JPS6218913B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and a recording medium for holography. More specifically, it relates to four-level materials useful for two-photon holography. These materials are particularly useful for holographic recording using continuous wave lasers. One example of such a recording medium is formed from a transparent resin containing a tetrazine compound.

The state of the art is described in the following references:

1 “Multiphoton Photorefractive Processes
for Optical Storage in LiNbO ₃ ,” D. von der
Linde, AMGlass and KFRodgers, Appl.
Phys. Lett. 25 , p. 155 (1974). 2 “High-sensitivity Optical Recording in
KTN by Two-photon Absorption, “D.von
der Linde, AMGlass and KFRodgers,
Appl.phys.Lett. 26 , p.22 (1975). 3 “Optical Storage Using Refractive Index
Changes Induced by Two-step
Excitation,” D. von der Linde, AMGlass
and KFRodgers, Jour.of Appl.Phys. 47 ,
p.217 (1976). 4 “Formation of Integrated Optics
Components by Multiphoton Photorefractive
Processes,” C.M. Verger, N.F.Hartman and
AMGlass, 30 , p.272 (1977). 5 U.S. Patent No. 3922061 “OPtical
Storage Apparatus and Process”, A.M.
Glass and D. von der Linde, Bell Telephone Laboratories The two-photon recording medium used in this prior art was limited to single-crystal pyroelectric materials.
These materials have the disadvantage that the hologram will disintegrate within a few months unless a heat fixing step is performed after exposure. Moreover, the efficiency of hologram formation by two-photon processes in these materials is relatively low. In addition, all of these conventional techniques require the use of high-power pulsed lasers to form holograms, and GaAs lasers and HeNe
It has not been possible to record holograms using convenient continuous wave (CW) lasers such as lasers. Finally, because these materials are single crystals, it has been difficult to produce samples of arbitrary size. No prior art suggests using a two-photon four-level system to record holograms. Nor does any prior art suggest the use of tetrazine compounds in the production of holograms.

In accordance with the present invention, a recording method for holography in systems exhibiting photochemical or photophysical changes as a result of four-level two-photon processes will now be described. One example of such a four-level two-photon system is provided by a transparent resin containing a tetrazine compound. The four-level two-photon system has three novel features that do not exist in any other recording medium currently available for two-photon holography. First, unlike the previously described three energy level two-photon systems, the present system has four energy levels involved in the photochemical process. As explained below, this
It allows a significant number of intermediate state particles to be formed using modest illumination powers, making two-photon holography possible at milliwatt cw power levels. The second is the insensitivity of two-photon materials to erasure by the reading beam. This enables the accumulation of recordable information.
Third is the possibility of gating the holographic recording process on or off using an additional light beam.

The recording medium of the present invention is formed from a system exhibiting a four-level two-photon process. The energy levels of such a system are shown in FIG. The hologram is
It is recorded as a result of photochemical or photophysical changes in the recording medium that occur after two-photon excitation to state B ^* . These changes result in corresponding changes in the refractive index or absorption coefficient. State B ^* is excited by the following two-step process. In the first stage, the system in the ground state A absorbs a photon of frequency ω ₁ and is converted to the state A ^* . This condition undergoes cascade relaxation,
Create a metastable state B. In the second stage, state B can relax to the ground state A with a lifetime τ or absorb a photon of frequency ω ₂ . Absorption of this second photon creates state B ^* , from which photochemical or photophysical processes arise.

Two methods of recording holograms are possible. The simplest configuration uses two interfering laser beams to form interference fringes. The laser frequency is selected such that both the first and second stages are pumped efficiently. In this case, ω ₁ =ω ₂ . The speed of the writing process in this case is proportional to the square of the laser intensity. This is because two laser photons must be absorbed for a photochemical reaction to occur. The second method uses three beams. A hologram is formed by the interference of two of these beams.
These two beams must be coherent. They may be selected to maximize the speed of either the first or second stage. Assume that the hologram is created in the second stage. A third beam, which does not need to be coherent, also irradiates the sample. This beam is optimally selected to maximize the first stage. FIG. 2 shows the case just described, in which the hologram interference fringes are created at ω ₂ and the recording medium 1 is uniformly illuminated with a beam at ω ₁ . From the standpoint of radiation of _ω2 , the recording medium 1 is ω
₁ radiation is "sensitized" or "turned on" or "gated" by the presence of radiation.

The two-photon nature of photochemical reactions provides additional degrees of freedom for holographic applications, allowing the fabrication of temporally gated or spatially modulated holograms with little sensitivity to the readout beam. do. ω ₁ ≠ ω ₂ and ω ₁ and ω ₂ are chosen such that ω ₁ efficiently pumps only the first step and ω ₂ efficiently pumps only the last step of the photochemical process. Assuming that , a hologram can only be efficiently recorded when both frequencies of radiation are present simultaneously. The hologram resulting from the interference of two ω ₂ beams is therefore efficiently recorded only when the recording medium is exposed to the ω ₁ beam. Once the hologram is recorded and the ω ₁ beam is turned off, the hologram becomes relatively insensitive to further deleterious exposure by the ω ₂ low power readout beam. For example, ω ₁ = ω
Even if ₂ = ω and ω is chosen to pump both stages of the optical process, the hologram can be made substantially insensitive to exposure during readout. This is the intensity of the photochemical reaction 2
This is due to the use of a low-intensity readout beam that takes advantage of the power dependence. Time gating is achieved by cutting the ω ₁ beam, and spatial modulation is achieved by utilizing the non-uniform ω ₁ beam.

The scheme outlined in FIG. 1 can be used to define a set of criteria for selecting a two-photon four-level system for holographic applications using CW lasers. The following criteria must be met: That is, (1) the intermediate level A ^* must be efficiently excited by light at ω ₁ ; A molecule in state A must therefore have a large cross section (σ>10 ⁻¹⁸ cm ² ) for the absorption of ω ₁ radiation.

(2) A fast (10 nsec or less) decay mechanism with high quantum efficiency must exist between levels A ^* and B. When ω ₁ radiation pumps the A→A ^* transition, a significant cascading excitation of level B must occur.

(3) Level B must be metastable with respect to decay back to ground state A and have a lifetime τ of at least 10 μsec. The gradual decay of the excitation of this level acts as a bottleneck, and a significant fraction of the total number of molecules can be temporarily accumulated in this level.

(4) The final state B ^* must be efficiently excited by light at _ω2 . Therefore, molecules in state _B have a large cross section (σ>
10 ^-18 cm ² ).

(5) Levels A, A ^* and B must be relatively inert within the medium.

(6) Level B ^* must undergo photophysical or photochemical changes that result in holographic recording with reasonable quantum efficiency.

For specific applications, more specific requirements can of course be given. For example, in applications requiring gate operation, it is desirable that ω ₁ ≠ ω ₂ .
In the case of semiconductor laser applications, it is desirable that ω ₂ be in the near-infrared region.

An example of the recording medium of the present invention that satisfies these requirements regarding four-level two-photon photochemistry is a transparent resin containing a tetrazine compound. Polyvinyl carbazole has proven convenient as the transparent resin, although other known transparent resins also give good results.
Items that fall into this category include polystyrene,
There are novolac type phenolic resins and acrylic resins such as polymethyl methacrylate. What is contained in the transparent resin is a compound having a tetrazine ring structure. Particularly useful are 3,6-dimethyl-1,2,4,5-tetrazine and 3,6-dimethyl-1,2,4,5-tetrazine.
3, such as dibutyl-1,2,4,5-tetrazine;
It is a 6-dialkyl-substituted tetrazine. The concentration of the tetrazine compound in the transparent resin is about 0.1 to about
Within the range of 10 ^-5 g/cc, preferably 0.05 to 0.1 g/cc
It is within the range of cc. Tetrazine should be uniformly distributed throughout the resin. The interference fringes of the hologram are recorded by spatial modulation of the tetrazine concentration after exposure to two or three laser beams of frequencies ω ₁ and ω ₂ . Note that ω ₁ may be equal to ω ₂ .

The tetrazine-transparent resin recording system has two novel features. The first is the sensitivity of the intermediate state to red light. This allows the hologram to be written at a frequency close to that of the HeNe cw laser. The hologram can thus be read using a HeNe laser, minimizing distortions due to differences in read and write frequencies. The second novel feature is the organic nature of the material and its compatibility with room temperature polymeric hosts. This makes it possible to inexpensively and easily manufacture recording media of any size.

Photodissociation of tetrazine occurs as a result of a four-level two-photon photochemical process illustrated in FIG. The tetrazine molecule in the ground state 2 absorbs a photon of ω ₁ and moves to the excited singlet state 3 in the normal configuration 6. This excited state 3 is a new intermediate molecular configuration.
configuration) 7 without radiation. The lifetime τ of this intermediate configuration is approximately 1 msec, so this level 4 acts as a bottleneck and collects a significant number of molecules even when pumped at modest cw power levels at frequency _ω1 . When the ground state 4 of the intermediate configuration 8 absorbs a photon of ω ₂ and moves to the dissociatively excited state 5, a permanent photochemical reaction occurs. That is, when tetrazine is N ₂ and dimethyl dielectric, it is converted to CH ₃ CN. This photochemical reaction causes permanent changes in the absorption coefficient and refractive index of the system. In this way, the hologram
It is produced by forming an interference pattern on the sample using either _{ω 1} or ω ₂ frequencies.

The absorption band corresponding to the first step in this process is shown in FIG. It will be apparent that the first stage, the regular configuration transition, is most effectively pumped by light of wavelengths between 500 and 580 nm, and can be pumped by light of wavelengths up to 600 nm. It is known that the absorption for the second or intermediate configuration transition is similar to that of the first stage.

By comparing Figures 1 and 3, it can be seen that the photochemistry of tetrazine provides an example of a four-level two-photon photosystem. Holograms are recorded on tetrazine-containing media using a cw laser and milliwatts of power. Sensitivity to cw red light is
It enables the production of holographic optical elements compatible with cw laser light sources such as HeNe lasers. for example,
2 to record the interference fringes formed by the interference of the raw output beam of a cw laser with a focused beam of the same frequency as the HeNe laser starting at 632.8 nm.
Using photon processes it would be possible to create focusing elements. Since long coherence lengths can only be obtained using a CW laser, the CW recording capability makes it possible to record large holograms or obtain a large depth of focus in the hologram image. Of course, holograms can also be recorded with this system using similarly high-power pulsed lasers.

The organic nature of the material, its compatibility with room temperature polymeric hosts, and the permanence of two-photon photochemistry make it useful for storing large amounts of archival information. A large amount of recording media can be manufactured inexpensively and easily as tapes, disks, drums, etc.

The recording medium described in this invention is clearly superior to conventional two-photon materials. The permanence of photochemistry makes it possible to produce permanent holograms without a fixing step, and in the case of tetrazine, polymer
The host makes sample production flexible and economical. However, the main advantage is the highly efficient two-photon photochemical process that allows two-photon holography at cw power levels. This increase in efficiency is due to the novel structure of the material's energy levels. In particular, in the case of tetrazine, the existence of an intermediate configuration with a lifetime of about msec, which is created by decay from the excited state of the regular configuration, is a significant difference between the lifetime of the intermediate state and the first
The two-photon process is greatly enhanced by eliminating the usual trade-off between the absorption cross-section for photons of

In summary, the present invention provides for the first time a two-photon four-level holographic recording method. The method consists of the following steps. That is, (1) A single beam of light with a frequency ω ₁ is irradiated onto the four-level holographic recording medium.

(2) Expose the recording medium to light with a frequency of ω ₂ , thereby forming a holographic interference pattern in the recording medium and changing the refractive index or absorption coefficient of the recording medium.

A hologram can be formed using only two interfering laser beams when ω ₁ =ω ₂ .

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of a general four-level two-photon process,
FIG. 2 shows a situation in which the hologram is created by two coherent laser beams of frequency ω ₂ and the recording medium is uniformly illuminated by a third beam of frequency ω ₁ ; The figure is a diagram of a four-level two-photon photochemical process related to photodissociation of tetrazine, and FIG. 4 is a diagram showing the absorption band of tetrazine corresponding to the first stage of the photochemical process. A...ground state, A ^* ...excited state, B...metastable state, B ^* ...state that causes photochemical changes.

Claims (1)

[Scope of Claims] 1. A ground state, a first excited state that can be generated from the ground state by absorption of light with a frequency of ω ₁ , and a first excited state that quickly arises from the first excited state and returns to the ground state. a second excited state with a slow transition; and a third excited state that is generated from the second excited state by absorption of light with a frequency of ω ₂ and spontaneously undergoes an irreversible chemical or physical change. A holographic interference pattern is formed in the recording medium by irradiating a holographic recording medium containing a compound having a two-photon four quasi state with a light beam of frequency ω ₁ and a light beam of frequency _{ω 2} . Holographic recording method. 2 A recording medium for two-photon holography made of a transparent resin containing a tetrazine compound in an amount of 10 ^-5 to 0.1 g/cc.