N.Kukhtarev, T.Kukhtareva, P.Land, J.H. Caulfield, and J.Wang Holographic Photo Electro-Motive Force in CdTe:V using Visible and IR Lasers N.Kukhtarev, T.Kukhtareva, P.Land, J.H. Caulfield, and J.Wang

OVERVIEW ● Abstract ● Photo-Electro Motive Force (EMF) ● Experimental Materials ● Modeling of optical schemes ● Experimental Setup ● Results and Conclusions ● Applications and Future Studies 4/28/2019

ABSTRACT Holographic photo-EMF experiments were conducted on CdTe:V crystals using a HeNe laser with 632 nm in wavelength and between 10-50 mW of power, and IR CW laser with 1064nm in wavelength and 100 mW of power. Crossing two beams using a beamsplitter and a radial grating we produce the interference pattern between the beams. This interference pattern will induce in the CdTe:V crystal photo-ionization which lead to the modulation of conductivity and the electric field inside of the crystal and results in the holographic photocurrent. This technique can be used for fast optical phase modulation sensing and can be applied for improvement of biomedical imaging. 4/28/2019

Experimental Motivation and Method/Procedure Experiments with interferometers using IR radiation. Usage of CdTe:V Crystal. For realization of the interferometric scheme we have used our standard optical components for visible light. Dynamic holographic grating was recorded in the photorefractive CdTe crystal by the parallel components of the two beams. Materials, sensitive to the spectral regions (IR and red) and with fast response will be analyzed and tested. 4/28/2019

Experimental Setup 4/28/2019

Experimental Setup and Materials Experimental setup for Photo EMF registration 1 – HeNe/IR laser 2 – Diffraction grating 3 – filter 4 – vibrating mirror 5 – functional generator 6 – mirror 7 – photorefractive CdTe:V crystal 8 – amplifier 9 – oscilloscope 1 2 3 4 5 6 7 8 9 8 7 9 6 1 2 3 4 4/28/2019

Photo-Electromotive Force (EMF) Light induced electromotive force or the energy per unit charge that is converted into electrical energy. The Interference pattern produced photo-ionization of impurities that result in the spatial modulation of conductivity and electric field inside the crystal. 4/28/2019

Basic Theoretical Equations The system of equations for photogenerated mobile charged carriers has the following form: n is the electron concentration N is the photosensitive ionized centers fixed in space E is the electric field 4/28/2019

Basic Theoretical Equations e is the effective charge of the carrier µ is the mobility D is the diffusion coefficient of the mobile carriers N0 is the total concentration of photosensitive centers NA is the concentration of the compensating centers (acceptors) g is the optical generation rate  is the thermal generation rate r is the recombination coefficient, 0 is the dielectric permittivity of vacuum  is the relative dielectric constant. 4/28/2019

Small Contrast Approximation Basic equations simplify for the case of small contrast interference pattern: m is the modulation index (intensity contrast). k is the grating vector. I0 is the average spatial intensity.  is the frequency detuning between laser beams. 4/28/2019

Theoretical Approach For the pulsed step-like phase modulation the solution will be (for the front-edge, phase jump): For the rear end of the pulse we should change sign of phase jump δ. : 4/28/2019

Theoretical Approach For the ramp phase modulation ( ) we get: 4/28/2019

Theoretical Approach The total current and the electric field obeys Maxwell’s equations: The gradient of the current, J, is 0 and the curl of the Electric field, E, is 0 4/28/2019

Results Modulation value = 250 mV, f = 5 Hz, amplification = 1010 4/28/2019

Results Holographic photo-EMF with the step-like phase modulation with different pulse rate a) I Hz,b)5 Hz. 4/28/2019

Results Holographic DC current produced by the moving fringe pattern in CdTe:V crystal using HeNe laser (632 nm). 4/28/2019

Conclusions The EMF signals rapidly decreases with the growing of the modulation frequency mainly due to the bandwidth of the amplifier for large amplification (x 10^9) . Holographic DC current depends on the frequency shift between two beams, or on the speed of the fringe movement proportional to the applied voltage on the driving motor. And electrical (holographic EMF) may be used for detection of small phase modulation. 4/28/2019

Applications and Future Studies Detection of the electrical signal (holographic EMF) may provide information about vibration spectra of different objects, vibrating mirror and function generator. Radial grating, being beam splitter in this experiment, may be also used for generation of DC-current. This technique can be used for fast optical phase modulation sensing and can be applied for improvement of biomedical imaging. We will model both all-optical (beam-coupling schemes) and holographic photo-EMF methods of improving sensitivity of acousto-photonic imaging to find an optimal experimental scheme. We plan to test different materials, suitable for operation in visible: in green with BSO crystal (532 nm), and in IR (1064 nm) with the photorefractive semiconductor CdTe crystal. 4/28/2019