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46 Photonics Spectra March 2014 www.photonics.com

Materials Evolving for Lighter, Stiffer OpticsBuilding better optical materials will impact applications

in high-energy lasers, consumer devices and more.

BY HANK HOGANCONTRIBUTING EDITOR

For optical materials, less can be

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RI�0LGGOH¾�HOG��&RQQ���ZKLFK�PDNHV�RSWL�FDO�FRPSRQHQWV�DQG�PHDVXUHPHQW�LQVWUX�PHQWV��DQG�RIIHUV�GHVLJQ�DQG�PDQXIDFWXU�LQJ�VHUYLFHV�/LJKW�\HW�VWLII�RSWLFV�DUH�²SDUWLFXODUO\�

LPSRUWDQW�IRU�DLUERUQH�DQG�VSDFH�DSSOLFD�WLRQV��RI�FRXUVH��EXW�DOVR�VHPLFRQGXFWRU�applications where lower mass translates

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High-energy laser applications$QRWKHU�DUHD�RI�IRFXV�LV�KLJK�HQHUJ\�

ODVHU�DSSOLFDWLRQV��7KH�PLOLWDU\�LV�LQWHU�HVWHG�EHFDXVH�KLJK�HQHUJ\�ODVHUV�FRXOG�

By repeatedly withstanding laser pulses scores of times more intense than sunlight, fused silica optics enable high-energy laser applications.

Zyg

o C

orp.

March 2014 Photonics Spectra 47

replace conventional weapons. Also inter-

ested are those researching fusion power,

since high-energy lasers are being used

to ignite the reaction. For this scenario,

=\JR�KDV�GHYHORSHG�DQG�TXDOL¾HG�PDWHUL-DOV�FDSDEOH�RI�ZLWKVWDQGLQJ�DV�PXFK�DV� ��-�FP2��RU�QHDUO\����WLPHV�WKH�LQWHQVLW\� of sunlight. This is done for pulses of

a few nanoseconds at a wavelength of

����QP�The need for relatively stiff yet light-

ZHLJKW�PDWHULDOV�LV�OHDGLQJ�UHVHDUFKHUV�WR�look again at single-crystal silicon. The

stuff of chips and solar panels has the

PHFKDQLFDO�DQG�WKHUPDO�SURSHUWLHV�WKDW�PDNH�LW�DQ�H[FHOOHQW�FRPSURPLVH�PDWH-

rial, Tricard said.

$QRWKHU�ZHOO�NQRZQ�VHPLFRQGXFWRU��$O*D$V��DOXPLQXP�JDOOLXP�DUVHQLGH���LV�DOVR�SURPLVLQJ��SDUWLFXODUO\�IRU�ORZ�QRLVH�optical coatings, according to Dr. Garrett

Cole, co-founder of Crystalline Mirror

6ROXWLRQV�*PE+�RI�9LHQQD��7KH�VWDUWXS�PDQXIDFWXUHV�KLJK�TXDOLW\�VLQJOH�FU\VWDO�$O*D$V�WKLQ�¾OPV�RQ�DUELWUDU\�EXON�optics.

7KLV�LV�RI�FRPPHUFLDO�DSSHDO�EHFDXVH�of the frontiers of science: As optical

PHDVXUHPHQWV�EHFRPH�PRUH�SUHFLVH��WKH\�GHPDQG�KLJKHU�RSWLFDO�DQG�PHFKDQLFDO�V\VWHP�TXDOLW\��,WµV�WKH�ODWWHU�WKDW�QRZ�FRQVWUDLQV�SHUIRUPDQFH��)RU�LQVWDQFH��gravitational wave detectors are kilo-

PHWHU�ORQJ�LQWHUIHURPHWHUV��EXW�D�VPDOO�FRPSRQHQW�VHWV�WKH�QRLVH�¿RRU�²7KHUH�DUH�WZR�PLUURUV�KHOG���NP�

DSDUW��DQG�WKH�V\VWHP�LV�ODUJHO\�OLPLWHG�E\�WKH����P�WKLFN�¾OP�PDNLQJ�XS�WKH�UH¿HF-

WLYH�HOHPHQW�RI�WKRVH�PLUURUV�³�&ROH�VDLG�Because they are crystalline in nature,

WKH�WKLQ�¾OPV�SURGXFHG�E\�&ROHµV�FRP-

SDQ\�KDYH�IHZ�GHIHFWV�DQG�ORZ�PHFKDQ� LFDO�ORVV��-XVW�DV�LPSRUWDQWO\��WKH�ORVV�shrinks upon cooling, which is not true

IRU�DOO�FRDWLQJV��7KHUHIRUH��DV�FRPSRQHQWV�DUH�FRROHG�DQG�PRWLRQ�GHFUHDVHV��WKHUHµV�DQ�DGGHG�ERQXV�EHFDXVH�WKH�PHFKDQLFDO�SHUIRUPDQFH�RI�WKH�¾OPV�LQFUHDVHV�2QH�FRQVHTXHQFH�RI�OHVV�PHFKDQLFDO�

ORVV�LV�WKDW�GHYLFHV�FDQ�EH�PDGH�VPDOOHU��EHFDXVH�XQZDQWHG�PRWLRQ�DV�D�SHUFHQW-age of the total cavity length is a critical

FRQVWUDLQW��,I�WKDW�PXVW�EH�EHORZ�D�JLYHQ�YDOXH��WKHQ�D�ORZHU�ORVV�PDWHULDO�DOORZV�GHYLFHV�WR�EH�PDGH�VPDOOHU��7KDW�SRVVLELO-ity is one reason why crystalline coatings

have attracted the interest of defense

researchers and contractors.

To produce the coating, Crystalline

Mirror Solutions begins by growing an

$O*D$V�PXOWLOD\HU�RQ�D�ZDIHU�YLD�PROHF-XODU�EHDP�HSLWD[\��7KH�¾OP�LV�WUDQVIHUUHG�LQ�FHQWLPHWHU�VL]HG�VZDWKV�XVLQJ�D�WHP-

SRUDU\�¿H[LEOH�PRXQW�DQG�WKHQ�ERQGHG�WR�D�¾QDO�VXEVWUDWH��ZLWK�FDUH�WDNHQ�WR�HOLPL-QDWH�YRLGV��7KH�UHVXOWLQJ�¾OPV�DUH�DW�OHDVW�DQ�RUGHU�RI�PDJQLWXGH�EHWWHU�LQ�WHUPV�RI�PHFKDQLFDO�SHUIRUPDQFH�ZKLOH�RIIHULQJ�RSWLFDO�FKDUDFWHULVWLFV�FRPSHWLWLYH�ZLWK�WKRVH�FUHDWHG�E\�LRQ�EHDP�VSXWWHULQJ��DQ�alternative approach, Cole said.

$�IXVHG�VLOLFD�VXEVWUDWH�ZLWK�D�WKLQ�ÀOP�RI� $O*D$V��FHQWHU�GLVF��ERQGHG�WR�LW��'XH�WR�LWV ORZ�PHFKDQLFDO�ORVV�DQG�KLJK�RSWLFDO�TXDOLW\��$O*D$V�LPSURYHV�PHDVXUHPHQWV�LQ�D�QXPEHU� RI�VFLHQWLÀF�UHVHDUFK�DUHDV�

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48 Photonics Spectra March 2014 www.photonics.com

Q Optical Materials

Dr. Jun Ye, a fellow at NIST and the

University of Colorado at Boulder’s JILA

research lab, makes the world’s most

stable laser, with a 30-mHz-wide spectral

output. Partly due to this extreme stabil-

ity, Ye’s group in January demonstrated

an atomic clock that’s accurate to a sec-

ond over 5 billion years.

In a quest for even better lasers, Ye was

involved in testing the crystalline mirror

coating developed by Cole and a team at

the University of Vienna. He participated

because the mechanical quality of an

optical system determines its response to

thermal noise. “That limits how stable a

laser can be made or how precise an inter-

ferometer can be used to read out length

information,” Ye said of what is known as

mechanical Q.

For his part, he sees the potential for

a big impact from these crystalline coat-

ings. There’s a need for high-quality opti-

cal and mechanical components across a

QXPEHU�RI�VFLHQWL¾F�GLVFLSOLQHV��DFFRUG-

ing to Ye.

Consumer applicationsAnother sector where high-perfor-

mance optical materials are attracting

interest is consumer devices. A case in

point comes from JDSU, the Milpitas,

Calif.-based optical technology company.

Today, consumer needs drive innova-

tion, noted Fred Van Milligen, vice presi-

dent of research and development. For

optical materials, an example of this can

be seen in smartphones, which have ever-

more-capable cameras, despite severe size

and power constraints.

In general, the goal in consumer goods

is to collect as much light over as wide a

¾HOG�RI�YLHZ�DV�SRVVLEOH��8VXDOO\��WKLV�LV�done in a bandpass, with only the desired

part of the spectrum getting through. Pre-

viously, this might have been for 2-D im-

aging in the visible, but now there’s in-

creasing demand for 3-D and IR imaging.

Most of JDSU’s coatings involve sput-

tering of inorganic and organic materi-

als onto substrates, with this often done

directly on device-bearing semiconduc-

tor wafers. The coatings are carefully

WDLORUHG�WR�\LHOG�VSHFL¾F�RSWLFDO�SHUIRU-mance, and they can be complex.

“We tend to have a fair amount of lay-

ers in our coatings,” Van Milligen said.

“We deliver some products with thou-

sands of layers in them.”

Multilayer coatings are becoming

more common, said Dr. E. Fred Schubert,

a professor of electrical engineering at

Rensselaer Polytechnic Institute in Troy,

1�<���KH�FR�DXWKRUHG�DQ�LQ¿XHQWLDO������Nature Photonics�SDSHU�RQ�DQWLUH¿HFWLRQ�coatings.

In part, this interest in coatings is being

driven by advances that allow the tailor-

ing of a material’s optical properties by

manipulating its physical characteristics,

Schubert said. “We are able to control the

refractive index by changing its porosity

or nanoporosity.”

As a result, coatings can be designed

based on calculations. Finding the sweet

spot that optimizes a multilayer coating

can be challenging. To do that, research-

ers and developers may make use of ge-

netic algorithms, which evolve a coating’s

properties from a starting point to arrive

at the best solution.

Metamaterials and moreFinally, new optical materials have

appeared on the horizon. For instance, Dr.

Withawat Withayachumnankul, a post-

doctoral researcher at the University of

Adelaide in Australia, was part of a team

that etched cavities into the surface of

slightly conductive silicon, as described

in a 2013 Advanced Optical Materials

paper. The resulting metamaterial trapped

DQG�FRQ¾QHG�WHUDKHUW]�ZDYHV�LQ�ZD\V�WKDW�RIIHUHG�VRPH�VLJQL¾FDQW�DGYDQWDJHV��RQH�of which was improved imaging contrast,

Multilayer coatings can be designed based on calculations. To find

the sweet spot that optimizes a coating, researchers and developers

may make use of genetic algorithms, which evolve the coating’s

properties from a starting point to arrive at the best solution.

March 2014 Photonics Spectra 49

thanks to minimal cross-coupling be-

tween each pixel, Withayachumnankul

said.

“Moreover, since each optimized cav-

ity can absorb nearly 100 percent of the

incident power, the imaging contrast is

improved further,” he added.

The next step in the research will be to

form a complete imaging system by inte-

grating a thermal detector to read out the

temperature change when terahertz waves

strike the cavity array. The technique has

attracted commercial interest, Withaya-

chumnankul said.

Metamaterials in general allow the

engineering of properties and so could

enable new applications, he added. An

H[DPSOH�PLJKW�EH�DQ�RSWLFDO�WXQDEOH�¿DW�lens that actively responds to incoming

light with polarization changes.

Another way to make new optical mate-

rials is to grow them. A research team

from New York University, Harvard Uni-

versity and Dow Chemical Co. revealed

in a 2012 Nature paper a self-assembly

technique based on small particles sus-

SHQGHG�ZLWKLQ�D�¿XLG��6XFK�FROORLGDO�manufacturing could be very inexpensive

and suitable for fabricating materials on

SODVWLFV�RU�RWKHU�¿H[LEOH�VXEVWUDWHV�WKDW�can’t tolerate high temperatures.

Dr. Vinothan Manoharan, a chemical

engineering and physics professor at Har-

vard who was a member of the research

team, is investigating the technique as a

way to produce photonic inks – materi-

als inspired by bird feathers that generate

FRORU�WKURXJK�LQWHUIHUHQFH��6LQFH�OLJKW�isn’t absorbed, photonic inks might retain

their hue longer than traditional pigments.

Also, this structure-based approach relies

on ambient light, making photonic inks

potentially useful in low-energy-consum-

ing electronic displays.

At present, the colloid-created photonic

inks can change color in response to

changes in their chemical environment,

although Manoharan noted that doing so

electronically would be best for displays.

Being researched is how to make photonic

inks mimic the appearance of traditional

pigments. Red, in particular, has proved

GLI¾FXOW�WR�DFKLHYH��,Q�DGGLWLRQ��EHFDXVH�these colors depend upon diffraction and

interference, another area of improvement

is being researched.

“You also have to get a color that

doesn’t change as you change the viewing

angle,” Manoharan said.

hank.hogan@photonics.com

An illustration of annular cavities etched into the surface of conductive silicon. Depending on their dimensions, the cavities trap terahertz waves at different frequencies and enhance imaging. These and other metamaterials form the basis for new optical materials.

With

awat

With

ayac

hum

nank

ul, U

nive

rsity

of A

dela

ide

54 Photonics Spectra March 2014 www.photonics.com

diSPIMDual Inverted Selective Plane Illumination Microscopy

Dual iSPIM from ASIGenerate 3D volumes with isotropic resolution (330 nm in all directions)Axial resolution is ~2x better than confocal or spinning disk systemsAchieve a ~7-10 fold reduction in photobleachingAcquisition rates up to 200 images per second or 2-5 volumes per second

For More Information Visit: www.asiimaging.comEmail: info@asiimaging.comCall: (800) 706-2284 or (541) 461-8181

Visit Us At: Focus on Microscopy 2014 - April 13-16th, 2014, Sydney, Australia www.crystallinemirrors.com

largements of monolithic Galileo tele-

scopes are relatively small as a result of

the limitation in center thickness. How-

ever, as these are afocal beam expansion

systems, they can be connected in series

to successively enlarge the incoming beam

one after the other in the beam course

(Figure 4).

This opens up new opportunities. Just

three of these elements can enlarge the

EHDP� E\� HLJKW� WLPHV�� ¾YH� HOHPHQWV�� ���times. If only individual elements with

M =���DUH�XVHG��WKH�LQFUHPHQWV�RI�WKH�SRV-sible enlargements are very approximate at

M =�����������������������7KLV�LV�DQ�RSWLPDO�solution if one requires strong enlarge-

ment with minimal space used and a high

wavefront quality.

,I�� KRZHYHU�� ¾QHU� LQFUHPHQWV� EHWZHHQ�the enlargement levels are desired, it is

necessary to introduce other individual

element enlargements, which also lie very

close together. Two lower levels are of-

fered here at M =�����DQG�M =�������HVSH-

cially for the version in glass. Because of

the afocal dimensioning of the individual

elements, the meniscus lenses can be ori-

ented in the course of the beam however

one likes, as shown in Figure 4c. This

means, when combined, there are not only

three but actually six individual element

HQODUJHPHQWV� DYDLODEOH�� ZKLFK� VLJQL¾-

cantly increases the combinatorics level.

If, for instance, one element is available

for each basic enlargement, this produces

���SRVVLELOLWLHV�IRU�WKH�RYHUDOO�HQODUJHPHQW�with just these three meniscus lenses. If

we push these combinatorics further with

additional elements, an above-average

number of combination options are opened

up by certain lens groupings.

What is common to all these groups

is the presence of one element each with

M =�����DQG�M =������DQG�DQ�LQFUHDVLQJ�number of elements with M =� ��� )LJXUH� 4b shows an example of an overall en-

largement of M =� ��� FRQVLVWLQJ� RI� ¾YH�LQGLYLGXDO� HOHPHQWV�� 8VLQJ� WKH� VSHFL¾F�JURXS�VKRZQ�WKHUH����× M =��������× M

=���������× M� �����LW�LV�SRVVLEOH�WR�UHDOL]H����HQODUJHPHQW�OHYHOV�ZLWK�WKH�PD[LPXP�at M =����

In practice, to implement such an aspher-

LFDO�FDVFDGH�V\VWHP�IRU�¿H[LEOH�EHDP�H[-

pansion, very high surface qualities for the

individual elements are required. To pre-

vent restrictions on combinatorics for later

use, each individual element must be sig-

QL¾FDQWO\�EHWWHU�RYHU�WKH�ZKROH�IUHH�DSHU-ture than the “diffraction limited” require-

ment (i.e., wavefront error RMS <�����For a Ti:sapphire laser wavelength at

���� QP�� IRU� LQVWDQFH��506�<��� QP�� IRU�λ =�����QP��LW�LV�HYHQ�MXVW�506�<���QP��If the center thickness and the decentra-

tion of the surfaces are also produced very

precisely for these requirements, this sys-

tem is adjustment-free. This means that

the attachment of additional monolithic

elements to change the enlargement level

also takes place completely adjustment-

free and is thus quick and easy.

Meet the authorsDr. Ulrike Fuchs is head of applications at

asphericon GmbH in Jena, Germany; email:

u.fuchs@asphericon.com. Sven R. Kiontke

is general manager at asphericon; email:

s.kiontke@asphericon.com.

Beam ExpansionTech Feature