FW6C.7.pdf Frontiers in Optics Postdeadline © OSA 2013
Scanning 3-D IR Imaging With a GaN Photodiode Using
Nondegenerate Two-photon Absorption
Himansu S. Pattanaik, Matthew Reichert, Honghua Hu, David J. Hagan, and Eric W. Van Stryland
College of Optics & Photonics: CREOL & FPCE, University of Central Florida, 4000 Central Florida Blvd., Orlando, Florida 32816-2700, USA
hagan@creol.ucf.edu
Abstract: We demonstrate scanning 3-D IR imaging using non-degenerate two-photon absorption
(2PA) in an uncooled GaN photodetector. The SNR of the technique should improve for longer
imaging wavelengths due to increased nondegeneracy, which enhances 2PA.
OCIS codes: (190.0190) Nonlinear optics; (190.4180) Multiphoton process; (110.3080) Infrared imaging
1. Introduction
Gated detection of IR pulses using nondegenerate (ND) two-photon absorption (2PA) in semiconductors occurs
through instantaneous absorption of two-photons of very different energies. We have observed theoretically [1] and
experimentally [2] large increases of the 2PA coefficient with increasing nondegeneracy in several direct-gap
semiconductors. Based on this, we reported sensitive, room temperature, infrared (IR) detection using the enhanced
2PA in an uncooled GaN photodiode (
= 3.39 ) with extremely nondegenerate (END) pairs of photons [3].
In this paper, we extend our work to realize three-dimensional (3-D) IR imaging based on gated detection of IR
pulses in an uncooled GaN p-i-n photodetector. The voltage on the photodetector is obtained from the 2-photon
generation of carrier density N, and is linear in the irradiance of each input beam as seen from Eq. 1, [3]
=2
2
�
;
�
ℏ
=2
�
1
4
2
�
ℏ
;
ℏ
�
(1)
2
(
1
;
2
)=
(
1
+
2
−1)
3
2
�
2
7
1
2
2
2
�
1
1
+
1
2
�
2
(2)
where
, ℏ
, and
, ℏ
are irradiances of the gating and signal pulses, respectively,
2
�
;
� is the ND-2PA
coefficient for optical frequencies
and
,
is the bandgap energy,
and
are refractive indices at
and
, respectively,
is the Kane energy, and K = 3100 cm GW
-1
eV
5/2
is a material independent constant.
2. Experimental results and discussion
The effect of 2PA in the photodetector is simultaneous absorption of photons of largely different energies (ND-2PA)
which creates free carriers. In our experiment a “gate” pulse is a high-energy photon of wavelength 390 nm
(3.18 = 0.94
) sensitizes the detector to low-energy photons (signal pulse), provided the sum of the two
photon energies exceeds the band gap. In this case, the signal wavelength is 1600 nm (0. 78 = 0.23
).
Figure 1 shows a schematic of this single-pixel 3-D IR imaging with a GaN p-i-n photodetector. The 1600 nm signal
pulse is focused onto the object and the
collected scattered light from the object is
imaged onto a GaN p-i-n photodetector
where it is spatially and temporally
overlapped with the 390 nm gate pulse for
the 2PA. Both wavelengths are derived from
the same laser source, which runs at 1 kHz.
To suppress the unwanted signal from
carriers due to degenerate two-photon
absorption (D-2PA) of the gate, we
modulate the IR beam at 285 Hz and use
synchronous detection to detect only the
photogenerated carriers due to ND-2PA.
The voltage recorded is obtained from the
cross-correlation measurements of the gate
and signal pulse when they temporally
overlap (zero delay) at the detector. To
obtain the image, the object is raster scanned
Figure 1. Experimental configuration for single-pixel 3-D IR imaging. L, lens;
P, polarizer; BS, beam splitter; M, mirror.