Photonic mode recording

The next stages in the development beyond 20 Gb will be to move away from heat mode recording to photon mode recording. This is discussed elsewhere in this book, under photochromies (Chapter 1, section 1.2.8.3) and under holography, optoelectronics and photonics in Chapter 5. 

Photochromic materials have been developed in order to dramatically increase the memory density. These materials can be used for photon-mode recording, which is based on the photochemical reaction of the medium. In photon-mode recording, light characteristics such as wavelength, polarization, and phase can be multiplexed in data storage and thus can, in a potentially dramatic ways, increase the memory density. 

T. Nagamura, H. Sakaguchi, S. Muta, and T. Ito, Ultrafast photon-mode recording by novel photochromic polymer via photoinduced electron transfer, Appl. Phys. Lett. 63, 2762-2764 (1993). 

Comparison between heat-mode and photon-mode processes is given in Table I. The main differences are the superior resolution and the possibility of multiplex recording in photon-mode systems. Because of the diffusion of heat, the resolution of heat-mode recording is inferior to that of photon-mode systems. Furthermore, photons are rich in information such as energy, polarization and coherency, which can not be rivalled by heat-mode recording. 

Table I. Comparison between heat- and photon-mode image recording...

Research on liquid crystalline polymers(LCP) is a fashionable subject with the goal of developing speciality polymers of superior mechanical and thermal properties. Besides these properties, other interesting properties of LCP have not been fully utilized. We are trying to use thermotropic LCP for photon-mode image recording material. 

The great merit of thermal irreversibility is the permanent nature of the states. Therefore, fulgides have long been viewed as potential candidates for photon-mode optical recording materials. In addition, fulgides have been used as prototypes to demonstrate their potential applicability as photoswitchable functional materials. Those switch models that had appeared up until the end of 1999 are described in this chapter. 

A photochromic compound is characterized by its ability to alternate between two different chemical forms having different absorption spectra in response to light of appropriate wavelengths. Photochromic materials are promising as recording media for optical memory, because the media store erasable/ rewritable data in photon mode. Because the data-recording mechanism is based on the photochemical reaction of each molecule, extremely high spatial resolution is expected. 

Tsujioka, T, Tatezono, F., Harada, T., Kuroki, K., and Irie, M. Recording sensitivity and superiow-power readout of photon-mode photochromic memory. Jpn. f. Appl. Phys. 33, 5788, 1994. 

HEAT-MODE AND PHOTON-MODE IMAGE RECORDING (7)... 

Comparisons between heat-mode and photon-mode processes are given in TABLE 1. The main differences are the superior resolution and the possibility of multiplex recording in photon-mode systems. 

Because of the diffusion of heat, the resolution of heat-mode recording is inferior to that of photon-mode systems. 

Furthermore, photons are rich in information such as energy, polarization and coherency, which cannot be rivalled by heat-mode recording. 

LCP is a suitable candidate for this purpose. The merits of photon-mode and heat-mode recording may be combined by the aid of... 

Sequential recording, also known as double kinetic mode [353] or time-lapse recording , adds one or two additional dimensions to the photon distributions recorded by multidetector operation and multiplexing. Controlled by its internal clock oscillator, the sequencer switches through a specified number of memory blocks. Each memory block contains the photon distributions of all detectors and multiplexing channels. Sequential recording in a multidetector system is illustrated in Fig. 3.7. For sake of simplicity, multiplexing has been omitted. 

Spectroscopy of single molecules is based on fluorescence correlation, photoncounting histograms, or burst-integrated-lifetime techniques. Each case requires recording not only the times of the photons in the laser period, but also their absolute time. Modem time-resolved single molecule techniques therefore use almost exclusively the FIFO (time-tag) mode of TCSPC. The FIFO mode records all information about each individual photon, i.e. the time in the laser pulse sequence (micro time), the time from the start of the experiment (macro time), and the number of the detector that detected the photon (see Sect. 3.6, page 43). 

A mueh higher burst resolution ean be obtained by reeording the photons in the FIFO or time-tag mode. The time-tag mode is deseribed under Sect. 3.6, page 43. From the time-tag data, BIFL results with a burst resolution down to the laser pulse period ean be obtained. MCS traees are available, and FCS and PCHs can be ealeulated. Beeause the full information about all photons is recorded time-tag data are extremely flexible. Conformational dynamics, rotational relaxation, and intersystem erossing ean be investigated at almost any time scale [108, 154, 155, 295, 419, 500]. However, time-tag data are also voluminous. For each photon four or six bytes are reeorded, and file sizes of a gigabyte per measurement are not unusual. 

Tamaoki N, Song S, Moriyama M, Matsuda H. 2000. Rewritable full color recording in a photon mode. Adv Mater 12(2) 94 97. 

The single photon detector - typically an avalanche photo diode (APD) driven in counting mode- detects the arrival of fluorescence photons. The recorded photon trace is finally evaluated according to the chosen FFS method, i.e. the sequence of detection events is numerically processed for yielding information about the investigated sample. 

Since then, fulgides have been regarded as prime candidates for apphcation as photon-mode rewritable recording media. ... 

A number of requirements, such as durability, diode laser sensitivity, efficient photoreactivity, etc, are necessary to make use of the thermally irreversible photochromic compounds as photon-mode rewritable optical recording media. Among these requirements, the challenging problem is to make the recording system have a nondestructive readout method. Several methods using fulgides have been proposed to date. 

Soft X-ray absorption measurements are done at low-energy synchrotron X-ray facilities such as the UV ring at NSLS or the Advanced Photon Source (APS) at Lawrence Berkeley National Laboratory (LBNL). The beam size is typically 1 mm in diameter. The electron yield data are usually obtained in the total electron yield (EY) mode, measuring the current from a channel electron multiplier (Channeltron). Sometimes a voltage bias is applied to increase surface sensitivity. This is referred to as the partial electron yield (PEY) mode. Huorescence yield (EY) data are recorded using a windowless energy dispersive Si (Li) detector. The experiments are conducted in vacuum at a pressure of 2 X 10 torr. 

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