illustrative embodiments of the invention are described below . in the interest of clarity , not all features of an actual implementation are described in this specification . it will of course be appreciated that in the development of any such actual embodiment , numerous implementation - specific decisions must be made to achieve the developers &# 39 ; specific goals , such as compliance with system - related and business - related constraints , which will vary from one implementation to another . moreover , it will be appreciated that such a development effort , even if complex and time - consuming , would be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure . in the course of studying thermochromic nlm reflective limiters described above , it was discovered that placing the thermochromic nlm behind a substrate with particular characteristics of conduction band gap energy and melting temperature would provoke a different physical mechanism resulting in a new class of limiters — the plasma absorption wave limiter (“ pawl ”). in general , the incident energy passes through the substrate to a trigger layer ( e . g . the thermochromic nlm ). if sufficiently intense , the energy will heat the trigger layer . the heat energy then conducts from the trigger layer into the substrate . the substrate material is chosen to have a low energy band gap between its bound state electrons and the conduction band ( free ) electrons . however , the substrate must have a high enough band gap to allow the electrons in the substrate to be bound at normal use conditions so that the material is transmissive in the desired optical pass band of the sensor . when heat conducts into the substrate , its electron population density in the conduction band will increase rapidly creating a free electron gas or “ plasma .” thermally induced conduction band electrons are free to reflect incident radiation just as the conduction band electrons do in a metal . these electrons also absorb incident radiation slightly , further heating the substrate material . the substrate material is chosen to have a high enough melting point that it is not damaged by this initial heating . as the absorbed energy heats the substrate , heat conducts from the plasma region to adjacent transparent dielectric region that is closer to the threat and begins to create plasma in front of the initial plasma region . thus the plasma absorption region grows and propagates toward the impinging radiation like a wave from the trigger layer . since the trigger layer of the pawl element is preferably located near a focal plane , as the absorption wave propagates toward the source , the free electron population moves into increasingly lower - intensity , less - focused position in the incident radiation pattern . this movement continues until the wave reaches the forward surface of the element or the energy absorbed from the incident radiation is balanced by the conductive heat losses from the plasma into the cooler substrate and related mounting materials . at that point the sensor is protected and the energy is distributed over sufficient material to prevent melting , vaporization or other forms of damage . thus , the detectors are protected by the trigger layer while the substrate provides the limiting function to protect the trigger layer and thereby increase the dynamic range of the limiter . the pawl mechanism thus reflects and absorbs the incident radiation before the trigger layer is permanently damaged . subsequent study has shown that the “ triggering ” heat provided by the thermochromic nlm can also be provided in alternative ways . consider the pawl 100 shown in fig1 . fig1 is a conceptualized cross - sectional view of a pawl 100 in accordance with the present invention . the pawl 100 comprises a trigger layer 103 — e . g ., a thermochromic nlm layer - and a limiting layer 106 — e . g ., a semiconducting substrate . if the incident energy 109 is sufficiently intense , the trigger layer 103 becomes opaque to protect the detector ( s ) ( not shown ) of the associated sensor ( also not shown ) then continues to absorb slightly and heat the limiting layer 106 . this heat generates a plasma wave 112 of free electrons in the limiting layer 106 . as heat is conducted away from the plasma and toward the source of radiation , the plasma wave 112 will propagate into the limiting layer 106 , which will dissipate the absorbed energy from the incident radiation 109 through conduction . damaging levels of optical energy will thus be reflected and absorbed to prevent damage to the trigger layer that has already protected the sensitive elements of the sensor system ( e . g ., its detector array ). the absorption of the plasma wave 112 can be tailored to applications by adjusting the band gap and thermal characteristics of the limiting layer 106 . these characteristics can be adjusted by choice of materials , doping ( either the bulk material or a thin layer ), thermal biasing , and alloys , for example . in general , design tailoring for specific implementations will include considerations such as threat characteristics , ambient operating temperatures , desired reaction time , and sensor performance / design characteristics . note also that the front surface 127 of the pawl 100 may be curved to adjust refraction in some embodiments . more technically , the pawl 100 is an optical element placed at or near a focal surface 115 in a sensor not otherwise shown . in the illustrated embodiment , the focal surface 115 is a focal plane . however , in alternative embodiments the focal surface 115 may be non - planar , for example , spherical , parabolic , or cylindrical . the lines 116 illustrate the converging rays of the focused threat radiation . as used herein , “ threat ” means incident energy sufficiently intense to damage the detector ( s ) of an associated sensor . at the focus 118 , incoming energy 109 will be concentrated in a focal pattern ; e . g . an airy diffraction pattern . the trigger layer 103 ( primarily transmissive ) absorbs some of the incident energy 109 . absorbed energy heats the trigger layer causing it to switch to an opaque state and protect the sensor &# 39 ; s detector ( s ). if the radiation continues intensely and long enough ( a few milliseconds for some high power lasers ), damage , such as melting and vaporization of the trigger layer 103 , will begin . however , before this occurs , heat conducts into the limiting layer 106 and thereby rapidly increases the population of charge carriers 200 , shown in fig2 ( only one indicated ), in the conduction band of the limiting layer 106 , making it more “ metallic .” fig2 illustrates a portion 121 of the pawl 100 in fig1 in greater detail . some incident energy 109 absorbed by the charge carriers 200 causes further heating of the pawl substrate near the plasma 212 . the resulting “ plasma region ” 203 of thermally - induced free charge carriers is highly reflective and slightly absorbing such that the previously insulating material of the limiting layer 106 becomes conducting , like a metal . the plasma region 203 then blocks the transmission of the incident radiation 109 to the focus spot 118 in the trigger layer 103 . the heat absorbed in the region 206 is quickly conducted , as represented by the arrow 209 , into the adjacent , cooler volume of the pawl 100 ; i . e ., the zone 212 . this causes charge carriers 200 of the plasma 203 to increase in front of the already heated region 203 ; i . e ., the heat conduction induces a free electron population density increase in the zone 212 . this newly heated zone 212 is slightly forward of the region 206 where the previous heating occurred , so the incident energy 109 is less concentrated in the newly heated region 212 . this process of thermally induced absorption of the incident energy 109 in the enlarged region of plasma 203 subsequently causes heat that propagates further into the limiting layer 106 toward the source ( not shown ) of the incident energy 109 . the plasma 203 blocks threat transmission to the previously heated region 206 . thus a wave 112 of thermally induced plasma 203 propagates from the triggering layer 103 into the limiting layer 106 ; i . e ., away from the focus 118 and toward the threat . this absorption wave 112 continues to build and propagate until it reaches the most forward face 124 , shown in fig1 , of the pawl 100 or to a region 215 in the pawl 100 where the threat radiation is defocused enough that heat conducted away from the plasma 203 is in equilibrium with the energy absorbed from the threat . note that , since the limiting layer 106 absorbs the plasma , the dynamic range of the pawl 100 can be increased by thickening the limiting layer 106 , since the threat intensity decreases away from the focal plane and there is more material to absorb threat energy loads . when the threat is removed , the pawl 100 cools back to ambient conditions and the absorbing plasma 203 dissipates so that the sensor functions without degradation . note that active means of cooling the pawl 100 may be incorporated to expedite the sensor &# 39 ; s return to full function . the pawl 100 trigger layer 103 may be implemented using , for example , an oxide of vanadium or titanium . the limiting layer 106 is a low - band - gap material that is transmissive in the pass band of the sensor at normal use temperature conditions . it may be made of any material where the band - gap energy of the conduction band is adequately above the energy of the photons in the sensor &# 39 ; s pass band . the melting point and strength of the material is selected to be high enough to prevent damage to the pawl 100 from threat radiation . for example if the sensor is designed for the 8 to 12 micron wavelength region like many infrared (“ ir ”) imagers , the pawl 100 limiting layer 106 might be made out of germanium (“ ge ”), either pure or slightly doped to tailor its limiting properties . many materials are sufficiently transmissive to be used for refractive elements and function in the manner desired . materials that meet these criteria are numerous and include not only ge , but also : ( i ) for long wave infrared (“ lwir ”) and medium wave infrared (“ mwir ”) sensors , limiting layer materials such as gasb , znsnas 2 , inas , insb , cufes 2 , cufese 2 , agalte 2 , aginte 2 , xnsnas 2 , cdgeas 2 , cdsnas 2 , hgln 2 se 4 , snte , pbse , pbs , pbte , bise , agsbse 2 , agsbte 2 , ag 19 sb 29 te 52 , cdsb , znsb , bi 2 se 3 , mg 2 sn , mg 3 sb 2 , cd 3 as 2 , tlse , hg 5 ln 2 te 8 , cualte 2 , cugase 2 , cugate 2 , cuinse 2 , cuinte 2 , agalse 2 , zngeas 2 , hgin 2 te 4 and zn 3 as 2 can be considered . ( ii ) for shorter wavelength sensors for near infrared (“ nir ”) and visible applications , higher band gap limiting layer materials such as si , zns , znse , znte , gap , may be appropriate , ( iii ) for millimeter wave (“ mmw ”) and microwave sensor applications , lower band gap materials such as insb , sn , bi 2 te 3 , hgte , pbse , cufese 2 , and pbte , can be considered . ( iv ) for uv and x - ray applications , high band gap materials like c ( diamond ), bn , bp , gan , aln , sic , and srs are applicable ; thus , as is implied above , the choice of materials as well as some other details will be implementation specific depending upon intended use and design constraints . turning now to fig3 , in one particular embodiment 300 , the trigger layer 303 may be implemented as a layer of thermochromic nlm , as is implied above . in this particular embodiment , the limiting layer 306 comprises a ge — or silicon (“ si ”)- based semiconducting substrate . the triggering layer 303 may be implemented in , for example , a thermochromic coating of a vanadium oxide deposited on the surface 310 near the focus 318 and its temperature biased below but near the phase change temperature of the nlm . after slight heating , the thermochromic nlm and switches from transmissive to reflecting before the detector is damaged . heat from the trigger layer 303 then conducts into the pawl 100 substrate causing a plasma 203 as described above . this heat conductance then protects the trigger layer 303 from damage such as fracture , melting , vaporization , delamination , etc . the trigger layer 303 may be fabricated on the limiting layer 306 using solid state material fabrication and thin film deposition techniques as are commonly known in the semiconductor and optical component fabrication arts . in general , techniques used for depositing thermochromic nlms on the forward face of the semiconducting substrates described above for conventional reflective limiters may be readily adapted to fabricating the trigger layer 303 on the rear face of the substrate in this particular embodiment of the present invention . one particular form of deposition that may be used is known as epitaxial growth , and is illustrated in fig4 . epitaxial growth describes a process by which a film or layer of one material is “ grown ” on a substrate . one suitable technique for this process is known as “ chemical vapor deposition ,” wherein a substrate is placed in a chamber and a chemical vapor is introduced into the chamber . over time , under proper temperature and pressure , the chemical vapor will deposit on the substrate in a crystalline film . an overview of this and other epitaxial growth techniques may also be found in any of several thin film and microchip fabrication handbooks . any suitable epitaxial growth process known to the art may be used . alloys of silicon and germanium (“ si — ge ”) or materials doped with impurities to adjust band gap may also be used depending on the threat characteristics , required reaction time and other sensor performance or design trade issues . fig5 illustrates an embodiment 500 in which a trigger layer 503 is formed by doping a si — or ge - based semiconducting substrate that is the limiting layer 506 . doping techniques are also well known in the semiconductor fabrication arts . for instance , well known ion implantation techniques are commonly used for doping purposes . an overview of this and other doping techniques may also be found in any of several thin film and microchip fabrication handbooks . any suitable doping techniques known to the art may be employed . as was mentioned above , the pawl 100 is preferably located at or near the focal surface 115 . to block the incident energy 109 quickly ( before damage to the detector ) the pawl 100 should be placed either immediately forward of the detector array or in a secondary focal plane ( reimager ) between the sensor &# 39 ; s objective aperture and detector . this largely results from the desire to maximize the dynamic range in a given embodiment and the fact that the intensity of the incident energy will be highest at the focal point 118 . however , this is not necessary to the practice of the invention . all that is required is that the pawl 100 be located at a position at which the intensity of the incident energy is strong enough to generate the plasma as described above before the sensitive elements of the sensor or the pawl 100 trigger layer damage . fig6 illustrates a one particular implementation of a pawl 600 in a conceptualized cross - sectional view . the pawl 600 includes a trigger layer 603 — e . g ., a thermochromic nlm layer — and a limiting layer 606 — e . g ., a semiconducting substrate . the trigger layer 606 residually absorbs the incident energy 609 , laser radiation , for example to generate a plasma wave ( not shown ) of free electrons that propagates into the limiting layer 606 . the limiting layer 606 then dissipates the plasma wave through absorption . the pawl 600 is placed at the focal plane 615 . the pawl 600 also includes optional anti - reflective coatings 622 on the front and rear surfaces 625 , 626 to reduce element transmission losses . fig7 illustrates but one exemplary use for the present invention in partial cross - section , in which an optical assembly 700 employs a pawl limiting layer 724 . the pawl limiting layer 724 protects the detector array 706 , which comprises an array of detector elements 709 ( only one indicated ), of a detector assembly 710 . note that the pawl limiting layer 724 is positioned so that it is in contact with the detector array 706 that also serves as the pawl triggering layer . the detector ( trigger layer ) 706 is positioned at or near the focal surface 715 of the optical sensor ( not shown ). the assembly 710 is housed in a thermal control apparatus 718 to control the operating temperature of the assembly 710 . the thermal control apparatus 718 may be any suitable means known to the art , such as a cryogenic temperature - controlled dewar . note that the front surface 721 of the limiting layer 724 may be configured to function as a cold window , a band - pass filter , and / or a field lens . as will be appreciated by those skilled in the art having the benefit of this disclosure , the optical assembly 700 will include additional , routine features such as support components , electronics , thermal conditioning components , and thermal isolation components . these features have been omitted for the sake of clarity and so as not to obscure the present invention . those in the art may realize further variations on the embodiments disclosed above that are also within the scope of the invention as claimed below . for example , referring now to fig8 , one particular embodiment 800 includes a trigger layer 803 and multiple limiting layers 806 a and 806 b . the trigger layer 803 is a layer of thermochromic nlm near the focus 818 and the limiting layer 806 a is a semiconducting substrate with a low band gap , e . g . germanium . the embodiment 800 furthermore includes a second limiting layer 806 b , which may also be a semiconducting substrate with a band gap higher than layer 806 a ; e . g . silicon . the temperature of the trigger layer 803 is biased below but near the phase change temperature of the nlm . after slight heating by threat radiation 109 , the thermochromic nlm that is the trigger layer 803 switches from transmissive to reflecting before the detector is damaged . the trigger layer continues to heat but then heat from the trigger layer 803 conducts into the limiting layer 806 a causing a plasma ( not shown ) as described above . the plasma protects the trigger layer from damage and if there is enough heat ( from a severe threat 109 ), the plasma wave in the limiting layer 806 a may expand to the front surface of layer 806 a . heat from layer 806 a then conducts into 806 b to induce a plasma in the second limiting layer 806 b . thus , the limiting layer 806 a may also function as a trigger layer for the second limiting layer 806 b . thus , a thermally induced plasma in both the first limiting layer 806 a , and subsequent limiting layers 806 b , etc . then protects its respective trigger layer from damage such as fracture , melting , vaporization , delamination , etc . thus , in its many manifestations and aspects , the present invention uses a thermally - induced conduction - band plasma wave in a solid - state material to passively block intense radiation . it thereby provides a number of benefits over and above the state of the art , including : it provides an automatic , low - loss means to protect optical sensors from damage by high - intensity light from a laser ; it provides sensor protection from other damaging sources within the wavelength range that the sensor is designed to detect and that would cause thermal damage to a sensitive component such as the sensor &# 39 ; s detector or focal plane array ; it provides protection from threats in the pass band of the sensor without degrading sensor performance when a threat source is not present ; it reacts to any wavelength in the sensor pass band and is thus more robust to evolving threats than a spike filter for a specific laser wavelength ; it is passive and requires no sensors , actuators and control electronics as does a mechanical shutter ; it can be designed to work over a large range of ambient temperatures from cryogenic to refractory ; it is tolerant of wide variation in ambient acceleration , shock and vibration unlike fluid cells , resonant etalons or pellicles ; it is unobservable from outside the sensor ; it is quick reacting , compact and light weight compared to shutters ; it can be tailored to a wide range of sensor bands from the microwave to x - ray ; it does not require prior knowledge of the threat wavelength like notch filters or notch absorbers ; it protects against extreme threat levels that would damage other protection equipment like thermochromic limiters ; and it is easier to design and fabricate than many other technologies . note that not all embodiments of the present invention will necessarily exhibit all these advantages . this concludes the detailed description . the particular embodiments disclosed above are illustrative only , as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein . furthermore , no limitations are intended to the details of construction or design herein shown , other than as described in the claims below . it is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention . accordingly , the protection sought herein is as set forth in the claims below .