Abstract:
A method and apparatus for enhancing the energy of THz radiation in the form of photon pulses comprising synchronously reflecting photon pulses using at least one pair of facing optical mirrors such that the reflected photon pulses encounter an electron bunch or another photon pulse travelling in the same direction and energy is transmitted from the electron bunch or the other photon pulse resulting in an increase in the energy of the reflected photon pulse.

Description:
The United States of America may have certain rights to this invention under Management and Operating contract No. DE-AC05-84ER 40150 from the Department of Energy. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to terahertz (THz) radiation and more particularly to methods and apparatus for the power enhancement of such radiation. 
     BACKGROUND OF THE INVENTION 
     The THz region (1 THz=33 cm −1  or 4 meV) lies in the far infrared spectral range where conventional thermal sources are very weak. For example, a blackbody source at 2,000° K. provides less than 1 μW per cm −1  of spectral power density for a typical spectroscopy application. A system for the production of multiwatt THz radiation is described in Nature, Vol. 420, Nov. 14, 2002, pp. 153-156. While the production of such high power THz radiation constitutes a significant step forward in the art, even further enhancement, i.e. increase in the THz radiation power for subsequent use in medical diagnostic, security and other similar applications would be highly desirable. Thus, the availability of a method and apparatus that permits increasing the energy of the THz pulse to a value higher than would be available from as single pulse, produced as described in the aforementioned “Nature” article, would be highly desirable. 
     OBJECT OF THE INVENTION 
     It is therefore an object of the present invention to provide a method and apparatus that permits enhancement of THz radiation to an energy level higher than that available from a single THz pulse. 
     SUMMARY OF THE INVENTION 
     According to the present invention there are provided a method and an optical mode based apparatus that permit enhancement of the energy level of THz radiation. Such an apparatus comprises a cavity defined by a pair of facing mirrors oriented to receive and reflect a THz radiation pulse derived from an electron beam comprising a series of short electron bunches such that the interval between subsequent electron pulses or bunches in the beam matches the time it takes for the optical mode (mirrors) to return a reflected THz photon pulse to its point of origin where it encounters a subsequent electron bunch acquiring energy therefrom. Enhancement of the THz radiation occurs through the encounter of the reflected THz radiation pulse and an incoming newly generated electron pulse with energy being transferred from the electron pulse to the THz radiation photon pulse. An apparatus comprising such a pair of facing mirrors that provides multiple reflection of photon pulses therebetween for several synchronized intervals and permits removal of the enhanced (higher energy) THz radiation is also described. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of one preferred embodiment of the THz radiation enhancement apparatus of the present invention. 
     FIG. 2 is a schematic representation of a second preferred embodiment of the THz radiation enhancement apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, the apparatus  10  of the present invention comprises a cavity defined by the volume  12  between a pair of mirrors  14  and  16 . As is well known in the art, THz radiation is generated in an arcuate pattern  18  as photon pulses  19  are extracted from a high energy, short bunch electron beam  21  comprising a series of electron bunches or pulses  20 . According to the present invention, as an electron pulses  20  reaches the apex  22  of its arcuate trajectory  18 , photon pulse  19  is launched in the direction of mirror  14  along trajectory  17 . Photon pulse  19  is reflected from mirror  14  toward mirror  16  as reflected photon pulse  19 A. Photon  19 A is then in turn reflected from mirror  16  back toward apex  22  as photon pulse  19 B. Synchronizing the reflection of photons  19  such that the interval required for a launched photon to leave trajectory  18  at apex  22 , be reflected from mirrors  14  and  16  and be reflected back to apex  22  equals the interval or some integral multiple of the interval between electron pulses  20 , results in a reflected photon pulse  19 B arriving at apex  22  at the same instant that a newly generated electron pulse  20  arrives at apex  22 . This action results in a “reinforcement” or increase in the energy or power exhibited by photon pulse  19  (now photon pulse  19 B) to yield a higher energy photon pulse that can be further circulated within cavity  12  or removed from the apparatus of the present invention and applied. Stated differently, through this relatively simple mechanism, energy from the incoming electron beam (as a short electron bunch) is imparted to the photons with which the electron bunch meets at apex  22  resulting in a the production of higher THz power. As will be apparent to the skilled artisan, although in the preferred embodiment depicted in FIG. 1, only two or a pair of mirrors are used to obtain the required reflection, a plurality of mirror pairs could be used to obtain the same result and such a configuration of multiple mirror pairs is clearly contemplated as within the scope of the appended claims. 
     In the embodiment depicted in FIG. 2, enhancement or increase in photon beam power or energy is achieved through a somewhat different arrangement. In accordance with this embodiment, a photon pulse beam  24  comprising a series of photon pulses  26 , also schematically shown in FIG. 2 as curves  25 , produced externally to cavity  28  that is defined by a pair of facing photon reflecting mirrors  30  and  32  is introduced into cavity  28  through, for example, a one way mirror or other partially transmitting or reflecting mechanism  27  located in an aperture  29  in mirror  32 . Photon pulses  26 / 25  produced by particle beam  24  impact mirror  32  from whence they are reflected back toward mirror  30 , then from mirror  30  back toward mirror  32  etc. etc. until they are removed from cavity  28  through the use of a partially transmitting or reflecting mechanism  34  located across aperture  36  in mirror  30 . In this case, synchronization of the reflected photons such that the interval required for first reflected photons  25  (moving in the direction indicated by arrows  25 A) to reach, for example, the surface  38  of mirror  30  equals the interval or some integral multiple of the interval required for another incoming photon pulse  26  or second reflected photon pulse  25  to traverse the distance between mirrors  30  and  32  such that an incoming photon pulse  26  or a reflected photon pulse  25  arrives at surface  38  at precisely the same instant as a reflected photon pulse  25  resulting in a “reinforcement” or increase in the energy or power exhibited by photons  26  to yield higher energy photons, i.e. higher energy THz radiation, that is extracted from cavity  28  through a partially transmitting or reflecting mechanism  34  in mirror  30 , controlled by a shutter or otherwise. Preferably mechanism  34  is swithchable. Through this relatively simple mechanism, higher energy THz radiation is produced. Since photons pass through each other with little or no interference, the system works in an extremely efficient manner. As will be apparent to the skilled artisan, while a single pair of mirrors is depicted in FIG. 2, a plurality of mirrors or mirror pairs that produce synchronized encounters of reflected photons with incoming photon pulses  26  and/or reflected photon pulses  25  would be equally useful and is intended to be within the scope of the appended claims. 
     There has thus been described a method and apparatus for the production of enhanced or higher power THz radiation. The apparatus is of relatively simple construction and can readily be designed and constructed by those skilled in the particle beam/photon transmission arts. 
     As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.