency modulation (FM) spectroscopy (described in United States Patent Specification No. A - 4,297,035) phase-modulates a probing laser beam before passing the beam through the absorbing sample. In fact, FM spectroscopy has been applied to the detection of spectral holes with zero background. Because the signal appears as amplitude modulation of a laser beam at MHz frequencies where laser noise fluctuations are only due to shot-noise, this method can show quantum-limited sensitivity. However, FM spectroscopy requires an electro-optic modulator for the production of frequency modulated light to probe the sample transmission. In addition, residual amplitude modulation hampers the application of FM spectroscopy in some cases. External modulation methods like FM spectroscopy, including amplitude modulation, wavelength modulation, frequency modulated polarisation spectroscopy (described in United States Patent Application Serial No. 06/511593 filed July 7, 1983) and polarisation spectroscopy also suffer the following shortcoming. Any perturbation of the carefully prepared probing beam by any optical element in the system other than the sample will produce spurious background signals. The frequency modulated polarisation spectroscopy technique overcomes this problem to an extent, but this method is complex and requires anisotropic holes. For example, FM spectroscopy suffers from spurious signals due to any frequency-dependent transmission present in all optical elements between the modulator and the detector other than the sample such as Fabry-Perot resonances in windows, lens coatings, etc. Polarisation spectroscopy is sensitive to low frequency laser power fluctuations and background birefringence. The basic fact is that with the external modulation detection methods described above, all frequency-dependent absorptions and dispersions are detected including effects that arise from reasons other than the property of the sample under study. Ultrasonically modulated electron paramagnetic resonances have been reported in J. Phys. C. Solid State Vol 13, p 865 (1980). In this non-optical technique, effects of 40 KHz strain fields were compared with 40 KHz magnetic fields. Incoherent, non-phase-sensitive ultrasonic modulation of persistent spectral holes has also been reported in Appl. Phys. Letter Vol 43, page 437 (1983). The method used incoherent pulses of ultrasound to modulate persistent holes. The paper emphasised that this technique can be used as a phase-insensitive optical detector for ultrasound in solids. A paper entitled "Detection of persistent spectral holes using ultrasonic modulation" by A L Huston and W E Moerner in the Journal of the Optical Society of America, Volume 1, No 3, June 1984 pages 349 to 353 describes a method and apparatus for detecting a strain-sensitive spectral feature in a sample. In the arrangement described a first modulating ultrasonic field produced by chopping or gating an 8MHz rf signal at a rate of 5-10kHz is used to irradiate the sample. A second field of radiation consisting of a light beam is propagated through the sample and is modulated by the first field of radiation. The modulated radiation emerging from the sample is analysed so as to detect modulations representing the existence of strain-sensitive features in the sample. The size of the light beam is not described in this paper. The object of the invention is to provide an improved method and apparatus for detecting strain-sensitive spectral features in a sample. The method of the invention comprises the steps of providing a sample containing a strain-sensitive spectral feature, generating in the sample a modulating field of ultrasonic radiation of a frequency greater than 1 Mhz, propagating a light beam with a spot size smaller than the ultrasonic radiation wavelength and with a spectral linewidth narrower than that of the spectral feature expected to be detected, through a region of the sample containing a well defined wavefront of the ultrasonic radiation whereby the amplitude or phase of the light beam is modulated, and analysing the modulated light beam emerging from the sample so as to detect modulations representing the existence of strain-sensitive features in the sample. The method of the invention is characterized in that the analysis of the modulated light beam includes phase-sensitive detection at the frequency of the ultrasonic radiation. The apparatus of the invention comprises first means for generating in the sample a modulating field of ultrasonic radiation of a frequency greater than 1Mhz, second means for propagating a light beam with a spot size smaller than the ultrasonic radiation wavelength and with a spectral linewidth narrower than that of the spectral feature expected to be detected, through a region of the sample containing a well defined wavefront of the ultrasonic radiation whereby the amplitude or phase of the light beam is modulated, and means for analysing the modulated light beam emerging from the sample so as to detect modulations representing the existence of strain-sensitive features of the sample. The apparatus of the invention is characterised in that the means for analyzing the modulated light beam employs phase-sensitive detection at the frequency of the ultrasonic radiation. The method and apparatus of the invention utilize direct modulation of the features in synchrony with an ultrasonic strain field. This phase-sensitive ultrasonic modulation method involves the generation of an ultrasonic field with well-defined wavefronts in a sample containing a strain-sensitive spectral feature. The spectral feature may be, for example, a spectral hole formed by photochemical hole-burning or nonphotochemical hole-burning. The ultrasonic field may be generated by a transducer which is bonded to the side of the sample so that the wavefronts of the ultrasonic wave are oriented parallel to the light beam direction. The spot size of the light beam must be smaller than the ultrasonic wavelength in order to achieve phase sensitivity, and the linewidth of the light beam must be narrower than that of the spectral feature. Modulation of the probing light beam occurs when the ultrasonic wave shifts or splits the absorption line shapes of the various centres contributing to the spectral feature thereby changing the shape or wavelength of the feature in synchrony with the ultrasonic field. The resulting modulation of the frequency or phase of the light beam or of emitted fluorescence is detected, signifying the presence of the spectral feature. In order that the invention may be more readily understood an embodiment will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic view of the essential elements in apparatus embodying the invention used to detect spectral features in a sample, and Figure 2 illustrates the signals that are generated by the presence of photochemical holes in a sample examined using apparatus as illustrated in Figure 1, in particular the sensitivity of the signals to the phase of the ultrasonic field used in the apparatus. The essential elements of apparatus in accordance with the invention are shown in Figure 1. The output beam 10 of a tunable light source 11 impinges on a sample 12 which includes a strain-sensitive spectral feature. The strain-sensitive spectral feature may be a spectral hole or a narrow absorption line of an impurity that shifts, splits, or broadens in an ultrasonic field. For example, the strain-sensitive spectral feature may consist of a spectral hole in the absorption line of colour centres, ions, or molecules in crystals, glasses, or polymers at low temperatures. The light source is a laser or another source of light: the only requirement is that the linewidth of the light source is less than the width of the spectral feature of interest. A well-defined ultrasonic field is be generated in the region of the sample 12 so that it can be probed by the light beam 10. This ultrasonic field is generated either by a piezoelectric transducer 14 that has been bonded to the sample 12 by methods known in the art, or by any other device or method that generates ultrasonic waves, such as photoacoustic generation. In fact, the ultrasonic wave may be generated directly in the sample itself if the sample is made of a piezoelectric material, for example, poly(vinylidene fluoride). The critical requirement is that during any instant of the ultrasonic period, only one phase of ultrasound is present in the region of the sample probed by the light beam 10 . For instance, if the ultrasound is longitudinal, the light beam 10 must overlap with compression and rarefaction alternately during each cycle of the ultrasonic wave. Similarly, if the ultrasound is a shear wave, the light beam 10 must overlap with positive and negative shear alternately during each cycle of the ultrasonic wave. To achieve this, the beam 10 impinges on the sample 12 with a spot diameter less than the ultrasonic wavelength. For example, for sodium fluoride crystals and 8 MHz ultrasound, the laser spot diameter should be smaller than 760 microns. For optimum signal, the laser spot should in fact be less than one-half the ultrasonic wavelength; however, phase-sensitive signals will result from any spot size less that the ultrasonic wavelength. As a specific example, an ultrasonic transducer 14 can be bonded to the side of the sample 12 so that the wavefronts of an ultrasonic wave generated in the transducer 14 are oriented parallel to the laser beam direction. To achieve maximum signal in this geometry, reflections of the ultrasound from the opposite side of the sample must be prevented from altering the well-defined phase in the region of the sample probed by the light beam. This may be achieved in any of several ways: for example the entire experiment may be performed in a time less than the round-trip time of ultrasound in the sample. Another possibility would be to prepare the sample with ultrasonic matching materials to prevent ultrasonic reflections from the opposite side of the sample. A third possibility would be to establish a standing wave resonance of the ultrasound in the sample. A further possibility would be to utilise a lossy sample material so that the ultrasonic field decays before reflections can reach the region of the sample probed by the light beam. Apparatus other than that shown in Figure 1 can also be used to implement the invention. For example, if the sample 12 consists of an ultrasonically lossy thin film such as a polymer, the light beam 10 can be propagated through one of the smaller faces of a transparent nonabsorbing prism, with the sample bonded on to the hypotenuse of the prism. The ultrasonic field can then be generated by a transducer bonded to the other small face of the prism. Another possible arrangement involves bonding a thin sample directly to the polished reflecting face of the transducer. The light beam can then be propagated through the sample, reflected off the face of the transducer, and propagated through the sample again before travelling to the detector. To excite the transducer 14 or other ultrasonic source and provide a phase reference, an RF oscillator 16 generates an RF signal, for example an 8 MHz RF signal, which is transmitted to the transducer 14 along a line 18. An example of the transducer 14 is a 8 MHZ x-cut quartz transducer. When spectral holes or other strain-sensitive spectral features are present in the sample 12 at the wavelength of the light beam 10, the ultrasonic wave passing through the transducer 14 shifts, splits, or broadens the absorption line shapes of the various centres in the sample 12 contributing to the spectral feature, thus changing the shape or the wavelength of the feature. This leads to a modulation of the light beam 10 at the ultrasonic frequency, for example at 8 MHz. A modulated light beam or a beam of fluorescence emerging from the sample 12 is detected by a high speed detector 24, for example a photodiode or a photomultiplier. The amplitude modulation is phase sensitively detected using a double balanced mixer 20 and phase shifter 22 as shown in Figure 1, which is known in the art. The current output of the detector 24 is combined with a phase-shifted local oscillator signal in the mixer 20 to yield a resultant phase-sensitive signal 26. A key feature of the apparatus described is that, since the detection of the amplitude modulation of the light beam is performed at RF frequencies, the technique can be quite fast, limited only by the acoustic transit time across the light beam spot and the period of the ultrasonic field. For example, with 8 MHz ultrasound travelling at 0.5 x 10 sup 6 cm/s and a light beam spot diameter of 500 microns, the ultrasonic wave completely traverses the laser beam in 100 ns. In this case, the ultrasound traverses the light beam in a time comparable to the period of the 8 MHz ultrasound, 125 ns. As is usual, the modulation cannot be detected in a time less than the ultrasonic period. So, in this example, the spectral feature can be detected in a time of the order of 125 ns. Example 1 Since spectral holes in inhomogeneously broadened lines are by their very nature strain sensitive, such spectral holes can be easily detected by using the ultrasonic modulation method described below. A sample was composed of X-ray irradiated NaF containing F₃⁺ colour centres, and spectral holes were produced with a single-frequency laser. The apparatus shown in Figure 1 was used with one minor modification. The RF signal at 8 MHz driving the transducer 14 was pulsed at 10 KHz at duty cycles from 5% to 50% and the output of the mixer 20 was detected with a boxcar averager with 100 m sec time constant. The ultrasound was pulsed because no effort was made to reduce reflections from the opposite side of the sample.The ultrasonic reflections reduced the size of the signal for times longer than 20-50 microseconds after the start of the ultrasonic wave. This pulsing of the ultrasound was not essential. Continuous wave excitation of the transducer also produced signals representing the detection of strain sensitive spectral holes at the output of the mixer 20. The resulting signals representing photochemical holes are shown in Figure 2. The horizontal axis corresponds to one laser scan over 2 GHz in frequency. The vertical axis indicates the signal in arbitrary units, for example volts at the output of the boxcar averager. Curve 40 is the detected hole signal with 0 phase shift as selected by the phase shifter 22. Curve 42 is the detected hole signal with 180° phase shift, where the inversion in sign of the signal proves that the signal is sensitive to the phase of the ultrasonic wave. Curves 40 and 42 have been offset for clarity; they both have a zero direct current value. Similar inversions in signs can be observed if the experiment is repeated with the laser beam translated on the sample in a direction parallel to the ultrasonic propagation vector. The irradiation of the holes with ultrasound does not appreciably erase the holes, even with irradiation periods of time lasting 10-20 minutes or more. It has been observed that the background due to acousto-optic interactions in the host crystal is negligible due to the extremely low ultrasonic powers required for these tests. The detection method described above has a number of advantages. The method uses direct internal modulation of only the effect under study, has zero-background, high sensitivity and simplicity , and is applicable to all types of spectral holes simply because spectral holes are intrinsically sensitive to strain. Although a preferred embodiment has been described, it is understood that numerous variations may be made in accordance with the principles of the invention as claimed.

Claims: A method of detecting a strain-sensitive spectral feature in a sample comprising the steps of providing a sample (12) containing a strain-sensitive spectral feature, generating in said sample a modulating field of ultrasonic radiation of a frequency greater than 1 Mhz, propagating a light beam (10) with a spot size smaller than the ultrasonic radiation wavelength and with a spectral linewidth narrower than that of the spectral feature expected to be detected, through a region of said sample containing a well defined wavefront of said ultrasonic radiation whereby the amplitude or phase of said light beam is modulated, and analysing the modulated light beam emerging from said sample so as to detect modulations representing the existence of strain-sensitive features in said sample, characterised in that said analysis of the modulated light beam includes phase-sensitive detection at the frequency of said ultrasonic radiation. A method as claimed in Claim 1 characterised in that said light beam spot size is less than half the ultrasonic radiation wavelength. A method as claimed in claim 1 or claim 2 characterised in that said light beam is propagated parallel to the ultrasonic wavefronts. A method as claimed in any one of the preceding claims characterised in that the amplitude or phase modulation is detected in a time less than the ultrasonic round-trip transit time in the sample. A method as claimed in any of the preceding claims characterised in that said sample (12) is prepared so that no reflections of the ultrasound occur. A method as claimed in any of the preceding claims characterised in that said sample (12) emits fluorescence in response to the incident light beam, and modulation of the emitted fluorescence caused by the ultrasonic field is detected. Apparatus for detecting a strain-sensitive spectral feature in a sample (12) comprising first means (14, 16, 18) for generating in said sample a modulating field of ultrasonic radiation of a frequency greater than 1 Mhz, second means (11) for propagating a light beam (10) with a spot size smaller than the ultrasonic radiation wavelength and with a spectral linewidth narrower than that of the spectral feature expected to be detected, through a region of said sample containing a well defined wavefront of said ultrasonic radiation whereby the amplitude or phase of said light beam is modulated, and means (20, 22, 24) for analysing the modulated light beam emerging from said sample so as to detect modulations representing the existence of strain-sensitive features in said sample, characterised in that said means for analysing the modulated light beam employs phase-sensitive detection at the frequency of said ultrasonic radiation. Apparatus as claimed in Claim 7 characterised in that said light beam spot size is less than half the ultrasonic radiation wavelength. Apparatus as claimed in claim 7 or claim 8 uncharacterised in that said first means comprises an external transducer (14) bonded to said sample (12). Apparatus as claimed in any one of claims 7, 8 or 9 characterised in that said sample (12) is of a piezoelectric material and said first means generates the ultrasonic field directly in said sample.

Inventors: HUSTON, ALAN LEE, MOERNER, WILLIAM ESCO
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: G01L1
IPC codes: G01L1, G01N29