Dr Iskandar Yahya Universiti Kebangsaan Malaysia 2013 KKKC 6014: Microelectronic Devices Electro-Optics Physics, Materials and Applications

Course Outline TopicsDetail Electro-OpticsSemiconductor nonlinear characteristics, Electro-optics effects and applications. Optical switches and modulators. Nonlinear materials. Light Crystal Display, LCD. MEMSMicroelectromechanical systems – Structure, Sensors, Actuators, Applications and Fabrication Gunn DiodeSemiconductor microwave device. Generator and detector. Data StorageData storage mediums and devices. Working principles and examples Optical FibersOptical fiber technology. Optical fiber structure and working principles. Applications Starts in week 10, finish in week 14 Exams: Most material will be asked, but not all. Better to study all Office: Room 25, Cabin, second floor (opposite FKAB café) KC I Yahya - Universiti Kebangsaan Malaysia

Electro-Optics: Introduction Reference: Saleh & Teich, Fundamentals of Photonics, 2nd ed., ch. 20 & ch. 21 What is electro-optic effect? Certain transparent materials change optical properties when subjected to an electric field. Distortion of position, orientations or shapes of molecules of anisotropic material. Change in the refractive index that results from steady or low-frequency electric field. The electro-optic (E-O) effect is a 2nd- order nonlinear optical effect that results in a refractive index that is a function of the applied electric field (voltage) KC I Yahya - Universiti Kebangsaan Malaysia

Electro-Optics: Introduction Dependence of refractive index, n, on the applied electric field, E: n α E : Linear electro-optic effect or Pockels effect. n α E 2 : Quadratic electro-optic effect or Kerr effect. ∆n is small, but can modify phase of optical wave significantly if distance of travel exceeds λ. Eg. n increased by by E, wave travelling a distance of of the wavelengths will experience additional phase shift of 2 Applications Optical switch or “intensity modulator” = LCVR between 2 crossed polarizers; Optical phase modulator = crystal with voltage-controlled refractive index; Optical scanner with voltage-controlled refraction in a prism made from Electro-Optics crystal; Variable-focus lens made of material with voltage-controlled refractive index… KC I Yahya - Universiti Kebangsaan Malaysia

Refractive index depends on electric field n of a medium as a function of E i.e. n(E) The function n(E) varies slightly with E so that it can be expanded in a Taylor series about E: Where the coefficients of expansions are: For convenience, we write the impermeability tensor :- And rewrite (1-1) as :- ….(1-1) KC I Yahya - Universiti Kebangsaan Malaysia ….(1-2)

Refractive index depends on electric field ….(1-3) KC I Yahya - Universiti Kebangsaan Malaysia

Pockels and Kerr effects Pockels effect and medium Materials for which r dominates are called “Pockels media” and devices made from this material are often referred to as Pockels cells. Common crystals used as Pockels cells are: Ammonium dihydrogen phosphate, ADP (NH 4 H 2 PO 4 ) Monopotassium phosphate, KDP (KH 2 PO 4 ) Lithium niobate (LiNbO 3 ) Lithium tantalate (LiTaO 3 ) Cadmium telluride (CdTe) Named for Friedrich Pockels ( ), who discovered this effect in From (1-2), the third term is negligable so that: KC I Yahya - Universiti Kebangsaan Malaysia ….(1-4)

Pockels and Kerr effects KC I Yahya - Universiti Kebangsaan Malaysia

Pockels and Kerr effects Kerr effect and medium If the material is centrosymmetric, as is the case for gases, liquids and certain crystals, n(E) must be even symmetric function since it must be invariant to the reversal of E. From (1-2) the first derivative vanishes, so that r is 0 whereupon: Named for John Kerr ( ), who discovered this effect in Kerr medium - most gas and liquids and some crystals e.g. nitrotoluene (C 7 H 7 NO 2 ) and nitrobenzene (C 6 H 5 NO 2 ).nitrotoluenenitrobenzene These are frequently used to modulate light, since the Kerr effect responds very quickly to changes in electric field Kerr cell may require voltages as high as 30 kV to achieve complete transparency KC I Yahya - Universiti Kebangsaan Malaysia ….(1-5)

Pockels and Kerr effects Kerr effect and medium The refractive index as a function of E can be visualised as: KC I Yahya - Universiti Kebangsaan Malaysia

Electro-Optic Modulators and Switches KC I Yahya - Universiti Kebangsaan Malaysia Phase Modulator ….(1-6) ….(1-7) ….(1-8)

Electro-Optic Modulators and Switches E can be applied perpendicular to the light propagation direction (transverse modulators) or parallel to the light (longitudinal modulators), in which case d = L r depends on directions of both light propagation and electric field (anisotropic medium). Speed is limited by capacitance and the finite transit time of light (you can’t switch until the previous light pulse has passed through the modulator) Practical bulk devices can achieve speeds up to several GHz … integrated optical devices can go even faster, to about 100 GHz. KC I Yahya - Universiti Kebangsaan Malaysia Phase Modulator

Electro-Optic Modulators and Switches KC I Yahya - Universiti Kebangsaan Malaysia Phase Modulator

Electro-Optic Modulators and Switches KC I Yahya - Universiti Kebangsaan Malaysia Dynamic Wave Retarder ….(1-8) ….(1-9)

Electro-Optic Modulators and Switches KC I Yahya - Universiti Kebangsaan Malaysia Dynamic Wave Retarder ….(1-10)

Electro-Optic Modulators and Switches Quarter wave retarder/ waveplate as circular polarizer: KC I Yahya - Universiti Kebangsaan Malaysia Dynamic Wave Retarder

Electro-Optic Modulators and Switches Mach-Zehnder interferometer is a device used to determine the relative phase shift variations between two collimated beams derived by splitting light from a single source. Used to measure phase shifts between the two beams caused by a sample or a change in length of one of the paths. A light beam is first split into two parts by a beamsplitter and then recombined by a second beamsplitter. Depending on the relative phase acquired by the beam along the two paths the second beamsplitter will reflect the beam with efficiency between 0 and 100%. KC I Yahya - Universiti Kebangsaan Malaysia Mach-Zehnder interferometer

Electro-Optic Modulators and Switches A slight tilt of one of the beam splitters, or light interaction from sample in one of the branch will result in a path difference and a change in the interference pattern. The Mach-Zehnder interferometer can be the basis of a wide variety of devices, from sensors to optical switches. KC I Yahya - Universiti Kebangsaan Malaysia Mach-Zehnder interferometer

Electro-Optic Modulators and Switches Phase delay or wave retardation alone does not affect light intensity By placing a phase modulator in one branch of a Mach-Zehnder interferometer, it can function as an intensity modulator. KC I Yahya - Universiti Kebangsaan Malaysia Intensity Modulator: Use of a Phase modulator in an interferometer

Electro-Optic Modulators and Switches KC I Yahya - Universiti Kebangsaan Malaysia Intensity Modulator: Use of a Phase modulator in an interferometer ….(1-11) ….(1-12)

Electro-Optic Modulators and Switches KC I Yahya - Universiti Kebangsaan Malaysia Intensity Modulator: Use of Retarder Between Crossed Polarizers ….(1-13)

Electro-Optic Modulators and Switches By changing V can be varied between 0 (shutter closed) and 1 (shutter open). Or by operating the device at transmittance around 0.5, it can act as linear modulator as well. Done by controlling the phase retardation via voltage. Practically, max transmittance is smaller than 1 due to losses by reflection, absorption and scattering. The min transmittance is more than 0 because of misalignment between polarizers and crystal axes. The ratio between max and min transmittances is called the extinction ratio. Ratios higher than 30 dB is possible (1000:1) KC I Yahya - Universiti Kebangsaan Malaysia Intensity Modulator: Use of Retarder Between Crossed Polarizers

Electro-Optic Modulators and Switches KC I Yahya - Universiti Kebangsaan Malaysia Optical Scanners ….(1-14)

Electro-Optic Modulators and Switches 1. Directional Couplers 2. Spatial Light Modulators 3. Optically Addressed Electro-Optic Spatial Light Modulator 4. Pockels Readout Optical Modulator 5. Incoherent-to-Coherent Optical Converters KC I Yahya - Universiti Kebangsaan Malaysia Other Electro-Optics Devices

Photorefractivity The photorefractive effect is a nonlinear optical effect seen in certain crystals and other materials that respond to light by altering their refractive index. The effect can be used to store temporary, erasable holograms and is useful for holographic data storage. Different but similar to Photodetectors from photoconductive materials When photorefractive material exposed to light, free charge carriers are generated like in photoconductor. These carriers diffuse away from positions of high photo intensity, leaving behind fixed charges of the opposite sign. The free carriers are trapped by ionized impurities at other locations, depositing their charges there as they recombine The result is the creation of an inhomogenous space-charge distribution that can remain in place for a period of time. The charge distribution creates internal electric field pattern that modulates the local refractive index via Pockels electro-optic effect. KC I Yahya - Universiti Kebangsaan Malaysia

Photorefractivity The image may be accessed optically by monitoring the spatial pattern of the refractive index using a probe optical wave. Illumination by uniform light or by heat will erase the image. Can be used to record and store images. Important materials: barium titanate (BaTiO 3 ), bismuth silicone oxide (Bi 12 SiO 20 ), lithium niobate (LiNbO 3 ), potassium niobate (KNbO 3 ), gallium arsenide (GaAs) and strontium barium niobate (SBN). Applications Real time holography – use reference wave to reproduce stored images Wave mixer – use in interferometer. KC I Yahya - Universiti Kebangsaan Malaysia

Photorefractivity KC I Yahya - Universiti Kebangsaan Malaysia

Electroabsorption Electroabsorption – change of the absorption characteristics of a medium in response to an externally applied electric field. Bulk semiconductor – external E results in tunneling, which extends to the absorption edge into the forbidden gap. The bandgap energy of the material is thus reduced below that provided by the band tail and the Urbach tail so that hv 2 < hv 1 Known as the Franz-Keldysh effect. Shift absorption spectrum to longer wavelentghs. KC I Yahya - Universiti Kebangsaan Malaysia

Electroabsorption In semiconducting multiquantumwell (MQW), the effect is more pronounced. Similar to Franz-Keldysh, but E applied in the direction of the confinement gives rise to additional phenomena known as Quantum-Confined Stark Effect (QCSE) Energy difference between the conduction and valence band energy levels decreases with increasing E (hv 2 < hv 1 ) The band tilt causes the locations of the wavefucntions to shift toward the edges of the well Exciton ionization is inhibited and exciton energy levels remain unbroadened even at high field levels, since the electron and hole remain in proximity by virtue of the confinement. Excellent characteristics: High speed, large extinction ratios, low drive voltages, and low chirp. KC I Yahya - Universiti Kebangsaan Malaysia

Electroabsorption Quantum-confined Strak Effect KC I Yahya - Universiti Kebangsaan Malaysia

Electroabsorption Electroabsorption effects can be used in optical electroabsorption modulators, switches and retarders. KC I Yahya - Universiti Kebangsaan Malaysia

Liquid Crystal Display A liquid crystal variable retarder (LCVR) uses a variable voltage to control the polarization state of light passing through a birefringent crystal. Birefringent - optical property of a material having a refractive index that depends on the polarization and propagation direction of light KC I Yahya - Universiti Kebangsaan Malaysia Liquid Crystal Variable Retarders (LCVR)

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia Liquid Crystal Variable Retarders (LCVR)

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia Liquid Crystals – Structure or Phases

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia Liquid Crystals Structure or phases

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia Twisting Liquid Crystals

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia Twisted Nematic Liquid Crystal Modulator/Switch

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia Twisted Nematic Liquid Crystal Modulator/Switch

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia Electro-Optic Response of twisted nematic Liquid Crystal cell

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia Transient response of twisted nematic Liquid Crystal cell

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia Reflective LCD (Passive)

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia LCD Adressing

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia LCD - Active Display

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia LCD - Active Display

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia LCD – Active Display Backlight

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia LC Modulator – Privacy Glass and Electrochromic Windows

Liquid Crystal Display KC I Yahya - Universiti Kebangsaan Malaysia LC Modulator – Privacy Glass and Electrochromic Windows

Integrated Electro-Optic Circuit KC I Yahya - Universiti Kebangsaan Malaysia