Photonics

Photonics studies the interactions between light and insulators or semiconductors [1]. It basically incorporates light generation, propagation (or manipulation) and detection. Traditionally people utilizes ray optics such as reflection and refraction to describe and engineer light. However, when it comes to micro- or nano-scales, modern photonics have to work in the regime determined by diffraction and interference. In another word, wave optics comes into play. The tremendous development of photonics over the past decades is largely attributed to both the explosion of microelectronic industry and the invention of lasers. Photonics have successfully expanded its territory to imaging, sensing, astronomy, new energy, healthcare, scientific computing, integrated circuits, communications and display [1].

Materials for photonics

  • Metals: They have been exploited in traditional optics for centuries, primarily offering a flat, highly reflective surface, essentially mirrors. Silver, gold and aluminum coatings are commercially available. Not long ago, they were picked for another exciting application – metamaterials. Periodic sub-wavelength patterns of metal show negative real permittivity and negative real permeability simultaneously, leading to negative refractive index, which is promising for super-lens and invisibility cloaking [2].
  • Semiconductors: Semiconductors are ubiquitous in modern microelectronic devices. They possess bandgaps from 0.2eV to 5eV. Silicon, germanium and compounds such as gallium arsenide, gallium aluminum arsenide, gallium phosphide and indium phosphide are commonly used in on-chip optical communication and signal processing. High refractive index of these materials provide tight photonic mode confinement in integrated waveguides and highly-ordered crystalline structures offer reduced losses within the spectrum of interest. Semiconductors also find important applications in photovoltaics, where photon energy greater than bandgap is absorbed and converted into electricity [3]. Likewise, array of semiconductor patterns can work as imaging sensors. On the other hand, semiconductors, especially III-V and II-VI compounds are utilized in light generation, such as light-emitting-diode (LED). Indirect bandgap materials, such as GaAs and InP are optimal choices for on-chip source (laser for instance) owing to momentum match.
  • Dielectrics: Lens is the most important component in both imaging and non-imaging optics. Glass has been used to make lenses for centuries. Crown and flint glasses are two major types that exhibit opposite dispersion properties. Silicon oxide, silicon nitride, silicon carbide, aluminum oxide and titanium oxide are commonly used in photonics. They often serve as substrate for supporting integrated photonic devices, barrier layer for preventing diffusion of species in thin film deposition process, surrounding media providing refractive index contrast or sacrificial layer for fabricating free-standing structures. Another application of dielectrics is fiber optics [4], where either glass or polymer (PMMA most popular) function as base materials because of their excellent optical transparency and inexpensive mass production. In glass fibers, germanium doping modifies refractive index and hence confines and guides optical mode over extremely long distance. Fibers doped with rare materials (such as erbium or ytterbium) have special applications including fiber laser and sensor (Raman scattering). Recent studies are focused on nano-wires as photonic waveguides [5].
  • Conducting oxides: This category of materials are useful for optoelectronic devices in which both good photon transparency and electron conductivity are expected. Front covers of solar cells and displays are two well-known examples. Indium tin oxide and aluminum-doped zinc oxides are most studied. They have high transmission in visible and near-IR spectrum while maintaining reasonable electron collection and transportation efficiency. In addition, they can be deposited with less defects.
  • Non-linear optical materials: Non-linear optical phenomena open a new area where light-matter interactions do not follow linear relationship [6]. Frequency mixing, Kerr effect and Pockels effect are three examples. Barium borate (BBO) and lithium niobate (LiNbO3) are two most common choices. These materials have specific orientation and symmetry in their crystalline structures, and thus give non-linear optical responses.
  • Organics: Organic contains a huge library of materials that can be utilized in photonics.
    • Most photoresist used in patterning photonic structures are mixtures of organic molecules. SU-8 is a popular epoxy-based negative photoresist. Non-photosensitive base phenolic resin in conjunction with photosensitive diazonaphthaquinone-derived compounds make a common positive photoresist.
    • Organic solar cell (OSC) is another hot research topic. Poly(3-hexylthiophene) with phenyl-C61-butyric acid methyl ester (P3HT:PCBM) is the best candidate for solar power harvesting [7]. Additionally, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is usually includeed in OSC to assist charge collection.
    • A lot of polymers are used in organic light-emitting-diode with relatively high efficiency and high flexibility, including cyano-polyphenylene vinylene (CN-PPV), poly(phenylene ethynylene) (PPE), polyfluorene (PFO) and polyphenylene vinylene (PPV).
    • Liquid crystal, due to its highly structural anisotropy, is commercially deployed in displays. Molecule orientation can be altered electronically so that the polarization state of light is changed to modulate light transmission states. In such sense, a spatial pixel can be turned on or off. Reference 8 summarizes some organic materials usually used.
    • Another important application of molecules is in microscopy. An electron from lower energy level may be excited to higher energy level (laser excitation) and spontaneously returns to the initial state at the expense of emitting a photon [8]. By attaching the non-fluorescent target by a fluorescent molecule, these natively ‘dark’ objects can be observed and distinguished [8]. Green fluorescence protein (GFP), rhodamine, 4′,6-diamidino-2-phenylindole (DAPI) and fluorescein are commonly used to label a variety of bio and non-bio samples. Recent discoveries of photo-switchable molecules enable the fluorescence microscopy with resolution beyond the far-field diffraction limit [9].

References

[1] YouTube video on introduction to photonics: https://www.youtube.com/watch?v=_DZHqedyYWY

[2] D. R. Smith, J. B. Pendry and M. C. K. Wiltshire, “Metamaterials and negative refractive index,” Science 305, 788-792 (2004). [10] M.  A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, “Solar cell efficiency tables (Version 45),” Prog. Photovolt: Res. Appl. 23, 1-9 (2014).

[3] G. Keiser, Optical fiber communications (John Wiley & Sons. Inc., 2003).

[4] L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Maxwell and E. Mazur, “Subwavelength-diameter silica wires for low-loss optical wave guiding,” Nature 426, 816-819 (2003).

[5] Website on nonlinear optics: http://en.wikipedia.org/wiki/Nonlinear_optics

[6] M. T. Dang, L. Hirsch and G. Wantz, “P3HT:PCBM, best seller in polymer photovoltaic research,” Adv. Mater. 23, 3597-3602 (2011).

[7] Website on a list of liquid crystal materials: http://www.sigmaaldrich.com/materials-science/material-science-products.html?TablePage=16378837

[8] YouTube video on fluorescent microscopy: https://www.youtube.com/watch?v=AhzhOzgYoqw

[9] E. Betzig, G. H. Patterson, R. Sougrat, O. W.  Lindwasser, S. Olenych, J. S. Bonifacino, M. W. Davidson, J. Lippincott-Schwartz and H. F. Hess, “Imaging intracellular fluorescent proteins at nanometer resolution,” Science 313, 1642-1645 (2003).