The new concept of molecular computing based on spectral hole burning, the interaction of molecular energy levels with an externally applied electric field and the interfero-metric properties of holography, is presented. Data stored in the form of 2-D arrays are directly combined in parallel without the use of an external processor.
Spectral hole burning is an important method in high resolution spectroscopy for the investigation of physical properties of molecules embedded in solid hosts at low temperatures.1 Narrowband laser light causes frequency selective photochemical or photophysical transformations: a modified absorption spectrum with a spectral dip—a spectral hole—is created in the inhomogeneously broadened absorption band of the dopant. In most amorphous hosts the lowest allowed optical transitions of organic conjugated molecules have an inhomogeneous width of the order of several hundreds of wavenumbers. At cryogenic temperatures the ratio between inhomogeneous and homogeneous widths may be as high as 106. This number determines the possible increase in experimental resolution, illustrating the potential of this method for scientific research. Spectral holes have been shown to be very sensitive molecular probes and allow, in combination with a suitable detection technique, the study of very small changes in the guest-host systems.
Hole burning spectroscopy has been developed through the application of the holographic technique for the recording and detection of spectral holes.2 Holography as a background-free detection technique has already been applied to study photochemical and photophysical processes in condensed matter. It has also been used to study homogeneous linewidths and the influence of an electric field on the spectral hole shape in different host-guest systems.3-4
Furthermore, spectral hole burning offers fascinating technical applications, especially with respect to future optical storage technology. It appears to be possible to burn 104
or more spectrally well resolved holes within such a broad absorption band. The presence or absence of a spectral hole in a pixel area of ~1 μm2 can be associated with binary
information. The storage capacity of hole burning optical memories can thus be improved by a wavelength multiplexing factor of 104-106 with respect to standard optical memories. To increase the optical storage density beyond 1012 bit/ cm2 would be very attractive, and such an enormous gain in storage capacity has initiated research activities to develop optical storage devices based on spectral hole burning. The effect of an externally applied electric field on hole burning and data storage has also been investigated,5,6 and it has been shown that the data storage capacity is further increased by the addition of the electric field dimension. The frequency and electric field multiplexing properties of spectral hole burning have been combined with the imaging capability of holography to produce image storing devices.7 It has been reported8 that twenty-five holograms can be stored in a single spot of 5-mm diameter, within the range of a single wavenumber. The time and space-time aspects of holography have been explored by Mossberg9 and Rebane et al.10
So far spectral hole burning has been discussed only as a means for future storage technology. The scope of this Communication is to describe the application of spectral hole burning in developing a parallel information processor, a molecular computer, based on the interaction of light with matter and subsequently on the interaction of matter with static electric fields. In such an experiment data are stored in a 4-D space consisting of the two geometrical dimensions of the sample as well as the wavelength and electric field dimensions. Specific data retrieval occurs at a specific wavelength and electric field values and consists of images which represent 2-D data arrays. No moving parts are involved in either writing or reading the information.
An experimental setup for holographic recording and retrieval of image information is shown in Fig. 1. The output from a tunable single-mode dye laser (CR 599-21) operating with the laser dye DCM was expanded by a telescope and split into a reference and object beam. Slides with a size up to 30 mm could be inserted into the object beam. The object wave was focused on the photocathode of an image intensifi-er interfaced to a video camera (Hamamatsu C-2400-25). The object beam was overlapped with the reference beam at the sample, which was immersed in liquid helium in a bath cryostat (Oxford MD 10). Holograms were recorded by simultaneously exposing the sample to object and reference beams and by adjusting the laser frequency and electric field applied to the sample to specific values. Each hologram is thus associated with a specific value of the electric field and of the laser frequency. Reconstruction of an image was performed by illuminating the sample with the reference beam alone. Specific images were selected by choosing the laser frequency and electric field strength used during recording. A small part of the diffracted light was directed to a photomultiplier PMT2 to measure the integrated diffracted image intensity. The output of the video camera was digi-
Fig. 1. Experimental setup for holographic image recording: BS, beam splitter; S1-S5, electronically controlled shutters; HV, high voltage. Image reconstruction is performed by stopping the object
beam (S2) and opening S3,S4, and S5.
Fig. 2. Arrangement of two holograms in the frequency-electric-field plane used for image superposition. The images were recorded at the different electric field positions marked by solid circles. An electric field splits the holes, and the Stark components overlap at
the regions marked by the rectangles.
tized by a DATACUBE image processing system connected to a Sun 3/150 work station.
A polyvinylbutyral(PVB) film doped with chlorin (2,3-dihydroporphyrin) was prepared as described previously.3
Chlorin in PVB was chosen as a guest-host system because a splitting of the hole is obtained when the electric field is applied in the direction of light propagation. Samples of excellent optical quality could be prepared. Furthermore, the homogeneous linewidth11 of the S1 ← S0 transition at 635 nm is comparatively narrow for this class of material, ~160 MHz at 1.7 K. The doped polymer film was annealed between two glass plates with conducting layers. The thickness of the polymer film was 90 μm, and the resulting optical density was 0.8 in the maximum of the absorption band. A voltage range of ±1000 V could be applied to the sample corresponding to an electric field range of approximately ±100 kV/cm.
The behavior of spectral holes in an electric field is well understood.3 If an electric field Es is applied to the sample, the shape of a spectral hole burnt at Es = 0 is changed due to the transition frequency shifts caused by interaction of the dipole moments and polarizabilities of the guest molecules with the externally applied field. The frequency-electric field plane allows pairs of holograms to be stored at the same optical frequency and at slightly differing electric fields as schematically represented in Fig. 2. The separation of the burning coordinates is such that the hole contours overlap within an accessible frequency and electric field range. The maxima of the Stark components are indicated as dashed lines and the burning coordinates as solid circles. At the burning coordinates the recorded images can be reconstructed separately. In the regions where the dashed lines intersect, marked by squares, the individually recorded holograms are reconstructed simultaneously. This results in a coherent superposition, the nature of which depends on the relative hologram phases selected during the recording process. Thus, by controlling the hologram phase during recording, constructive or destructive interference of the hologram is observed. The concept is based on the properties of holography, especially the recording of phase information and its significance during recording and reconstruction of spectral holes in an electric field. The fundamental principles for holograms created by two plane waves have been described in detail previously.13,14 Here we show experi-
Fig. 3. Superposition of image information by means of spectral hole burning and holography. Images (A), (B) reconstructed at the recording positions at different values of the electric field E1,E2 as shown in Fig. 2. The image reconstructed at the overlap region shows the result of the superposition: constructive interference (C) for a phase difference of zero and destructive interference (D) when a phase difference of π was chosen during the initial recording of the
holograms.
mental results for holograms of 2-D objects; in this case interference between neighboring holograms results in logical operations between data arrays appearing as the image.
Two different images have been recorded at different positions of the electric field. A horizontal bar was stored at the position (v,E1) and a vertical bar at (v,E2). Both images can be reconstructed individually by adjusting the experimental parameters used during recording. The experimental results are shown in Figs. 3(A) and (B). The reconstructed images correspond exactly to the originals, and no crosstalk is observed. The superposition of the images can be con-
structed at the frequency, v1 or v2, and the electric field (E1 + E2)/2. The logical operation performed depends on the relative phase used during the recording process, and the result shown in Fig. 3(C) is the image superposition for a phase difference of 0. Constructive interference leads to an increase in the intensity where the images overlap. In Fig. 3(D) the result is shown for a phase difference of Π. Due to destructive interference a decrease in the intensities is observed in the overlapping region. The intensities in Fig. 3 are given by the following four tables representing 3- X 3-pixel arrays of normalized hologram efficiencies:
A value of 4 appears in C since the amplitudes rather than the intensities add in the interferometric holographic operation. From these efficiencies it is straightforward to derive truth tables by looking at corresponding pixel elements and employing suitable discrimination. The first two truth tables were obtained from C and D with a discriminator level of 0.5, the third is derived from C with a level of 2.5.
The logical operations performed correspond to the OR, XOR, and AND functions. It is easy to see that these operations are performed in a fully parallel way on the basis of the individual pixels. Experiments with arrays of 4 X 5 elements have already been successfully performed, and operations involving more than 1000 pixels simultaneously are at present being studied. Whereas the electronic processing of data is based on the properties of electrons in an electric field, the molecular processor introduced here relies on spectroscopy—the behavior of molecular energy levels in an electric field. The molecular system becomes a parallel information processor—a molecular computer.
These experiments demonstrate clearly that inhomogen-eously broadened systems bear a huge potential—not only with respect to data storage—but also in information processing.
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