Optical Filters Go Deeper

OpticalOptical FiltersFilters GoGo DeeperDeeper A new generation of optical filters benefits optical applications in the mid-UV. by Dr. Turan Erdogan and Dr. Atul Pradhan, Semrock Inc. he importance of optical ap- and/or multiple laminated absorb- other type of ion source and directed plications in the middle-ultra- ing and transparent glass substrates at a metal or metal-oxide target. With T violet spectral region — be- cannot withstand the intense ener- sufficient energy, atoms are sput- tween about 200 and 320 nm — con- gies of illumination at mid-UV wave- tered from the surface of the target, tinues to increase with the advent of lengths. Hybrid metal-dielectric fil- where they subsequently migrate and new excitation light sources in this ters also have poor optical damage then deposit onto a nearby glass sub- range, also identified as the UVC (200 thresholds, and inadequately low strate by random diffusion (Figure to 280 nm) and UVB (280 to 320 nm) transmission and edge steepness. 1). The deposition is further assisted bands. Examples of new sources in- Optical filters with good perfor- by a secondary ion gun, which ac- clude more efficient mid-UV lasers, mance in the mid-UV are crucial for celerates a mixture of inert ions di- AlGaN LEDs and more conventional enabling important new instruments rectly onto the substrate to densify based on applications including bio- and compact the coating films. O + broadband sources such as high- 2 pressure mercury and xenon arc chemical absorption, fluorescence ions often are introduced into the lamps with improved efficiency and and spectroscopy, as well as for re- deposition region near the surface longer lifetimes. alizing the full potential of industrial of the glass substrate to control the A major impediment that contin- applications in areas including UV stoichiometry of the oxide films. ues to make the mid-UV an optically sterilization, and semiconductor and Deposition of thin-film materials challenging spectral region is the lack electronics manufacturing. with transmission in the UV presents of durable optical filters with adequate challenges such as a greater degree performance. However, optical filters Deposition of UV filters of absorption of refractory materials constructed from thin-film coating Modern thin-film interference fil- at these wavelengths and a high vari- materials that have relatively low ab- ters made with dense hard-oxide di- ation in refractive index. These prop- sorption and spectral dispersion yet electric coatings and with transmis- erties are true of all oxide coating high reliability at these short wave- sion bands in the visible and infrared materials that may be used in the wavelength regions are now well es- mid-UV, including Al O , HfO , Ta O , lengths would have significant ad- 2 3 2 2 5 tablished (see “Thin Film Filters Sc O and Y O . vantages over components such as 2 3 2 3 the diffraction gratings used in mono- Come of Age,” Photonics Spectra, July To attain the desirable attributes of chromators and spectrophotometers. 2003, page 94). The highest-perfor- high transmission, deep out-of-band In general, grating-based systems mance filters are made using ad- blocking and sharp cut-on and are not as selective and do not vanced coating deposition tech- cut-off passband edges, many lay- achieve as much throughput as fil- niques, such as ion-assisted ion- ers must be deposited. High-perfor- ter-based systems, and only filters beam sputtering, and the materials mance filters in the visible and near- allow direct imaging. Until recently, are hard refractory oxides with in- infrared are made now with many optical filters for the mid-UV have dexes of refraction in the range of hundreds of layers. Even minute lev- exhibited poor performance in terms about 1.45 to 2.35. els of absorption and index variation of transmission, spectral selectivity Ion-beam-sputtering deposition in- errors can result in severely degraded and durability. Optical filters made volves a high-energy beam of ions transmission and spectral profiles with soft-coated thin-film materials that is generated by a gridded or relative to the desired target values. Reprinted from the March 2008 issue of PHOTONICS SPECTRA © Laurin Publishing Filters Worldwide Coverage: Optics, Lasers, Imaging, Fiber Optics, Electro-Optics, Photonic Component Manufacturing ® Figure 1. Advanced optical filters for mid-UV wavelengths now can be made with ion-assisted ion-beam sputtering (left). One ion gun is directed at a target to sputter the target material to the substrate, and a second is directed at the substrate itself to control the surface chemistry and densify the hard-oxide coating. In this way, multilayer thin-film coatings can be deposited with high durability and low loss on a single glass substrate to produce complex optical filters (right). Controlling these variables requires careful consideration and monitoring of the stoichiometry of the de- 100 posited materials. Deposition chal- Hg01-254 Filter lenges at UV wavelengths are further 90 253.7-nm Line exacerbated because of the physical 350-W “254” Line thinness of the film layers, requiring 80 tighter tolerances on layer deposi- ) % tion timing. ( 70 r UV radiation is particularly chal- e lenging for optical systems because w 60 o P the transmission and refractive index , n 50 of many optical materials can de- o i s grade rapidly when exposed to in- s i 40 tense UV illumination, as a result of m s solarization. Colored glasses are par- n 30 a r ticularly susceptible to photodark- T ening and even cracking under typ- 20 ical high-intensity illumination con- ditions, so optical system designers 10 often must resort to significantly ex- 0 panded UV beam sizes with oversize 230235240245 250 255 260 265 270 optical filters to reduce the intensity of light and to minimize pho- Wavelength (nm) todegradation of the filters. These measures greatly increase the cost of an optical system. In contrast, Figure 2. Typical spectra of both low- and high-pressure mercury arc lamps at hard-oxide UV filters made by ion- 254 nm are overlaid on the spectrum of a mercury line filter. The filter has greater beam sputtering have been shown than 70 percent transmission and excellent discrimination of all other mercury to exhibit no significant change in lines as well as the broad background radiation. PS March 08 Feature Semrock Figure 2 Jules Filters transmission or index of refraction communications; monitoring of poly- tion and classification of trace levels even after thousands of hours of ex- cyclic aromatic hydrocarbons at of chemical hazards. When excita- posure to intense UV illumination in petrochemical facilities; and flame tion wavelengths are near or below the mid-UV-wavelength range. and combustion chemistry analysis. 250 nm, Raman and native-fluorescence emission bands occur in dif- Mercury line illumination UV Raman spectroscopy ferent spectral regions for nearly Hard-oxide UV filters can benefit Some of the most challenging ap- all materials, enabling sensors that applications that involve mercury arc plications for optical filters in the employ exclusive detection of Raman lamps. Low-pressure mercury arc mid-UV are Raman and native-fluo- signals with no fluorescence inter- lamps emit radiation at powerful res- rescence “standoff” (remote) detec- ference, or even simultaneous de- onance lines, such as 254 nm in the mid-UV. As shown in Figure 2, prac- tical spectra of the 254-nm mercury line in higher-power (higher-pressure) light sources can exhibit a sig- LP01-266RU LWP Edge Filter nificant amount of collisional and Laser Line Doppler line broadening as well as Autofluorescence line-center absorption leading to self- reversal. These lamps are used for Raman a variety of annealing and curing Signal applications, for electroless plating, and for electrochemical oxidation processes in electronics and printed circuit board manufacturing. 200 225 250 275 300 325 350 375 400 A broadband source featuring the 254-nm mercury line with a suitably Wavelength (nm) wide filtered bandwidth (Figure 2) also is useful for prototype, research-ori- Figure 3. A new steep and highly transmitting long-wave-pass edge filter blocks ented and small-scale photolithogra- the 266-nm line of a quadrupled Nd:YAG laser with optical density greater than 6, phy applications, particularly those while transmitting Raman signals from wavelengths as short as 268 nm to about involving catadioptric or largely re- 300 nm and fluorescence at near-UV and visible wavelengths. flective systems and direct-contact- mask photolithography. These optical systems can take ad- 100 vantage of the wavelength-dependent FF01-280/20 filter 90 improvement in resolution despite the ) UV 280-nm LED % fact that the 254-nm line is signifi- ( Tryptophan abs. cantly less intense than the tradi- n 80 o i Naphthalene abs. tionally utilized 365-nm mercury “i- t p r 70 line.” o s Filters that pass UV light in the b 60 A Hartley bands of ozone (230 to 290 , r nm) and that block longer-wave- e 50 w length contaminating solar radiation o PS March 08 P are important for so-called “solar , 40 n Feature Semrock o blind” UV detection. Because solar i s Figure 3 s 30 radiation in the Hartley bands is so i Jules m strongly absorbed, essentially no s n 20 solar radiation reaches the Earth’s a r T surface at these wavelengths. Filters 10 for this application traditionally have been metal-dielectrics, which have 0 very poor transmission. The high 230240 250 260 270 280 290 300 310 320 330 transmission of the new class of ion- beam-sputtered hard-oxide coating Wavelength (nm) filters, a substantial performance improvement, is now possible in a wide Figure 4.

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