Abstract:
The invention is a method and apparatus for generating terahertz radiation. The terahertz source is a versatile terahertz device that can be configured to transmit a plurality of wavelengths, thereby facilitating the detection of multiple contaminants using a single source device. In one embodiment, the Smith-Purcell radiation effect is exploited by passing an electron beam over a modulated conducting surface, wherein the spacing of the periods of the modulated surface is varied. The variations in the modulated surface enable the source to produce light of varying wavelengths.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/471,381, filed May 16, 2003 (titled “Terahertz Source and Applications”), and of U.S. Provisional Patent Application No. 60/530,508, filed Dec. 18, 2003 (titled “An Autonomous Rapid Facility Chemical Agent Monitor Via Smith-Purcell Terahertz Spectrometry”), both of which are herein incorporated by reference in their entireties. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention generally relates to the detection of chemical and biological contaminants, and more particularly relates to the generation of terahertz radiation to detect contaminants.  
       BACKGROUND OF THE INVENTION  
       [0003]     There is an increasing demand for systems for military, private or individual use that are capable of detecting and analyzing chemical and biological contaminants, such as explosives (e.g., TNT or DNT). One method of detecting such contaminants uses rotational microwave spectroscopy. The rotational and vibrational modes of molecules (e.g., contaminant molecules) have energies that naturally correspond to energies of photons in a spectrum of radiation. A source generates radiation that interacts with contaminant molecules present in a “target” to be analyzed, so that specific frequencies of emitted light are absorbed by the molecules. A detector is positioned to identify the frequencies that fail to transmit through the target, and the failure of a particular frequency to transmit can indicate the presence of a specific absorbing contaminant.  
         [0004]     The far infrared (or terahertz) spectrum of radiation is particularly well-suited for use in systems such as that described above, because the spectrum corresponds to the vibrational and rotational modes of many chemicals, including explosives, and contains a great deal of signature information. Unfortunately, the effectiveness of conventional terahertz source devices is limited because the devices tend to be fixed such that they can only be used to transmit a very narrow range of frequencies of radiation. This can lead to confusing or incomplete results, because contaminants may often be capable of absorbing a number of frequencies. Thus, the lack of a sufficient spectrum of radiation for analysis purposes may cause some contaminants to be overlooked or misidentified. Generation of a sufficient spectrum of frequencies can therefore require several devices, and this limitation can make the use of terahertz sources complicated and costly.  
         [0005]     Therefore, there is a need in the art for a terahertz source that can produce a spectrum of terahertz radiation for use in the detection of contaminants.  
       SUMMARY OF THE INVENTION  
       [0006]     The invention is a method and apparatus for a terahertz source. The terahertz source is a versatile terahertz device that can be configured to transmit a plurality of wavelengths, thereby facilitating efficient detection of contaminants in a target using a single source device. In one embodiment, the Smith-Purcell radiation effect is exploited by passing an electron beam over a modulated conducting surface, wherein the spacing of the periods of the modulated surface is varied. The variations in the modulated surface enable the source to produce light of varying wavelengths. In another embodiment, a method for generating terahertz radiation using a mode-locked semiconductor laser is provided. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0008]      FIG. 1  illustrates a perspective view of a modulated conducting surface that may be used to produce Smith-Purcell radiation;  
         [0009]      FIG. 2  illustrates a plan view of a variable-period modulated conducting surface according to the present invention, in which the period is “deformed”;  
         [0010]      FIG. 3  illustrates a perspective view of the modulated conducting surface of  FIG. 2 , wherein the modulated conducting surface is mounted on a rotatable cylinder;  
         [0011]      FIG. 4  illustrates top schematic view of a variable-period modulated conducting surface according to the present invention, in which the conducting surface comprises at least two sections having different periods;  
         [0012]      FIG. 5  illustrates a top view of a variable-period modulated conducting surface according to the present invention, in which the conducting surface is tapered; and  
         [0013]      FIG. 6  illustrates a flow diagram of a method for generating terahertz radiation using a mode-locked semiconductor laser. 
     
    
       [0014]     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
       DETAILED DESCRIPTION  
       [0015]     A method and apparatus are provided for generating a spectrum of terahertz radiation. The terahertz source is a continuously tunable electromagnetic wave device that, in several embodiments, exploits the phenomena of Smith-Purcell radiation to produce multiple wavelengths of light. In one embodiment, the Smith Purcell radiation produced is in the far infrared region of the spectrum.  
         [0016]     Smith-Purcell radiation is produced when an electron beam is passed in a vacuum close to the surface of a periodically modulated conducting surface (e.g., a “grating”). The grating essentially “bunches” the beam and causes the beam to radiate. This produces light having a wavelength that is a function of the periodicity of the grating, the velocity of the electrons, and the angle at which the light is observed relative to the direction of the electron beam. At low voltages, light is typically emitted at an angle normal to the grating.  
         [0017]      FIG. 1  illustrates a perspective view of a conventional sinusoidal grating  100  that may be used to produce Smith-Purcell radiation. The conventional grating  100  is static—that is, the periodicity is constant, as is evidenced by the peaks  102  that are each spaced a substantially equal distance d from the next peak  102 . The distance d is substantially constant over the entire length l of adjacent peaks  102 . Furthermore, the peaks  102  are all of substantially equal height h, and the height h is substantially constant along the entire length l of the peaks  102 . Thus, for example, an electron traveling from (z, y)=(0, 3) to (z, y)=(20, 3) along the z axis will encounter substantially the same conditions as an electron traveling from (z, y)=(0, 2) to (z, y)=(20, 2). Therefore, as long as the velocity of a group of electrons passing over the grating  100  remains constant, light having a single, substantially constant wavelength (as observed along a given viewing angle) will be produced.  
         [0018]      FIG. 2  illustrates a plan view of one embodiment of a grating  200  that may be advantageously adapted for use with the present invention. The periodicity of the grating  200  is varied in the y direction to create a “deformed” period. That is, a periodic variation is created in the heights of the peaks  202  along their lengths l and in the distances d n−1 , d n , d n+1 , etc. between the peaks  202 . Thus, for example, an electron traveling from (z, y)=(0, 3) to (z, y)=(20, 3) along the z axis will not necessarily encounter the same conditions (i.e., the same period of grating) as an electron traveling from (z, y)=(0, 2) to (z, y)=(20, 2). Therefore, the varying periodicity of the grating  200  allows a source into which it is integrated to produce light having multiple (or “swept”) wavelengths. This is desirable because swept wavelength outputs can be used to perform spectroscopic identification (i.e., by measuring the differential absorption of a target).  
         [0019]      FIG. 3  illustrates a perspective view of one embodiment of a tunable terahertz source device  300  in which the periodically varied grating  200  illustrated in  FIG. 2  may be used to advantage. The device  300  comprises a modulated cylinder  302 , a drive shaft  304 , and a motor  306 . The modulated cylinder comprises the periodically varied grating  200  illustrated in  FIG. 2 , mounted to and wrapped around a first end  308  of the drive shaft  304 . The motor  306  is coupled to an opposite second end  310 .of the drive shaft  304 . Although the periodically varied grating  200  is mounted on a cylinder  302 , rotatable surfaces having other shapes may also be used to advantage. In use, the motor  306  slowly rotates the drive shaft  304  and cylinder  302  so that when an electron beam passes closely to the rotating cylinder  302 , the electron beam encounters a grating  200  whose periodicity varies with the rotation of the cylinder  302 . As the electron beam passes over the periodically varying grating  200 , the Smith-Purcell light that is produced will vary periodically and continuously in time. Thus, the light generated by the device  300  may be “tuned” to produce various wavelengths of light, while the velocity of the electrons passing over the device  300  remains substantially constant.  
         [0020]      FIG. 4  is a schematic diagram illustrating a second embodiment of a tunable terahertz source device  400  according to the present invention. The device  400  comprises an electron beam source  402 , a deflection yoke  404  and at least two sets of substantially uniform-period gratings  406   a - g  (hereinafter collectively referred to as “gratings  406 ”). Although the embodiment illustrated in  FIG. 4  depicts seven sets of gratings  406 , any number of gratings  406  numbering two or more may be used. In one embodiment, the gratings  406  radiate outward from a common starting point P at various angles, and the periodicity of each grating  406  is different (e.g., in the embodiment illustrated in  FIG. 4 , the “peaks” of grating  406   a  are spaced closely together, while the peaks of grating  406   g  are spaced further apart). The yoke  404  is positioned between the electron beam source  402  and the starting point P of the gratings  406 . The deflection yoke  404  has at least one aperture (not shown), and the deflection yoke  404  is movable so that the aperture may be positioned along the axis of any one of the gratings  406 .  
         [0021]     Thus, when an electron beam is emitted by the electron beam source  402 , it is received by the deflection yoke  404 , and the beam is deflected along a chosen grating  406  (depending on how the deflection yoke  404  is positioned). Thus terahertz source device  400  is tunable to produce electromagnetic radiation in a broad spectrum. In one embodiment, the device  400  produces tunable electromagnetic radiation in the ten micron to one millimeter range of the electromagnetic spectrum.  
         [0022]      FIG. 5  is a top view of a third embodiment of a terahertz source device  500  according to the present invention. The device  500  comprises a grating  502  and an array of emitters  504 . The grating  502  is tapered so that the periodicity of the grating  502  gradually increases from a first end  508  of the grating  502  to a second end  510  of the grating  502 . The array of emitters  504  is configured laterally, so that the emitters form a line that is substantially coplanar with the grating  502 . The array of emitters  504  emits several small electron beams (not shown) that pass closely to the grating  502  in the form of a sheet. In one embodiment, all beams emitted from the array of emitters  504  are emitted at the same voltage. When the electron beams simultaneously encounter the grating  502  having a varying periodicity, each beam radiates at a different frequency so that the device  500  produces a polychromatic spectrum (i.e., the device  500  has output at substantially all frequencies simultaneously).  
         [0023]     In one embodiment, the device  500  includes an optional lens  506  positioned between the array of emitters  504  and the grating  502 . Each individual electron beam produced by the emitters requires a different focusing parameter depending on the periodicity of the grating that it encounters (e.g., a more rapid focus is needed for a beam traveling over a short period than for a beam traveling over a longer period). The lens  506  focuses the electron beams produced by the emitters so that the beams are maintained in close proximity to the grating  502 . The lens  506  has varying optical properties (e.g., focal length) over its surface, and in one embodiment, the lens  506  is an electrostatic or magnetic lens. Therefore, as the electron beams pass through the lens  506 , each beam encounters a different strength lens. Therefore, the lens  506  provides the correct focusing to maintain close proximity between the electron beams and the grating  502  for all periods of the grating  502 , so that maximum output from the device  500  is obtained.  
         [0024]     As illustrated in  FIG. 5 , the lens  506  is angled to provide the correct focusing for each electron beam passing therethrough. To achieve proper focusing for all beams emitted by the array of emitters  504 , the waists for all beams must be at the same z location. The focal length f(x) necessary to properly focus a particular beam is dependent upon the distance from the emitter at which the beam originates to the lens  506 , or o(x), and upon the distance from the lens  506  to the image produced by the beam, or i(x). The proper focal length f(x) may be computed as  
         -         M   ⁡     (   x   )       ⁢           ⁢     z   f           (     1   -     M   ⁡     (   x   )         )     ⁢   2         ,       
 
 wherein 
 
 M(x) is demagnification and is defined as  
       -       i   ⁡     (   x   )         o   ⁡     (   x   )             
 
 and z f  is the z location of the waists of each electron beam. M(x) is always less than zero, so f(x) will always be a positive value. 
 
         [0028]      FIG. 6  is a flow diagram illustrating one method for a fourth embodiment of the present invention, in which multiple wavelengths in the far infrared spectrum are produced by a mode-locked semiconductor laser. Mode-locked semiconductor lasers are chip-based lasers that produce ultra-short optical pulses. The pulses are sufficiently energetic to produce terahertz emissions when the pulses are incident on a target (e.g., an absorbing semiconductor). In general, there are two mechanisms by which these optical pulses can produce a broadband, single cycle terahertz pulse: photoconductive generation of transient current and optical rectification via instantaneous nonlinearity.  
         [0029]     The first mechanism utilizes application of a photoconductive switch (e.g., fabricated on GaAS or silicon substrates), where the generated THz waveform is proportional to the time-derivative of the generated photocurrent. The second mechanism produces a waveform proportional to the second-derivative of the laser pulse. For optical rectification, phase-matched non-linear material (e.g., ZnTe or DAST, among others) is needed for the generation of THz emission. The ultrashort optical drive pulse generates a THz pulse with a broadband spectrum extending from near DC to a value proportional to the inverse of the optical pulse duration (THz range).  
         [0030]     One example of a mode-locked semiconductor laser that may be used to advantage with the method illustrated in  FIG. 6  is an external cavity semiconductor laser such as that disclosed by Gee et al. (“High-Power Mode-Locked External Cavity Semiconductor Laser Using Inverse Bow-Tie Semiconductor Optical Amplifiers”,  IEEE Journal of Selected Topics in Quantum Electronics , Vol. 4, No. 2, March/April 1998).  
         [0031]     As illustrated in  FIG. 6 , a short optical pulse is emitted from a mode-locked semiconductor laser in step  602  to illuminate a target. At step  604 , the pulse reaches the target, and, depending on target geometry, generates a single-cycle THz pulse (e.g., broadband emission extending into the THz range). To achieve high frequency terahertz radiation, the shortest possible pulses are emitted from the laser source at step  602 . The highest achievable frequency is given approximately by the inverse of the optical pulse duration. In one embodiment, compensation optics, such as grating, prisms or grisms, may be incorporated into the laser (at optional step  601 ) to shape either the gain spectrum or the spectral phase (to compensate for gain narrowing or dispersion, respectively), thereby producing a more nearly Fourier transform limited pulse.  
         [0032]     A mode-locked semiconductor laser such as that disclosed by Gee et al. is based on integratable solid-state components and may replace lasers having separate components (e.g., mirrors, gain crystal, etc.) “connected” by use of free-space propagation of light. Therefore, the use of a mode-locked semiconductor laser in the method illustrated in  FIG. 6  can produce significant advantages over conventional ultra-short pulse laser-based terahertz systems, which are typically quite large and consume a great deal of power. A mode-locked semiconductor laser and target can be produced in a more compact and portable form than conventional short-pulse laser systems. For example, in one embodiment, the components of the laser are integrated into a single-chip scale device. Thus, mode-locked semi-conductor lasers also typically consume less power than laser systems typically used for the generation of terahertz radiation. Furthermore, because a mode-locked semiconductor laser is a solid state source (as opposed to the vacuum-based electron sources such as those illustrated in  FIGS. 2-5 ), it may offer advantages for a number of other applications including imaging, communications and spectroscopy.  
         [0033]     Thus the present invention represents a significant advancement in the field of terahertz source technology. A terahertz radiation source is provided that substantially more compact and efficient than existing terahertz sources. Furthermore, in several embodiments, the invention may be tuned or configured to produce multiple wavelengths of radiation, both individually and simultaneously, thereby facilitating more accurate and efficient detection of contaminants in an analyzed target. The present invention may have further advantages in the fields of imaging, communications and spectroscopy.  
         [0034]     While foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.